This paper describes the development of a cost-effective floating production system, comprising a conventional tanker fitted with a dynamic positioning (DP) system to maintain the riser connection to the well head.
Reduced costs in the DP system are achieved by using a control strategy based on thrusters at the forward end of the vessel thus allowing the vessel to rotate freely, i.e. weathervane, to the environment. A description of the principles of this control strategy is given.
Application of this DP system to a conventional tanker is briefly outlined. Special attention was required in the location of the thrusters in the forward cargo tank and a description is given of the numerical studies and model tests used to optimise thruster location.
In order to gain insight into vessel behaviour under such a DP system, simulation studies were carried out and the results correlated with model tests in a wind, wave and current environment. These results indicated the need for a thruster at the aft end of the vessel under certain environmental conditions.
The paper concludes with an economic assessment of the operational costs of such a DP system, which together with reduced thruster and power plant costs presents an attractive solution to production from marginal fields.
There is a growing need for more cost effective solutions to development of marginal oil fields particularly in the light of the current depressed oil prices and the uncertainty attached to them. Floating production systems can provide such a solution using existing technology which is evidenced by the growing trend of units being put into operation during the past decade. A tanker based system is ideally suited to this role since it has the capability for in-built storage of the produced crude oil combined with a high deck load capability. The ability of the tanker to weathervane to ambient environmental conditions provides a relatively stable platform, which is comparable with the motion performance of a semi-submersible, for the process plant to function.
Large tankers are subject to large drift forces from the waves which place substantial demands 011 mooring systems. Common practice is to provide a catenary anchored buoy or tower to which the tanker is linked by a yoke structure. The vessel is permanently on station and discharges produced oil in reasonable weather conditions to shuttle tankers in a tandem mode.
The moored system is suitable for many applications but the costs can rise significantly with increasing water depth and in areas with severe environmental conditions.
An alternative solution for location of the vessel is use dynamic positioning (DP). This offers a large degree of flexibility. Risers can be disconnected quickly and the vessel moved off station in the event of environmental hazards, such as typhoons or ice. In certain circumstances where discharge ports are nearby the vessel can transport the cargo alleviating the need for shuttle tankers.
Dynamic positioning is generally not dependent on water depth. Recent developments in satellite navigation, such as GPS, will soon provide position measurement accuracy comparable with existing position reference systems.
