Investigation into application of environmental contours for analysis of floating systems

Environmental contours are used to analyze extreme combinations of the metocean of response variables in the design of structures. Several methods exist for their generation and each uses a different mathematical model which results in different statistical meanings. This study compares the exceedance contour method, a non-standard approach, to the established Constant Probability Density (CPD) and Inverse First Order Reliability Method (IFORM) approaches to construct contours of extreme sea-states. Contour of extreme sea states are presented for three locations which show that the exceedance method generally produces narrower contours compared to the CPD and IFORM contours. Three offshore systems were considered: heave response of semi-submersible, wave induced bending moment of a ship shaped vessel and flexible riser supported by an FPSO. These studies showed that for the concave shapes of the metocean probability density (PDF) isolines the exceedance contour predicts lower maximum responses than the CPD and IFORM. For the convex PDF isolines, this is not always the case. The differences in the predicted extreme responses are further dependent on whether the response is more sensitive to shorter or longer periods. Strengths and limitations of the application of the environmental contours to offshore systems coupled to floating structures are presented. The assumption that risers or other coupled systems' design responses occur simultaneously with the design responses of the floating structure is critically reviewed.