Small Water Plane Area Solutions for Access of Offshore Wind Turbines
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- Institutt for marin teknikk 
In 2014, 536 offshore wind turbines were installed on European shelf, connecting an average of5.9 MW to the grid every day. About two thirds of this were installed in the North Sea. Thisnew and emerging market represents an opportunity for Norway to make use of our knowledgeabout marine operations in the North Sea from the petroleum industry. In this thesis the op-eration of accessing the wind turbine to transfer personnel and parts is investigated. Due tomaintenance and unexpected repair the wind turbine needs to be accessed by technicians aboutthree times per year. Hence, for a wind farm of some size, the access operation is done a signicant amount of timesper year. The lack of a sufficient robust and cheap way to do this has proven a costly problemfor the industry. Another problem is how to analyse and compare different access concepts.Time domain simulations as widely used in the oshore petroleum industry are time consumingand expensive, as you in principle should find one limiting Hs for all combinations of peakperiod and wave direction each concept will encounter to do a fair comparison. Therefore,MARINTEK's MingKang Wu in 2014 proposed a efficient way to calculate the limiting signifi-cant wave height for all combinations of peak period and wave direction in the frequency domain. At Dogger Bank location two, the moderate sea states were access is realistic have peak pe-riods in the area of 5 to 9 seconds. Having a small water plane area vessel designed to havelow responses in this frequency area, such as a SWATH or a mini semi-submersible might bea good access solution on such a location. There is different strategies on how to access thewind turbine, fender docking is a popular choice today due to its simplicity and lack of vul-nerable expensive parts. Considering its superior velocity to the semi-submersible, a SWATHconcept with fender docking was chosen for further analysis. The concept was inspired by theFOB SWATH used by Oddfjell Wind AS and has been created and analysed in VERES. Fur-ther, fender docking with this vessel was analysed with a MATLAB program containing thefrequency domain method proposed by Wu (2014) and in the time domain simulation softwareSIMO. This thesis have three focal points, which this SWATH concept has been used to investigate.Firstly, to explore what parameters that should be included in the accept criteria for initiating the access operation. It was found that the limiting significant wave height depend on both peak period and direction of wave environment. Hence, it is recommended to step away from the industry standard of considering limiting Hs as a constant value, and consider limiting Hs as a function of peak period and wave direction. The second were to explore the potential of small water plane area solutions. It was foundthat it is feasible to design a SWATH to maximize its performance in a specic wave environ-ment. The producers of classical work boat catamarans claims that their vessels can access anoffshore wind turbine in Hs up to 1.5 m without considering Tp nor wave direction. Comparingthis with the results from the Matlab program and SIMO, the SWATH concept analysed cannot outperform this. One should nevertheless have in mind that as this is not a design thesis, the concept investigated is not optimized and that a optimized vessel surely would outperform the SWATH considered here. As well there is a chance that the limit of 1.5 m Hs typically given by the manufacturers is somewhat optimistic. It might for instance only be valid in favorable combinations of Tp and wave direction, this belief is supported by wind farm owners reporting of work boats not being able to perform as promised in all sea states. The last focal point were to verify the method proposed by Wu (2014) by time domain simula-tion in SIMO. The method has not been verified, the results obtained by the use of Wu (2014)and time domain simulation in SIMO had large deviations. It was found that the simplification done by Wu that the propeller thrust works in the global x-direction directly in the fender point, at least is one of the reasons why the method underestimate the risk of slip. In the end of chapter three a frequency domain method where the propeller thrust is directed along the local x-axis is proposed. To improve the modelling of fender docking,one should improve the understanding of how thefenders dynamic and static coefficient depend on pressure, temperature,slip velocity and hu-midity. Another improvement would be to investigate whether diffraction effects from the windturbine needs to be included in the analysis.