Novel Subsea Production System for Shallow Water Oilfield Development in China Bohai Bay

Abstract Bohai Oilfield of CNOOC is the largest crude oil production base with annual production of 265 million barrels; however, as the Bohai Bay is an internal bay with an average water depth of 60ft, the overlapping proportion of fishery, channel area to oilfield mining area reaches 78%, the commonly used jacket platforms are prohibited in this area. The production rate of shallow water is relatively low compared to deep water, and the economy of underwater production system mode is low. Meanwhile, the mature underwater Christmas tree products are almost unable to meet the development requirements of oil and gas as the formation energy is incapable of achieving self-jetting. The subsea wellhead and production system suitable for shallow water are manufactured to develop the overlapping reserves. Since 2019, CNOOC has established a research team to integrate over a hundred domestic and international supply chains to complete a series of design, processing, assembly, and testing work for underwater wellhead and production systems. Based on the characteristics of operations in the Bohai Bay, the design principles of integration, simplification, and low cost are adopted to overcome more than 10 industry challenges such as wellhead stability and flow safety assurance. Moreover, desktop simulations are fully conducted to ensure technical feasibility and operational safety. The "slimming" underwater wellhead system has been developed to replace traditional mud line hangers, and the multi-layer casing tieback procedures are saved, and the costs have been reduced by 30% while the drilling and completion efficiency have been improved by 20%. On the premise of ensuring the configuration of oil extraction functions and safe production, the design of small-sized, lightweight, and low-cost Christmas tree products has been achieved. Compared to products on the market, the height, weight and price of newly developed Christmas tree have been reduced by 25%, 35% and 40% respectively. It has a safe and reliable service life of 20 years. Moreover, multiple sensors for real-time monitoring of fluid temperature and pressure, as well as underwater production nozzles with adjustable opening, can remotely adjust oil and gas well production. The fully enclosed underwater protective facilities can effectively avoid the risks caused by fishing net towing and ship anchoring hitting the underwater production system. As the visibility of shallow water is poor, the divers and underwater manipulators are employed to conduct the auxiliary installation of underwater equipment instead of the ROV commonly used in deep water. The underwater wellhead and production system suitable for shallow water oil and gas development have been firstly manufactured and applied in China Bohai Oilfield, which fully validate the functionality and reliability of the underwater production system. It will be widely applied to more restricted blocks in next three years, to increase production.