Abstract
Offshore wind offers a great potential to replace fossil fuel energy sources, the latter being the major source of greenhouse gas emissions contributing to global warming. Size of offshore turbines is increasing (better efficiency), with it the size of supporting monopile (MP) structure increases, and turbines are being installed further offshore (stronger and sustained winds available). These factors can cause water motion of the internal free surface (sloshing) inside the MP to be relatively strong. Here, a 3D sloshing analysis is performed to study the effect on the MP and on internal structures such as inter-array cables (IACs) using a Computational Fluid Dynamics (CFD) solver. Sloshing resonance periods/frequencies are calculated using a theoretical formulation. Forced oscillatory motions of the MP in surge direction are simulated at selected sloshing resonance periods to obtain pressure and velocity on the internal free surface. It is found that loads on the MP and internal structures are within limits. A check for IACs fatigue due to sloshing-induced velocities is performed. Fatigue analysis is performed using an in-house tool Flexstress for Umbilicals (FFU). FFU calculates fatigue by combining global analysis from a specialized slender structures time-domain solver (OrcaFlex) for the IAC with a local cross-section analysis of IAC (UFLEX2D). It is found that sloshing has little effect on cable fatigue life (high fatigue life found).
