Fatigue loads on large diameter offshore wind monopile foundations in non-operational situations
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- Institutt for marin teknikk 
The objective of this master thesis is to perform fatigue analysis on a large diameter offshore wind turbine with monopile foundation in non-operational situations. Offshore wind turbines are fatigue dominated structures, and the results from the fatigue analysis are therefore of great importance. Offshore wind turbine projects are very expensive, and reducing the cost is always a priority. The foundation is one of the main cost-drivers. This thesis will investigate if performing more accurate wave load calculations and modeling of the sea state will reduce the calculated total fatigue damage on the monopile. The offshore wind turbine is exposed to the variable stochastic environmental loadings from wind and waves. In non-operational situations the rotors are not rotating, which results in a negligible aerodynamic damping. The aerodynamic damping is the biggest contributor to the total damping during operation, and therefore the damping is significantly lower in non-operational situations. Low damping causes large dynamic responses, which leads to the non-operational situations being important for the fatigue analysis of offshore wind turbines. This thesis will focus on the three non-operational situations where the wind speed is below cut in, above cut out and the non-availability situations where the wind speed is within the operational range. Load cases are chosen based on recommendations from DNV GL and the metocean report from the chosen site, Doggerbank. A wind speed with a corresponding sea state, is chosen for each load case. The chosen sea states have high probability of occurring along with the chosen wind speeds. Simulations are run with the software FEDEM Wind Technology. The model of the offshore wind turbine is made according to the NREL 5MW reference wind turbine, with a monopile foundation according to the OC3 project. The simulations were run with different considerations given to the modeling of the sea state and the wave load calculations to make them more accurate. This was to investigate if any of them would reduce the total fatigue damage. In the first four methods, the analysis were done using short and long crested waves. Both of these were analyzed with the wave load according to Morison's and according to MacCamy and Fuchs equation. JONSWAP spectrum was applied in all four methods. In the fifth and sixth method, the wind driven and swell sea were separated by modeling the sea state with Torsethaugen spectrum instead of JONSWAP. Long crested waves were used. The difference between the two methods is the wave load calculation. The calculated wave load on the monopile was reduced when the wave load calculation were done according to MacCamy and Fuchs equation instead of Morison's, especially in the sea states with low peak period. This is because MacCamy and Fuchs equation includes the effect of diffraction of waves with wavelength shorter than five times the diameter of the monopile in the wave load calculation. Two hypothesis are tested in this thesis. The first one is that the short crested waves will reduce the calculated total fatigue damage on the monopile, compared with long crested waves. The second hypothesis is that the calculated total fatigue damage is reduced when the swell and wind driven sea is separated, compared with total sea. The main result investigated is the total fatigue damage accumulated over the design life of 20 years. The analysis methods that calculated the wave load according to MacCamy and Fuchs always resulted in less total fatigue damage than the methods that calculated the wave load according to Morison's equation. Based on this, MacCamy and Fuchs equation should be used to calculate the wave load on a large diameter offshore wind turbine with monopile foundation. The short crested waves reduced the total fatigue damage, compared with long crested waves, for both wave load calculation methods. The reduction of the total fatigue damage, due to the short crested waves, was greater when the wave load was calculated according to MacCamy and Fuchs than Morison's. The short crested waves, based on JONSWAP spectrum with wave load calculated according to MacCamy and Fuchs resulted in the lowest calculated total fatigue damage. The confidence in these results are acceptable. This shows that, based on the results here, the first hypothesis seems valid. The short crested waves did reduce the calculated total fatigue damage. To apply Torsethaugen spectrum with the wave load calculated according to Morison's equation, increased the total fatigue damage compared to when JONSWAP spectrum was used. When Torsethaugen spectrum was applied with the wave load calculated according to MacCamy and Fuchs equation, the calculated total fatigue damage was reduced, compared with when JONSWAP spectrum was applied. This method, Torsethaugen with wave load according to MacCamy and Fuchs, resulted in the second lowest calculated total fatigue damage. The confidence in these results are low, due to the simulation length and number of seeds. However, the results indicate that the second hypothesis is valid as well, as long as the wave load is calculated according to MacCamy and Fuchs. The goal of the thesis have been reached, since the results show that by modeling the sea state more accurately the calculated total fatigue damage is reduced. Further simulations are needed to ascertain a higher confidence in the results, and to further investigate the effect of short crested waves and separated swell and wind driven sea on the fatigue damage. A damping sensitivity study and availability sensitivity study have been run as well. The total fatigue damage is highly dependent on both of these variables. Increased damping and increased availability both decrease the calculated fatigue damage.