Long-term analysis using a scatter diagram key region to evaluate the extreme response of steel risers

Abstract

This work presents a new procedure named Key Region Long-Term Analysis (KRLTA) for the estimation of long-term responses of offshore structures subjected to the effects of wave actions.The procedure is based on the selection of the key region of the joint probability distribution of significant wave heights and wave periods that most contributes to the long-term response. Response surfaces fitted to response parameters of a reduced number of short-term sea states, usually between 12 and 16, are used to define the parameters of other sea states inside the key region, allowing a fast long-term response evaluation. The selection of the scatter diagram key region is bounded by extreme environmental contour lines. The suitability of the KRLTA results is evaluated by inspecting the contribution coefficients for the long-term response of the sea states contained in the key region.The application of the KRLTA is demonstrated in the estimation of the long-term extreme generalized load effect (or utilization ratio) of three sections of a steel catenary riser connected to a FPSO under North Sea environmental conditions. The results obtained with the KRLTA using only numerical simulations of 12 sea states are compared with those from a full long-term analysis (FLTA), which demands about 500 sea states to be analyzed, and with results obtained by the currently used environmental contour method (ECM).The obtained results indicate that KRLTA can achieve similar estimates of a FLTA, with computational effort similar to the ECM. Thus, the new KRLTA procedure allows long-term analyses of complex structures such as riser systems, mooring lines and tension leg platforms tendons, with computational efforts comparable to the approximate methods currently applied on the design of these systems.