Simplified fatigue assessment of offshore wind support structures accounting for variations in a farm

Abstract

The optimal design and preliminary strength assessment of offshore wind support structures gain growing interest given the potential to drive the costs further down. This study develops a framework for Fatigue Limit State (FLS) estimations of monopiles in a simple and quick manner so as to address site variations in an offshore wind farm (OWF). Additionally, it serves the need for optimisation of all structures in the farm in the early design phase. The framework consists of two elements: (a) a stand-alone model that predicts in a simplified way the damage caused by the varying loading and (b) correction factors that increase its reliability. The concept of the model relies on the analytical approximation of the dynamic response, thus by-passing time consuming numerical processes and advanced software. The above step renders it a simplified version of the conventional frequency-domain. Its benchmarking against the time-domain aeroelastic code Bladed yields sufficient accuracy but also certain systematic errors. Effectively, these are tackled by the correction factors that are generated at a reference position where time-domain detailed assessment is necessary. Once calculated, they are transferred to the positions of interest in the farm. A case study examining the variations in a site shows an efficient performance of the proposed scheme: particularly at the parts of the structure close to the seabed with errors lower than 5 % with respect to the outcome of Bladed. Finally, given the fatigue estimations at every location, the foundation piles are re-designed individually in order to fulfil the target of mass reduction. By using the outcome of the case study as input for the tailoring of the geometry, it is shown that a considerable amount of steel, up to 16 %, can be saved.