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

One of design criteria that have been used for the mooring system design for floating platforms in the oil and gas industry is the Ultimate Limit State (ULS). The 100-year level response in the mooring line should be applied for this ULS design check, which is ideally estimated by taking into account the dynamic mooring line tension in all sea state available in the operational site. This is called a full long-term response analysis using the all sea state approach. However, this approach is time consuming. Therefore, it is proposed to use the contour line method to estimate the 100-year response by primarily studying the short term response for the most unfavorable sea states along the 100-year environmental contour line. Experiences in the oil and gas industry confirmed that the method could give good prediction if the response at higher percentile than the median is used. In this paper, the mooring system of a two-body wave energy converter (WEC) is considered. Since this system involves interaction between two bodies, the estimation of the ULS level response by using the all sea state approach will be even more time consuming. Therefore, the application of the contour line method for this case will certainly be beneficial. However, its feasibility for a WEC case needs to be documented first. In the present paper, the ULS level response in the mooring tension that is predicted by the contour line method is compared with that estimated by taking into account all sea states. It is achieved by performing the coupled time domain mooring analyses using SIMO/Riflex for six cases with different mooring configurations and connections between two bodies. An axi-symmetric Wavebob-type WEC is chosen as the object of investigation and the Yeu site in France is assumed to be the operational site of the WEC. Hydrodynamic loads including 2^nd order forces are determined using WAMIT. Finally, the applicability of the contour line method to predict the ULS level mooring tension for a two-body WEC is assessed and shown to yield accurate results with proper choice of percentile level for the extreme response.