Influence of soil and structural stiffness on the design of jacket type substructures

The detailed design of jacket type substructures for offshore wind turbines involves an iterative load calculation process in which structural properties and loads are exchanged between the turbine vendor and the substructure designer. Structural and soil stiffness in particular play an important role as they directly influence the magnitude of loads in the ultimate and fatigue limit state. The well established procedures for monopiles cannot be directly adopted for jackets. Ramboll has conducted extensive sensitivity studies on how global and local loads change when stiffness is adjusted during the design process. The main findings are presented in this paper. These form the basis for some recommendations on how the design and load iteration process for jackets can be planned in order to achieve an optimal substructure design with a limited number of full load iterations. So-called "Mini-Load-Iterations", in which only a reduced number of time series is simulated, play an important role in this process.