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.
Environmental contours are a known method for establishing extreme combinations of several variables of interest in the design of offshore structures. Several methods exist for their generation which use different mathematical formulations and result in different statistical meaning. The data sets to which the contours are applied are described by joint probability density functions (PDF), commonly generated for metocean parameters such as significant wave height, peak period, wind speed, and others (Bitner-Gregersen 2005).
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.
The offshore system design is usually based on the prediction of extreme system responses, which are most accurately predicted by a long term analysis, also referred to as response based analysis. Because such analysis may be time consuming and expensive, especially at the initial design stage, the environmental contours were developed as an approximation, which enabled the long term responses to be estimated by a short-term analysis over a set of metocean combinations (Winterstein et al. 1993) (Haver and Kleiven 2004). After the short term response analysis has been performed for all combinations on the environmental contour, the maximum response can be identified and the critical environmental condition established as that point on the contour that generated the overall maximum.