Galloper Wind Farm Project - Galloper Wind Farm proposal

Galloper Wind Farm Project 

Preliminary Environmental Report – Chapter 6: Project 

Details 

Galloper Wind Farm Limited

Document title Galloper Wind Farm Project 

Preliminary Environmental Report – Chapter 

6: Project Details 

Document short title Galloper Wind Farm PER 

Status Final Report 

Date 3 June 2011 

Project name Galloper Wind Farm Project 

Client Galloper Wind Farm Limited 

Reference 9V3083/R01/303424/Exet 

Drafted by Peter Gaches, Jon Allen et al. 

Checked by Rob Staniland, Peter Thornton 

Date/initials check RS PT 30.05.2011 

Approved by Dr. Martin Budd (Royal Haskoning) 

Date/initials approval MB 30.05.2011 

GWFL Approved by Kate Tibble (GWFL) 

Date/initials approval KT 1.06.2011 

A COMPANY OF 

Galloper Wind Farm PER - i - 9V3083/R01/303424/Exet 

Final Report 3 June 2011

CONTENTS 

Page 

6 PROJECT DESCRIPTION 1 

6.1 Introduction 1 

6.2 Outline Project Description 1 

6.3 Site Location 1 

6.4 Wind Resource 2 

6.5 Offshore Physical Characteristics 3 

6.6 Wind Farm Layout Options 4 

6.7 Foundation Systems 9 

6.8 WTG Support Structures 29 

6.9 WTG Support Structure Ancillary Equipment 32 

6.10 Wind Turbine Generators 34 

6.11 Ancillary Infrastructure 39 

6.12 Inter, Intra-array and Export Cables 42 

6.13 Cable Landfall and HDD Works 53 

6.14 Onshore Transition Pits 57 

6.15 Onshore Cabling 58 

6.16 Onshore Substation 61 

6.17 Project Programme 70 

6.18 Offshore Pre-construction and Construction 72 

6.19 Onshore Construction 75 

6.20 Commissioning 78 

6.21 Offshore Operations and Maintenance 79 

6.22 Onshore Operations and Maintenance 83 

6.23 Repowering 85 

6.24 Decommissioning 86 

6.25 References 87 

Galloper Wind Farm PER 9V3083/R01/303424/Exet 

Final Report - iii - 3 June 2011

6 PROJECT DESCRIPTION 

6.1 Introduction 

6.1.1 This Chapter presents the details of the Galloper Wind Farm (GWF) scheme 

and describes the construction, operation, maintenance and 

decommissioning components of the project, which would primarily comprise: 

� Wind turbine generators (WTG) and supporting tower structures; 

� WTG foundations with associated support and access structures; 

� Offshore platforms to support offshore substation(s), collection station 

and potentially accommodation facilities; 

� Meteorological mast(s); 

� Subsea inter and intra-array and export cables; 

� Cable landfall; 

� Onshore transition pits; 

� Onshore cabling from the landfall to the GWF substation; 

� Horizontal Directional Drilling (HDD) under two roads and across 

foreshore habitats; 

� 132kV onshore GWF compound and 132kV/400kV onshore 

transmission compound, which together are referred to as the “GWF 

substation”; 

� Onshore cabling from the 132kV/400kV transmission compound to the 

sealing end compounds; 

� Transmission sealing end compounds adjacent to existing electricity 

transmission towers (pylons); and overhead line connections to the 

towers; 

� Alterations to existing electricity transmission towers; 

� Temporary works and laydown areas; 

� Permanent and temporary access roads; and 

� Onshore cabling from the 132kV/400kV transmission compound 

connecting into the existing Greater Gabbard Offshore Wind Farm 

(GGOWF) 132kV cables (which run from Sizewell B to the GGOWF 

substation). 

The diagram on the following page shows the high level components of 

Galloper Wind Farm. 


Final Report Chapter 6 - Page 1 3 June 2011



6.1.2 The details provided below are based on the latest knowledge and 

information available at the time of production of the Preliminary 

Environmental Report (PER). In some cases, specific information relating to 

GWF is not available (for example, the exact method of construction will not 

be confirmed until contracts have been tendered and appointed). As such, 

construction methodology provided herein is based on similar projects. 

Complex and extensive detailed design and procurement processes take 

place over a lengthy period closer to construction on a scheme of this scale. 

6.1.3 For the purposes of the consent documentation, Galloper Wind Farm Limited 

(GWFL) will therefore provide a range of possible options with regard to 

certain aspects of construction methodology and project components. GWFL 

has provided sufficient flexibility in this project description to ensure that all 

realistic development scenarios are captured within this Preliminary 

Environmental Report (PER). Furthermore, it is noted that the design 

information provided is representative of the maximum scenario (in terms of 

size, number, depth etc). These two facets serve to ensure that consultees 

can have confidence that the initial assessments made within the PER will be 

on a scenario that represents the upper limit (or realistic worst case) of what 

may actually take place (as discussed in Chapter 5 EIA Process). It is 

noted, that whilst these upper ranges have been assessed, in the final design 

GWFL may seek to develop less than these maximum values. 

6.2 Outline Project Description 

6.2.1 As outlined in Chapter 1, the GWF project comprises a development of up to 

140 WTG, with a maximum capacity of up to 504MW encompassing an area 

of 183km 2 comprising up to three distinctly identifiable areas and inter-array 

cabling (Development Areas A, B and C – see Figure 1.1 in Chapter 1). 

6.2.2 Detail of the specific project components currently under consideration are 

provided throughout this Chapter, along with descriptive information on the 

methods associated with the construction, operation and decommissioning of 

these components. This information has been used to inform the technical 

chapters contained within this PER and is considered to represent the 

Project’s ‘consent envelope’. 

6.2.3 The technical elements of the Project give rise to the potential for multiple 

options in accordance with the Project design parameters (e.g. for WTGs and 

foundations). The implications for the PER are discussed within Chapter 5 

under the Rochdale Envelope principle. 

6.3 Site Location 

6.3.1 Located approximately 27km off the Suffolk coast, at the nearest point, the 

GWF project lies immediately adjacent to the existing Greater Gabbard 

Offshore Wind Farm (GGOWF) project (see Figure 1.1 in Chapter 1). 



6.3.2 The export cable corridor would connect from the offshore development to a 

landfall south of Sizewell on the Suffolk coast, adjacent to the route of the 

existing GGOWF export cables. The corridor shown in Figure 1.1 shows the 

export carroidor to the North of GGOWF, but the export cables will come into 

shore in the GGOWF export cable corridor. 

6.3.3 The onshore GWF substation would comprise a new GWF 132kV compound 

and also a new 132kV/400kV transmission compound, as described in 6.1.1. 

The two compounds would be located alongside each other and together are 

referred to as the GWF substation. 

6.3.4 The GWF substation is located near Sizewell, approximately 1km inland on 

the Suffolk coast. It would be situated to the north of Sizewell Gap, 

immediately to the west of the existing GGOWF substation site (see Figure 

1.2 in Chapter 1). 

The onshore transition pit(s) would be located in land to the south of Sizewell 

Gap with onshore cabling from there to the proposed GWF substation. There 

would be a need for additional cabling between the transmission compound 

and the sealing end compounds and also between the transmission 

compound and the existing GGOWF cables (see Figure 1.2). 

6.4 Wind Resource 

6.4.1 To minimise initial intrusive works, it was established that the current 

meteorological mast located on the GGOWF site (4km from the western tip of 

GWF Area A) presented current and historic data which would be 

representative of the GWF site. 

6.4.2 The first measurements were recorded by the mast in August 2005. The 

mast continues to collect data and to date has recorded over five years of 

data. Measurements are at a range of heights between 42.5m and 86m 

above mean sea level (amsl). 

6.4.3 Using the data set from August 2005 to July 2010, a mean wind speed of 

9.4m/s was calculated at 86m amsl (which is indicative of conditions at hub 

height), using the mean of monthly means method to account for seasonal 

variation. 

6.4.4 Wind direction over this period was predominantly from the southwest (Plate 

6.1). 



Plate 6.1 Long term wind rose for GGOWF and GWF 

6.5 Offshore Physical Characteristics 

6.5.1 GWFL has developed a sound appreciation of the physical conditions 

(bathymetry, seabed sediments and shallow geology) of the GWF site. This 

has been achieved by site investigations including a geophysical survey 

campaign and utilising the detailed knowledge gained from developing the 

adjacent GGOWF project. Chapter 10 provides a detailed account of these 

physical parameters. 

6.5.2 The data is sufficiently detailed to enable GWFL to have confidence that the 

conditions present are conducive to the development of an offshore wind 

farm and associated cable infrastructure. Water depths vary significantly 

across the development area which would give rise to consideration of 

multiple foundation solutions to accommodate different depths, turbine types 

and ground conditions as presented in Section 6.7. 

6.5.3 The Outer Gabbard Bank (an open shelf linear tidal sand bank) is present 

within Area A (see Figure 1.1 in Chapter 1 Introduction). The sand bank 

would be avoided for environmental reasons. The presence of 

Paeolochannels (see Chapter 10) may also have a bearing on the final 

layout of the WTGs within Development Areas A, B and C. 

The current level of information is sufficiently detailed to enable a robust EIA 

to be undertaken. However, additional survey will be required to facilitate 

final detailed design in key areas including: 

� For foundation structures and scour protection (if required); 

� Subsea cables within the wind farm site and along the export cable 

route; and 



� Any jack-up vessel operations (vessels with legs that can be lowered 

to the seabed allowing the hull of the vessel to be raised clear above 

the water). 

6.5.4 With the adoption of a Front End Engineering Design (FEED) approach it is 

anticipated that these investigations would be conducted prior to the start of 

construction and would include: 

� Boreholes at a select number of foundation locations and along the 

export cable route; 

� Cone penetrometer testing (CPT) at a select number of foundation 

locations and along the export cable route; 

� Vibrocore sampling along the export cable route; 

� Plough trials along the export cable route; 

� Pre-lay grapnel runs along the export cable route; 

� Unexploded Ordnance (UXO) survey; and 

� Investigation of any obstructions to construction that are identified 

through geophysical survey. 

6.5.5 Geotechnical investigations both within the wind farm site and cabling areas 

would be to a depth below the design penetration depth of the foundation 

options and potential cable burial depths to give confidence in results, design 

and construction approach 

6.5.6 Geophysical survey investigations would be carried out in accordance with 

relevant Maritime and Coastguard Agency (MCA) guidance (MGN 371) and 

International Hydrographic Organisation (IHO) standards (IHO Order 1a) and 

encompass the project footprint. Further pre-construction survey may 

include further higher coverage (200%) multibeam surveys around each 

foundation location followed by a Remotely Operated Vehicle (ROV) 

inspection if anything relevant for UXO assessment is identified. Ultrasonic 

investigations and acoustic coring local to foundation locations would be 

undertaken for scour assessment. 

6.5.7 Further detail of this pre-construction activity (in terms of vessel types and 

durations) is provided in Section 6.19. 

6.6 Wind Farm Layout Options 

6.6.1 The maximum turbine population of the wind farm for any given turbine size 

would be primarily driven by optimising the space between WTGs to gain 

maximum effect from the wind resource. In the prevailing wind direction, the 

WTGs would be spaced at a minimum of 8 rotor diameters apart. At 90 

degrees to the prevailing wind the WTGs would be spaced at a minimum of 6 

rotor diameters apart (termed as an 8 by 6 grid). 

6.6.2 WTGs generating capacity is driven by their swept area (determined by blade 

length) and internal generating components. Hence turbines of the same 



otor diameter may not have the same MW capacity. The WTG under 

consideration (Table 6.4 later in this document) in this PER range from 107m 

to 164m rotor diameter and hub height 79m to 120m. These have different 

technical and dimensional characteristics but the site would not incorporate 

any more than 140 WTGs nor have a greater total output than 504MW. 

6.6.3 Example indicative layouts to show how rotor diameters of 107m, 120m and 

150m might be laid out across the site area are presented in Figures 6.1 to 

6.3. These layouts are based on the physical, environmental and human 

information obtained for the site to date and take into account the known 

constraints posed by these parameters. These indicative layouts have been 

used to inform the relevant studies carried out within the EIA for the GWF 

project. Parameters which are used to influence layouts include: 

� Water depths; 

� Seabed geology; 

� Ship wrecks and other obstructions; 

� Wind resource assessment; 

� Ornithological recommendations; 

� Stakeholder feedback; 

� Proximity to the future East Anglia Offshore Wind Farms; and 

� Proximity to the existing GGOWF. 

6.6.4 As a result of the above the final layout would not be fixed until FEED work 

and tendering has been completed post consent. Therefore these layouts 

represent reference layouts that may be used when considering the multiple 

variations available. 

6.6.5 In practice, it is unlikely that any of these indicative layouts would be 

constructed exactly as shown in Figures 6.1 to 6.3, as the necessary 

detailed work post-consent will inevitably produce a different layout. The 

Development Consent Order will not contain any layout plans for the offshore 

infrastructure, simply the red line areas within which the infrastructure can be 

built, together with various restrictions (maximum number of turbines, 

maximum height etc). 

6.6.6 The final built layout will not necessarily be evenly spread across entire 

development areas. Also, not all of the development areas will necessarily 

be used, for example Area C. This is discussed in more detail in Chapter 5 

EIA Process. 









6.7 Foundation Systems 

6.7.1 There are four foundation options under consideration for supporting the 

WTG structures; monopiles, space-frame structures (Jacket/Tripod), gravity 

base structures (GBS) and monopod suction-buckets. The requirement for 

this level of flexibility in options is principally driven by the large variation in 

water depths across the site, differing geological conditions across the site 

and potential for use of different turbine types. 

6.7.2 Given the varying site factors it is possible that multiple foundation solutions 

would be utilised to address commercial uncertainty on the project. 

6.7.3 Determination on the final foundation type or types to be used for the project 

would be made following FEED work post consent. Consideration would be 

given to: 

� Assessment of foundation impact on receiving environment; 

� Geological profile of the seabed; 

� Water depth; 

� Geotechnical properties of soil; 

� Site metocean conditions (wind, wave, current and tidal regime); 

� WTG selection; 

� Access and maintenance requirements; 

� Foundation material, fabrication, transportation and installation costs; 

and 

� Availability of foundation supply chain components (apparatus / lifting 

vessels etc). 

6.7.4 The following sections provide an overview of the different foundation options 

under consideration together with their typical installation process. 

Furthermore, for each foundation type, discussion is provided on the 

situations in which each option might be deployed to enable a realistic worst 

case scenario to be developed for the EIA (see Chapter 5 EIA Process). 

Monopile foundations 

The assessments presented in the PER are on the basis of monopiles in water depths 

of up to 35m. At the time of writing GWF are also considering the possibility of using 

monopiles in up to 45m water depth. This is likely to represent the ‘worst case’ for 

assessments relating to underwater noise impact (for example, those detailed in 

Chapter 14 – Fish and Shellfish Resource and Chapter 15 – Marine Mammals). The 

PER, as written, has assumed a maximum water depth of up to 35m for monopiles. 

The final submitted ES will include assessments based on up to 45m. 



General overview 

6.7.5 Monopile foundation systems have been the mainstay of the UK offshore 

wind industry to date and are the foundation solution that has been adopted 

for the adjacent GGOWF project. 

6.7.6 Monopile foundations (Plate 6.2) comprise a single, large diameter hollow 

steel pile that relies on the soil to provide lateral resistance to loading. A 

separate Transition Piece (TP) would be installed on top of the monopile to 

provide a horizontal levelled platform for the WTG structure support. The TP 

would be lifted onto the monopile and a grout injected into the interface to 

bond the TP to the monopile structure. 

6.7.7 The size (diameter and length) of the monopile depends upon the water 

depth, metocean conditions and ground conditions as well as the size of 

WTG that it supports. Based on the current knowledge of physical conditions 

at the GWF site and experience from the adjacent GGOWF project, GWFL 

consider that the WTG options under consideration would have a maximum 

size of monopile of 7m in diameter by 90m in length. 

Plate 6.2 Indicative monopile foundation 

Work 

platform 

Intermediate 

platform 

External 

J tubes 

Grouted 

connection 

Scour protection 

(if required) 

Turbine tower 

Boat landing 

/ ladder 

Transition piece 

Monopile 



Installation process 

6.7.8 Installation of monopiles would be carried out using a dedicated heavy lift 

vessel (HLV) or a jack-up barge. 

6.7.9 A crane would be used on the installation vessel/jack-up barge to manoeuvre 

the monopile into a guide frame that supports the monopile during the 

installation process (see Plate 6.3). 

Plate 6.3 Monopile installation 

Source: GGOWL, 2011 

6.7.10 There are two methods of installation for the monopile that are considered to 

be applicable for GWF: 

� Driven to full penetration; and 

� Drive / drill / drive. 

Driven to full penetration 

6.7.11 The driven method initially allows the pile to sink under its own weight and 

then the required penetration depth into the seabed would be achieved using 

a hydraulic hammer installed on top of the monopile as depicted in Plate 6.4. 



