17.04: Design of offshore wind turbine jacket foundations: On the influence of subsequent modifications on fatigue performance

Abstract

The design of jacket foundations for offshore wind turbine typically relies on design loads from an integrated aero-elastic model of the full turbine structure and a reduced (superelement) representation of the foundation. In this paper the influence of introducing subsequent design changes to the foundation that potentially invalidate the design loads is studied based on a generic jacket foundation subject to combined wind and wave loading. The fatigue performance is considered for the reference case as well as a number of structurally modified configurations covering changes in braces, legs, soil conditions and footprint. The modified jackets are examined for both inconsistent design loads based on the reference geometry and updated loads associated with a consistent, aero-elastic load iteration with the updated geometry and/or soil conditions. Ratios between the fatigue lives based on the inconsistent and the updated forces are presented, indicating whether an additional load iteration is essential or not. By having an overall understanding of allowable structural modifications a foundation designer is able to provide a more efficient jacket design by independently optimizing the structure in the post-processing stage.It is shown that the subsequent jacket modifications may significantly affect the quality of the fatigue results. Some modifications, especially the ones associated with the boundary conditions, e.g. soil modifications significantly modify the global dynamic behaviour, thus indisputably require an additional aero-elastic load iteration. On the other hand, modifications resulting solely in local changes such as minor brace or leg size modifications, especially when increasing the stiffness, may be allowed in the post-processing phase without committing a significant error.