Moment loads on a two-dimensional turret-moored vessel under forced motion

This paper investigates the moment loading and heading response of a turret-moored vessel subjected to surface currents using two-dimensional Computational Fluid Dynamics. To investigate the hydrodynamic loads on the vessel, the moment load was first calculated when the vessel was held fixed at different headings such that the angle between the long axis of the vessel and the current ranged from 0° to 360°. At any fixed angle of attack, the moment load showed 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° to 120°). The instantaneous moment is dependent on the vortex flows around the structure. Following this, the vessel was modelled with one rotational degree-of-freedom and forced to rotate at a fixed angular speed. Several angular speeds were simulated, indicating that the moment loads correlate with the ratio of the steady current velocity to the linear velocity of the vessel at its far end. These comparisons illustrate the complex dynamics of forces with and without vessel rotation. Ongoing work will compare these results to predictions based on a simplified dynamic model for the vessel heading and extends the results to a three-dimensional vessel, where the dynamics remain complicated, but the role of vortex shedding is less significant.