The caisson vessel system is a deep-water floating production concept which is currently being developed for use in water depths of 600 to 1500 meters. Its operational capabilities include the following aspects of oil production: development drilling, subsea production, surface processing, well maintenance, storage, and off-loading. All the subsystems necessary to provide these capabilities are integrated into the design of the 400,000-tonne moored concrete vessel. Constraints of vessel stability, environmental loading, equipment accessibility, safety, ease of construction, and cost govern the design of such a vessel. The project has now progressed through conceptual and preliminary stages, and a detailed site-specific design can proceed when a field development need is confirmed.
The search for oil is now taking place in deep-water sites that will have to be developed with technology beyond the reach of bottom-supported structures. If a significant oil discovery is made in these locations, a new production system will be needed in order to recover the crude economically. The technology behind such a deep-water production system must be developed well in advance of any potential application to help in assessing the economic viability of the basins during lease sale, exploration, and delineation drilling phases.
The caisson vessel system was conceived in an effort to provide the technology required to produce oil in water depths from 600 to 1500 meters. It has been developed from feasibility studies through preliminary design and is now ready for final design and construction for a specific location. The system consists of an integrated drilling and production vessel with storage and offloading capability (Fig. 1).
A 400,000-tonne spar-shaped, reinforced concrete vessel is conventionally moored over a submerged production system (SPS). A drilling riser and a production riser connect the vessel to the seafloor template. Storage is provided in cells surrounding the base of the spar by means of a seawater displacement system. The drilling and production facilities are housed on and within a steel superstructure. The vessel will be capable of drilling and completing wells through the SPS, then processing and storing the oil. Dedicated shuttle tankers will be moored to the caisson and the stored oil will be off-loaded.
The objectives, capabilities, and design criteria for the caisson vessel system will first be discussed. A description of each major subsystem will follow. The remainder of the paper will concentrate on the development of the vessel hull design to meet the specified conditions and capabilities.
The design objectives of the caisson system are similar to those of most offshore oil projects. They involve maximizing economic attractiveness while minimizing risks to the personnel, the environment, and the system. Maximization of economic attractiveness depends on choosing the least costly system configuration consistent with meeting functional requirements, then minimizing costs in two areas, namely, component investment and operating expense.
