MOTION SIMULATION AND DYNAMIC STABILITY OF AN ANCHORED TANKER SUBJECT TO CURRENT, WIND AND WAVES

motions and anchor chain forces of a supertanker when anchored in a steady current, with or without wind and waves, were simulated. Additionally, locally linearised stability analysis of equilibrium states were carried out. Five sets of forces were modelled: 1) nonlinear quasi-steady hydrodynamic response and control forces based no a four-quadrant manoeuvring model, 2) linear memory effects due to radiated waves, 3) nonlinear anchor chain force, 4) variable wind forces and 5) first-order forces and drift forces due to waves. Current speed, wind speed, significant wave height, hawsepipe location, anchor chain length and rudder deflection were varied systematically. Results are presented as stability domains in parameter space for constant wind forces and wave drift forces and confirmed by complementary simulations. Selected simulations were also performed in variable wind and irregular waves. Three practical measures to stabilise vessel motions and to reduce tensile force peaks in the anchor chain were examined: selective use of either port or starboard hawsepipe, rudder deflection and choice of anchor chain length.