An optimisation methodology for suspended inter-array power cable configurations between two floating offshore wind turbines

Abstract

Floating offshore wind turbines (FOWTs) convert wind energy to electrical energy in deeper water areas where installing bottom-fixed offshore wind turbines is not feasible. However, technological obstacles have to be overcome to make them cost-effective, with inter-array power cable designs being one of the potential areas to be investigated. This study presents a novel configuration of suspended inter-array power cables using subsea buoys between two 5MW OC3-Hywind FOWTs. In this configuration, the inter-array cable is kept afloat between the seabed and the sea surface by attaching several buoys distributed along the cable. The effects of marine growth at a water depth of 320 m are considered. First, steady-state analyses are performed in the Start of Life (SOL) and End of Life (EOL) states to select the configurations which satisfy the minimum breaking load (MBL) and minimum bend radius (MBR) acceptance criteria. The selected configurations are then further investigated by considering a fitness optimisation criterion. Subsequently, full dynamic analyses are performed on the best configurations identified through the fitness optimisation criterion, considering the operating and feathered modes of the FOWTs. Copper and aluminium cables with two differently sized buoys are evaluated. The results indicate that the suspended inter-array power cable concept is feasible using the proposed buoy setup and optimisation methodology. Smaller buoys yield better designs for both cable types than larger buoys. The aluminium cable configuration has a lower effective tension but also a lower safety margin to its MBL limit than the copper cable configurations. The copper cable configurations result in smaller horizontal excursions. The configurations with closely spaced buoys result in large bend radii.