Mooring design for F(P)SOs in West of Africa offshore environment is in many cases governed by the squall driven condition. In the past, the squall condition was typically analyzed by using the peak wind speed with associated wind direction. However, due to its inherent transient nature, the squall event formulated in the time history with varying wind speed and direction is more appropriate and could be potentially more critical for the mooring system design. This approach has been adopted in the design and analysis of recent F(P)SO mooring systems. The F(P)SOs are turret-moored in various water depths in offshore West of Africa. A series of squall time histories have been applied to predict the global responses of the F(P)SO in the time domain. Each squall time history, which provides a unique combination of wind speed and direction variations, is analyzed in five nominal directions covering a sector of 90 degrees from East to West. Squall time histories are also applied to analyze the tandem offloading operation. The results are compared with those of the conventional constant wind speed approach and a few interesting observations are made. The paper also provides some insights into the F(P)SO yaw motions, as well as their relations to the changing wind direction. Analysis results demonstrate that using the squall time series with changing wind speed and direction is more critical than the conventional constant wind speed approach in the tandem offloading scenario. It is therefore recommended that mooring analysis using squall time series should at least be used for the tandem offloading simulations.
This paper presents the key results and conclusions of the study of FPSO transient responses in the cyclonic storm. The measured wind, wave and current data of recent cyclonic events are utilized to simulate the FPSO responses in terms of mooring loads, vessel yaw motion and relative FPSO heading to waves as it weathervanes in the wind, waves and current, input as time series. The primary objectives are to check the FPSO responses as the cyclone (eye or fringe) passes over it, causing rapid changes in the intensity and the direction of environmental loads, and to confirm the adequacy of the present mooring system design criteria. The results of the study serve as a good benchmark of the current industry standard for mooring design and address industry’s concern of the safety of FPSO platforms in the event of cyclonic storm. This study has used the hindcast data to inspect the event of a strong cyclonic storm passing over an FPSO. Despite the fact that the wind direction changes for about 140° in only one hour in the path of the cyclone eye, higher mooring line tension has not been observed due to reduced wind speed in the eye of the storm. The extreme mooring line tension is still governed by the responses in the path of cyclone fringe due to its maximum wind speed. Note that the transient analysis has shown that, during the rapid change of wind direction, the vessel can potentially be exposed to beam sea waves. Although this does not correspond with the highest tension in mooring legs, it can lead to critical green water impact.
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