Prediction of the mean heading of a turret moored FPSO in bi-modal and bi-directional sea states

Ship-shaped platforms like Floating Production Storage and Offloading (FPSO) vessels are deployed in many offshore locations where the sea states are dominated by wind driven local seas combined with long period swells that are incident from different directions. Many of these vessels are held in station by a mooring system that is connected to the platform by a rotating turret. The heading stability of the vessel and its ability to self-align with respect to oncoming environmental forces is studied in this paper. Experiments at 1:120 scale conducted in a wave basin facility for two storm conditions offshore Brazil and West of Africa show that the heading angle is self-limiting to a range of ±20° with respect to head sea direction. Time domain simulations conducted using industry-standard software by modeling the wind-sea and swell with suitable spectrum models approaching from different directions show almost double the heading range seen in experiments. If similar numerical analyses were part of a design process, it is conceivable that this could result in incorrect predictions of the weathervaning of the platform, and hence the latter’s global motions in response to design storm conditions.Previous studies by the authors have confirmed that the delicate balance between various mean wave drift forces and moments is the main contributor to the heading stability of a single-point moored ship shaped vessel. It is shown here that in a bi-directional seastate, the interaction between the two wave components can significantly alter the drift loads on the platform. A simple cross-wave correction is added to the time domain simulations to account for the interaction. Implementing this correction brings the numerical results more in line with the experimental findings. The cross-wave correction term is quantified and recommendations are made for better simulations and experiments of such platforms.


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  • Accession Number: 01678933
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Jul 3 2018 3:09PM