Benefits of using Assisted DP for Deepwater Mooring Systems
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.