Application of the Contour Line Method for Estimating Extreme Responses in the Mooring Lines of a Two-Body Floating Wave Energy Converter

The ultimate limit state (ULS) is one of the design criteria used in the oil and gas industry in mooring system design for floating platforms. The 100 year level response in the mooring line should be applied for the ULS design check, which is ideally estimated by taking into account the dynamic mooring line tension in all sea states available at the operational site. This approach is known as a full long-term response analysis using the all-sea-state approach. However, this approach is time consuming, and therefore, the contour line method is proposed for estimation of the 100 year response by primarily studying the short-term response for the most unfavorable sea states along the 100 year environmental contour line. Experience in the oil and gas industry confirmed that this method could yield good predictions if the responses at higher percentiles than the median are used. In this paper, the mooring system of a two-body wave energy converter (WEC) is considered. Because this system involves the interaction between two bodies, the estimation of the ULS level response using the all-sea-state approach may be even more time consuming. Therefore, application of the contour line method for this case will certainly be beneficial. However, its feasibility for application to a WEC case must be documented first. In the present paper, the ULS level response in the mooring tension predicted by the contour line method is compared to that estimated by taking into account all sea states. This prediction is achieved by performing coupled time domain mooring analyses using SIMO/RIFLEX for six cases with different mooring configurations and connections between two bodies. An axisymmetric Wavebob-type WEC is chosen for investigation, and the Yeu site in France is assumed as the operational site. Hydrodynamic loads including second-order forces are determined using WAMIT. Finally, the applicability of the contour line method for prediction of the ULS level mooring tension for a two-body WEC is assessed and shown to yield accurate results with the proper choice of percentile level for the extreme response.