On the use of constrained focused waves for characteristic load prediction

Abstract

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.

Laboratory investigation on short design wave extreme responses for floating hinged-raft wave energy converters

“…The main peak of each SDW occurs at 45 s, and the constrained waves are embedded within a 120 s random irregular sea state. As proposed by Tosdevin et al (2022), the response-conditioned SDW approaches (MLER, CRRW) considered in this work are scaled to the 99th percentile of the EVDs for the targeted response based on a 3-h exposure time. Scaling in this way helps to alleviate the importance of history effects, which are unaccounted for in any of the methods, since the profiles will be more likely to produce high-percentile responses (Tosdevin et al, 2021;Tosdevin et al, 2022).…”

“…As proposed by Tosdevin et al (2022), the response-conditioned SDW approaches (MLER, CRRW) considered in this work are scaled to the 99th percentile of the EVDs for the targeted response based on a 3-h exposure time. Scaling in this way helps to alleviate the importance of history effects, which are unaccounted for in any of the methods, since the profiles will be more likely to produce high-percentile responses (Tosdevin et al, 2021;Tosdevin et al, 2022). The NW approach, however, is scaled to the 70th percentile of the wave amplitude EVD, due to difficulties in physically producing the higher percentile profile.…”

UK perspective research landscape for offshore renewable energy and its role in delivering Net Zero

“…Progress has been made in investigating ways of improving the efficiency of the design process through probabilistic approaches and their applicability in wave and floating offshore wind cases. A constrained wave approach for predicting characteristic loads has been developed whereby a procedure for quickly calibrating constrained focused waves combined with scaling to a high percentile target response using the response spectrum has been tested for point absorber and hinged raft type WECs (Tosdevin et al 2021, Jin et al 2022a and for a semi-submersible floating wind turbine (Tosdevin et al 2022). The procedure is shown to produce characteristic load estimates in line with traditional irregular wave methods in a shorter time.…”