Plate 6.4 Monopile driven installation 

Source: GGOWL 2011 

Drive / drill / drive 

6.7.12 The monopile would be sunk under its own weight, and then driven into the 

seabed using a hydraulic hammer until a pre-determined refusal point is 

reached. The first refusal would be gauged by the blow count and travel of 

the pile. The pile hammer would be then removed and a drilling rig system 

would be installed within the monopile. 

6.7.13 The drill continues down to a location typically 0.5m above the final design 

elevation of the toe of the monopile. The drilling rig would then be removed 

and the pile hammer placed on the monopile once again. The monopile 

would then be driven into the drilled cavity until the required penetration is 

obtained. 

6.7.14 The cutting action of the drill would create spoil or ‘arisings’. These arisings 

would be lifted from inside the monopile by using a suction pump unit. The 

arisings would then deposited and left in-situ on the seabed around the 

monopile. 

Deployment philosophy 

6.7.15 As detailed in Table 6.1, GWFL’s assumption based on site conditions and 

previous experience is that GWF would promote the deployment of 

monopiles in water depths up to - 45mb LAT. 

6.7.16 Up to two piling vessels and rigs may be present and operational at the site 

at any one time. 



Gravity base structure foundations 


6.7.17 Gravity Base Structure (GBS) foundations are typically constructed in steel or 

concrete and use their weight to remain stable on the seabed. The GBS 

foundation holds position on the seabed through frictional forces, which is 

often enhanced by the provision of grout under the base of the structure and 

skirts around the structure’s perimeter. These skirts (see Plates 6.5 and 6.6) 

move the friction plane downwards from the relatively weak surficial 

sediments into a stronger undisturbed soil layer below. The skirts also serve 

to ensure that any scour that may occur around the perimeter does not 

undermine the structure. 

6.7.18 GBS foundations would either be conical or column based in shape, as 

depicted in Plates 6.5 and 6.6. Maximum dimensions for these structures 

are provided in Table 6.1. 

6.7.19 GBS foundations are used extensively in European Offshore Windfarms 

(such as Vindeby, Middelgrunden and Nysted in Denmark). GBS 

foundations are foreseen as part of a diverse foundation supply chain 

required to meet the offshore wind industry’s contribution to UK government’s 

renewable energy targets. 

6.7.20 The GBS foundations will have a transition structure, (see Section 6.8), 

which would be an integrated part of the foundation and upon which the 

tower would be mounted. This could be a flanged bolted connection. 

Plate 6.5 Indicative conical GBS foundation 

Intermediate 

Platform 

Work Platform 

Boat Landing / Ladder 

Skirt 

Under-base Grout 

Shaft 

Turbine 

Tower 

Internal J-tube & 

Ballast 

Scour Protection 



Plate 6.6 Indicative column GBS foundation 

Work 

Platform 

Intermediate 

Platform 

External J-tubes 

Skirt 

Turbine 

Tower 

Boat Landing 

Shaft 

Scour protection 

Under-base 

grout 


6.7.21 GBS foundations may require a degree of seabed preparation prior to their 

installation to ensure they are laid on a surface capable of supporting the 

structure adequately. Grout is used to help bond the foundation base to the 

seabed. The maximum volume of grout used in this process would be 

estimated at approximately 1,590m 3 per foundation. 

6.7.22 Should seabed preparation be required, a maximum of 2m in thickness of 

material (2,000m 3 ) would be removed from the seabed at the location of each 

foundation. Inert levelling material (comprising stone or aggregate) would 

then be deposited in place prior to installation of the foundation. 

6.7.23 The arisings produced from the seabed levelling (achieved through dredging) 

would be disposed of in-situ. The arisings likely to be produced from the 

seabed preparation for GBS foundations comprise deposits of sand and 

gravel with occasional potential for London clay where present close to the 

surface. 

6.7.24 The GBS foundations can either be transported to site on a dedicated barge 

or as a self floating tow. 



6.7.25 Installation would typically be undertaken through the use of a heavy lift 

vessel where the gravity base is lowered on to the prepared seabed, or if 

floated to site under their own buoyancy, then ballasted onto the seabed 

using tug-boats to control operations. Material used to ballast the foundation 

would typically comprise rock, gravel, shingle or sand (depending on the 

weight required). 

6.7.26 Following deployment of the GBS, a fall pipe vessel would install scour 

protection about the base of the foundation to prevent seabed scour about 

the base of the structure (a maximum of 1,600m 3 per foundation). 


6.7.27 As detailed in Table 6.1, GBS may be deployed in water depths up to -45mb 

LAT where ground conditions are suitable. 

Plate 6.7 Indicative GBS installation process 

Space-frame foundations 


6.7.28 Space-frame foundations encompass structures that are commonly lattice 

type or consist of main legs and braces. The most common space-frame 

structure is a space frame (jacket) foundation that has been widely deployed 

to successfully support offshore oil and gas platforms. Tripods are a 

monopile/space frame hybrid in which a large central column is supported at 

its base by a frame piled at its 3 corners. 



6.7.29 This system is used to support WTGs in deeper water (typically greater than - 

45mb LAT) or to support heavier WTGs i.e. 5MW to 7MW class. Spaceframes 

are generally stiffer than a monopile and more transparent to wave 

loading. Space-frames are typically installed without scour protection given 

their small pile dimensions and therefore have low influence on tidal and 

wave induced currents. 

6.7.30 The space-frame consists of a steel frame with slender leg members 

(typically four legs but three legs would also possible), with steel cross 

bracings. The members are steel tubes with bracings and would be prefabricated 

onshore. The space frame can be attached to the seabed by long 

cylindrical piles approximately 40m to 60m in length. Plate 6.8 shows a four 

leg post piled space frame structure. Suction cans may also be used under 

the leg of each support structure, resisting force by friction and active suction 

with the seabed. They are normally made of steel, cylindrical, have a closed 

top and are typically approximately 10m in diameter. Plate 6.9 shows the 

indicative suction can substructure. 

6.7.31 The transition structure for space-frame foundations would be an integrated 

component of the foundation as described under GBS. 

Plate 6.8 Indicative Space frame lattice substructure 

Work 

External J- 

Pile 

Transition 

Tower 

Central column 

Intermediate 

Boat landing 

Mud mats 


Final Report Chapter 6 - Page 16 3 June 2011 

Pin

Plate 6.9 Indicative suction can substructure 


6.7.32 A degree of seabed levelling may be required prior to the installation of the 

space-frame and this would be undertaken using dredging equipment with 

high resolution sonar to ensure that the seabed level is within the required 

tolerances after the intervention works and prior to the installation of the 

space frame. The maximum volume of arisings per turbine depending on 

local conditions is detailed in Table 6.1. 

6.7.33 Installation is usually carried out using a dedicated HLV or a jack-up barge 

(as detailed for monopiles). However, as this option is very much in a 

development phase it is highly possible that a combination of vessels would 

be used. 

6.7.34 The installation process for this system would be expected to take up to one 

day dependent on the number of legs associated with the space-frame 

foundation. 



Plate 6.10 Installation of tripod structure from HLV 

6.7.35 There are two options for the installation of the cylindrical pin piles for a 

space-frame structure. The pins are significantly smaller diameter than those 

used for the monopile concept (Table 6.1). For pre-piled space-frame 

solutions, pin piles may be installed prior to the space-frame, with the 

foundation then lowered onto the piles with the legs being inserted into the 

piles (Plate 6.11). Alternatively, the foundation would be placed onto the 

seabed and the piles driven through sleeves connected to the space-frame 

legs, as shown in Plate 6.12. Installation of suction can piles would be 

carried out using a jack-up barge or similar, that lifts and places them onto 

the seabed. Pumps would be connected to the suction cans and operated 

until the can ‘lid’ is flush with the seabed. Multipile suction can foundations 

may require rock ballast to meet design requirements. 

6.7.36 In both designs, the piles would be typically attached to the foundation by a 

grouted connection, although other appropriate connections may be used. 



Plate 6.11 Indicative pre-installed pile space-frame option 

Plate 6.12 Indicative driven pile space-frame option 

Side view of the spaceframe 

foundation on the 

seabed prior to the 

piles being driven in 

Driving Piles 

Space-frame foundation with 

the piles being driven into the 

seabed to secure the structure 


6.7.37 As detailed in Table 6.1, space-frame systems can be deployed in any water 

depths across the site where ground conditions are suitable. 

6.7.38 The maximum number of piles installed with a space-frame foundation 

system at any one time would be two. 



Monopod suction-bucket foundations 


6.7.39 Suction-buckets are tubular steel foundations that utilise the hydrostatic 

pressure difference and their deadweight to enable the bucket to penetrate 

the soil. This installation procedure allows the buckets to be connected to 

the rest of the structure before installation, enabling a reduction in the 

number of steps to the installation procedure. The system has been tried in 

practice in the Norwegian oil and gas fields in the North Sea (DOWEC, 2003) 

as well as at Horns Rev 2 offshore wind farm for the met mast foundation 

(see Plate 6.13). 

Plate 6.13 A typical suction-bucket foundation 

Source: DONGenergy (2009) 




6.7.40 Suction monopods may be floated to site under their own buoyancy using 

tug-boats or transported on a dedicated barge. Installation would be carried 

out using a dedicated HLV or a jack-up barge that lifts the monopod into the 

upright position and positions it onto the sea-bed. Pumps would be 

connected to the suction caisson and the ‘bucket’ penetrates into the seabed 

until the top surface of the base is flush with the seabed. Small volumes of 

seabed sediment may be extracted and deposited locally during this process. 


6.7.41 Suction-bucket foundations can be installed into water depths up to -45mb 

LAT. The maximum number of foundations installed at any one time would 

be two. 

Foundation detail summary 

Table 6.1 Summary of foundation parameters 

Foundation detail Monopiles Gravity 

base 

systems 

Space-Frame 

(tripod / jacket) 

Suction 

buckets 

Maximum water depth 45m 45m >45m 45m 

Maximum diameter at seabed 7m 45m 3m per pile 25m 

Column diameter 7m 10m N/A 9m 

Maximum No piles per 

foundation 

Maximum seabed footprint 

(per foundation) 

1 None Space frame: 8 

(2 piles per leg, 

up to 4 legs) 

38.5m 2 

Maximum No. legs 1 1 

1,590m 2 Space frame: 

57m 2 (7.1m 2 *8) 

Space frame: 4 1 

None 

490m 2 

Maximum penetration depth 50m 5m 70m 20m 

Maximum number of piling 

driven at any one time 

Maximum volume of grout (per 

foundation) 

Maximum volume of arisings 

(per foundation) 

1 None 2 None 

23m 3 1,590m 3 188m 3 N/A 

1,600m 3 

7,200m 3 1,300m 3 500m 3 



Table 6.2 Foundation build-out scenarios 

Build-out scenarios – Foundations 

Single foundation type (A) 

Single foundation type (B) 

Mixed foundation type 

Optimised smaller project on monopiles in 

shallow water 

Project on either space frame or suitable 

foundation for all water depths 

Up to two types of foundation used to 

enable development across all water 

depths 

Fabrication and transportation 

At the time of writing, the number of foundation fabrication plants and skilled 

labour force, whilst emerging, are still limited within the UK. Therefore such 

structures would commonly be brought in from overseas either directly to site 

or to a holding port. 

6.7.42 If the foundations are shipped to a holding port, there are two principal 

methods for transporting the foundations to their installation location: 

� Floating transportation procedure; or 

� Direct lift procedure. 

6.7.43 Floating transportation procedure requires sealing the toe of the pile and 

fitting a sealed lifting head at the top of the pile. The sealed lifting head 

serves as the towing point. The foundation system would then towed to site 

after being lifted into the water using a crane from the holding port key side. 

6.7.44 The foundation system would then be upended, using a combination of 

controlled ballasting and lifting to ensure stability and positioned in a guiding 

or gripper system. These systems are used to guide the foundation system 

to the pre-defined location on the seabed whilst maintaining verticality. 

6.7.45 The alternative transportation method is the direct lift, where the foundations 

are lifted from the quayside onto either a dedicated transportation vessel or 

the installation vessel itself, which would be used to transport the foundations 

to the offshore location. The foundations would then be lifted from the 

transportation vessel by the crane on the installation vessel and pitched into 

the installation guiding system. 



Foundation scour protection 

6.7.46 Scouring of soft surficial sediments may occur around foundation structures 

where localised effects on the hydrodynamic regime take place (see Chapter 

10). Such scouring erodes sediment leaving depressions (known as scour 

holes or scour tails) around the foundation structures. 

6.7.47 Experience from the adjacent GGOWF project, where similar seabed 

sediments and hydrodynamic conditions exist, would suggest that scour 

events sufficient to require dedicated protection are unlikely to be widespread 

(only 5 out of 140, or 3.5% of foundations at GGOWF will have scour 

protection installed to date). For the purposes of the GWF project a 

conservative approach has been applied to the likely percentages of 

foundations that may require scour protection (see Table 6.3). 

Table 6.3 Foundation scour detail summary 

Foundation detail Monopiles 

Maximum scour protection 

(radius per foundation/leg 

base) 


area (per foundation) 


depth (per foundation) 


volume (per foundation) 

Approximate percentage of 

foundations requiring scour 

protection 


volume 

50m 

Gravity base 

systems 

Space-frame Suction- 

buckets 



10m 

1,900m 2 790m 2 

2m 

3,900m 3 1,600m 3 

5% 

2m 2m 

10m 10m 

500m 2 

100% 10% 

27,300m 3 132,000m 3 

450m 2 

2m 

1,000m 3 950m 3 

100% 

8,500m 3 80,000m 3

6.7.48 The pre-construction geophysical survey (see Section 6.5) will ascertain the 

level of scour protection required for the GWF project. Surveying for any 

scour would continue beyond the construction phase of the project and would 

form part of the ongoing inspection regime of the wind farm. 

6.7.49 Should scour protection be deemed necessary, the following procedure 

would typically be adopted: 

� A filter layer of gravel would be installed prior to placement of the 

foundation 

� Rock or slate would be deposited at the base of the foundation 

structures after installation of the foundation. The rock placement 

would infill any scour pit, which may have developed, as well as 

building a profile above the seabed, referred to as a rock berm. The 

rock berm would be designed to remain stable for the full life of the 

structure under all forms of predicted environmental loading 

� The rock or slate would be placed by a vessel using a side tipping 

system or placed using a grab device; and 

� The base of the structure would be resurveyed to confirm that the 

required coverage and rock profile has been achieved. 

Foundation installation noise 

6.7.50 All marine construction activity generates some level of noise. The 

installation of the foundation systems, however, has the potential to generate 

significant noise levels. Underwater noise behaves very differently to 

airborne noise largely due to the high sound transmission speed within water 

(1,500m/s, as opposed to 340m/s for air). Given these considerations 

underwater noise is therefore specifically discussed in this Chapter. The 

effects of this noise on the sensitive ecological receptors (namely ornithology, 

fish and shellfish and marine mammals) are discussed in Chapters 12, 14 

and 15 respectively. 

6.7.51 The two foundation systems which can be expected to result in the highest 

noise levels would be those that have impact piling associated with their 

installation, namely monopiles and specific space-frame foundation 

structures that use pin piles. 

6.7.52 Impact piling involves a large weight or “ram” being dropped or driven onto 

the top of the pile, driving it into the ground. Usually double-acting hammers 

are used in which compressed air not only lifts the ram, but also imparts a 

downward force on the ram, exerting a larger force than would be the case if 

it were only dropped under the action of gravity. 

6.7.53 Airborne noise would be created by the hammer, partly as a direct result of 

the impact of the hammer with the pile. Some of this airborne noise would be 

transmitted into the water. Of more significance to the underwater noise, 

however, is the direct radiation of noise from the surface of the pile into the 



water as a consequence of the compressional, flexural or other complex 

structural waves that travel down the pile following the impact of the hammer 

(Subacoustech, in prep). Due to the high transmission properties of sound in 

water (1,500m/s, as opposed to 340m/s for air), noise generated from pile 

installation will transmit efficiently into the surrounding water column. 

Consequently these waterborne sound waves usually provide the greatest 

contribution to underwater noise (Subacoustech, In prep). 



6.7.54 At the end of the pile, force would be exerted on the substrate not only by the 

mean force transmitted from the hammer by the pile, but also by the 

structural waves travelling down the pile inducing lateral waves in the 

seabed. These may travel as both compressional waves, in a similar manner 

to the sound in the water, or as a seismic wave, where the displacement 

travels as Rayleigh waves (Brekhovskikh, 1960). The waves can travel 

outwards through the seabed, or by reflection from deeper sediments. As 

they propagate, sound will tend to “leak” upwards into the water, contributing 

to the waterborne wave. Since the speed of sound is generally greater in 

consolidated sediments than in water, these waves usually arrive first as a 

precursor to the waterborne wave. Generally, the level of the seismic wave 

is 10 to 20 decibels (dB) below the waterborne arrival, and hence it would be 

the latter that dominates the noise impact. 

