Squalls can be the major design driver for FPSO systems in offshore West Africa where other environmental loadings are relatively benign. The measured squall time series indicate a transient change in the wind speed, starting with a sudden increase to a peak wind speed followed by a rapid decay, all within a total duration of about an hour. In the design of FPSOs for squall loadings, careful attention needs to be given to the transient characteristics of squalls.
The main objective of this investigation is to characterize the response of FPSO systems subjected to squalls and develop a robust approach for estimating the design value. For this purpose, first, the dynamic behavior of an FPSO in squalls is studied and then the significant squall parameters that affect the dynamic response of the FPSO are identified. The results of this study are utilized to define the upper bound of the dynamic amplification factor and the upper limit of the extreme response of the FPSO. Next, three different response-based approaches: a) based on the long-term statistical analysis of the response, b) based on the long-term statistical analysis of the squall parameters and c) based upon the dynamic amplification limitations, are utilized to estimate the design values. Finally, the design value estimates obtained from the response-based approaches are compared with those estimated from scaling squall time traces to the 100-year peak wind speed. The study is mainly focused on spread moored systems; however, the proposed methods are also tested on turret moored FPSOs, and their application for these dynamically complicated systems is evaluated.