Session: 06-12-02 Ship Hydromechanics II

Paper Number: 81219

81219 - Moment Loads on a Two-Dimensional Turret-Moored Vessel Under Forced Motion 

This paper investigates the hydrodynamics of a ship-like structure using a simplified two-dimensional numerical study. The vessel is turret moored about a single point away from the vessel's geometrical centre and is subjected to steady current. The motivation of the work is to investigate the dynamics of vessel heading, as may be relevant to an offshore facility such as an FPSO.

To initially investigate the hydrodynamic loads on the vessel, the moment on the vessel (about the mooring point) was calculated at selected fixed angles from 0° to 360° between the direction of the vessel alignment and the current. At any fixed angle of attack, the moment load shows highly nonlinear time-dependent behaviour, where the amplitude of the load is larger at angles when the ship is aligned normal to the current (approx. 90° or 270°). The instantaneous moment is dependent on the vortex flows around the structure. However, the time-averaged moment load appears to be a close-to sinusoidal function of the alignment angle in general, except at low attack angles (-10° ~ +10°) where the moment shows minor variation with angle.

The vessel was also modelled with one-degree-of-freedom rotation and forced to rotate at a fixed angular speed or allowed to spin freely. Several forced angular speeds were studied, which indicate that the moment loads correlate to the ratio of the steady current velocity to the linear velocity of the vessel at the far end. The free motion of the vessel was investigated by (a) holding the vessel at a fixed attack angle for a period of time for the flow around the vessel to develop and then (b) releasing to let the vessel adjust its position with the current. The results from these simulations are compared to predictions based on a simplified dynamic model for the vessel heading based on the fixed moment loads.

Overall the results illustrate the complex dynamics of vessel rotation. Ongoing work is extending the results to a three-dimensional vessel, where the dynamics remain complex, but the role of vortex shedding is less significant.

Presenting Author: Feifei Tong The University of Western Australia

Authors:

Feifei Tong The University of Western Australia
Hugh Wolgamot The University of Western Australia
Scott Draper The University of Western Australia