6.7.55 Studies carried out to date (Nedwell et al., 2003; Nedwell et al., 2007; Parvin 

et al, 2006) indicate that the source level of the noise from impact pile driving 

operations is primarily and strongly related to the pile diameter. This 

probably results largely from the increased force that is required to drive 

larger piles and the improved noise radiation efficiency of larger piles 

(Subacoustech, In prep). 

6.7.56 For GWF the maximum sized piles considered are 7m in diameter (for 

monopiles) and 3m in diameter for space-frame foundations. There is 

currently no measured data for piles of 7m diameter, but using extrapolation 

from other pile diameters, it is possible to establish indicative noise levels at 

GWF piling operations (Subacoustech, In prep). 

6.7.57 Sound may be expressed in many different ways depending upon the 

particular type of noise, and the parameters of the noise that allow it to be 

evaluated in terms of an environmental effect. Those relevant to the GWF 

project are described in more detail below: 

� Peak to peak level - Usually calculated using the maximum variation 

of pressure from positive to negative within the wave. This represents 

the maximum change in pressure as the transient pressure wave 

propagates. Peak to peak levels of noise are often used to 

characterise sound transients from impulsive sources such as 

percussive impact piling. Measurements during offshore impact piling 

operations are commonly expressed as having a peak to peak source 

level of dB’s re 1μPa @ 1m, which describes the noise level 

experienced at 1m from the source (Subacoustech In prep). 

� Sound Pressure Level (SPL) - Normally used to characterise noise 

and vibration of a continuous nature such as drilling, boring, 

continuous wave sonar, or background aquatic noise levels. The SPL 

method measures the average unweighted level of the sound over the 

measurement period (Subacoustech In prep). 

� Sound Exposure Level (SEL) - When assessing the noise from 

transient sources such as blast waves, impact piling or seismic airgun 

noise, characterising the time period of the pressure wave is often 



addressed by measuring the total acoustic energy (energy flux 

density) of the wave. Sound Exposure Level (SEL) sums the acoustic 

energy over a measurement period, and effectively takes account of 

both the SPL of the sound source and the duration the sound is 

present in the acoustic environment (Subacoustech In prep). 

� M-Weighting – A method used to represent the levels of underwater 

noise perceived by marine mammals through filtering underwater 

noise data using generalised frequency weighting functions, which are 

designed to match the frequency response of different groups of 

marine mammals (Subacoustech In prep). 

6.7.58 In instances where a single defined noise is emitted underwater, sound 

pressure measurements may be expressed using the peak to peak level (i.e. 

dB re 1μPa @ 1m), which represents the noise level at a distance of one 

metre from the source. The actual level at the source (Source Level) may be 

different from that experienced at 1m. The Source Level may be quoted in 

any of the measures described above, for a piling source this is typically 

expressed as having a “peak to peak Source Level of dB re 1μPa @ 1m”. 

6.7.59 Based on this approach it is predicted that noise levels of 254dB re 1 µPa @ 

1m for 7m diameter piles and approximately 240dB re 1 µPa @ 1m for 3m 

diameter piles may be expected for the GWF project (see Plot 6.1). Note 

modelling to date has been carried out on 2m pin piles, but approximate 

values for 3m piles are drawn from interrogation of the Cubiclaw Fit line (see 

Plot 6.1) 

Plot 6.1 Predicted noise levels from GWF piling operations (Subacoustech in prep) 



6.7.60 The implications of the noise generated from construction activity on the 

receiving environment are assessed and discussed within the relevant 

technical Chapters of this report (namely, Chapter 13 Fish and shellfish 

resource, and Chapter 14 Marine Mammals). 

Foundation corrosion protection 

6.7.61 Any offshore metal structure (typicall steel), below the water level or in the 

splash zone will corrode freely without corrosion protection. Corrosion can 

reduce the structural integrity of the WTG support structure, hence corrosion 

protection is required. The design of the internal and external corrosion 

protection systems (CPS) utilised at GWF will be agreed with the Certification 

Bodies, namely Det Norske Veritas (DNV), Germanischer Lloyd (GL) or 

Lloyds Register, who will be responsible for certifying the subsea structure. 

Internal corrosion protection 

6.7.62 Internal corrosion prevention measures are required in subsea foundation 

types comprised of steel (i.e. all excluding concrete GBS foundations) as a 

result of the sea water that would be trapped inside the structures after 

installation. Corrosion in this instance would be predominately caused by 

microbial activity, both aerobic (with oxygen) and anaerobic (without oxygen) 

in nature. Sealing the tops of the foundation structures and removing the 

oxygen (by filling the space with an inert gas) would be the primary method to 

limit the growth of microbes. 

6.7.63 If through investigation the rate of microbial corrosion is deemed to be high, 

then additional measures could be undertaken after sampling the sea water 

captured in the foundation during the installation process. A biocide could 

then be chosen to inhibit the growth of the microbes responsible for 

corrosion. However the foundations would be designed to ensure that 

biocides do not need to be used initially, although in time internal 

investigations and water sampling could identify the need for biocide use. 

Biocides are used extensively in the offshore industry to control internal 

corrosion. If biocides are used the necessary licences would be sought with 

risk assessments and method statements put in place. 

External corrosion protection 

6.7.64 The external CPS, selected for the TP and foundation would be required to 

ensure protection against corrosion and to ensure that the design life of the 

project would be met. The external CPS would comprise two primary 

systems; an external coating system and a cathodic protection system 

provided by one of the following forms: 

� Galvanic CP, commonly known referred to as a sacrificial anodes; or 

� Impressed current cathodic protection (ICCP). 



6.7.65 The basic principle behind CPS is electrolysis which requires an anode (a 

source of negative ions), a cathode (a source of positive ions) and an 

electrolyte (a liquid which would conduct the ions, i.e. the sea). A CPS only 

operates on the outermost exposed surfaces of the steel structure at the 

molecular / atomic level, simplistically a protective force-field to prevent 

corrosion. 

6.7.66 The Galvanic CP or sacrificial anode system is used heavily in the oil and 

gas offshore industry. This system, as the name suggests, uses fixed 

anodes, usually zinc, magnesium or aluminium (or alloys of these metals) 

placed about the base of the submerged structure which remains there for 

the life of the equipment. As the anode is more easily corrodible, there is a 

continual electron flow from the anode to the cathode (steel structure) 

causing polarisation. The driving force for the CP current flow is the 

difference in electrochemical potential between the anode and the cathode. 

Galvanic corrosion protections are generally specified to provide adequate 

protection for the design life of the foundation. Due to the challenging nature 

of Cathodic Protection design, it may be necessary to replace the anodes 

within the foundation design life. 

6.7.67 The ICCP system is commonly found on boats and submarines. ICCP uses 

anodes connected to a directional current (DC) power source; current is 

supplied to the anodes causing the cathode (the steel structure) to become 

more electronegatively charged and therefore reduces its rate of corrosion. 

In this case reference electrodes on the structure are also required to monitor 

the electrical potential. 

6.8 WTG Support Structures 

Transition piece 

6.8.1 The TP connects the foundation to the WTG. The TP serves several 

different purposes as it can be used to house the necessary electrical and 

communication equipment and provide a landing facility for personnel and 

equipment from marine vessels. 

6.8.2 For space frames and GBS foundations the TP are often integrated to the 

foundation and hence discussed in the foundation section. TP are primarily 

used for monopiles where the secondary structures cannot be on the 

monopole (as it is driven) and, as such, need to be installed on a TP that also 

assists verticality via adjustment permitted in the grouted connection. 



Transitional piece: monopile and suction can structures 

6.8.3 The TP (Plate 6.14) would be 

Plate 6.14 Typical monopile transitional piece 

marginally larger in dimensions 

than the foundation pile and 

would be fitted once the 

foundation is in place. The TP 

would be craned into position 

over the exposed top and 

carefully lowered into position. 

Once located in situ on top of 

the foundation a jack system 

would be used to level the TP. 

A level top would be essential 

as the WTG tower section is 

bolted directly onto the TP. 

6.8.4 To ensure the TP remains in the 

level position, grout would be 

applied between the foundation 

and TP to bond the structures 

together and provide a load 

bearing connection. 

6.8.5 The external corrosion 

protection system used for the 

TP would be as described in 

detail for the foundation 

systems in Section 6.7. 

Source: www.kentishflats.com 

Transition piece: space frame structure 

6.8.6 The TP associated with a space frame structure (see Plate 6.15) would be 

shorter then those used on the other types of foundation structures, primarily 

as the space frame structure extends far enough above sea level and would 

also be fitted with landing facilities. The TP can be welded into position 

onshore prior to the space frame being transported to its offshore location. 



Plate 6.15 Typical space frame transition piece 

Transition piece: gravity base structure 

6.8.7 A gravity based structure may or may not be designed to accommodate a 

TP. If a TP is to be used the design and installation process would be 

identical to the monopile TP. It should be noted that if this foundation system 

uses a TP then less seabed levelling preparation would be required as the 

jack system between the GBS and TP can be used to level the top surface. 

6.8.8 If no TP is to be used then the GBS would extend further above the sea level 

and require landing facilities and auxiliary equipment to be installed. 

6.8.9 The WTG tower is connected directly onto the GBS structure via a flanged 

connection. With this option all the necessary electrical equipment would be 

housed within the WTG tower. 

Corrosion projection 

6.8.10 The corrosion protection utilised for the TP section, is likely to be in line with 

that considered for the foundation options. 

Electrical equipment 

6.8.11 The electrical equipment contained within WTG would typically be housed 

within either the TP or the base of the tower (see Section 6.10), and 

comprises: 

� Converters or power electronics; 



� Low voltage (LV) circuit breakers; 

� Transformers (occasionally, for instance a Vestas WTG, would house 

the transformer in the nacelle rather than the TP); and 

� Medium voltage (MV) circuit breakers. 

6.8.12 Cabinets would also be installed to house control equipment, telecoms and 

emergency power supply units. 

6.8.13 The electrical equipment requires a controlled atmospheric environment, 

along with the regulated dissipation of the heat generated. Therefore, 

dehumidifiers and air conditioning units would also be installed to ensure a 

suitable atmosphere is maintained. 

6.8.14 The precise composition of electrical equipment, housing and its location 

(within TP or tower) would depend on the WTG selected. 

6.9 WTG Support Structure Ancillary Equipment 

Introduction 

6.9.1 This section details the ancillary equipment that would normally be located 

externally on the WTG foundation and the transition piece. 

6.9.2 The ancillary equipment would be defined as: 

� J-tubes; 

� Access and rest platforms; 

� Access ladders; 

� Boat access system; and 

� Corrosion Protection Systems. 

J-tubes 

6.9.3 J-Tubes comprise the metal tubes that protect the inter or intra-array 

electrical cables, as they travel up the foundation structure to the TP. The 

metal tubes at the bottom of the foundation structure would generally be 

curved to support the inter or intra-array cable as it transitions from a 

horizontal to a vertical position. Each J-tube would house one inter or intraarray 

cable, therefore, more then one J-tube would be required per 

foundation structure to facilitate the incoming and out going array cables. 

6.9.4 The J-tube can extend from the seabed all the way up the foundation 

structure to the TP. If the foundation structure is a monopile or space-frame 

type the J-tubes can be housed externally or internally to the structure. Jtubes 

located inside the foundation structures have an additional level of 

protection. External attachment of J-tubes would be more likely with GBS 

foundation structures. 



Boat landing system access 

6.9.5 The design of boat landing facilities, access ladders and subsequent 

platforms would be driven by the type and largest size of boat anticipated to 

be used in the maintenance programme. It would be reasonable to assume 

that marine grade floodlights would be fitted to illuminate the boat landing as 

required during hours of darkness. Typically the lights would be controlled 

from the wind farm control room. 

Access ladders 

6.9.6 Experience on GGOWF has shown that two vertical access ladders would be 

required and it is anticipated that a similar system would be used on the 

GWF project where TP solutions would be adopted. The ladders would be 

approximately 600mm wide with rungs at 300mm vertical intervals. To 

protect the lower ladder a permanent fender system would be located either 

side, which also provides a safety zone for personnel. The fender system 

provides further guidance or a buffer system for the landing craft as it 

maintains its position in the water next to the TP to allow the transfer of 

personnel. 

6.9.7 To ensure the safety of personnel climbing up the ladder a fixed inertia reel 

safety system is used. Personnel would attach themselves to the fixed safety 

system which allows them to climb the ladder freely, however in the event of 

a fall or slip it locks preventing movement backwards. 

6.9.8 The initial ladder would typically be approximately 11m long and extends 

below the lowest astronomical tide (LAT) to ensure access at all tidal states. 

A rest platform would be located between the two ladders. The second 

ladder would typically be approximately 5m long and would also be fitted with 

a fixed inertia reel safety system but not the fixed fender system. 

6.9.9 If space frame structures are selected the upper ladder may be replaced by a 

stairway. This would also be the case if GBS foundations are selected 

without a TP. 

Access platform 

6.9.10 All structures whether TP’s, space frames, GBS would have to have access 

platforms of a minimum 1m width. The access platform would also include 

an integral laydown area adjacent to the access door. 

6.9.11 A small davit crane capable of lifting up to approximately 250kg, would be 

mounted on the main platform, beside the lay-down location. This would be 

used to lift the necessary equipment from the boat onto the platform, or vice 

versa. Additionally some WTG would have cameras located on the main 

platforms to allow remote observation to take place. These would allow an 

onshore based O&M team to assess the sea state and weather conditions at 

the WTG, whilst also maintaining visual contact with maintenance operations 

in the field. 



6.9.12 During the WTG installation phase there is the potential that temporary 

generators would be positioned on the main platforms, as the process would 

require electricity which might not be available from the shore. If this 

situation arises then the correct environmental mitigation would be 

introduced, such as ensuring the generator would be effectively bunded. 

6.10 Wind Turbine Generators 

Concept summary 

6.10.1 The WTG consist of three primary components (see Plate 6.16): 

� The tower; 

� The nacelle; and 

� The rotor. 

Plate 6.16 WTG component overview 

Source: The Crown Estate (2010) 



WTG tower 

6.10.2 The WTG tower is the component which gives the rotor the necessary height. 

The structure is likely to consist of up to four tapering steel tubular sections, 

which would be lifted into place and secured together by bolting. Each tower 

section would arrive on location with pre-installed internal fittings, thus aiding 

the secondary installation phase. 

6.10.3 The bottom tower section would be bolted via a flange to the TP. Similarly 

the top tower section would be flanged to facilitate the connection with the 

nacelle component. 

The nacelle 

6.10.4 The nacelle houses the electro-mechanical elements of the WTG, or more 

simply a machine that can turn rotational motion into electrical energy. 

Depending on the WTG supplier additional equipment such as the WTG 

transformer would also be housed on the nacelle. 

The rotor 

6.10.5 The rotor is the device which, through circular motion, extracts the energy 

from the wind. Increasing the blade length allows more energy to be 

extracted from the passing wind through a greater ‘swept area’. The blades 

can be feathered or twisted (i.e changing their pitch) to maintain a particular 

speed. Three-bladed rotors are the most common type and are envisaged 

for use on the GWF project. 

WTG manufacturing and transportation 

6.10.6 The primary components of the WTG are typically fabricated in smaller units 

for transportation purposes, for example the blades for the rotor are 

manufactured individually. 

6.10.7 The method of transportation to site would depend on WTG manufacturer’s 

recommendations, the type of installation vessel and the timing of installation. 

The GGOWF WTG components were transported to the offshore location 

then lifted into position, as opposed to pre-assembly onshore of the tower 

and nacelle, followed by shipment to site. Either method may be adopted by 

the GWF project. 

Fluids and oils 

6.10.8 The volume of fluids and oils required by a WTG is greatly dependent on the 

drive train and pitching technology of the specific WTG. If the oil in the 

nacelle (i.e. the gearbox and hydraulic tanks) were to leak under any 

circumstances then it would most likely leak down the walls of the tower, on 

the inside, and would accumulate under the access platform at the base of 

the transition piece. 



6.10.9 During operation the fluids may be under high pressure, and mechanical 

failures could lead to high pressure fluid release. In such an event the 

machine would be shut down automatically due to low pressure/ low level 

alarms etc. A hydraulic failure in the hub would have a similar outcome. 

6.10.10 WTGs are designed to prevent any significant accidental leakages of such 

fluids and oils, as the bottom of the tower/ transition pieces are typically air 

tight with sufficient containment incorporated into the structure to prevent 

accidental fluid release. Furthermore, containment would also be included in 

the design and construction of the nacelle and the top tower platform. All of 

the main bearings including the slew, hub and pitch would also be equipped 

with grease catchers. 

WTG installation 

6.10.11 Installation methods of WTG vary and include assembly of the turbine tower, 

nacelle and rotors individually whilst at sea, through to transfer of complete 

turbines from land. Most typically the towers would be mounted vertically on 

a dedicated heavy lift installation vessel and one or more blades joined to the 

rotor hub before shipment, with the final blade/s installed once the tower and 

rotor are in place. 

