Tidal Current Effects on Support Structure Loading
Tidal Current Effects on Support Structure Loading
Tidal Current Effects on Support Structure Loading
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Sub-group <strong>on</strong> ‘Calculati<strong>on</strong> Methods’<br />
WG03(Olais<strong>on</strong>)10068<br />
<str<strong>on</strong>g>Tidal</str<strong>on</strong>g> <str<strong>on</strong>g>Current</str<strong>on</strong>g> <str<strong>on</strong>g>Effects</str<strong>on</strong>g> <strong>on</strong> <strong>Support</strong> <strong>Structure</strong> <strong>Loading</strong><br />
Introducti<strong>on</strong><br />
Offshore wind turbines are subjected to tidal currents. These currents lead to a low-frequency<br />
fatigue cycle and will influence the dynamic behaviour of the wind turbine support structure.<br />
Two aspects of the effect of tidal currents <strong>on</strong> wind turbine support structure were studied. The<br />
first was the effect <strong>on</strong> fatigue loading, while the sec<strong>on</strong>d was the phenomen<strong>on</strong> of vortex<br />
induced vibrati<strong>on</strong> (VIV).<br />
Fatigue loading<br />
The effect of the tidal current <strong>on</strong> fatigue damage was studied in two ways. One was to do a<br />
simple hand calculati<strong>on</strong> to investigate the Damage Equivalent Load (DEL) caused by the tidal<br />
current al<strong>on</strong>e (very low frequency cycle; loading due to drag al<strong>on</strong>e). The other was to<br />
simulate a full set of fatigue calculati<strong>on</strong>s using Bladed with and without tidal current to<br />
examine the effect of current <strong>on</strong> wave loading. This was d<strong>on</strong>e for two generic turbines, of<br />
5MW and 2MW ratings, with hub heights of 80m and 65m respectively.<br />
A maximum mean spring tidal current for the British Isles was estimated to be 1.2 m/s. This<br />
depth-averaged value equates to approximately 1.4 m/s at the surface using a standard subsurface<br />
current profile. This value was used as input in both the hand calculati<strong>on</strong>s and the<br />
Bladed simulati<strong>on</strong>s.<br />
A good estimate of fatigue damage is the Damage Equivalent Load (DEL), defined as below:<br />
DEL<br />
∑ = m<br />
RangeOfLoadCycle<br />
1/ m<br />
⋅ NumberOfCycles<br />
ReferenceFrequency⋅TotalTime<br />
The DEL is the load range that at a certain reference frequency gives the same fatigue damage<br />
as the real dynamic load, i.e. obtaining the same fatigue life.<br />
The period of the tidal cycle was assumed to be 12 hours, i.e. 730.5 cycles per year. The static<br />
loads <strong>on</strong> the tower structure caused by a tidal current, applying a current surface velocity of<br />
1.4 m/s, were calculated with Bladed for the two generic turbines. The load ranges caused by<br />
the tidal current were calculated by using quasi-static simulati<strong>on</strong>s which modelled the current<br />
running in two different directi<strong>on</strong>s.<br />
Table 1 below presents the load range at the seabed calculated with Bladed, and the DEL at an<br />
inverted SN-slope of 4 for the steel secti<strong>on</strong>s of the wind turbine support structure.<br />
Load range<br />
DEL<br />
(m=4)<br />
2MW<br />
F x (thrust) 102.0 kN<br />
M y<br />
898.8 kNm<br />
F x<br />
7.1 kN (2.1%)<br />
M y 62.3 kNm (1.1%)<br />
Table 1 Tower loads at seabed<br />
5MW<br />
183.5 kN<br />
2043.7 kNm<br />
12.7 kN (1.7%)<br />
141.8 kNm (0.7%)<br />
In Table 1 above, the values in brackets are the fracti<strong>on</strong> of the damage due to the tidal current<br />
al<strong>on</strong>e compared with the total damage due to wind and waves. The maximum c<strong>on</strong>tributi<strong>on</strong> of<br />
the tidal current is 2.1%. It should be noted that a relatively extreme current was chosen and<br />
the same amplitude was assumed over the full year (i.e. the spring-neap cycle was ignored).<br />
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WG03(Olais<strong>on</strong>)10068<br />
In order to investigate the impact of the interacti<strong>on</strong> between waves and current, Bladed was<br />
used to calculate lifetime fatigue loads in terms of DELs for the two turbines. Two sets of<br />
simulati<strong>on</strong>s per turbine were c<strong>on</strong>sidered: <strong>on</strong>e including currents; <strong>on</strong>e excluding currents.<br />
Wind and wave loading were included in both sets. Table 2 presents the external input data<br />
for the Bladed simulati<strong>on</strong>s.<br />
Wind speed<br />
[m/s]<br />
Number of hours<br />
per year<br />
Turbulence<br />
intensity [%]<br />
J<strong>on</strong>swap/Piers<strong>on</strong> Moskowitz<br />
wave spectrum<br />
T p [s] H s [m]<br />
4 12.0 5.54 0.52 1139.1<br />
6 12.0 5.43 0.66 1397.2<br />
8 12.0 5.46 0.86 1628.6<br />
10 12.0 5.73 1.11 1601.9<br />
12 12.0 6.17 1.45 1112.4<br />
14 12.0 6.59 1.86 774.3<br />
16 12.0 6.98 2.28 534.0<br />
18 12.0 7.35 2.70 293.7<br />
20 12.0 7.82 3.17 169.1<br />
