FPSO-Turret system stability and wave heading

Turret system positioning is a crucial task in the design of an FPSO. A common approach is to avoid equilibrium bifurcation when the system is subject to current action, in order to assure low levels of mooring forces as well as undesired dynamic behaviour that could lead to excessive loading both on the bearings and on the risers. Installing the turret near the midship section is a desired trend and doing it whilst preserving the ship's directional stability can be made possible with the use of passive current stabilisers. This move, on the other hand, has to be investigated for the case when waves and wind are present, as, in this case, the stabiliser may produce an adverse effect. The present work addresses this point through a case study where the dynamics of a typical VLCC converted into a FPSO and moored to the seabed through a turret system is numerically simulated with a model where wave-current interaction is taken into account. It has been found that a small degree of instability with respect to current action may reduce wave heading which tend to improve the system overall behaviour, as far as mooring forces are concerned. The effect of rudder-type stabilisers on equilibrium bifurcation and on wave heading is discussed and exemplified. A brief discussion on other aspects involved in moving the turret towards midships or installing stabilisers is carried out.