WTG operational noise 

6.10.12 When a windfarm is operational the main source of underwater noise is 

mechanically generated vibration from the turbines transmitted into the sea 

through the structure of the support pile and foundations (Nedwell et al. 

2003). Subacoustech (Nedwell et al., 2007) have undertaken a review of 

four operational wind farms (North Hoyle, Scroby Sands, Kentish Flats and 

Barrow). The available data indicated that the noise generated by a working 

turbine is significantly lower than the noise created during construction by 

piling. However, while construction noise may only span a period of a few 

months, operational noise would span the lifetime of the windfarm (Nedwell 

et al., 2007). 

6.10.13 The study findings revealed that the level of noise from operational 

windfarms was very low and was not considered to pose any risk to fish and 

marine mammals (Nedwell et al., 2007). In some instances, operational 

noise could be recognised by the tonal components caused by rotating 

machinery and by its decay with distance. However, the noise generated by 

the operating WTG, even in the immediate vicinity, only dominated over the 

background noise in a few limited bands of frequency and equated to a rise 

of only a few dB above background noise levels. In some cases, the tonal 

noise caused by the WTG was dominated by the tonal noise from distant 

shipping (Nedwell et al., 2007). 



WTG options 

6.10.14 There are several WTG models of varying rotor diameters which are being 

considered. The wind farm would consist of up to a maximum of 140 WTGs, 

with an output not exceeding 504MW, and a rotor diameter range of 107m to 

164m. These rotor diameters represent WTGs in the order of 3.6MW to 

7MW capacity based on currently available models. All options would have a 

minimum clearance distance of 22m above mean high water springs 

(MHWS). 

6.10.15 WTGs normally operate within a predetermined range of wind speeds, 

typically starting at 3.5ms -1 and producing their maximum power at 

approximately 12ms -1 . WTGs typically shut down at wind speeds greater 

than 25ms -1 , in order to avoid damage. 

6.10.16 For the assessment of the worst realistic case, three turbine examples were 

chosen to represent the greatest levels in a number of different parameters, 

such as hub height, tip height and maximum number. Details of the 

dimension and key criteria of the maximum and minimum size ranges 

considered for the GWF assessment are provided in Table 6.4. These are 

not intended to be exhaustive of all the turbine examples that may be utilised, 

however they provide a sound basis for identifying the realistic worst case 

based on upper and lower size limits in each of the areas of environmental 

assessment. 

Table 6.4 Summary of the WTG parameters 

WTG detail 107m rotor 120m rotor 164m rotor 

Typical MW rating (for 

easy referencing) 

Minimum clearance above 

MHWS 

Maximum No. WTG in 

array 

3 - 3.6MW 3.6 - 4MW 6 - 7MW 

22m 22m 22m 

140 140 72 

Hub height (LAT) 79.5m 86m 120m 

Maximum tip height 135m 146m 195m 

Rotor diameter 107m 120m 164m 

Cut in/out speed 

Maximum rotor speed 13rpm 

Maximum tip velocity 73ms -1 

3-5ms -1 cut in, 

25ms -1 cut out 





13rpm 11.5rpm 

81.7ms -1 90.3ms -1 



WTG detail 107m rotor 120m rotor 164m rotor 

Total project swept rotor 

area 

Minimum grid downwind 

(8D) 

Minimum grid crosswind 

(6D) 

1.264km 2 

1.6km 2 

1.3km 2 

856m 960m 1,212m 

642m 720m 984m 

6.10.17 The final decision on the WTG type taken forward by GWF would depend on 

the WTG available in the market place at the time of procurement, the 

economics associated with the manufacture, transportation and installation of 

the available options, and the outcome of detailed FEED and optimisation 

studies post consent. 

6.10.18 Table 6.5 provides a summary of the possible WTG build-out scenarios. 

Table 6.5 Example WTG build-out scenarios 

Build-out scenarios – WTGs 

One size class only 

One size class only 

Two size classes, different phases 

Optimised smaller project with smaller 3 – 

3.6MW or 3.6 - 4MW WTG on monopiles in 

shallow water 

Project utilising larger WTG class machine 

(e.g. 6 – 7MW) on either space-frame or 

suitable foundation for all water depths 

First stage of development with 3-3.6MW or 

3.6-4MW on monopile foundations, second 

stage utilising larger turbines, e.g. 6-7MW 



6.11 Ancillary Infrastructure 

Offshore substation platform(s) 

6.11.1 The principal purpose of the Offshore Substation Platform (OSP) is to house 

the transformers required to increase the distribution voltage (typically 66kV 

or above) of the inter and intra-array cables to a higher export voltage 

(132kV) for the export cables. 

6.11.2 It is envisaged that between one and three OSPs would be required for the 

GWF project. The flexibility in the number considered is reflective of the 

potential to develop a range of layouts utilising one, two or three of the three 

distinct Development Areas. 

Topsides 

6.11.3 The topside is the name for the structure which would be placed on top of the 

foundation structure and completes the OSP. It may be configured in either a 

single or multiple deck arrangement. Decks would either be open with 

modular equipment housing or the structure may be fully clad. All weather 

sensitive equipment would be placed in environmentally controlled areas. 

6.11.4 The offshore substation(s) would be up to 75m in height (30m from LAT to 

topside and then 45m for height of substation unit). 

6.11.5 The OSP would be fabricated at a quayside facility to enable the transfer of 

the topside structure onto a barge for transportation offshore. Whilst at the 

quayside the topside would be fitted out internally with all the necessary 

equipment. As far as possible the equipment would be made ready for 

operation prior to being moved offshore. Environmental mitigation measures, 

such as transformer bunding, would be fully operational prior to the OSP 

transportation phase. 

6.11.6 The OSP would typically accommodate the following: 

� Helicopter landing facilities; 

� Refuelling facilities; 

� Potable water; 

� Black water separation; 

� Medium (MV) to high voltage (HV) power transformers; 

� MV and/or HV switch gear; 

� Instrumentation, metering equipment and control systems; 

� Standby generator; 

� Auxiliary and uninterruptible power supply systems; 

� Marking and lighting; 

� Emergency shelter, including mess facilities; 



� Craneage; and 

� Control hub. 

Discharges 

6.11.7 The OSP’s drainage system would collect the waste water as well as 

connecting the bunded areas. The drainage system would incorporate a 

separation unit which would separate any contamination from the collected 

water. The collected water would be re-circulated through the separator until 

the water complies with the Environmental Agency’s requirements, prior to 

discharging to the sea. 

6.11.8 The collected contamination would be discharged into a storage facility, for 

transporting to shore and the appropriate disposal. 

Foundations 

6.11.9 The offshore platform(s) foundation would most likely comprise a spaceframe 

foundation system analogous to that described in Section 6.7 for WTG 

space-frame foundations, only larger in dimensions (Table 6.1). However, 

at this stage GWFL may consider all foundation types as detailed for the 

WTGs in Table 6.1. 

6.11.10 The existing GGOWF project uses a square lattice type foundation with 

cylindrical piles, driven through sleeves at each of the four space frame legs 

into the seabed to secure in place. The advantages of the lattice space 

frame are that it provides the most protective solution for the incoming inter 

and intra-array and export submarine cables and also enables the OSP to be 

deployed in any of the water depths within the array. Details of the maximum 

space frame dimensions are provided in Table 6.1. 

6.11.11 As an example, the lattice space frame would be fabricated onshore and 

subsequently loaded onto a transportation barge that would deliver it to the 

required location, before being placed onto the seabed by an HLV or jack-up 

barge. The pilling options would be the same as a space-frame to support a 

WTG (i.e. pre or post installed cylindrical piles or suction piles (Section 6.7)). 

6.11.12 Corrosion measures for the space frame would be a mixture of pre-coating 

and either sacrificial anodes or an impressed CP system, again following the 

oil and gas industry guidelines and certification channels. The CPS methods 

for the space frame would be the same as those discussed in the previous 

Section 6.7. 



Accommodation platform 

6.11.13 An accommodation platform may be required to provide accommodation and 

suitable landing points (for vessels and or helicopter) for any onsite 

coordination of emergency marine activities. 

6.11.14 The foundation for an accommodation platform would be similar to the OSP 

(namely space-frame), but smaller in dimension. The topside would be large 

enough to contain emergency shelter and facilities for crews undertaking the 

necessary work offshore. Power and communication links would need to be 

installed, with a standby generator in case mains electrical supply was lost. 

Fabrication of the space frame and topside would take place onshore, with 

transportation to the offshore location before being lifted into position. 

Collection station 

6.11.15 Once the location of the turbines has been finalised, then the design to link 

the turbines together can be undertaken. The electrical design study would 

determine if a collection station is needed. Subsequent development of the 

initial design proposal would then determine the location, substructure and 

dimensions of the necessary collection station. 

6.11.16 The role of the collection station would be to facilitate the electrical 

connection of several turbine electrical strings so that the total generated 

power from them can be exported on a single cable. 

6.11.17 The collection station would be housed on a similar substructure to that of the 

WTG. The foundation type for the collection stations would either be a 

monopile or a space-frame type structure. Either of these structures would 

contain a number of J-tubes 

6.11.18 Transformers, if required, would be located on the collection stations to step 

up the voltage, for instance from 33kV to 66kV. This would assist the energy 

efficiency of the site as higher transmission voltages incur less electrical 

power loss. This must be offset with the fact that transformers themselves 

consume electrical energy. Consequently the installation of transformers 

would be a less attractive option. 

6.11.19 The topside of the collection station would comprise electrical switchgear, 

thus dimensionally small and lighter than a complete WTG. If the collection 

station was required to house a transformer the floor plate would be larger. 

Meteorological mast(s) 

6.11.20 Up to three permanent meteorological (met) masts are envisaged for the 

GWF project. The met masts would be installed in key locations within the 

GWF licensed area. Information detailing the wind direction and strength 

would be recorded continuously via the SCADA system, with the information 

retrieved bi- monthly via a visit to the offshore location. 



6.11.21 The met masts are used to verify the wind turbine’s MW output, and 

additionally feed the real time data into the wind forecasting module to help 

predict the next day’s wind pattern. The met mast would be placed on a 

foundation structure of similar type to those described in the previous section 

i.e. monopile, space frame, gravity base or suction bucket, which would be 

appropriate to the location of the installation. A topside, or deck protected 

from the elements, which is large enough to house electrical switch gear and 

communication equipment, along with back-up systems would be required. 

Ancillary structure summary 

6.11.22 Table 6.6 provides a summary of the offshore ancillary infrastructure. 

Table 6.6 Summary of the ancillary infrastructure 

Detail 

OSP / Collection station / 

Accommodation platform 

Maximum No. of infrastructure 4 3 

Maximum infrastructure 

height 

Foundation options 

6.12 Inter, Intra-array and Export Cables 

Met-mast 

75m 120m 

Monopile, GBS, spaceframe 

and suction-bucket 

(as detailed in Table 6.1) 

Monopile, GBS, spaceframe 

and suction-bucket 

(as detailed in Table 6.1) 

Inter and Intra-array cables 

6.12.1 Inter-array cables would be laid between the different areas and intra-array 

cables would be laid between turbines as shown in Figures 6.1 to 6.3. 

6.12.2 The inter and intra-array cables collect and transfer power generated in WTG 

to the OSP(s), potentially in different key areas of the project. The cables 

connect the WTG together into strings, with the maximum number of WTG 

connected together depending on WTG size and cable rating. The strings of 

turbines would then in turn be connected to the offshore platform, possibly 

via a collection station. 

6.12.3 The intra-array cables between adjacent WTG would be relatively short in 

length, typically in the range of 600m to 2,000m. However, some cables (e.g. 

those between the last turbine in the string and the OSP) could be 

significantly longer and may pass between arrays, i.e. becoming inter-array. 



6.12.4 The precise inter and intra-array cable layout would be defined following 

detailed FEED studies. In addition, the decision as to whether a radial, 

looped or branched arrangement is adopted would be made following further 

site investigations and would be influenced by a combination of ground 

condition and economic factors (as for the WTG layout determination). For 

the purposes of the PER, indicative inter and intra-array cable layouts are 

provided in Figures 6.1 to 6.3. 

6.12.5 The cable size would be expected to increase from the far end of the strings 

to the OSP to accommodate the increasing power that would be carried. It is 

possible that up to five different cable sizes would be used throughout the 

wind farm. 

6.12.6 The inter and intra-array cables would typically be rated at 33kV, and would 

carry the electrical energy generated by the WTG to a central location. The 

inter or intra-array cable would be a single armoured submarine cable, 

containing 3 electrical conducting cores and an optical fibre cable. Plate 

6.17 shows the typical make-up of a single armoured submarine cable. 

Plate 6.17 Typical single armoured submarine cable 

Source: GGOWL, (2009) 

Export cables 

6.12.1 Export cables would be three phase Alternating Current (AC) cables with a 

rating of 132kV. The cable would be extruded cross linked polyethylene 

(XLPE) insulated and wire armoured for erosion protection. 

6.12.2 Up to four export cables would be required to transfer the wind farm output to 

shore. The final number of export cable circuits would depend on the final 

wind farm design. It is possible that the export cables will also allow for data 

to be transferred using spare optical fibres. 



6.12.3 The export cables comprise the cabling from landfall to the wind farm site 

and any inter-connecting cabling between OSP within the potential separate 

array Areas (see Figures 6.1 to 6.3) 

Proposed export cable route 

6.12.4 A dedicated geophysical survey has been undertaken on a corridor between 

500m and 1,000m wide within which the export cable route would be 

established (see Chapter 10). A corridor of this width would be required to 

account for spacing between the four cables and any amendment of the 

cable route due to the navigation around obstacles including aggregate 

extraction (see Chapter 19 Human Activity), marine archaeology / wrecks 

and other magnetic and sonar contacts (see Chapter 20 Archaeology) and 

sandwaves (Chapter 10 Physical Processes): 

6.12.5 The philosophy behind this route selection is provided in Chapter 7 Site 

Selection and Alternatives. It should be noted that the export cable 

corridor, which has been agreed with The Crown Estate, reaches shore to 

the north of Sizewell; however the actual landfall will be to the south of 

Sizewell. 

6.12.6 The precise export route within this corridor would be established following 

detailed FEED studies and further site investigation post consent. 

Cable manufacture & transportation 

6.12.7 The estimated length of each export cable, depending on the final route, 

would be approximately 50km. This is established from the nearest offshore 

substation to the landfall at Sizewell. Additional export cables, up to two 

circuits (20km each), would be expected to connect the first OSP to the most 

distant OSP. Therefore, it is estimated that the total length of subsea export 

cable required would be up to 240km. Table 6.7 indicates the detail of 

typical AC export cables. 

6.12.8 Plate 6.18, below shows an AC export cable being loaded into a carousel - a 

circular rotating cage mounted onto the back of an installation vessel. The 

vessel transports the export cable to the required location from where it is 

pulled initially into position and then installed slowly into the seabed as 

described in the following sections. 



Plate 6.18 Export cable being loaded into a carousel 

Cable installation 

Pre-installation works 

6.12.9 The preferred cable route would be surveyed (via the pre-construction 

geophysical survey) to locate any obstacles that may obstruct cable laying 

(e.g. rocks, wrecks, metal objects, unexploded ordnance). If an obstruction is 

located it would be assessed and an appropriate strategy would be 

established to remove or avoid the obstruction. Typically a Pre Lay Grapnel 

Run (PLGR) and ROV survey would be conducted to clear the obstruction. 

Where the obstacle is suspected to be UXO specialist mitigation would be 

employed to either avoid or make safe the obstruction. 

6.12.10 The geophysical surveys would also serve to identify the location of sand 

waves along the cable route so that an assessment can be made as to 

whether such features can be avoided or, if not, what level of seabed 

preparation (pre-lay sweeping) is required to ensure an appropriate burial 

depth is achieved in stable (i.e. non mobile) seabed conditions. 

6.12.11 Prior to cable installation, cable burial trials may be conducted in advance of 

the main installation programme to ensure that the chosen equipment would 

be suitable for the ground conditions encountered and that an appropriate 

burial depth can be achieved. If undertaken, any such trial may involve trials 

of lengths of up to 1km in each of the soil types likely to be encountered 

along the export cable route. 

Cable installation methods 

6.12.12 There are several different methods available for the installation of submarine 

cables: 

� Simultaneous lay and burial using a cable plough 



� Post lay and burial using a jetting ROV 

� Simultaneous lay and burial/post lay and burial with a mechanical 

trencher. 

6.12.13 The final decision on installation method would be made on completion of the 

pre-construction geotechnical site investigation surveys. However, it is 

noteworthy that the GGOWF project has utilised a combination of ploughing, 

jetting and trenching to accommodate for the variation in sedimentary 

conditions along the cable routes. 

Cable burial by ploughing 

6.12.14 Cable burial ploughs cut through the seabed, lifting the soil from the trench. 

Cable ploughs are designed to cut a narrow trench, with a slot of material 

temporarily supported before falling back over the trenched cable. 

