Wind squalls are known as a design driver for FPSO systems in areas where other environmental loadings are relatively benign, e.g. offshore West Africa. These mesoscale phenomena are characterized as a dramatic variation in the wind speed with a relatively short duration. The transient variation of wind speed and direction introduces a challenging design issue for turret-moored FPSO systems considering that routine steady-state approaches are not fully applicable for these loadings.
In the open literature, most studies on squalls are performed on spread-moored FPSOs which have limited dynamic response to the transient characteristics of the wind squalls. The response of turret-moored FPSOs in squalls due to their weathervaning capabilities is significantly more complex and is a function of initial vessel heading, ambient environment, vessel rotational resistance, and squall wind speed and directionality timeseries characteristics. Specification of the squall wind environment is typically a set of measured time series, scaled to match the estimated peak 100-year squall velocity. Typically the number of scaled time series provided ranges from 10 to 100.In this paper, the challenges of analysis of turret-moored FPSOs in squalls will be addressed by looking at the entire design and analysis cycle, highlighting issues or criteria not properly addressed in current design practice. This will range from the specification of squall time histories to be used for design, development of the representative turret-moored FPSO model, and the use of extreme statistics to develop design values for design. The paper shall also provide a design methodology guideline for turret-moored FPSOs in a squall environment that is consistent with those for other extreme environmental conditions.