Determination of design metocean conditions by response based analysis

Response Based Analysis (RBA) aims at prediction of the long term distributions of critical responses such as motions, accelerations, wave loads, which have significant impact on the design of a floating system. As compared with the conventional analysis, which predicts the responses of the facility to the N-year return period metocean conditions, RBA provides directly the N-year return period responses by analysing the statistics of their long term time histories. Another outcome of RBA is the Design Metocean Conditions (DMCs) which are combinations of sea state, wind and current causing the corresponding N-year response. The knowledge of the DMCs enables the more detail time domain analysis and model tests to be performed for a set of critical combinations of metocean parameters. It also enables all the associated responses to be determined.

The RBA framework is generally well addressed in the literature, but the DMC identification methods are not necessarily clearly established. The objective of this paper is to present the theoretical background, numerical method and an example for the determination of the N-year return period responses and the associated DMCs for a floating facility. The method includes prediction of the N-year response, identification of the metocean combinations within the available time history which produce this response, determination of the required percentile of the short term response to match the N-year response, and the search for the most probable DMC within the joint probability density of the metocean parameters. Features of the method are discussed and results are presented for several critical responses of a weather vaning vessel.