Print Email Facebook Twitter Hydrodynamic loading on the shaft of a gravity based offshore wind turbine Title Hydrodynamic loading on the shaft of a gravity based offshore wind turbine Author Smaling, H.W. Contributor Jonkman, S.N. (mentor) Molenaar, W.F. (mentor) Labeur, R.J. (mentor) Segeren, M.L.A. (mentor) Ten Oever, E. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Programme Hydraulic Engineering Date 2014-06-03 Abstract Gravity based support structure By a joint venture of BAM and Van Oord an innovative type of Gravity Based Structure (GBS) for an offshore wind turbine was developed. The GBS consist of a concrete caisson and a steel shaft. The applicable water depth range is between 35 and 60m. Hydrodynamic loading on the shaft In order to optimize the support structure, the hydrodynamic loading on the steel shaft is investigated. Previous studies with a CFD package showed an increase of the load on the shaft with a factor 2 compared to the Morison equation. The hypothesis is that the presence of the caisson is responsible for this increase. Therefore the (numerical) CFD model FinLab is used to investigate the influence of the caisson on the wave forces on the steel shaft. The influence of the caisson is investigated within FinLab for the extreme wave (ULS) and for two relative moderate waves (FLS). The presence of the caisson leads to an increase of the maximal horizontal force on the shaft of about 20%. The bending moment cycle (important for fatigue) does not change significantly. It is concluded that the caisson is not responsible for the increase of a factor 2. However, while investigating the influence of the caisson, for the shaft a comparison with the Morison equation is made. It is found that FinLab gives much higher forces for the ULS wave (a 75% higher maximal horizontal force on the shaft) than the Morison equation. By also interpreting the results of the FLS waves there seems to be a relation with the degree of non-linearity of the wave. Different possible causes for the difference are discussed. Those causes give a possible explanation why the Morison equation would be less accurate for a highly non-linear wave. It is however recommended to calibrate the outcomes of FinLab with experimental data. Influence of the hydrodynamic loading analysis on the design The influence of the higher wave forces obtained with FinLab on the design of the shaft is investigated by means of a case study. By performing a FLS and ULS analysis the required dimensions of the shaft can be determined. The FLS analysis is based on a simplified method to be used for pre-design only. With respect to fatigue (FLS), the higher loads found by FinLab for highly non-linear waves do not result in a higher fatigue load. This has to do with the small probability of occurrence of highly non-linear waves. With respect to the extreme event (ULS) the strong increase in bending moment due to waves found by FinLab (+120%) results in a 26% higher total bending moment (wind+waves). Conclusions The most important conclusions of this research are that 1) The caisson does not significantly influence the ULS and FLS analysis 2) The Morison equation gives lower wave forces than an analysis with FinLab. For further design these conclusions have the implication that the caisson does not have to be part of the structural schematisation when the wave loads are determined. For the FLS waves most likely the Morison equation can be used, which has large benefits for the calculation time. For the ULS wave another method than the Morison equation is suggested. Subject hydrodynamicoffshore wind turbineFinLabgravity base structureGBS To reference this document use: http://resolver.tudelft.nl/uuid:7b697465-7e26-48c4-b53f-efbd835f1997 Part of collection Student theses Document type master thesis Rights (c) 2014 Smaling, H.W. Files PDF Thesis_HWSmaling_main.pdf 3.42 MB PDF Thesis_HWSmaling_appendices.pdf 9.02 MB Close viewer /islandora/object/uuid:7b697465-7e26-48c4-b53f-efbd835f1997/datastream/OBJ1/view