Tidal Current Effects on Support Structure Loading

Sub-group on ‘Calculation Methods’ 

WG03(Olaison)10068 

<strong>Tidal</strong> <strong>Current</strong> <strong>Effects</strong> on Support Structure Loading 

Introduction 

Offshore wind turbines are subjected to tidal currents. These currents lead to a low-frequency 

fatigue cycle and will influence the dynamic behaviour of the wind turbine support structure. 

Two aspects of the effect of tidal currents on wind turbine support structure were studied. The 

first was the effect on fatigue loading, while the second was the phenomenon of vortex 

induced vibration (VIV). 

Fatigue loading 

The effect of the tidal current on fatigue damage was studied in two ways. One was to do a 

simple hand calculation to investigate the Damage Equivalent Load (DEL) caused by the tidal 

current alone (very low frequency cycle; loading due to drag alone). The other was to 

simulate a full set of fatigue calculations using Bladed with and without tidal current to 

examine the effect of current on wave loading. This was done for two generic turbines, of 

5MW and 2MW ratings, with hub heights of 80m and 65m respectively. 

A maximum mean spring tidal current for the British Isles was estimated to be 1.2 m/s. This 

depth-averaged value equates to approximately 1.4 m/s at the surface using a standard subsurface 

current profile. This value was used as input in both the hand calculations and the 

Bladed simulations. 

A good estimate of fatigue damage is the Damage Equivalent Load (DEL), defined as below: 

DEL 

∑ = m 

RangeOfLoadCycle 

1/ m 

⋅ NumberOfCycles 

ReferenceFrequency⋅TotalTime 

The DEL is the load range that at a certain reference frequency gives the same fatigue damage 

as the real dynamic load, i.e. obtaining the same fatigue life. 

The period of the tidal cycle was assumed to be 12 hours, i.e. 730.5 cycles per year. The static 

loads on the tower structure caused by a tidal current, applying a current surface velocity of 

1.4 m/s, were calculated with Bladed for the two generic turbines. The load ranges caused by 

the tidal current were calculated by using quasi-static simulations which modelled the current 

running in two different directions. 

Table 1 below presents the load range at the seabed calculated with Bladed, and the DEL at an 

inverted SN-slope of 4 for the steel sections of the wind turbine support structure. 

Load range 

DEL 

(m=4) 

2MW 

F x (thrust) 102.0 kN 

M y 

898.8 kNm 

F x 

7.1 kN (2.1%) 

M y 62.3 kNm (1.1%) 

Table 1 Tower loads at seabed 

5MW 

183.5 kN 

2043.7 kNm 

12.7 kN (1.7%) 

141.8 kNm (0.7%) 

In Table 1 above, the values in brackets are the fraction of the damage due to the tidal current 

alone compared with the total damage due to wind and waves. The maximum contribution of 

the tidal current is 2.1%. It should be noted that a relatively extreme current was chosen and 

the same amplitude was assumed over the full year (i.e. the spring-neap cycle was ignored). 

1



In order to investigate the impact of the interaction between waves and current, Bladed was 

used to calculate lifetime fatigue loads in terms of DELs for the two turbines. Two sets of 

simulations per turbine were considered: one including currents; one excluding currents. 

Wind and wave loading were included in both sets. Table 2 presents the external input data 

for the Bladed simulations. 

Wind speed 

[m/s] 

Number of hours 

per year 

Turbulence 

intensity [%] 

Jonswap/Pierson Moskowitz 

wave spectrum 

T p [s] H s [m] 

4 12.0 5.54 0.52 1139.1 

6 12.0 5.43 0.66 1397.2 

8 12.0 5.46 0.86 1628.6 

10 12.0 5.73 1.11 1601.9 

12 12.0 6.17 1.45 1112.4 

14 12.0 6.59 1.86 774.3 

16 12.0 6.98 2.28 534.0 

18 12.0 7.35 2.70 293.7 

20 12.0 7.82 3.17 169.1 

22 12.0 8.24 3.64 80.1 

24 12.0 8.62 4.25 35.6 

Table 2 External input data for Bladed simulations 

Table 3 and 4 present the tower seabed DELs for the 2MW and the 5MW turbine 

respectively. To calculate these results the tower dynamics were not included in order to show 

the change in applied loads due to the tidal current. 