6.12.15 The advantage of this method is that burial can be achieved as the cable is 

laid, thus minimising risk to the cable. However, the number of vessels which 

can carry out this method and that have the required cable carrying capacity 

for “heavy” power cable is limited. 

6.12.16 The performance of a plough and the depth of burial which can be achieved 

are a function of plough geometry and seabed conditions, with dense / stiff 

soils providing the greatest challenge. 

6.12.17 A typical cable burial plough (such as the Sea Stallion 4, as shown in Plate 

6.19) excavates to a width sufficient to enable insertion of a cable up to 

280mm in diameter. The plough itself would typically be around 5m in width, 

although the operating footprint on the seabed would be much smaller than 

this. 



Plate 6.19 Example cable burial plough 

Source: www.VSMC.nl 

Cable burial by jetting 

6.12.18 Where seabed conditions are predominantly soft sediment material it may be 

considered appropriate to bury the array cables with a Directionally 

Positioned (DP) vessel post installation. 

6.12.19 Under this process the cable would be laid on the seabed first and an ROV 

(Remote Operated Vehicle) fitted with high-pressure water jets is 

subsequently positioned above the cable. The jets fluidise a narrow trench 

into which the cable sinks under its own weight. The jetted sediments settle 

back into the trench and with typical tidal conditions the trench coverage 

would be reinstated over several tidal cycles. 

Cable burial by trenching 

6.12.20 In locations where seabed conditions comprise very stiff soils (typically over 

100kPa) and/or bedrock, ploughing and jetting techniques may not be 

appropriate for cable burial. 

6.12.21 One approach for installing cables in very stiff/hard seabeds would be to use 

mechanical trenchers which can either be used to simultaneously bury the 

cable as it is laid or in a “post lay” mode where the cable is laid by one vessel 

and burial is achieved by another vessel spread following on behind. 

Simultaneous lay and burial of the cable tends to be preferred since this 

reduces risk to the cable from exposure. However, if a post lay burial 

solution is used then typically the length of time of exposure would only be a 

few hours (depending on the exact arrangements). During this time any 

unburied lengths of cable would be protected using a guard vessel. 



6.12.22 It should be noted that simultaneous lay and burial can also be achieved by 

ploughing in stiff materials to 140KPa and above (eg chalk) by use of 

specially designed “rock ripping” ploughs as well as certain types of 

“standard” subsea plough. For example, in the recent installation on 

GGOWF the export cable was installed through stiff/hard crag material using 

a standard “Sea Stallion” subsea plough. Trench spoil would be left to 

naturally backfill, which typically takes two or three tides 

Plate 6.20 Example cable burial trencher 

Source: Pharos offshore group 

Cable installation procedure 

6.12.23 A cable barge or specialist cable installation vessel is likely to be required to 

install the cable into the seabed. The array cables would be supplied on 

cable reels or loaded onto the vessel in one continuous length. Collection 

would take place from the manufacturer’s facilities or holding docks prior to 

transportation to the ploughing vessel. 

6.12.24 The vessel then travels to site and takes up position adjacent to the start 

location (WTG for inter and intra-array cabling or OSP / the shore for export 

cabling). The vessel would either hold station via a DP system or set 

anchors in a stationary mooring pattern. One end of the array cable would 

then be floated from the cable reel towards the substructure / shore. The 

cable would then be laid away from the substructure / shore in a direction 

towards the landfall / OSP. The cable installation vessel would either move 

under DP control or by hauling on its anchors; if the secondary method is 

used then redeploying the anchors would be required. 



6.12.25 Depending on the design of the relevant substructure, the cable is either 

sunk and fed through the J-tube and lifted / pulled into the transitional piece 

or pulled through a pre-installed J-tube attached externally to the 

substructure. 

6.12.26 The cable installation vessel’s ability to get close to shore is dependent on 

vessel draft, (but is typically around 10m) at which point water depths are too 

shallow to proceed. At this time the installation vessel would hold its position 

either by use of Differential Global Positioning System (DGPS) or anchors 

whilst the cable is brought (floated) to shore. If a cable barge is used then 

their draft is suitably shallow to enable access to shore. The process of 

connection to shore is discussed in detail in Section 6.13. 

Cable burial depths 

6.12.27 Appropriate cable burial depth would be determined by a detailed hazard 

identification survey, which would assess the different locations and the 

various shipping and dredging activities. It is possible that the hazard 

identification survey would identify places where the depth may vary due to 

local features, such as: 

� Sand waves 

� Erosion of the seabed 

� Intense demersal fishing 

� Existing infrastructure (such as cables) or observed seabed obstacles. 

6.12.28 Export, inter and intra-array cable target burial depth could be up to 1.5m. 

Typically though the burial success may be closer to 1m, although this would 

be highly dependent upon equipment used and ground conditions, and with a 

minimum aim of achieving 0.6m. 

Cable lay protection 

6.12.29 Achieving target burial depths for export, inter and intra-array cables would 

not be possible in close proximity to the WTG and OSP, where the cables 

rise up to connect to the J-tube of these structures. Typically these sections 

may be in the region of 20m in length (DOWL, 2009). 

6.12.30 There are three surface based protection measures which may be utilised for 

cable protection: 

� Extension of the scour protection rock dumping (if being used) to 

cover the final 20m of cable on the seabed 

� Use of concrete mattresses which are lifted and placed over the cable 

sections (see cable crossings section for further detail of this 

methodology). This methodology is sometimes supplemented with the 

use of sandbags to stabilise the edges of the mattresses 



� Use of grout bags which can be placed over the lengths of cable and 

then inflated with structural grout. The grout then cures to provide an 

effective cover protection system for the cables. This approach 

requires diver assistance. 

6.12.31 Alternative protection may also be required where it is impossible to achieve 

the target burial depth (e.g. where seabed conditions prevent access for the 

installation equipment). Experience from the adjacent GGOWF project would 

suggest that the likelihood for significant levels of cable lay protection under 

such circumstances would be limited. 

Cable separation 

6.12.32 Cables must be laid with a separation distance so that, in the event of a fault, 

repairs can be carried out without risk of damaging the adjacent cables. 

6.12.33 It is anticipated that a nominal spacing of 60m between cables would be 

utilised, which would be sufficient to avoid conflict with anchor spread, whilst 

decreasing the risk from damage through cable ploughing activity for 

adjacent cables. The cable separation would need to be reduced to a 

suitable width at the shoreline as the cables approach the HDD ducts at the 

landfall point, however this approach is subject to detailed design. Therefore 

whilst the export corridor is shown in Figure 1.1 (Chapter 1) approaching the 

shore at a consistent offset from the existing GGWOF export cables, the 

separation would reduce signficantly on the approach to shore to allow the 

cables to pass south of Sizewell, as shown in Figure 1.2 (Chapter 1). This 

would result in the GWF cables being in the GGOWF export cable corridor as 

the GWF cables approached the shore. Cable separation for the crossing of 

telecommunication cables would be subject to final agreement with the third 

party owners. 

Cable crossings 

6.12.34 There are four telecommunication cables that would require crossing (three 

by the export cable route and one within Areas A and B (see Chapter 19). 

Given that there would be up to four export cables, there would be up to 12 

crossings required on the export route. The number of crossings associated 

with the intra-array cabling would be determined following design 

optimisation and confirmation of final layout post consent. 

6.12.35 The International Cable Protection Committee (ICPC) has issued a 

recommendation for crossings arrangements between telecommunication 

cables and power cables. This recommendation outlines how, and why, a 

Crossing Agreement should be put together, but does not describe the 

physical construction of a crossing. 

6.12.36 There is no single universally accepted crossing design that would be 

applicable in all situations. Designs would vary with the seabed properties at 

the particular location. Each crossing would have a range of features 

possibly unique to that location, based on: 



� The physical properties of the crossing product, for example the cable 

size and weight, bend radius and armouring 

� Protection requirements relative to the hazard profile, including depth 

of burial or extent of mattress/rock cover 

� The physical properties and protection status of the crossed product 

� Seabed properties at the crossing point, for example substrate type, 

morphology and stability - presence of mobile bedforms 

� Any constraints placed by the crossed party, for instance location and 

burial determination standards, maintenance clearance zone, plough 

approach limits and notification zone. 

6.12.37 The minimum vertical separation distance between the two cables would 

likely be governed by the requirements of the crossed party and construction 

methodology. This is normally 0.3m (from a mechanical separation point of 

view). Assuming that the crossed cables are buried by 1.5m and a 0.3m 

mattress is placed underneath the GWF cables, a minimum separation of 

1.8m would be likely. 

6.12.38 The components most commonly used to protect telecommunication cables 

would be flexible mattresses and graded rock. These components may be 

used exclusively or in combination. 

6.12.39 The telecommunication cables on the export cable route would require 

multiple crossings (given that there would be four export cables). Crossing 

the cables at one point, via a single physical structure, has benefits through 

reducing installation time and expense. 

Mattresses 

6.12.40 Mattresses made from elements of concrete or bitumen are widely used to 

protect from seabed hazards where burial is not viable or is not effective. 

The most common form of mattress is that made of concrete elements 

formed on a mesh of polypropylene rope, which would conform to changes in 

seabed morphology. Bevelled elements are used on the periphery to create 

a lower profile to encourage hazards such as trawl gear to roll over the 

mattress. Typically, mattresses of 6m by 3m and 150mm to 300mm in 

thickness would be used for cable crossings. 

6.12.41 If the sediment dynamics are appropriate, mattresses fitted with 

polypropylene ‘fronds’ may be used to enhance the protection provided, as 

the fronds encourage transient sediments in the water column to be 

deposited, in the best case creating a protective sand bank. Where the burial 

depth of a cable is zero, shallow or ambiguous mattresses can be configured 

to reduce the risk of direct contact. Plate 6.21 shows various designs that 

have been proposed historically. 



Plate 6.21 Fully mattressed crossing designs 

Source: GGOWL, 2009 

6.12.42 A site specific geophysical survey would confirm the precise crossing point 

(with areas of level seabed and suitable cover over the existing cable being 

preferable). Mattresses are then lowered into place from a dedicated vessel, 

with divers to ensure accurate deployment. The export cable(s) would be 

laid on the primary layer of mattresses and a second layer of protective 

mattresses would subsequently be installed on top of the cable. The cable 

burial equipment would then bury the cable into the sediment, and extra 

mattressing or rock dump material applied to ensure suitable burial depth 

would be maintained where the cable re-enters the sediment. 

Rock placement 

6.12.43 Rock placement has long been established as a method for constructing 

crossings. The rock used would normally be imported from land quarries, 

although sea aggregates can also be used, the grain sizes being tailored to 

achieve the necessary protection. Rock would usually be deposited by a fall 

pipe vessel where the water depth is adequate as this would be the most 

efficient method of getting the material onto the seabed. In very shallow 

waters a specialist vessel fitted out with basic equipment for pouring the 

aggregate over the side may be used. 

6.12.44 On fall pipe vessels, the aggregate would be conveyed to the side of the ship 

and freefalls down a chain-mail pipe. At the end of pipe would be an ROV, 

which may be used to adjust the delivery point relative to the ship. The 

combined movements of the ship and ROV would be used to construct the 

necessary bridging and protection berms. The fallpipe ROV would be used 

to survey the position and shape of structures created, using acoustic 

profilers and other devices. 

6.12.45 Rock dumping is a relatively quick operation and is not as weather 

dependent as mattressing. 

Cables summary 

6.12.46 Table 6.7 provides a summary of the offshore cabling system. 



Table 6.7 Summary of the cable parameters 

Cable detail Inter and intra-array cables Export cables 

Maximum voltage 66kV 132kV 

Maximum external cable 

diameter 

213mm 258mm 

No. cables TBC 4 

Total length Up to 200km 60km * 4 (cables) 

Target burial depth Circa 1.5m Circa 1.5m 

Burial method 

No. of cable crossings 

Cable protection area 

Plough / jetting Plough / jetting / 

Trenching 

TBC following design 

optimisation 

TBC following design 

optimisation 

6.13 Cable Landfall and HDD Works 



12 

5,400m 2 

6.13.1 Export cables would be laid from a specialist vessel, most likely to be an 

anchored barge, due to the shallow nature of the shore approaches. The 

barge would require two attendant anchor handlers for positioning and a tug 

for transiting. 

6.13.2 The programme for each export cable lay is expected to be 25-30 days 

including a weather allowance. A few days before the vessel arrives, 

preparations would be made on the beach. This would include the creation 

of up to four HDD ducts (to enable the export cables to be brought through to 

the onshore transition pits) inland of the sea defences (see Plate 6.22). This 

would be to the north of the existing GGOWF works, as depicted in the 

footprint shown on Figure 1.2 in Chapter 1. The working area associated 

with these sites would be approximately 25m by 25m. The trench from the 

duct would be extended down to low water in order to bury the cables (see 

Plate 6.23). 

6.13.3 The end of the relevant HDD duct would be excavated during the low water 

period. The duct drawstring would be connected to a winch wire on its 

landward side and the wire then pulled in a seaward direction and connected 

to the cable end on board the lay vessel. The cable would then be pulled 

ashore by a land based winch behind the onshore transition pit. If a subsea 

plough is being used to bury the cable offshore, it would be pulled ashore at 

the same time and the cable loaded into it before it is pulled through the duct 

and into the onshore transition pit. It is anticipated that this process would

take at most two days to complete, from the arrival of the vessel carrying the 

export cable. 

6.13.4 Once pulled through the HDD duct the export cable would be buried in the 

intertidal and nearshore approaches (Plate 6.23). The void between cable 

and the duct wall would most likely be filled with bentonite to aid dissipation 

of heat away from the cable. 

Plate 6.22 Example HDD duct 

Plate 6.23 Example trench to enable intertidal cable burial at GGOWF 

Source: GGOWL 



6.13.5 The HDD works to create the connection between the onshore transition pit 

and the HDD ducts on the beach would involve drilling an arc between the 

two points, to pass underneath a feature to be avoided (namely the sensitive 

foreshore habitats), and exits at a predetermined completion point. Plate 

6.24 provides a simplified typical HDD arrangement. 

Plate 6.24 Typical (simplified) HDD arrangement 

Rig Site Pipe Site 

Source: Balfour Beatty Power Networks Limited 

6.13.6 HDD requires a working area at each side of the proposed drill. One working 

area would be required for the rig site and another for the pipe site. The pipe 

site would effectively be the cable landfall location, and the rig site would be 

the exit location (approximately the location of the onshore transition pit). 

6.13.7 The rig and ancillary equipment would be set up on a level, firm area 

approximately 20m by 15m in size. At the pipe site an area approximately 

20m by 20m would be required. There would need to be sufficient room in a 

direct line behind the drill exit point to accommodate the complete length of 

the fabricated product pipe string. Both the rig site and pipe site would 

require entry (or launch) and receiving pits. These would need to be 

approximately 2.5m by 1m and 1m deep. Following completion of the HDD 

exercise excavated materials would be replaced into the pits, where excess 

waste material is generated this would be re-used or disposed of in 

accordance with the site waste management plan. 

6.13.8 The rig site and the pipe site would be securely fenced to ensure a safe site, 

with safe access and egress for site staff, any visitors required and the 

emergency services. 

6.13.9 A non-saline water supply at the rig site would be necessary within 100m of 

the drilling rig to facilitate the installation of drilling mud (bentonite, which acts 

as a lubricant during the process). If there is no suitable water supply on site 

this can be provided by tanker. 



6.13.10 A mud lagoon would be required to capture and recycle drilling mud during 

the drilling process and to ensure it does not exit the site. The plan area of 

the lagoons may vary but would be a maximum of 5m by 5m. The depth of 

the lagoons would generally be 0.8m but this may vary according to local 

topography. In any event excavations would be plastic lined and protected 

with ‘Heras’ type fencing. Mud waste from this activity would be disposed of 

in accordance with the site waste management plan. 

6.13.11 The first stage of the drill involves a small pilot hole being drilled with a 

cutting / steering head to set the path of the arc from the rig site towards the 

pipe site. When the pilot bore is completed, the cutting / steering head would 

be replaced with an appropriately sized back-reamer at the pipe site and 

pulled through the pilot hole from the drill rig towards the rig site to enlarge 

the diameter of the hole. Depending on the final borehole diameter required, 

it may be necessary to carry out the back-reaming in several stages, each 

time increasing the borehole diameter gradually. Once the required diameter 

has been drilled, the back-reamer would be sent through the bore one or two 

more times to ensure that the hole is clear of any large objects and that the 

mud slurry in the hole is well mixed. 

6.13.12 On the final pass, the product pipe (the cable ducts in this case) would be 

connected onto the back-reamer and the drill string at the pipe site, using an 

extending sealed towing head. The drill string would then be pulled from the 

drill rig and retracted to the rig site cutting a larger diameter (clearance) bore 

whilst also installing the new pipe (the cable ducts). 

6.13.13 The drill process would be repeated until all required boreholes are drilled 

and all of the cable ducts are installed. The HDD exercise would typically 

require one week to drill each hole, and each drill string can be pulled 

through in a single day. 

