Effect of wind-wave interaction on the performance of SPAR type FOWT for varying mooring arrangements

The paper discusses the dynamic responses of a SPAR floating offshore wind turbine system under varying sea states. The performance of slack and taut mooring arrangements in restraining the motions of the system is also looked upon. A coupled aero-hydrodynamic analysis is conducted using a potential flow theory-based model and the dynamic motion response of the platform in surge, heave and pitch; power generation in these mooring arrangements are checked. The load cases consider random sea states under steady wind fields for below, rated and above-rated wind speeds. The manuscript aims to examine the effect due to wind-wave misalignment for SPAR FOWTs with taut moorings, which is still unexplored. The manuscript entails systematic studies focussed to understand the performance based on responses and power generation of the SPAR FOWTs. It is observed that the dynamic motions in surge and pitch are relatively lower for the taut mooring arrangement whereas, heave shows comparatively higher values in taut arrangement due to the distribution of loads and lower mooring line restoring force. The generated power shows less variation in accordance with the load cases irrespective of the mooring arrangements. Both wind-wave loads affect the wind turbine operating cases in 0°/180° headings, whereas in 90° wave, power generation is primarily altered by wind loads. There is a reduction in power generation in a 90° sea condition as compared to 0°/180° sea conditions. This is attributed to the prominent platform responses that are noticed in the sway and roll. Thus, it is observed that by adopting a taut mooring arrangement in deeper waters for SPAR FOWT not only improves the system responses, power generation, but also reduces the mooring line footprint and related costs.