Abstract
Accurate representation of wave spectrum is crucial for extreme response analysis of weathervaning vessels under tropical cyclone. It is a common practice to use unidirectional or symmetrical directional wave spectrum for response analysis of floating vessels. However, reported measurements of cyclonic sea states show that directional sea spectrum may be very different from the commonly assumed spectrum. Importantly, the directional wave spectrum of cyclonic sea state is typically skewed, with its higher frequency tail tending to align with predominant local wind, while the peak wave energy at lower frequencies propagates in a different direction, sometime at large angles to wind. Asymmetrical shape of the spectrum may affect responses of a floating structure in different ways. Given that responses in cyclonic conditions usually govern the structural design, there is a need to assess the significance of the directional spectrum for the behavior and extreme responses of a weathervaning vessel.
This paper examines the effect of directional wave spectrum on weathervaning behavior and responses of a generic turret-moored Floating Production Storage and Offloading (FPSO) vessel, exposed to tropical cyclones at the North-West Shelf of Australia. Cyclonic sea states chosen for the study is representative of design metocean conditions generating the 100-year wave load responses. The study compares the directional wave spectra from a hindcast cyclonic database with a commonly assumed spectrum. A parametric formulation for the directional wave spectrum in tropical storms, which was developed in oceanographic research in the last decades, is also considered in the comparison. Results of weathervaning behavior and several wave load responses of the vessel are presented and discussed.
