Modified Environmental Contours Approach for Long-Term Extreme Response Prediction of Marine Risers

Abstract

Abstract Accurate prediction of the long-term extreme response of offshore structures is challenging for complex systems under varying marine environments. The long-term uncertainties come from the varying environmental parameters such as significant wave height and spectral peak period, while the short-term uncertainties are due to the random wave amplitudes and phases. The conventional environmental contour lines method provides a fast approximate way to estimate the extreme response by performing simulations along the environmental contours. However, the method is unconservative if the short-term uncertainties are neglected. Current methods to account for the short-term uncertainties are approximate and require an empirical parameter that is case-dependent. This paper presents a modified environmental contour line approach, based on introducing an additional multiplicative random variable to account for the short-term variability. It is efficient as additional simulations are performed only in the critical direction determined by the conventional approach. The method is first tested on a single-degree-of-freedom model representing the surge motion of a floating offshore structure. Subsequently, it is applied on the top riser stress of a catenary moored floating production system. Monte Carlo simulation is impractical here as a validation tool due to the extensive simulation time. Hence, the results are compared with subset simulation, which is known to be asymptotically unbiased.