22 12.0 8.24 3.64 80.1<br />
24 12.0 8.62 4.25 35.6<br />
Table 2 External input data for Bladed simulati<strong>on</strong>s<br />
Table 3 and 4 present the tower seabed DELs for the 2MW and the 5MW turbine<br />
respectively. To calculate these results the tower dynamics were not included in order to show<br />
the change in applied loads due to the tidal current.<br />
Load m Wind and waves Wind, waves, and currents %Change<br />
Tower (seabed) Mx 4 2229.8 2229.8 0.0%<br />
Tower (seabed) My 4 4106.3 4151.7 1.1%<br />
Tower (seabed) Mz 4 469.6 469.6 0.0%<br />
Tower (seabed) Fx 4 286.4 292.4 2.1%<br />
Tower (seabed) Fy 4 27.9 27.9 0.0%<br />
Tower (seabed) Fz 4 27.7 27.7 0.0%<br />
Table 3 2MW Tower seabed DEL comparis<strong>on</strong> – No tower dynamics<br />
Load m Wind and waves Wind, waves, and currents %Change<br />
Tower (seabed) Mx 4 6009.2 6009.2 0.0%<br />
Tower (seabed) My 4 10218.8 10263.1 0.4%<br />
Tower (seabed) Mz 4 1394.2 1394.2 0.0%<br />
Tower (seabed) Fx 4 518.1 523.7 1.1%<br />
Tower (seabed) Fy 4 60.2 60.2 0.0%<br />
Tower (seabed) Fz 4 60.1 60.1 0.0%<br />
Table 4 5MW Tower seabed DEL comparis<strong>on</strong> – No tower dynamics<br />
It appears that the smaller wind turbine is more sensitive to the tidal current. The loads of the<br />
2MW turbine show an increase of Fx of 2.1% and an increase of My of 1.1%. These are<br />
relatively small values, but still not necessarily insignificant.<br />
It is more difficult to provide general results c<strong>on</strong>cerning the influence of the tidal current <strong>on</strong><br />
the dynamic behaviour of the wind turbine, hence to predict the influence <strong>on</strong> fatigue loading.<br />
Tables 5 and 6 presents the seabed DELs for the 2MW and the 5MW turbines respectively<br />
where the tower dynamics were included in the simulati<strong>on</strong>s.<br />
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C<strong>on</strong>sidering the 5MW turbine, tower Fx at the seabed decreased by 1.5% (see Table 6) when<br />
the tower dynamics were included, but the same load increased with 1.1% (see Table 4) when<br />
the tower dynamics were excluded. This indicates that due to interacti<strong>on</strong> between the different<br />
structural modes and the dynamic behaviour of the wind and sea this particular fatigue load<br />
decreases due to the currents.<br />
Load m Wind and waves Wind, waves, and currents %Change<br />
Tower (seabed) Mx 4 1286.5 1266.1 -1.6%<br />
Tower (seabed) My 4 5624.5 5671.1 0.8%<br />
Tower (seabed) Mz 4 465.8 465.8 0.0%<br />
Tower (seabed) Fx 4 341.2 344.5 1.0%<br />
Tower (seabed) Fy 4 37.1 36.6 -1.2%<br />
Tower (seabed) Fz 4 25.6 25.6 0.0%<br />
Table 5 2MW Tower seabed DEL comparis<strong>on</strong> – Including tower dynamics<br />
Load m Wind and waves Wind, waves, and currents %Change<br />
Tower (seabed) Mx 4 4087.1 4063.1 -0.6%<br />
Tower (seabed) My 4 18787.6 18960.3 0.9%<br />
Tower (seabed) Mz 4 1381.6 1380.6 -0.1%<br />
Tower (seabed) Fx 4 773.2 761.9 -1.5%<br />
Tower (seabed) Fy 4 162.0 157.5 -2.8%<br />
Tower (seabed) Fz 4 57.8 57.3 -0.9%<br />
Table 6 5MW Tower seabed DEL comparis<strong>on</strong> – Including tower dynamics<br />
Vortex induced vibrati<strong>on</strong><br />
Vortices which form and shed into the flow around the tower structure produce a dynamic<br />
loading up<strong>on</strong> the tower. Vortex induced vibrati<strong>on</strong> (VIV) may occur if the eigen frequency of<br />
the tower is close to the frequency of vortex shedding. The forces due to vortex shedding are<br />
the result of complex fluid dynamic phenomena and are sensitive to a wide range of<br />
parameters which describe the characteristics of the flow and the structure.<br />
The objective of this study was to investigate the possibility of vortex induced vibrati<strong>on</strong><br />
occurring for wind turbine support structures in sea currents, and to give a guideline for how<br />
to avoid it.<br />
The flow speed at which significant vortex induced vibrati<strong>on</strong> may occur generally depends <strong>on</strong><br />
the structural mass and damping and <strong>on</strong> the displaced mass. The n<strong>on</strong>-dimensi<strong>on</strong>al mass<br />
damping parameter k s , combines these influences, and is defined below [1]:<br />
k<br />
s<br />
2⋅<br />
m e<br />
⋅δ<br />
= ρ<br />
2<br />
⋅ D<br />
Where:<br />
m e = the equivalent mass per unit length of the structure<br />
δ = the logarithmic decrement of structural damping<br />
ρ = the density of the fluid<br />
D = the diameter of the structure<br />
Using data for the two generic wind turbines, and a density of the sea water of 1030kg/m 3 , the<br />
k s values for the two generic machines are 0.017 and 0.020 for the 2MW and the 5MW<br />
machines respectively.<br />
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WG03(Olais<strong>on</strong>)10068<br />
The <strong>on</strong>set of vortex induced in-line vibrati<strong>on</strong> occurs between U/f n D = 1.0 and 1.6, for small<br />
mass damping parameters (k s