6.13.14 To avoid damage to the sensitive foreshore habitats during the HDD 

exercise, a temporary road surface (gridded matting or similar) would be 

used for any beach access to avoid damage from heavy plant and other 

construction vehicles. It is anticipated that the temporary access route would 

follow a similar alignment to that used during the beach works undertaken by 

GGOWL. In addition, when the entry pit is excavated on the beach, the 

excavated shingle would be segregated and stored to ensure that the shingle 

layers are returned in the same orientation to minimise impact upon the 

integrity of the structure of the shingle. 



6.13.15 A temporary beach compound would be required at the landfall location to 

accommodate beach anchors and for the temporary storage of plant and 

machinery during landfall operations. This temporary compound would be 

approximately 80m x 50m and would be fenced off from members of the 

public during construction activities. The machinery required for the landfall 

works are likely to include up to three excavators, a tractor and a winch. The 

fenced compound would not extend to the public footpath located landward 

of the shingle, and the public would not be prevented from accessing other 

parts of the beach that are unaffected by the cabling operations. 

6.13.16 Beach anchors would be required during this phase of work, to pull the export 

cables to the shore. These would extend approximately 100m to either side 

of each cable being pulled. Members of the public would be kept away from 

these areas during cable pulling operations, and there would also be security 

patrols to ensure that members of the public do not enter these areas. These 

temporary beach areas would be located within the existing cable corridor 

area shown on Figure 1.2. 

6.13.17 Two further areas of HDD are anticipated on the cable corridor route: 

underneath the unnamed lane to the west of the transition pits and across 

Sizewell Gap. There is also potential to use HDD at other points on the cable 

corridor if appropriate. This will be ascertained during the detailed design 

phase. Construction methodology and requirements for temporary working 

areas will be similar to that set out above. 

6.14 Onshore Transition Pits 

6.14.1 An onshore transition pit is where each multi-core export cable is jointed to 

the single core onshore cables. GWF consists of a maximum of four export 

cables, and therefore requires up to four onshore transition pits located 

adjacent to each other. The location and dimensions of all four transition pits 

together is shown on Figure 1.2. The total footprint of all four transition pits 

together would be approximately 16m by 25m. Each pit would be excavated 

to a depth of approximately 2m with the only evidence above ground, during 

operation, being access covers at ground level for adjacent link boxes. 

6.14.2 Each transition pit would be sheet piled and structurally buffered whilst open. 

The floor of each transition pit would be concrete lined to provide a flat, clean 

working environment. The excavation of the pits would follow environmental 

recommendations, with the topsoil being stored separately. At the eastern 

end of each transition pit would be the HDD shaft. This would be sealed until 

the export cable is ready to be pulled into position. The other end (west) 

would be the location from which the four onshore cables would exit towards 

the new substation. 

6.14.3 The export cables and onshore cables would be jointed together in a 

controlled environment, requiring a purpose designed container to be placed 

on top of the transition pit. 



6.14.4 Adjacent to each transition pit there would be a link box (up to four in total), 

which would be required to be accessible during the operation of GWF. A 

link box contains removable links and represents a point where the onshore 

and offshore cables can be separated (electrically). This allows a cable fault 

to be more easily identified within the onshore or offshore cables. Link boxes 

would therefore have a number of surface level access covers and each one 

would be placed in the vicinity of its associated transition pit. The area 

around the transition pits and link boxes, approximately 30m by 30m, would 

be fenced and made inaccessible during operation. 

6.14.5 A platform would be required near the onshore transition pits to support 

equipment during the cable pull process from shore. This would consist of a 

concrete slab approximately 5m by 5m with a shallow structure visible at 

surface level. The exact location would be finalised during detailed design, 

however a position is likely to be required some 20m beyond the transition 

pits in a roughly westerly direction, dependent on the final alignment of the 

HDD ducting. It is anticipated that the structure would be removed after 

completion of cable installation, although it could be retained within the 

fenced transition pit area for operational access. 

6.14.6 The total working area for the onshore transition pits would be approximately 

75m by 75m and would include space for temporary portacabins, lay down 

areas, vehicles and other necessary construction and installation equipment 

required during the construction of the transition pit. 

6.14.7 A permanent new access track would be required between the transition 

pit(s) and Sizewell Gap. The proposed location of this is also shown on 

Figure 1.2. 

6.15 Onshore Cabling 

Onshore cable route 

6.15.1 The onshore cable corridor would run between the onshore transition pit and 

the GWF 132kV compound, as shown on Figure 1.2. 

There would also be two additional cable corridors: 

� Between the 132kV/400kV transmission compound and the two 

sealing end compounds; and 

� Between the 132kV/400kV transmission compound and the existing 

National Grid Electricity Transmission (NGET) 132kV cable corridor, 

which contains four cables from the Leiston A substation to the 

existing 400kV/132kV substation at Sizewell. This connection has 

some residual capacity, which is intended to be utilised in the first 

instance (i.e. for the first MWh generated) for GWF. Because of 

physical constraints it is not practical to extend the existing Leiston A 

substation. 



6.15.2 The onshore cabling between the transition pit and the GWF onshore 

substation would be approximately 900m in length. The aim would be to 

utilise one continuous cable over this distance. However it is possible that 

two sections would be required and jointed together. Consideration would be 

given to locate any link boxes close to a field boundary to minimise any 

impact to agricultural activities. Link boxes would be the only visible indicator 

after installation; for safety reasons these areas would be fenced off. The 

cable corridor between the 132kV/400kV transmission compound and the 

sealing end compounds would be approximately 300m to the western sealing 

end compound and 400m to the eastern sealing end compound. The cable 

corridor between the 132kV/400kV transmission substation and the existing 

NGET 132kV cables would be approximately 300m. The exact route of the 

cable corridors will be subject to detailed design and feasibility and will be 

subject to micrositing during construction. Preliminary cable corridor routes 

are shown in Figure 1.2. 

Working area and site preparations 

6.15.3 The majority of the cable corridor between the transition pit and the GWF 

substation passes through agricultural land under arable cultivation. A 40m 

wide working corridor would be required along the length of the cable corridor 

where open cut trenching takes place. Figure 1.2 shows the location of the 

corridor with a width of approximately 45m to allow for micrositing within the 

corridor route if required. The working width would comprise: 

� A set of four trenches with a total width of approximately 20m; 

� Construction access for vehicles - which needs to allow the safe 

tracking of construction vehicles in two directions; 

� Topsoil storage (up to 10m in width); and 

� Fencing. 

6.15.4 The cable corridor between the transmission compound and the sealing end 

compounds passes through the northern extent of the Sizewell Wents block 

of woodland. A working width of approximately 20m would be required, this 

would comprise: 

� Cable trenches; 

� Construction access for vehicles , which needs to allow the safe 

tracking of construction vehicles in two directions; 

� Topsoil storage; and 

� Fencing. 



6.15.5 The cable corridor between the transmission compound and the existing 

132kV cabling passes south of the existing GGOWF substation. A working 

width of approximately 20m would be required, this would comprise: 

� Cable trenches 

� Construction access for vehicles - approximately 8m wide to 

allow the safe tracking of construction vehicles in two directions 

� Topsoil storage (up to 10m in width) 

� Fencing. 

6.15.6 The working corridor would be suitably fenced; the type of which would 

depend upon the farming activities and discussions with the landowner. 

6.15.7 Protection of topsoil would be ensured and the ground reinstated to its former 

condition following the construction phase. Generally topsoil would be 

stripped from the working corridor using 20t tracked 360 degree excavators 

and stored to one side in the area allocated. Topsoil would not be stripped 

from the actual storage strip. 

6.15.8 In most cases the construction haul road along the working corridor would 

require no additional preparation other than topsoil stripping. However in 

certain circumstances, such as poor ground conditions, temporary surfacing 

such as hardcore, geotextiles or re-useable plastic surfacing may be 

necessary. 

6.15.9 In some locations it may only be necessary to strip the topsoil from the actual 

trench width, with the remaining working corridor being protected by means 

of temporary surfaces to protect the underlying soil structure. 

Open trench cable installation methodology 

6.15.10 Following the cable corridor preparation works, excavation of the cable 

trench would commence using a mechanical excavator. The cable trench 

width and depth would be approximately 2m by 2m. Dumper trucks would be 

used to transport material to and from the storage areas. Any surplus spoil 

would be taken off site and disposed of in accordance with the appropriate 

waste carrier licence as detailed within the site waste management plan. 

Parts of the cable corridor would require some tree felling (through Sizewell 

Wents). 

6.15.11 Cable installation may be undertaken using a mole plough. This allows the 

cable to be installed without the need to dig a trench. As the mole plough is 

dragged through the ground, it leaves a channel deep under the ground, 

within which the cable is laid. 

Reinstatement 



6.15.12 Following completion of the cable system installation, the working area would 

be reinstated to its previous condition. This would include: 

� Reinstatement of foreshore habitats, including shingle and dune 

slacks; 

� Reinstatement of topsoil; and 

� Reseeding of any fields of grassland, grass margins and ditch 

banks. 

6.15.13 Table 6.8 provides a summary of the onshore cabling system. 

Table 6.8 Summary of the onshore cable system 

Key onshore cable system characteristics 

Number of cables Up to 4 

Transition pit area Approximately 30m x 30m 

Total working footprint of transition pits 

and link boxes 

Onshore Cable length 

Link pits, additional to those at the 

transition pits (if required) 

Trenching technique 

Target cable trench depth 1-2m 

Cable trench corridor width 20m 

Cable trench corridor working width Up to 40m 

6.16 Onshore Substation 

Approximately 75m x 75m 

1,200m (land fall to GWF substation); 

300/400m between transmission compound 

and sealing end compounds; 300m between 

transmission substation and existing 132kV 

cables 

Up to 4, to be located at a field boundary 

Open trench, with HDD proposed for 2 road 

crossings and to avoid sensitive foreshore 

habitats 

6.16.1 The electrical engineering design has identified the need for electrical plant 

and equipment to control and facilitate the export of electricity from the wind 

farm to the 400kV national electricity transmission network. In order to 

achieve this, GWF would require a new 132kV compound to be built near 

Sizewell. 



6.16.2 Transmission infrastructure would also be required in order for electricity to 

convert from 132kV to 400KV and to reach the existing transmission network. 

As such, a 132kV/400kV transmission compound and sealing end 

compounds would also be built in the Sizewell area. 

6.16.3 The GWF compound and the transmission compound would be located 

adjacent to each other and together are referred to as the “GWF substation”. 

The GWF substation would be located approximately 1km inland on the 

Suffolk coast near Sizewell. It would be situated to the north of Sizewell Gap, 

immediately to the west of the existing GGOWF substation site (see Figure 

1.2). An aerial view of the existing GGOWF substation is shown in Plate 

6.25 to give an indication of the type of equipment that would be present 

(note this site does not incorporate lightning protection due to proximity to the 

adjacent transmission towers). 

6.16.4 The footprint of the GWF substation sits predominantly within arable land 

although it also includes part of a block of woodland (Sizewell Wents). In 

addition, part of the proposed landscape mitigation area encroaches into 

Broom Covert (a block of semi-natural grassland used predominantly for 

grazing). 

6.16.5 The dimensions of the GWF compound would be approximately 170m by 

125m (2.1ha) with a maximum height of approximately 14m, exluding 

lightning protection. The dimensions of the transmission compound would be 

approximately 70m by 130m (0.91ha) with a maximum height of 

approximately 14m, excluding lightning protection. Only a small proportion of 

the buildings and equipment in the compounds would be expected to be 14m 

in height. In addition, the substation (both compounds) may require a 

lightning protection system to be installed to protect the electrical equipment 

from lightning strikes. The design of this lightning protection system is 

dependent on the equipment that is included within the substation but, if 

required, could involve the erection of up to approximately 28 lightning 

protection masts, which would each be up to 22m in height, mounted at 

ground level. Each mast would have a footprint of approximately 1.5m by 

1.5m and would be most likely be in the form of a slim lattice tower design, 

although a protective net arrangement is also available. The western sealing 

end compound would have dimensions of approximately 32m x 40m (0.1ha) 

and the eastern sealing end compound would have dimensions of 

approximately 25m by 40m (0.1ha). Gantries would be located in each 

sealing end compound which would be used to connect the 400kV cables 

from the transmission compound to the adjacent transmission towers 

(pylons). The gantries would be approximately 13m in height. Some 

modifications to the existing transmission tower cross arms may be required. 



Plate 6.25 Aerial photo of existing GGOWF substation (looking west) 

Souce: GGOWFL 


Final Report Chapter 6 – Page 63 3 June 2011

6.16.6 The GWF 132kV compound would comprise up to four electrical bays. The 

typical equipment within each electrical bay would include: 

� 132kV SF6 switchgear 1 ; 

� Transformer; 

� Reactive compensation, to include; 

� Dynamic reactive compensation, e.g. SVC, STATCOM; 

� Mechanically switched capacitors; and 

� Mechanically switched reactors. 

� Harmonic filters. 

6.16.7 The 132kV/400kV transmission compound and associated infrastructure 

would include: 

� 132kV and 400kV switchgear; 

� Two 400kV / 132 kV super grid transformers; 

� Control, communication and monitoring equipment; and 

� Cable sealing compounds and gantries to connect to the 

existing overhead transmission lines. 

6.16.8 Each compound would be enclosed by a fence containing the external 

equipment outlined above, with the additional features: 

� Interconnecting cables; 

� Access tracks, gravel paths and hard standing; 

� Control buildings; 

� Internal substation roads 

� Car parking; 

� Communications mast; 

� Earth mats; 

� Lighting; 

� Dump tanks; 

� Water tanks; 

� Back up diesel generators; 

� Welfare facilities; and 

� Security fencing. 

1 switchgear for all bays may be combined and housed in a single building 



6.16.9 Further information regarding the design of the substation and the proposed 

landscaping around it is provided in Section 21 Landscape, Seascape and 

Visual Character. 

6.16.10 Tables 6.9 to 6.11 provide a summary of the onshore infrastructure. 

Table 6.9 Summary of the onshore GWF 132kV compound 

Key project characteristics 

Compound area Up to approximately 2.1ha 

Building/electrical equipment height Up to approximately 14m 

Number of bays Up to 4 

Typical components within each bay 

Other substation elements 

� 132kV SF6 switchgear 2 ; 

� Transformer; 

� Reactive compensation, to include; 

o Dynamic reactive 

compensation, e.g. SVC, 

STATCOM; 

o Mechanically switched 

capacitors; and 

o Mechanically switched 

reactors. 

� Harmonic filters 


� Access tracks, gravel paths and 

hard standing; 

� Security fencing 

� Up to 16 lightning protection masts 

of maximum 22m height 

� Earthing mat 

� Permanent and temporary utilities 

� Control buildings; 


� Communications mast; 

� Lighting; 


� Water tank; 

� Back up diesel generators; and 

� Welfare facilities. 

2 switchgear for all bays may be combined and housed in a single building 



Table 6.10 Summary of the onshore 132/400kV transmission compound and sealing 

end compounds 

Key project characteristics 

Compound area 

Building/electrical equipment height 

Typical substation components 

Sealing end compounds and gantries 

Approximately 0.9ha 

Up to approximately 14m 

� 132kV and 400kV switchgear; 

� Two 400kV / 132 kV super grid 

transformers; 

� Control building; 


� Up to 12 lightning protection masts 

of maximum 22m height; 

� Access tracks, gravel paths and 

hard standing; 

� Internal substation roads; 


� Earth mat; 

� Lighting; 


� Water tank; 

� Back up diesel generator; 

� Welfare facilities; and 


� Total area of 0.2ha; and 

� Maximum height of gantries 

approximately 13m 




Table 6.11 Summary of substation permanent and temporary footprint 

Development element 

Permanent footprint 

GWF substation (comprising both compounds) 3.1ha 

Sealing end compounds 0.2ha 

GWF transition pit 0.1ha 

New permanent access roads and turning area 0.3ha 

Drainage reserves adjacent to new permanent access roads 0.2ha 

Landscape screening area 1.4ha 

Security area (5m strip between substation fence and landscape mitigation 

area) 

Approximate 

total area 

0.5ha 

Total 5.8ha 

Temporary footprint 

Cable corridor between landfall and GWF compound (working corridor 

including transition pit and temporary laydown areas) 

400kV cable corridor between transmission compound and sealing end 

compounds 

132kV cable corridor between transmission compound and joint with 

existing Leiston A 132kV cables 

Substation and sealing end compounds temporary laydown areas 

(excluding land already covered within cable corridor) 

6.2ha 

0.9ha 

0.6ha 

10ha 

Access roads 0.3ha 

Service reserves 0.3ha 

Beach compound and access restrictions 1.5ha 

Total 17.4ha 

6.16.11 Further information regarding the design of the substation and the proposed 

landscaping around it is provided in Section 21 Landscape, Seascape and 

Visual Character. 



