Breaking wave loads and stress analysis of jacket structures supporting offshore wind turbines

• Main Author: Devaney, Louise Claire
• Other Authors: Stansby, Peter
• Published: University of Manchester 2012
• Subjects: 621.312136
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.554163

In terms of future power generation in UK and Germany, offshore wind is the next big player with 40GW and 32GW capacity planned for installation in both countries respectively by 2030. The latest Round 3 of sites owned by the Crown Estate explore deeper water depths of up to 78m in the Irish Sea. Foundations for offshore wind structures consume around 25% of the total project cost therefore the design of support structures is the subject of this thesis. The current state-of-the-art support structure options available for offshore wind turbines have been outlined in this thesis with an evaluation of the preliminary design of monopile and jacket solutions. This assessment resulted in further studies into the loading acting on a monopile foundation along with research into the fatigue design of multiplanar tubular joints for jacket structures. Mathematical modelling of linear and nonlinear waves combined with the Morison equation was completed to check the effects of breaking waves on a monopile foundation. Results indicated that measured forces were up to a factor of 2.5 times greater than calculated values, which suggests that loads could be under predicted if the effects of breaking are not considered. The theoretical maximum wave height before breaking was then linked to wind speed and a comparison of overturning moments from the two loads was made. Wave loads dominated at water depths of around 30m for lower wind speeds but this depth decreased to around 12m as wind speeds approached cut-out of 25m/s. For deeper water depths and larger capacity turbines, jackets are the preferred design solution. Joint design in FLS is the critical aspect of jacket design with castings often required to provide adequate capacity. A review of stress concentration factors (SCF) for tubular joints indicated that the coded approach, which uses SCF equations for uniplanar joints, could be missing the multiplanar effects. Finite element (FE) modelling of multiplanar tubular joints was completed using ANSYS Workbench to examine the effects of loading in out-of plane braces. Carry-over of stress from the loaded brace of the joint to unloaded neighbouring braces was observed which implies the importance of modelling joints as multiplanar geometries. A parameter study in ANSYS Workbench covering 1806 different geometrical configurations and loads was carried out with a regression of the data to give new sets of SCF equations for multiplanar tubular joints. SCFs from these equations were improved compared to Efthymiou but difficulties were encountered when superimposing the output (including Efthymiou). Further work on the superposition of individual load cases was therefore recommended for future work.