The application of Inverse First-Order Reliability Methodology (IFORM) to the determination of response maxima using environmental contours and response surfaces is presented in this paper. In the first part concerned by the environmental data modelling, the directional effects are analysed for the current, wind and wave parameters, and taken into account if necessary. The response modelling is presented in the second part. Current, wind and wave responses are derived using response surface models (RSM). Directional coefficients are obtained from harmonic analysis. Two types of response are considered in this study: the total offset and the total tension in one particular mooring line. The third part is devoted to the application of IFORM approach to the specific case of a moored FPSO in West Africa in ballast condition. Design points and response maxima are computed with a robust iterative gradient method. Directional effects on response maxima are analysed. Results are compared with statistical extreme values from a Direct Simulation Analysis (DSA) in similar loading condition. A reasonably fair agreement is obtained between statistical extreme values from DSA simulations and response maxima from the proposed method combining IFORM approach and RSM technique.
The traditional approach to the design of offshore structures has been to quantitatively assess the loads created by simultaneous and colinear extreme return values of wind, wave and current. It has now long time been recognised that this represents an overly conservative assessment of the environmental forces, and during the past fifteen to twenty years, Response-Based Design has been proposed as a more realistic and less conservative alternative because it aims at:
Achieving 1- to 100-year return periods for design parameters of interest (such as vessel excursions, mooring or riser design loads) rather than environmental parameters;
Accounting for joint probabilities of wind, wave and current;
Accounting for structural loading and response characteristics.
