Svetsaren 1/2008 - Esab

(Hartlepool). All are equipped with large prefabrication
and assembly halls for indoor
construction and are capable of handling large
projects, simultaneously.
HFG is part of the Heerema Group, together with
Heerema Marine Contractors (HMC) which transports,
installs and removes offshore facilities, and
INTEC engineering, which provides engineering
services to the energy industry. HFG Engineering,
a subsidiary of HFG with offices in New Orleans
and Houston, specialises in on- and offshore
facility designs.
All Heerema companies operate an integrated
management system that complies with ISO 9001:
2000 (Quality Management Systems), ISO 14001:
2004 (Environmental Management System) and
OHSAS 18001: 1999 (Occupational Health and
Safety Management System) standards.
The Tombua Landana template
The tower base template (TBT) has a surface area
of forty by forty metres, is 24 metres high and
weighs 3,000 tons. It includes 12 main foundation
piles with a total weight of 9,500 tons. It was completed
and shipped to Angola in December 2007.
Figure 1 shows a sketch of the TBT. The principal
components are the pile sleeve clusters, the rows
and the leveling jacks. Not indicated, but
discussed later in this article, are the lifting
trunnions – used for dual crane lifting with one of
HMC’s specialised barges.
The pile sleeve clusters form the cornerstones of
the TBT. They guide the twelve foundation piles
which are driven through them into the sea bed.
Crucial during installation of the 190 m long piles in
nearly 400 m deep waters, are the open cones on
top of the pile sleeves. They catch the foundation
piles hanging from the crane and guide them into
the sleeves, after which pile driving commences.
Part of the length of all foundation piles remains
extended above the pile sleeves. The Tower Base
Section – the lower part of the tower – is placed
over them and secured to the template.
The rows are a network of heavy pipes connecting
the four pile sleeve clusters to form a rigid
construction. Four leveling jacks, devices to
position the template horizontally with great
accuracy, are attached to the central columns of
the rows. The shim piles on the leveling jacks rest
on leveling piles in the sea bed. Leveling is
performed by jacking the template up or down
relative to the shim piles.
The entire Tombua Landana project is characterised
by very narrow construction tolerances, the
substructures being placed on top of each other,
in nearly 400 m deep waters - a particularly
unforgiving environment for any misalignment.
Also, the TBT was subject to strict dimensional
tolerances – up to three times more precise than
normally required in offshore fabrication.
Moreover, it was the first part of the tower and all
eyes were focused on Heerema. Two Daewoo
representatives and two representatives of
Chevron supervised the project and carried out
regular inspections.
Table 1. Mechanical requirements WPQ for type I and II steels.
Steel grades, mechanical requirements and
preheat temperatures
Steel grades were purchased according to the
“General Specification 1.14 Structural steels and
other materials”, issued by the Cabinda Gulf Oil
Company for the projects in block 14. In this
specification, material types are ranked I and I-X,
II and II-X, III, IV and V. Material types I are for structural
members and tubular joint cans which are
fracture critical and material types II are for structural
members and cans where failure would pose a
threat to the structure. Material types III, IV and V
are for non-critical components. A list of valid steel
classifications is given for each material type.
Heerema Vlissingen purchased various types of
plate according to EN 10225 Grade 355 (thermomechanically
controlled rolled) and API 2MT1 as
rolled, covering the demands of material types
I and II, and meeting special constructional
requirements such as “through thickness
properties”. All main steel was purchased from
Dillinger Hüttewerke in Germany.
Mechanical weld requirements are established by
Cabinda’s General Specification 1.15 – Structural
welding and inspection. Charpy V-notch impact
testing of both weld metal and heat affected zone
was required on all welding procedure qualifications,
with notch locations at the weld centre line, fusion
line and FL+2mm. CTOD testing of the WM and
HAZ was required for type I and II steels with a
thickness greater than or equal to 63 mm (2.5”),
to be performed on the thickest steel to be
welded while using the highest preheat and
interpass temperature permitted by the welding
procedure to be qualified.
Table 1 gives an overview of CVN and CTOD
requirements. An additional cross weld zone hardness
requirement was set at HV10 325 maximum.
In constructions such as these, involving thick
material, the prevention of hydrogen induced
cracking (cold cracking) is essential. This starts
with the purchase of steels with limited
hardenability. Cabinda’s General Specification
1.14 for structural steels and other materials
therefore specifies a maximum Pcm value of 0.23
(extended CE formula).
In welding, preheating, along with the use of
low-hydrogen consumables, is essential in the
prevention of cold cracking. Cabinda General
Specification 1.15 refers to AWS D1.1, for
preheat and interpass temperatures to be applied.
CVN
CTOD
Steel type minimum average minimum single thickness minimum
I 34J/-40ºC 27J/-40ºC 76mm (3”)
0.38mm/-10ºC
II 34J/-18ºC 27J/-18ºC 76mm (3”)
0.38mm/-10ºC
8 - Svetsaren no. 1 - 2008