Breaking Wave Loads and Stress Analysis of Jacket Structures Supporting Offshore Wind Turbines

Abstract

In terms of future power generation in UK and Germany, offshore wind is thenext big player with 40GW [2] and 32GW [3] capacity planned for installation inboth countries respectively by 2030. The latest Round 3 of sites owned by theCrown Estate explore deeper water depths of up to 78m in the Irish Sea [4].Foundations for offshore wind structures consume around 25% of the totalproject cost [5] therefore the design of support structures is the subject of thisthesis.The current state-of-the-art support structure options available for offshore windturbines have been outlined in this thesis with an evaluation of the preliminarydesign of monopile and jacket solutions. This assessment resulted in furtherstudies into the loading acting on a monopile foundation along with researchinto the fatigue design of multiplanar tubular joints for jacket structures.Mathematical modelling of linear and nonlinear waves combined with theMorison equation was completed to check the effects of breaking waves on amonopile foundation. Results indicated that measured forces were up to afactor of 2.5 times greater than calculated values, which suggests that loadscould be underpredicted if the effects of breaking are not considered. Thetheoretical maximum wave height before breaking was then linked to windspeed and a comparison of overturning moments from the two loads was made.Wave loads dominated at water depths of around 30m for lower wind speedsbut this depth decreased to around 12m as wind speeds approached cut-out of25m/s.For deeper water depths and larger capacity turbines, jackets are the preferreddesign solution. Joint design in FLS is the critical aspect of jacket design withcastings often required to provide adequate capacity. A review of stressconcentration factors (SCF) for tubular joints indicated that the coded approach,which uses SCF equations for uniplanar joints, could be missing the multiplanareffects. Finite element (FE) modelling of multiplanar tubular joints wascompleted using ANSYS Workbench to examine the effects of loading in out-ofplanebraces. Carry-over of stress from the loaded brace of the joint tounloaded neighbouring braces was observed which implies the importance ofmodelling joints as multiplanar geometries.A parameter study in ANSYS Workbench covering 1806 different geometricalconfigurations and loads was carried out with a regression of the data to givenew sets of SCF equations for multiplanar tubular joints. SCFs from theseequations were improved compared to Efthymiou but difficulties wereencountered when superimposing the output (including Efthymiou). Furtherwork on the superposition of individual load cases was therefore recommendedfor future work.

Date of Award	1 Aug 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Stansby (Supervisor)