Flow Assurance Challenges for Offshore Deepwater in Republic of Congo

摘要

Until now, offshore oil and gas production in the Republic of Congo has been restricted to the area covered by shallow water technology, up to water depths of around 200m. The Moho Bilondo development is the first deepwater development in these offshore Congolese waters and this paper presents the flow assurance issues that have been addressed during the design, construction, installation and start-up of this field. The development comprises of production from the two reserves of Mobim and Bilondo. To meet the challenging project schedule the flow assurance aspects of the process design had to play their part and had to be both functional and robust. This paper presents the flow assurance issues that have been addressed during the development of the project. The challenges for flow assurance fall into three categories. Firstly, the hydrodynamic issues had to be addressed. Bilondo oil is relatively heavy and requires gas lift injected downhole to achieve the desired recovery of reserves. At the same time, the down-sloping nature of the sea bed terrain from the Bilondo manifold can present the typical problems of induced slug flow regime. Management of the gas lift strategy was therefore key to the operating philosophy. The thermal insulation performance to provide the required cooldown time for preservation had to be achieved for the entire subsea production system, so as to avoid the formation of hydrates during prolonged shutdowns. Making matters worse is the downhole supply of cold lift gas via the Xmas tree, which has a cooling effect on the production fluid in the tubing. An additional risk identified here was that of wax formation on the inner tubing wall. Significant finite element studies were performed on the Xmas tree wet insulation to achieve the desired thermal performance. Thirdly, the risk to production by the formation of naphthenate soap deposits was also assessed. Steps were taken in the design of the topsides process and of the chemical injection strategy, dosing rates and injection points both subsea and on the topsides, to manage these risks.