On the use of constrained focused waves for characteristic load prediction

Physical experiments investigating the extreme responses of a semi-submersible floating offshore wind turbine were conducted to allow a comparison of design wave methods. A 1:70 scale model of the IEA 15MW reference turbine and VolturnUS-S platform was studied focusing on the hydrodynamics under parked turbine conditions. A comparison of characteristic load predictions was made between design standard recommendations by the IEC and DNV covering different design wave types and post processing methods. Constrained waves are permitted for predicting characteristic loads for fixed offshore turbines but the extent to which they are suitable for floating devices is questionable. A constrained wave method for characteristic load prediction is applied and it is concluded that in general characteristic responses related to pitch may be estimated well with single response conditioned focused waves but for response types where the low frequency surge is important, e.g. mooring loads, constrained focused waves need to be applied

Extreme responses of a hinged raft type wave energy convertor

Much attention has been paid in recent years to the determination of design loads for moored floating structures and the application of established methodologies for fixed structures have been found to be ineffective. This paper experimentally investigates extreme responses of a lazy S moored 1:20 scale model of Mocean Energy’s Blue Star wave energy convertor (WEC) along the 1 year return contour. The device is a hinged raft type WEC and the extreme responses studied include mooring loads and snatch load events. Long irregular wave time series are used in the estimation of extreme value distributions of the mooring load for particular sea states. Conditional random response wave and constrained new wave profiles are used to study and support the predictions. Wave calibration and the impact of wave breaking on the physical realisation of response conditioned focus waves and extreme value distribution (EVD) predictions are discussed