F(P)SO Global Responses in the West of Africa Squall Environment

Abstract

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.

Permanent versus disconnectable FPSOs

“…The FPSOs currently in operation cover a wide arrange of environment conditions, vessel sizes, production rates, operating water depths and number of risers FPSOs have been deployed in benign environment such as Offshore West of Africa [2] and in extreme hostile environment such as North Atlantic Margin; FPSOs operate in water depths ranging from 20 meters (Chang Qing Hao) in South China Sea to 1 853 meters (Seillean) Offshore Brazil. The vessel size varies with storage capacities ranging from a mere 50 000 bbl to 2 000 000 bbl (barrel).…”