Response based design metocean conditions for an FPSO by the extremum search within joint metocean PDF

The response based analysis of a floating system aims at the prediction of extreme N-year return period responses by analysing their statistics in site-specific metocean conditions. After the extreme responses have been determined the inverse problem is solved: the most probable design metocean conditions (DMC) should be determined which cause these responses. The most probable DMC associated with a given response can be determined by using the joint probability density function (PDF) of metocean parameters. Mathematically, the most probable DMC is equivalent to a “design point”, at which the joint metocean PDF has a conditional maximum. This paper presents a method for finding the most probable DMC by the extremum search. Several numerical examples are presented to demonstrate finding the most probable DMCs for typical responses of an FPSO in North Sea environment, including global motions, wave loads, turret motions and relative wave motions. The most probable DMCs are found by the search process implemented in the space of physical metocean variables, which allows for easy interpretation of the results and exploring the response sensitivity to variability of metocean parameters.