Comparison of environmental contour and response-based approaches for system reliability analysis of floating structures

Abstract

Long-term extreme design loads have typically been estimated using an environmental contour (EC) approach, in which the maximum extreme responses of a structural system associated with critical sea states along the contours are applied for mechanical design purposes, to avoid the tedious task of direct integration. The variabilities in short-term extreme response and the contributions of all sea states have been ignored, and this has caused unreliable results. Considering concerns over the sensitivity of the hydrodynamic performance of floating structures to wave data, the objective of this study is to evaluate the effect of environmental conditions on the short-term extreme response parameters and present a full long-term analysis for the failure probability evaluation of floating structures. Numerical simulations are conducted to establish a representative and reliable database which consists of wave variables and the corresponding short-term extreme response parameters for a semi-submersible platform based on a discrete scheme, using the specialised finite element software ANSYS/AQWA. Tension and offset extreme data are derived from time-domain simulations of the coupled system using the peak-over-threshold method to determine the short-term load distribution. Starting with the database, a linear interpolation scheme and intelligent algorithm are utilised to predict extreme response parameters for any other environmental conditions. Surrogate models are employed for three scenarios: (1) design load estimations obtained from various ECs as a basis for long-term load evaluation, (2) full long-term extreme response distribution calculation accounting for all possible sea states, and (3) system reliability analysis of floating systems. A simplified long-term analysis based on EC-based design loads is proposed, and the results are compared with those of the full long-term methodology. The system reliability of a floating system is calculated considering mooring line breaking, fatigue, and floater offset. These expressions provide a means of preliminary design evaluation of floating structures, accounting for all sea state contributions, which can be used to address the design loads and failure probability of an entire system.