Turret-Moored FPSO Yaw Motions in a Squall-Prone Region

The design of mooring systems in West of Africa is often dominated by squall events. These are episodes of rapidly increasing wind speeds accompanied with sudden and significant wind direction changes. The response of weathervaning systems (offloading tankers moored to an export system or turret moored FPSO’s) to such transient and violent environments is very complex and the subject of an on-going joint industry project. One important parameter in numerical simulations of weathervaning units to squall events is the yaw motion damping model. There is quite a large body of references and insights in yaw damping under steady environmental conditions but hardly any when it comes to responses in squall owing to the transient nature of the event and to its complexity. Field measurements of the yaw motions of a turret moored unit offshore Angola with concurrent on-board wind measurements are presented. Both quantities are sampled at 1-minute intervals. Time series of aft and fore drafts are also available on a daily basis. These measurements span one decade. A systematic scan of the wind speed time series has been performed to identify, based on the World Meteorological Organization definition, all squall events passing through the asset. The main particulars of the turret-moored FPSO are described along with its station-keeping system and the associated horizontal slow drift motion response time scales. Systematic analysis of measured yaw motions and velocities has been performed seeking correlations with the hull loading condition at the time of the squall event, its heading prior to the squall and the squall maximum speed and direction range. The main objective of these analyses is to provide factual information that can be used to firm up the yaw damping models used for mooring analyses in squall-prone areas.