For the exploration and production in deeper water use may be made of semi-submersibles and ship shaped types of structures. In the production phase the floaters are normally permanently moored and the mooring system is designed to withstand the local extreme environmental conditions.
Due to the risers and mooring lines in deep water a considerable amount of damping can be induced to counteract partly the low frequency motions. The mooring system however has to take the ultimate forces to keep the vessel on station. In these water depths the length of the mooring lines may be considerable. The cost effect by reducing the mooring system in terms of less mooring lines or a lighter system by applying assisted DP is worth to consider.
As an example a 200 kDWT tanker based FPSO system moored by means of a turret system has been considered. A water depth of 1200 m has been taken. The heavy mooring system may withstand survival conditions e.g. 100-year storm condition.
The same tanker is now provided with a class 3 assisted DP system, consisting of 4 azimuthing thrusters. The size of the passive mooring system (no DP) can be reduced and may withstand a much lower seastate e.g. 1-year storm condition.
It will be shown that the light mooring system including the DP system may be equivalent to withstand the same survival condition e.g. the 100-year storm as the heavy passive mooring system. The results show the effectiveness and benefit in using DP to assist the mooring system in survival conditions.
At present DP assisted tanker based FPSO units are used mostly for heading control. In these designs the location of the turret is located closer to midship than to the bow. Due to the osition close to midship the heading of the FPSO with regard to the environment may become unstable and heading control may be necessary, see Ref.1. For FPSO units with bow turrets permitting weathervaning, the application of DP assistance to reduce the mooring forces is worth to consider.
In deep and ultra deep water not only the costs of the long length of the mooring leg are large, but also the installation costs increase considerably with the water depth.
In applying assisted DP to reduce the mooring forces, the design of the mooring system may be reduced to a passive system to withstand the 1-year or 5-year storm. In higher seastate the DP system must be activated. The average percentage of times of the occurrence of seastates higher than e.g. a 1-year or 5-year storm is relatively small. If a passive mooring system is designed to cope with the 100-year storm, the mooring system will be a heavy one. This heavy system will encounter during the full life cycle a large number of oscillations and different load levels, effecting the strength and fatigue of turret and mooring legs.
In this paper an attempt is carried out to show the benefits of DP assisted turret moored FPSO units in deep water. For deep to ultra deep water computer programs are valuable design tools.