Dynamic Load Reduction and Station Keeping Mooring System for Floating Offshore Wind

Abstract

The mooring system for a floating offshore wind turbine ensures that the platform stays within pre-defined station keeping limits during operation, while it provides sufficient restraining forces in storm events to guarantee survival. This presents a challenge during the design process, since the cost of the mooring system is proportional to the peak loads, i.e. those that occur infrequently in extreme conditions. Mooring designs are governed by extreme and fatigue loads which determine the required Minimum Breaking Load (MBL) of the system. If uncertainties in the environmental loading or hydrodynamic coupled response exist, additional safety factors are required. This paper explores the application of a hydraulic based mooring system that enables a variable, non-linear line stiffness characteristic that cannot be achieved with conventional designs. This non-linear load-response behavior could function like a ‘shock absorber’ in the mooring system, and thus reduce the line tensions, enabling a more efficient mooring system that necessitates a lower MBL and thus lower cost. These claims are evaluated through numerical modelling of the NREL OC3 spar buoy and OC4 semi-submersible offshore wind platforms using the FAST-OrcaFlex interface. The simulations compare the dynamics with and without the inclusion of the hydraulic mooring component. The results suggest that mean mooring line loads can be reduced in the region of 9 – 17% through a combination of lower static and dynamic loads, while the peak loads observed in extreme conditions were reduced by 17 – 18%. These load reductions, however, come at the expense of some additional platform motion. The paper also provides an outlook to an upcoming physical test campaign that will aim to better understand the performance and reliability of the mooring component, which will provide the necessary evidence to support these load reduction claims.