Oceanographic and Motion Response Statistics for the Operation of a Weathervaning LNG FPSO

Abstract

An oceanographic database is used to estimate the operability of a weathervaning LNG FPSO (FLNG) in multidirectional seas with swells. The oceanographic data consist of directional wave data, current profile measurements, and a wind dataset. Response functions are used to convert the oceanographic data into an equivalent dataset of vessel motion response parameters and turret mooring line tensions. The resulting response data have enabled operational type statistics to be computed, as well as the environmental conditions that induce responses with a one-year return period. In addition to the analysis of the responses of the FLNG alone, the method has been applied to a system where a LNG shuttle carrier is moored alongside the FLNG for offloading of the products. In this case, important responses are the combined roll and pitch of the two vessels and tension in the mooring lines between the shuttle carrier and the FLNG. The same method can be applied for a stern-to-bow offloading system, where the shuttle carrier is moored with a hawser or soft yoke. The diversity of the directionality of the oceanographic variables makes this a particularly interesting exercise. The analysis included significant inertial currents, a high level of background swell and frequent local-wind seas that act at an angle to the prevailing swell direction. A method for separating a sea state into a sea and swell component has been derived and is reported, along with a programme for visualising the oceanographic conditions together with the induced responses corresponding to the existing conditions.