CONCEPTUAL STUDY OF A DEEPWATER OIL PRODUCTION AND STORAGE SYSTEM

Offshore oil production activities have been developed rapidly during the last twenty years and are expected to grow in the future. Techniques for deepsea oil production are some of the most significant possibilities that should be developed.; As far as the production is concerned, the conventional fixed platform technique is not cost effective in deepwater, and may become technically impracticable in very deep waters. The use of a floating production platform associated with subsea wellheads seems to be the most feasible.; Pipelines have been widely used to transport the oil in shallow and medium waters. In deepwaters, which are usually very far from the shore, offshore tanker loading is preferred from technical and economic viewpoints. In the open sea environment, however, a continuous tanker loading operation will become very difficult. Therefore, offshore storage is desirable as a buffer so that the produced oil can be stored continuously in a storage tank and then pumped from the storage tank into each tanker as soon and as rapidly as the weather permits.; The purpose of this study is to develop a deepwater oil production and storage system, and a new structural type 'DOPS' has been developed. The DOPS system is an integral system which contains all the subsystems in one structure: production, storage and tanker loading.; The unique feature of the DOPS system is the use of an underwater storage tank which is located deep below the sea surface. The tank is suspended from a surface production platform with vertical tension lines, and is connected to the seafloor with catenary mooring lines. The DOPS structure is a combined structure of theTLP and the semi-submersible systems, and will have the following structural advantages: (1) smaller wave forces, (2) intermediate support for the production riser, (3) availability of a conventional catenary mooring line system, and (4) less maintenance requirement of the catenary mooring lines. Buoyancy adjustment chambers filled with compressed air are provided in the storage tank structure to keep a constant underwater weight of the tank (constant draft concept). In the storage mechanism, a water displacement principle is used.; A computer program has been developed to analyze the dynamic response of the system under the various wave actions. The following characteristics of the deepwater oil field have been selected for a case study: 100,000 b/d production and 600,000 bbl storage capacity in a 1000 m water. The analysis results show that the DOPS system is a feasible and stable system in operational and survival conditions, and that the vertical tension line will be the most critical structural element. The storage tank location is the most important factor in determining reasonable and practical values for the vertical tension line force. For this study, it developed that the tank should be located below a 400 m depth for proper performances.; Reliable installation procedures of the storage tank and of the anchor blocks for the catenary mooring lines have been developed.; Operational aspects of the DOPS systems have been investigated. The unitized system of subsea wellheads will be used to provide early production to the project. The production riser will remain connected even during the survival conditions. A bow loading shuttle tanker will be moored directly to the production platform with the weathervaning system.; Consequently, it can be concluded that the DOPS system will be a very competitive system in deepwater oil production from structural and economic viewpoints.