Assessment of the Reliability of the Moorings of a Floating Structure Against the Extreme Cyclone Hazard

This paper describes the methodology adopted for an analysis performed to assess the reliability of the moorings of a free weathervaning FPSO against the cyclone hazard. A joint distribution characterizing the long-term cyclone metocean conditions was fitted to a database of environmental conditions at the site of interest. A series of archetype cyclone histories were selected from the cyclone database to capture the evolving relationships between key environmental parameters during a cyclone. A response-surface model for extreme tension in the vessel moorings was developed for the short-term extreme vessel response as a function of environmental conditions by fitting the surface to an extensive series of time-domain vessel response analyses. The reliability of the mooring system was evaluated using Monte Carlo simulation by sampling from the fitted joint distribution of long-term cyclone characteristic and generating simulated cyclone time histories by scaling the archetype cyclones. The response model for short-term exceedance probability is used to develop the probability of exceedance within each simulated cyclone, and the probability of exceeding a given response is derived from the simulation.