Fatigue Assessment and Resonance Effects in an Inter Array Cable due to Global Dynamic Deflections in a Bottom Fixed Monopile Wind Turbine Foundation

Inter Array Cable (IAC) hanging through the Monopile (MP) are critical components for fixed offshore wind turbines. Here, fatigue analysis for IAC is performed with focus on resonance modes of the IAC. The IAC is modelled using a specialized time-domain solver for offshore marine systems (OrcaFlex) with imposed motion time-series at IAC hang-off location. The IAC responses (tension and curvature time series) are extracted for further computation of component stress time series. The IAC cross-section is further modelled in detail considering friction between components using a tailor-made cross-section analysis tool (Uflex2D) for extracting stresses under various tension and curvature combinations. Finally, cable fatigue is calculated using an in-house tool, Flexstress for Umbilicals (FFU). FFU allows both non-linear stress calculations and processing a relatively large amount of load cases with significant computational efficiency. FFU further calculates fatigue damage based on Rainflow counting using appropriate SN curves. It is found for the current cable configuration that the most critical component with regards to fatigue damage is the copper tape screen with fatigue service life of 31 years considering a safety factor of 10. During fatigue assessment it is also observed that IAC resonance modes can be excited. The resonance effect is further investigated, and mitigation methods are discussed.