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dc.contributor.advisorKrokstad, Jørgen Ranum
dc.contributor.authorGrøtting, Heine
dc.date.accessioned2015-10-05T15:05:43Z
dc.date.available2015-10-05T15:05:43Z
dc.date.created2015-06-10
dc.date.issued2015
dc.identifierntnudaim:13738
dc.identifier.urihttp://hdl.handle.net/11250/2350952
dc.description.abstractIn 2014, 536 offshore wind turbines were installed on European shelf, connecting an average of 5.9 MW to the grid every day. About two thirds of this were installed in the North Sea. This new and emerging market represents an opportunity for Norway to make use of our knowledge about 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 to maintenance and unexpected repair the wind turbine needs to be accessed by technicians about three times per year. Hence, for a wind farm of some size, the access operation is done a signicant amount of times per year. The lack of a sufficient robust and cheap way to do this has proven a costly problem for 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 consuming and expensive, as you in principle should find one limiting Hs for all combinations of peak period 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 have low responses in this frequency area, such as a SWATH or a mini semi-submersible might be a good access solution on such a location. There is different strategies on how to access the wind 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 SWATH concept with fender docking was chosen for further analysis. The concept was inspired by the FOB 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 the frequency domain method proposed by Wu (2014) and in the time domain simulation software SIMO. 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 found that 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 an offshore wind turbine in Hs up to 1.5 m without considering Tp nor wave direction. Comparing this with the results from the Matlab program and SIMO, the SWATH concept analysed can not 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 the fenders dynamic and static coefficient depend on pressure, temperature,slip velocity and hu- midity. Another improvement would be to investigate whether diffraction effects from the wind turbine needs to be included in the analysis.
dc.languageeng
dc.publisherNTNU
dc.subjectMarin teknikk, Marin hydrodynamikk
dc.titleSmall Water Plane Area Solutions for Access of Offshore Wind Turbines
dc.typeMaster thesis
dc.source.pagenumber127


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