Load m Wind and waves Wind, waves, and currents %Change 

Tower (seabed) Mx 4 2229.8 2229.8 0.0% 

Tower (seabed) My 4 4106.3 4151.7 1.1% 

Tower (seabed) Mz 4 469.6 469.6 0.0% 

Tower (seabed) Fx 4 286.4 292.4 2.1% 

Tower (seabed) Fy 4 27.9 27.9 0.0% 

Tower (seabed) Fz 4 27.7 27.7 0.0% 

Table 3 2MW Tower seabed DEL comparison – No tower dynamics 


Tower (seabed) Mx 4 6009.2 6009.2 0.0% 




Tower (seabed) Fy 4 60.2 60.2 0.0% 


Table 4 5MW Tower seabed DEL comparison – No tower dynamics 

It appears that the smaller wind turbine is more sensitive to the tidal current. The loads of the 

2MW turbine show an increase of Fx of 2.1% and an increase of My of 1.1%. These are 

relatively small values, but still not necessarily insignificant. 

It is more difficult to provide general results concerning the influence of the tidal current on 

the dynamic behaviour of the wind turbine, hence to predict the influence on fatigue loading. 

Tables 5 and 6 presents the seabed DELs for the 2MW and the 5MW turbines respectively 

where the tower dynamics were included in the simulations. 

2



Considering the 5MW turbine, tower Fx at the seabed decreased by 1.5% (see Table 6) when 

the tower dynamics were included, but the same load increased with 1.1% (see Table 4) when 

the tower dynamics were excluded. This indicates that due to interaction between the different 

structural modes and the dynamic behaviour of the wind and sea this particular fatigue load 

decreases due to the currents. 


Tower (seabed) Mx 4 1286.5 1266.1 -1.6% 




Tower (seabed) Fy 4 37.1 36.6 -1.2% 


Table 5 2MW Tower seabed DEL comparison – Including tower dynamics 


Tower (seabed) Mx 4 4087.1 4063.1 -0.6% 


Tower (seabed) Mz 4 1381.6 1380.6 -0.1% 

Tower (seabed) Fx 4 773.2 761.9 -1.5% 

Tower (seabed) Fy 4 162.0 157.5 -2.8% 

Tower (seabed) Fz 4 57.8 57.3 -0.9% 

Table 6 5MW Tower seabed DEL comparison – Including tower dynamics 

Vortex induced vibration 

Vortices which form and shed into the flow around the tower structure produce a dynamic 

loading upon the tower. Vortex induced vibration (VIV) may occur if the eigen frequency of 

the tower is close to the frequency of vortex shedding. The forces due to vortex shedding are 

the result of complex fluid dynamic phenomena and are sensitive to a wide range of 

parameters which describe the characteristics of the flow and the structure. 

The objective of this study was to investigate the possibility of vortex induced vibration 

occurring for wind turbine support structures in sea currents, and to give a guideline for how 

to avoid it. 

The flow speed at which significant vortex induced vibration may occur generally depends on 

the structural mass and damping and on the displaced mass. The non-dimensional mass 

damping parameter k s , combines these influences, and is defined below [1]: 

k 

s 

2⋅ 

m e 

⋅δ 

= ρ 

2 

⋅ D 

Where: 

m e = the equivalent mass per unit length of the structure 

δ = the logarithmic decrement of structural damping 

ρ = the density of the fluid 

D = the diameter of the structure 

Using data for the two generic wind turbines, and a density of the sea water of 1030kg/m 3 , the 

k s values for the two generic machines are 0.017 and 0.020 for the 2MW and the 5MW 

machines respectively. 

3



The onset of vortex induced in-line vibration occurs between U/f n D = 1.0 and 1.6, for small 

mass damping parameters (k s

Tidal Current Effects on Support Structure Loading

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?