The GWF substation footprint 

6.16.12 Although GWF and GGOWF are expected to be of a similar installed capacity 

and are located in similar geographical locations there are significant 

differences between the two, necessitating an increase in the land included in 

the consent application for the GWF substation compared to the GGOWF 

substation. The most significant reasons for this necessary increase in size 

are listed in the following sections of this chapter. 

6.16.13 Unknown reactive power capability of Wind Turbine Generators 

(WTGs): GGOWF wind farm uses a significant amount of reactive power 

from its specific turbines reducing the rating/size of the onshore equipment to 

deliver the NGET code/STC obligations. Before final WTG selection for the 

GWF project it cannot be assumed that a similar capability will be available 

from turbines and that a similar technique will be possible. At this point it is 

assumed that turbines will always operate at unity power factor (as per 

minimum NGET Code requirement) and all necessary reactive power will be 

delivered from the plant onshore. It is expected that the rating of the reactive 

compensation for the GWF project may double as compared to the GGOWF 

project to cope with these increased requirements. 

6.16.14 Different cable parameters: Although GWF is of similar size and location as 

GGOWF there may be a significant difference in the final cable parameters 

used in both wind farms. The differences may be caused by cable design 

(e.g. insulation materials, armouring etc.) and potentially bigger cable size 

(i.e. to cope with increased reactive power flow from the wind farm). 

Alternative cable parameters may increase charging currents by up to 20% 

and necessitate a further increase of the rating of reactive compensation 

installations onshore. 

6.16.15 Increase in harmonic filters requirements: Accurate specification of 

harmonic filters to be installed in the GWF substation onshore can only be 

completed once the final parameters of electrical systems (turbines, 

transformers, cables, reactive compensation) are available and once the 

harmonic injection limits are provided by NGET - the system operator. The 

harmonic limits are allocated by the grid operator to projects on the first come 

first serve basis. Available harmonic injection limits in the Leiston/Sizewell 

location were largely ‘consumed’ by GGOWF project and the remaining 

headroom for GWF is expected to be much tighter. It is reasonable to 

assume that rating of harmonic filters may increase by 100% - 200% as 

compared to the GGOWF project. This will result in a larger area for 

harmonic filters in the GWF substation compared to the GGOW substation. 



6.16.16 Alternative technologies: There are various technologies available on the 

market to provide the characteristics and functionality required from the GWF 

onshore substation. The technologies may differ quite considerably in terms 

of size but also capital cost, O&M requirements, system availability, H&S 

hazards etc. The technology used in GGOWF project for the reactive 

compensation (Siemens SVC +) tends to be on the lower end of the available 

options in terms of the footprint requirements, also HIS switchgear used by 

Siemens is one of the most compact designs available on the market. 

Therefore it has not been assumed that only equipment with the smallest 

available footprint will be used as this would curtail both the onshore and 

offshore procurement options. 

6.16.17 Equipment make: Similarly as per the previous section, equipment of the 

same rating and class may differ in size when sourced from alternative 

vendors. Based on the experience from other projects, some suppliers tend 

to yield a smaller footprint than similar equipment from other manufacturers. 

6.16.18 Substation accessibility / maintainability: Given the limited space 

available in the GGOWF onshore substation the equipment within the site 

was very densely installed. GWF has to achieve acceptable consideration of 

health and safety aspects of the development on a risk basis and consider all 

practicable options to reduce risk. This includes allowing sufficient space 

between the plant, and width of paths and roads within the substation, to 

afford safer and easier access to all parts of the system without deenergising 

other system components (as has to occur in the GGOWF 

substation). 

6.16.19 As discussed above there are factors significantly affecting rating and size of 

the apparatus to be installed in the GWF onshore substation. It should be 

emphasised that the final sizes of equipment and hence the whole substation 

have a high degree of uncertainty and it is difficult to estimate accurately 

before the technical details of the project become available. Based on 

realistically conservative assumptions as to the expected ratings and quantity 

of the equipment, GWF has commissioned an indicative design from a major 

supplier to develop a draft onshore design for the GWF compound to allow a 

realistic estimate of the compound footprint to be calculated. Based on this 

work the expected land take for the onshore substation is approximately 1.8 - 

2ha (i.e. about 3.6 - 4 times footprint of the equivalent compound in the 

GGOWF onshore substation). 

6.16.20 The size of the transmission compound which will be used to connect GWF 

to the onshore transmission network is also expected to increase in size as 

compared to the substation serving GGOWF (called Leiston A). NGET 

utilises the pre-existing super grid transformers (SGTs) and 400kV 

switchgear located within Sizewell B power station for the GGOWF 

connection but this infrastructure has insufficient spare capacity to 

accommodate the expected output of GWF and therefore additional SGTs 

and switchgear would be required within the new transmission compound, as 

well as the 132kV switchgear that was required for GGOWF. 



6.16.21 The SGTs and 400kV equipment will be located within the same compound 

as the 132kV switchgear, increasing the size of the transmission compound 

by approximately 100 - 150%. 

6.17 Project Programme 

6.17.1 The key project programme details are detailed in Table 6.12. The dates 

may be subject to change, but provide the current best estimate to enable a 

meaningful assessment to be made at this juncture. 

6.17.2 As the offshore wind farm industry is developing rapidly, a number of the key 

components are in high demand. In addition, several components have long 

lead times such as the subsea export cables and the main transformers 

which can take up to 18 months to be manufactured. Therefore production 

slots have to be booked several years in advance to secure these items. 

This is an important consideration, as it sequentially shapes and drives the 

overall programming of works and the selection of components of the 

scheme. 



Table 6.12 Indicative project programme 

Timing 2012 2013 2014 2015 2016 2017 

Detail 

Achieve consent 

Grid construction 

Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 

3 

Offshore 

foundations 4 

Offshore cables 5 

Offshore 

topsides 6 

Commissioning 

and handover 

Project 

completion 

3 Encompasses all onshore sub-station construction, cable installation and NGET transmission system works 

4 

Includes all WTG and ancillary infrastructure foundations 

5 

Includes export, inter and interra-array cable works 

6 

Includes all WTG components and topsides of ancillary infrastructure 



6.18 Offshore Pre-construction and Construction 

6.18.1 Construction of the offshore works would generally be completed in a number 

of stages, which are as follows: 

� Pre-construction surveys 

� Prefabrication (structures constructed onshore) 

� Transportation (structures floated or transported by 

transportation vessels) 

� Offshore foundation structure installation 

� Offshore substation installation and commissioning 

� Inter and intra-array cabling 

� Transitional pieces installed 

� WTG installation 

� Cable landfall works 

� Export cabling 

� Commissioning. 

6.18.2 Installation of the offshore elements would primarily take place outside of the 

winter months due to the potential adverse weather conditions, which 

increases the risk of delays in activities and excessive costs. The main 

offshore construction season may extend from March to November each year 

depending on the vessels chosen by the construction contractor, although 

consideration would still be given to working outside of this envelope if 

practicable and safe. Offshore construction works would normally be carried 

out on a 24 hour operations basis. 

6.18.3 A two season programme is likely to be adopted. In the first season 

installation would also include all of the navigational aids required to ensure 

navigational safety, as laid down in the appropriate guidelines (see Chapter 

17). The commissioning programme would then take place throughout both 

construction seasons, as the WTGs would be brought online sequentially. 

6.18.4 The project timetable would ensure that any seasonal restrictions on certain 

activities identified during the consenting process are adhered to. Subject to 

all consents for the project being received during late 2012, it is anticipated 

that GWF project would be constructed in 2013 – 2016 though this would be 

influenced by a number of factors. 

6.18.5 Table 6.13 summarises the indicative suite of construction vessels and 

vehicles that would be utilised for the construction of offshore components of 

GWF. 



Table 6.13 Construction activity summary 

Construction 

aspect 

Pre-construction 

geophysical survey 

Pre-construction 

geotechnical survey 

Detail 

Dedicated geophysical survey vessel using side scan sonar, 

multibeam echosounder and magnetometer. Would survey GWF site, 

export cable corridor and landfall site. 

Dedicated geotechnical survey vessel taking a number boreholes, 

cone penetration tests (CPT) and vibrocores within the GWF site, 

export cable corridor and landfall site. 

Pre-lay grapnel run Dedicated vessel with PLGR device and ROV 

Plough trails Cable installation vessel along selected installation equipment 

(plough, jetting ROV and or trencher). 

WTG and ancillary 

infrastructure 

foundations 

Foundation installation HLV / jack-up barge, possible grouting vessel, 

possible foundation transportation vessel and opossible support 

vessels 

. 

Scour protection Construction barge or dedicated rock placement vessel 

WTGs HLV or jack-up barge 

Ancillary structures 

(OSP, collection 

station, 

accommodation 

platform and met 

mast) 

HLV or jack-up barge, substation installation vessel 

Cable lay Cable lay barge/ vessel 

6.18.6 The physical footprint on the seabed from the construction vessel activity 

would come from a number of sources including PLGR work, jack-up barge 

legs, anchor placement (if DP vessels not used), cable 

plough/trencher/jetting machine and would be dependent on the method 

utilised and the number of movements required (dictated by the WTG option 

taken forward and site layout). A jack-up barge would have between four 

and six legs with a footprint of around 30m 2 per leg. The anchor vessels 

would have between four and six anchors each, with the anchor size being 

around 2-4m 2 . 



6.18.7 In addition to these main vessel movements, there would also be significant 

levels of activity undertaken by smaller support vessels, including: 

� Tow barges 

� Anchor handling tugs 

� Offshore supply vessels 

� Crew vessels for personnel / equipment transfer 

� Standby vessels 

� Guard vessels. 

Construction logistics, management and security 

6.18.8 During marine operations a Safety Zone would be applied for the 

construction, commissioning and operational phases of the project. 

6.18.9 The purpose of a safety zone would be to manage the interaction between 

vessels and the wind farm in order to protect life, property and the 

environment. The fundamental principle is that vessels would be kept at a 

safe distance from construction, commissioning and operational activities 

related to the wind farm in order to avoid collisions. 

6.18.10 A 500 metre safety zone (the maximum permissible under international law) 

is very likely to be in place around each turbine during the construction 

phase. The safety zone would be monitored and controlled by the Project 

with the support of a Marine Control Centre. 

6.18.11 During construction, wind farm extremities are generally marked with 

standard cardinal marks, and in areas of high traffic density guard vessels 

may also be employed. The requirement for such measures are set out in 

the International Association of Lighthouse Authorities (IALA) 

Recommendation O-117 on ‘The Marking of Offshore Wind Farms’ (as 

detailed under Section 6.23 below). Jack-up barges and HLV whilst 

‘engaged’ in construction work would be lit in line with the requirement of 

IALA Recommendation O-114 on the Marking of Offshore Structures. 

Specific detail on that proposed for GWF is provided in Chapter 17 Shipping 

and Navigation. 

Pollution prevention 

6.18.12 Pollution prevention would be controlled and mitigated from the design stage 

onwards. For example, the WTG nacelle frame would typically be designed 

and manufactured with a bund incorporated which can hold the full oil content 

of the gearbox in the event of catastrophic failure. Additionally, if any oil filled 

transformers are used, again the area would be bunded to contain any oil 

leaks (as discussed in Section 6.9 and 6.10). 



6.18.13 The staff and vessel crew would be trained and equipped to use spill kits in 

the event of a break in containment occurring. This would be defined by 

GWFL at the appropriate juncture and would be governed by a full Risk 

Assessment and Method Statement process additionally the work relating to 

the WTG would specifically be controlled and managed via the Wind Turbine 

Safety Rules . 

6.18.14 There would also be a waste management procedure which would be 

administered and managed to ensure it is strictly adhered to by site staff, 

contractors and visitors to the wind farm. 

6.18.15 In the event of the safe system of work failing or a catastrophic incident 

occurring it is assumed that a spill response contract would be in place to 

control, manage, recover and dispose of any contaminants and dropped 

objects. 

Construction ports 

6.18.16 GWFL are not in a position to make commitments with regard to specific 

ports at this stage. The construction ports used for the GWF project would 

be driven by the contractual placements for the wind farm components and 

availability of sufficient port space. 

6.18.17 It is envisaged that a local port would be selected by the GWF project 

principal contractor based on suitability for facilitating the transportation 

vessels, equipment to move the wind farm components and skilled labour 

force, if secondary fabrication is required. The port would require the 

necessary space to layout a production line to fit the components, have a 

large volume of storage space and be able to accommodate deep water draft 

construction vessels. 

6.18.18 The location of other ports associated with the construction process would be 

driven by the location of the chosen manufacturing companies for the various 

components associated with the wind farm. Experience from Round 1 and 2 

projects indicates that this diversity may be at a global scale. 

6.19 Onshore Construction 

Construction sequence and timing 

6.19.1 Onshore, the construction programme is based on a likely 32 month 

programme. The construction sequence and approximate length of each is 

shown in Table 6.14. 



Table 6.14 Onshore construction timings 

Activity Duration 

Site preparation 6 months 

Construction of both substations 24 months 

Onshore cabling 8 months 

Transition pit 3 months 

HDD works 4 months 

Site demobilisation 3.5 months 

6.19.2 It is estimated that the onshore construction workforce would not exceed 200 

personnel during the peak construction period. Onshore working shifts are 

anticipated to be 08:00 to 19:00 from Monday to Friday; and 09:00 to 13:00 

on Saturday. Construction activity may occasionally require 7 days per 

week. Where the working hours are expected to extend beyond those given 

above, these periods would be agreed in advance with the Local Planning 

Authority to ensure that they do not clash with programmed community 

activities such as village fetes. The following activities could result in an 

extended working week: 

� Continuous concrete pours, which would generally occur nearer 

the start of the civil construction phase 

� Weather window during a spate of bad weather 

� Testing of equipment. 

6.19.3 It is noted that the above work timing does not apply to works related to cable 

landfall in the intertidal zone, where working hours would be dictated to a 

large extent by the tidal state. 

Site preparation and earthworks 

6.19.4 The site boundary would be securely fenced using a suitable steel mesh and 

panel fencing system. A construction compound would be located within the 

site boundary. This would include contractor’s offices / cabins plus an 

equipment storage area. 



6.19.5 Earthworks would be required in order to create a level area upon which the 

substation would be built, which would generate spoil. Excavated material 

would be used as part of the site landscaping wherever possible, however 

this may not account for all the material generated. Should additional spoil 

be taken off site this would be done in accordance with the agreed site waste 

management plan. 

6.19.6 It is estimated that this phase of the civil engineering works would take 

approximately 6 months to complete. 

Building construction works 

6.19.7 The substation buildings would most likely be constructed immediately 

following the initial civil engineering works, with an estimated timescale of 24 

months. 

Earthworks and planting 

6.19.8 The remaining programme of topsoil reinstatement and planting works would 

be implemented to visually contain the new substation and sealing end 

compounds. Full details are provided in Section 21 Seascape, Landscape 

and Visual Character. 

Traffic and access 

6.19.9 Table 6.15 provides a provisional estimate of the vehicle movements 

required during the construction of both the GWF and transmission 

compounds and associated infrastructure. This predicts not only the delivery 

of major plant items but also the daily travelling of the workforce, based on 

current assumptions. 

Table 6.15 Summary of construction related traffic 

Key traffic 

Site preparation 315 lorries (630 movements) 

GWF Substation 2650 lorries (5300 movements) 

Onshore cabling 300 lorries (600 movements) 

Transition pit 30 lorries (60 movements) 

HDD works 100 lorries (200 movments) 

Site demobilisation 213 lorries (426 movements) 

Max daily workforce 

200. Therefore assume 150 cars per day 

(300 movements) 



6.20 Commissioning 

6.20.1 To ensure that the electrical energy from GWF is delivered to the national 

400kV transmission system, three parties must ultimately operate the assets 

applied for in the GWF consent. These three parties are GWF, NGET (the 

operators of the 400kV network, and the offshore transmission owner 

(OFTO), which is the new regulatory regime for licensing offshore electricity 

transmission. 

6.20.2 The GWF project, as described within this chapter, can be split into the 

following systems or components, from which follows a description of the 

commissioning process and associated parties responsible: 

� Transmission grid connection; 

� Onshore GWF substation; 

� Export cables; 

� Offshore platform substation; 

� OFTO Supervisory Control And Data Acquisition (SCADA) system; 

� Array cables; 

� Equipment within transition pieces (if TPs are installed); 

� Balance of Plant SCADA system; 

� Wind Turbine Generators; and 

� Wind SCADA. 

6.20.3 NGET would be responsible for the transmission grid connection 

commissioning, whilst the OFTO would take responsibility for the onshore 

GWF substation, export cables, offshore platform substations and the 

associated OFTO SCADA system. 

6.20.4 The GWF project principal contractor would manage the delivery of the 

commission programme associated with the array cables, equipment within 

the transition pieces (if TPs are installed) and the balance of plant SCADA. 

