Characterization of tropical cyclones to identify the response based design metocean conditions for an FPSO mooring system. (1st March 2022) - Koohi Kheili, Ali Ghasemi
Abstract: Design metocean conditions (DMC) for a single-point moored weathervaning vessel exposed to the cyclonic environment are obtained by characterising the tropical cyclones on the basis of extreme responses induced in its mooring system. Determination of design metocean conditions, as combinations of wave, wind and current associated with the extreme response, is a multivariate problem that does not have a unique solution. Its probabilistic nature dictates that the design metocean condition should be a short-term sea state which is most probable to occur while at the same time inducing the extreme N-year response of the system. This study aims at the development of a methodology for obtaining the DMC for a mooring system of a weathervaning vessel by characterizing the tropical cyclonic environment based on the mooring responses. First, a method for identifying the DMCs within the metocean history is described, along with its limitations. Then, the storm characterisation approach is introduced, based upon the maximum responses of the system rather than the extreme sea states in a particular event. The metocean conditions at a specific location during the passage of a cyclone are described by a set of dimensionless shape functions, with their main parameters linked to the most probable mooring tension experienced during each cyclonic event. It is further determined that for a weathervaning floating system, cyclones should be separated into two distinct types according to the relative wave-wind direction highly affecting the mooring response. The cyclone shape functions are generalized to describe the characteristic cyclones at the N-year return period response, which are determined by considering a limited group of storms inducing the mooring responses close to the N-year response. By averaging the dimensionless shape functions for each of the two cyclone types, the characteristic cyclones can be synthesized to describe two types of events, where the N-year mooring response is most likely to occur. The robustness of the method is checked through a case study, in which the characteristic 5-year, 10-year and 100-year DMCs are produced for a mooring system of a floating production, storage and offloading vessel (FPSO) permanently moored offshore Australia. By using a 44-year hindcast time history of cyclonic metocean conditions, the DMCs were developed from the response-based synthesized N-year cyclones and compared with the specific short-term conditions retrieved from the same time history. An alternative averaging method for the cyclone shape functions is also considered and results are discussed. Highlights: In the present study, a methodology is developed for obtaining the design metocean conditions for a mooring system of a weathervaning vessel by characterizing the tropical cyclonic environment based on the mooring responses. The robustness of the method is checked through a case study, in which the characteristic 5-year, 10-year and 100-year DMCs are produced for a mooring system of an FPSO permanently moored offshore Australia. The main conclusion was that the proposed methodology captures the probabilistic nature of the problem by utilizing the concept of response percentile, which serves as a criterion to identify the DMC.