Session: 01-04-01 Design & Analysis-I

Paper Number: 127200

127200 - Effect of Directional Sea Spectrum in Cyclonic Conditions on Responses of a Weathervaning Vessel 

Accurate representation of wave spectrum is crucial for extreme response analysis of weathervaning vessels under tropical cyclone conditions. It is a common practice to use unidirectional wave spectrum (long crested waves) for response analysis of floating vessels, and sometimes, depending on the required accuracy, symmetrical directional spectrum formulations are used. However, reported measurements of cyclonic sea states show that directional sea spectrum may be very different from the assumed unidirectional or even symmetrical directional spectra. 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, with its higher and lower frequency wave energy components propagating in different directions, 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 behaviour and extreme responses of a weathervaning vessel.

This paper examines the effect of directional wave spectrum on weathervanning behaviour and responses of a generic turret-moored Floating Production Storage and Offloading (FPSO) vessel, exposed to tropical cyclones at the Northwest Shelf of Australia. A range of cyclonic sea states chosen for the study is representative of design metocean conditions generating the wave load responses of the 100-year return period, and the study compares the directional wave spectra from a hindcast cyclonic database with a commonly used assumption of a unidirectional 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 behaviour and several wave load responses of the vessel are presented and discussed.

Presenting Author: Michael Binsar Lubis The University of Western Australia

Presenting Author Biography: Michael in a research fellow at the University of Western Australia. After completing his master at NTNU, Norway, with research related to second-order irregular waves on jack-up platform, he continued doing research on ultra-deep water remotely operated vehicle at the University of Western Australia. In 2021, he worked as deputy project manager on the design and installation of 145MW floating solar panel project in Indonesia and then back to the University of Western Australia to do research on the responses of offshore structures under cyclonic environment.

Authors:

Michael Binsar Lubis The University of Western Australia
Yuriy Drobyshevski The University of Western Australia
Mehrdad Kimiaei The University of Western Australia