6.20.5 The array cables transport the electrical energy onto the offshore platform 

substations, were an assortment of MV equipment manages the electricity 

prior to it reaching the OFTO interface point. The GWF project principal 

contractor would also be responsible for the commissioning this MV 

equipment. This leaves the GWT WTG and the wind SCADA to be 

commissioned by the appointed wind turbine manufacture. 

6.20.6 Commissioning would generally be made up of the following process, with 

procedures formalising the different stages: 

� A mechanical, visual and electrical continuity assessment; 



� An energisation programme; 

� Testing mechanical, electrical and control functions; 

� Identification of faults; 

� Rectification of faults; 

� Re-testing; and 

� Certification. 

6.20.7 The commissioning of GWF would be in accordance with approved 

commissioning procedures and would include the NGET commissioning 

procedures where applicable. All commissioning activities would be the 

subject of an approved safe system of work. Commissioning activities would 

include the WTG’s performance and reliability testing and compliance with 

the grid code standard. 

6.21 Offshore Operations and Maintenance 

Overview 

6.21.1 All elements of the onshore and offshore GWF project would be designed to 

operate unmanned with the systems monitored and instructions issued from 

a central location 24 hours a day. 

6.21.2 The wind farm and associated plant and apparatus would be controlled and 

monitored centrally using SCADA systems, most likely located with in the TP. 

The SCADA systems are the means by which the monitoring is undertaken 

and commands relayed to the equipment. 

6.21.3 The facility would also exist for starting and stopping of WTG in events such 

as emergencies or access to the turbines by helicopter for hoisting personnel 

onboard. 

6.21.4 The WTG would normally shut down during severe weather conditions, when 

wind speeds exceed 25ms -1 to avoid damage to the turbine components. 

6.21.5 Planned outages for a WTG would be triggered primarily by routine 

maintenance requirements, but also occasionally at the request of the 

Maritime Rescue Co-ordination Centre (MRCC) in support of Search and 

Rescue (SAR) activities in the area. 

Operational safety zones 

6.21.6 It is likely, although to be confirmed, that an operational safety zone of 50m 

around each structure would be applied for. Furthermore, during 

maintenance operations this would be extended to 500m (the maximum 

permissible under international law) around the relevant structures. 



6.21.7 The control mechanisms and processes associated with the operational 

phase would be as specified under construction details. 

6.21.8 Once the Wind Farm is in operational use, an Automatic Identification 

System (AIS) as well as CCTV from control centre may be used to monitor 

vessel movements within the wind farm. 

WTG navigation aids and lighting 

6.21.9 As with the construction phase, the marking of the wind farm would be in 

accordance with the requirements set out in the IALA Recommendation O- 

117 on ‘The Marking of Offshore Wind Farms’ and IALA Recommendation O- 

114 on the marking of offshore structures. Under these recommendations 

the following would be of relevance for GWF: 

� Navigation aids would be fitted on any WTG below the lowest point of 

the arc of rotation of the turbine blades, and at a height above HAT of 

not less than 6m or more than 15m, typically at the top of the yellow 

section of the mast. 

� Corner or significant boundary point WTGs would be designated a 

Significant Peripheral Structure (SPS), with a minimum separation 

distance of 3nm between SPS’s. Each SPS would be fitted with lights 

that are visible from all directions in the horizontal plane, and the lights 

on a structure should be synchronised to show a yellow ‘special mark’ 

light characteristic with a range of not less than 5 nautical miles. 

� Intermediate Peripheral Structures (IPS) may be used between SPS. 

These would be within 2nm of SPS, and fitted with lights as per SPS, 

but with a distinct flash characteristic. They would be visible from a 

minimum range of 2nm. 

6.21.10 Additional aids to navigation would be at the discretion of the operator, and 

may include: 

� Racons, which may have morse letter ‘U’ or radar reflectors; 

� AIS; and 

� Sound signals may be fitted for restricted visibility with a range of not 

less than 2 nautical miles. 

6.21.11 Ancillary structures would be marked in accordance with IALA 

Recommendation O-114 on the Marking of Offshore Structures. 

6.21.12 Individual WTG would be marked with a unique alphanumeric identifier which 

would be clearly visible at a range of not less than 150m. At night, the 

identifier would be lit discretely (e.g. with down lighters). 



Operation and maintenance activities 

6.21.13 O&M of the wind farm after commissioning would comprise both scheduled 

and unscheduled maintenance events. Scheduled works on the WTG and 

onshore/offshore electrical infrastructure would include annual or bi-annual 

maintenance and inspection visits. In addition, necessary retrofitting and 

upgrading works may also take place. The scheduled works would normally 

be timetabled for the summer months, given the typically more settled 

weather and longer day light hours. During this period, O&M personnel at 

site would be expected to peak, consisting of up to 30 technicians and eight 

vessel crew aboard four vessels. 

6.21.14 Unscheduled repair activities would range from attendance on location to 

deal with the resetting of false alarms to major repairs. The frequency of 

unscheduled activities would be expected to be highest in the early years of 

operation (one visit per turbine, per month) when experience on other wind 

farms sites has shown the highest number of teething faults tend to occur. 

After the first two years of operation, unscheduled visits to turbines would be 

assumed to reduce in frequency to six month intervals. 

Access strategy 

6.21.15 Access to each installation offshore would be by boat or helicopter with at 

least two service personnel being on each offshore structure at any one time 

for safety reasons. In order to achieve the maintenance programme, it is 

anticipated that O&M teams would work simultaneously on several WTG 

(and potentially also on the offshore substations). It is therefore expected 

that, when boat access is required, at least two vessels would be on-station 

within the wind farm site at all times that O&M work is being undertaken. 

6.21.16 GWFL would consider the use of both boats (currently using aluminium 

catamarans known as windcats) and helicopters when establishing a suitable 

access strategy. The boats may be used for routine maintenance operations 

and in weather conditions up to approximately 2m wave height. Helicopters 

may be used in situations which are time crucial and for increasing access 

when vessels are unable to work. 

6.21.17 GWFL may also explore the potential need for the use of a mothership, such 

a vessel typically serves to accommodate the personnel, provides a control 

room facility and acts as the main stores location for the site. They are also 

commonly fitted out with a number of work boats and associated launch and 

recovery systems. Latest mothership vessel designs also incorporate ROV, 

diver operations and helicopter pads and are therefore able to service much 

of the post construction and asset management work that would be required 

over the life of the project. 

6.21.18 The primary means of transferring personnel would be via the workboats 

which may be stationed with the mothership (if utilised), but this would also 

be supplemented by helicopter support. 



6.21.19 Detailed evaluation is underway to identify the best access strategy for GWF. 

It would consider the extension of existing GGOWL services to cover the 

GWF project. 

Maintenance team 

6.21.20 GGOWF already have an Operational Team in place for the day to day 

management and control of their project. The GGOWF Operational Team 

are based in a purpose built facility situated on the key side at Lowestoft. 

This is also the location of the maintenance transportation vessel. The 

operation and maintenance activities have already been scoped, plans 

developed and necessary services procured. In addition, call off orders are 

already in place with companies who can provide specialist vessels or 

equipment that might be required at short notice in the event of a failure. 

6.21.21 Further investigation is presently being undertaken to assess the suitability of 

the GGOWF work remit and services being extended to include the GWF 

project as it enters its working phase. 

6.21.22 The OFTO would be responsible for the offshore substations and the export 

cables. Once appointed it would be their decision on the exact numbers of 

full time service personnel, the marine access options to be used and their 

base location. 

Offshore accommodation 

6.21.23 The addition of an accommodation platform could facilitate the mobilisation of 

maintenance crew for short durations or in an emergency, adding a degree of 

further flexibility to the maintenance strategy. Further detailed analysis would 

be undertaken to evaluate the benefits of the accommodation platform before 

the final decision is taken on its appropriateness. GWFL would explore all 

possible accommodation options, if required, including a fixed platform, 

floating hotel and jack-up vessel for transferring and accommodating 

maintenance staff. 

6.21.24 The supply logistics for the fixed platform/floating hotel/jack-up operating as 

the operations and maintenance hub, would be managed by the onshore 

O&M base, with an offshore supply vessel used to replenish the 

accommodation facility with food, water (if necessary), fuel, spares and 

equipment on a regular basis to meet operational requirements 

Operation and maintenance port and facilities 

6.21.25 The GGOWF project has utilised Lowestoft, in Norfolk, and it is noted that 

this facility has additional space which could be converted to accommodate 

GWF. However, as for construction, the O&M port is under consideration, 

with an independent port study current being undertaken. 



6.21.26 The O&M base would need to be able to provide facilities for the O&M crew, 

control centre for the operations, storage space for maintenance equipment 

and spare parts, and some strategic spare parts such as gearboxes, 

generators, and possibly blades. 

6.21.27 It is also likely that a telecommunication mast would have to be erected as 

part of the O&M base infrastructure to communicate with the wind farm 

personnel and the associated transport. Typically the mast could be 

approximately 18m high and be used for VHF telecommunications and 

microwave line of site communications if used. 

6.21.28 The chosen O&M facility would be equipped with a chart indicating the GWF 

projects WTG displaying their unique identification numbers, in accordance 

with MGN 371 (M+F). 

6.21.29 It would be possible for each individual WTG to be remotely controlled from 

the O&M facility. This would enable WTG to be controlled and shut down at 

the request of the MRCC in support of SAR activities in the area. These 

procedures would be tested on a regular basis, in accordance with MGN 371 

(M+F). Furthermore, the WTG would be remotely monitored on a 24 hour 

basis from the WTG manufacturer’s control room. The above is discussed in 

more detail in Chapter 17. Any port works falling under the remit of the 

Planning Act, the Town and Country Planning Act or other relevant planning 

legislation do not form part of the GWF application. 

6.22 Onshore Operations and Maintenance 

Cable route overview 

Operation 

6.22.1 Once operational the onshore cable system would be primarily beneath the 

ground surface and buried to a sufficient depth to allow ploughing and other 

agricultural practices to continue. The only above ground features would be 

related to the onshore transition pit if required. This would preferentially be 

located at a boundary feature, which would minimise the impact to farming 

activities. 

Maintenance 

6.22.2 A full check of the cable system would be carried out on an annual basis. 

Access would normally be along the agreed cable route wayleave by foot. 

Fault repairs 

6.22.3 In the unlikely event that there is any failure of cables, a fault finder with test 

gear would locate the fault along the cable section. Once located, the area 

around the fault would be excavated and the fault repaired. If the cable 

cannot be repaired, a new length of cable would be inserted and jointed to 

replace the failed section. 



Substation overview 

Operation and maintenance 

6.22.4 It is not anticipated that the new substation would be permanently manned 

but maintenance and inspection visits would occur. 

6.22.5 In summary, the main equipment at the site would consist of transformers, 

electrical reactors, capacitors, and switchgear. Best practice guidelines for 

the frequency of maintenance of this equipment is summarised in Table 6.16, 

and would be adhered to during the lifetime of the operational phase. 

Table 6.16 Maintenance required at substation 

Equipment Maintenance 

frequency 

Transformer / 

electrical reactor 

Routine – annually 

Intermediate – four 

years 

Major – 12 years 

Type of work 

� Check silica gel and ventilator, 

check for oil leakage, check cooling 

equipment, general visual 

inspection; 

� Test for water content, dielectric 

strength, acidity and dissolved gas 

analysis; and 

� Clean bushings and grease 

ventilator. 

Capacitors Routine – annually � General visual inspection. 

GIS / AIS switchgear Routine – annually 

Intermediate – six 

years 

� Check gas pressure, check and 

clean mechanisms and hydraulics; 

and 

� Test protection and control 

equipment. 

6.22.6 It is envisaged that the maintenance works would normally only require a site 

visit by a light goods vehicle (LGV). Any major equipment failure 

necessitating removal would require the use of suitable mobile cranes. 

Utilities and environmental issues 

6.22.7 Potable water for drinking and washing would be provided by a permanent 

connection to the local mains water network. Wastewater disposal from site 

would be either facilitated by a permanent connection to the local sewer 

network or via a septic tank. 



6.22.8 Surface water drainage options would be developed based on an 

understanding of the area of impermeable surfaces being created and known 

soil infiltration rates. The drainage plan would ensure that the surface water 

run-off rate of the new development would be maintained at the existing 

‘greenfield’ rate. Sustainable Drainage Systems would be preferred and 

would be expected to include, permeable paving, swales, and other water 

attenuation techniques. The risk to water quality from pollution sources at 

the station is considered to be low. Surface water quality would be protected 

by the following measures: 

� Oil retention bunds around the transformers, reactors and any 

other oil filled equipment that may be used; and 

� Oil interceptors on the surface water drainage gullies. 

6.22.9 The transformers and electrical reactors would be filled with mineral 

insulating oil and would be located within bunds to contain any leakage. The 

capacitors are sealed units and contain non-PCB, biodegradable insulation 

liquid. The switchgear would be constantly monitored for gas pressure to 

detect any leakage. 

6.22.10 Transformers, electrical reactors and associated cooling equipment would 

comply with NGET noise level specifications measured in accordance with 

IEC 60076-10. Where additional noise mitigation measures would be 

required, and where these would not be achieved through distance 

attenuation or baffling by site screening, consideration would be given to 

employing noise attenuation housings around equipment where practicable. 

Noise impacts are considered within Chapter 27 Noise. 

6.22.11 It is not envisaged that the site would be provided with permanent lighting 

around the internal roads or equipment. Lighting may be provided at the 

main entrance doors for safe ingress / egress to the buildings. In the event of 

any essential works, temporary task lighting would be provided by the 

maintenance staff in addition to any installed lighting use. 

Operational safety zones 

6.22.12 The onshore substation site would have a security fence surrounding its 

perimeter. Access beyond the entrance at Sizewell Gap would not be 

possible. Beyond this there are no formal operational safety zones 

envisaged for GWFL. 

6.23 Repowering 

6.23.1 The wind farm’s operational life is defined (by The Crown Estate) as up to 25 

years, an additional two years would be granted to the lease to allow 

decommissioning to take place. All elements of the wind farm would be 

designed with a minimum operational life of 25 years. 



6.23.2 Following this a decision would be made on whether the operating company 

wish to proceed with decommissioning or apply to the relevant Regulatory 

Authority at the time, to repower the wind farm. 

6.23.3 Should repowering be sought then an investigation would be undertaken as 

to the possible options for this. It is envisaged that any such repowering 

activity would require a new agreement for lease with The Crown Estate and 

would be subject to an additional planning consent application. 

6.23.4 It is acknowledged within the Scoping Opinion for GWF that the statutory 

nature advisory body (the Joint Nature Conservation Committee, JNCC) has 

requested (on Pg 5 of their response) that: 

“It is important to be clear on what repowering entails and whether there 

is likely to be any relocation of subsea infrastructure or alteration of the wind 

farm layout. This includes whether further scour protection is required for 

foundations in the same, or in new, locations across the wind farm site. Any 

alterations to the locations of offshore elements for repowering may require 

an update to the benthic survey work and assessments that have previously 

been carried out”. 

6.23.5 GWFL are not able to make such detailed statements with regard to 

repowering at this stage. GWFL do however, commit to working closely with 

the Government’s advisory bodies throughout the life cycle of the project and, 

should at any stage repowering be considered, progressing detailed dialogue 

covering how such matters would take place. 

6.24 Decommissioning 

6.24.1 A full Decommissioning Plan for the project would be drawn up and agreed 

with the Department for Energy and Climate Change (DECC) before 

construction commences. 



6.25 References 

DONG energy, (2009). The Monopod Bucket Foundation; Recent experience 

and challenges ahead. Presentation at Offshore Wind 2009 

DOWL, (2009). Dudgeon Offshore Wind Farm: Environmental Statement. 

DOWEC, (2003). Suction bucket foundation. Feasibility and pre-design for 

the 6 MW DOWEC 

GGOWL, (2009). Cable laying and landfall plan 

GGOWL, (2011). Greater Gabbard Construction Method Statement 

Ibsen, L.B., Liingaard. M., Nielsen, S. A, (2005). Bucket Foundation, a status. 

Jim Hodder Associates. (2010). Galloper Wind Farm: Cable Desktop Study. 

Report C1001\101130 

Subacoustech, (in prep). Underwater noise modelling for the Galloper Wind 

Farm 

Nedwell J R, Turnpenny A W H, Lovell J, Langworthy J W, Howell Dm and 

Edwards B. (2003). The effects of underwater noise from coastal piling on 

salmon (Salmo salar) and brown trout (Salmo trutta). Subacoustech report to 

the Environment Agency 

Nedwell J R, Parvin S J, Edwards B, Workman R, Brooker A G and Kynoch J 

E (2007) Measurement and interpretation of underwater noise during 

construction and operation of offshore windfarms in UK waters. 

Subacoustech Report No. 544R0738 to COWRIE Ltd 

Parvin S J and Nedwell J R. (2006). Underwater noise survey during impact 

piling to construct the Barrow Offshore Wind Farm. COWRIE Project ACO- 

04-2002 

The Crown Estate, (2010). A guide to an Offshore Wind Farm.

Galloper Wind Farm Project - Galloper Wind Farm proposal

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