New measuring sensor for level detection in subsea separators

BB launched a development project called SUBSIS 1) at the beginning of 1996 with the goal of finding a subsea technology that would increase recovery and allow more economic offshore oil and gas production while reducing the environmental impact of such operations [1]. Based on the initial results, Norsk Hydro awarded in June 1997 the world's first commercial contract for such a system. Delivery of the installation – for the Troll Pilot subsea separation project in the North Sea – is scheduled for late 1999. Norsk Hydro specified for this first SUBSIS application the use of two independent level measuring principles. Sub-sea environments make special demands on the installed instruments, and as a result their specifications differ in several important respects from those used in top-side separators. Several measuring principles were subsequently evaluated. 1 For the project it is planned to use a nucleonic densitometer and an inductive level monitoring system (ILMS) to measure the conductivity. The ILMS was developed at ABB Corporate Research in Norway in close cooperation with ABB Offshore Systems, who owns the system and has overall responsibility for the project. It is currently being prepared for industrial use. Both instruments are due for delivery in 1999. Subsea separators rely on accurate level monitoring Gravitational separators rely on the principle that immiscible fluids separate if left to rest, with the lightest fluids on top and the heaviest ones at the bottom. In separators of the kind used in SUBSIS [1], a mixture of oil, water and gas enters the tank from the reservoir. At the other end of the tank the gas and oil float freely over a weir-plate into the production line, while the water collects in front of the weir and is pumped back into the reservoir. Although its working principle is simple, the separator needs to be able to control the mixed fluid inflow and the separated outflows of the separator tank. Closed-loop control is being used for the SUBSIS installation for the Troll project. The data measured by the level instrument in the separator tank are fed via fiber-optic cable to the platform , where they are used to automatically adjust the speed of the water injection pump and so maintain the required levels in the tank. The inflow from each well is adjusted by means of a choke at the well-head. It is extremely important to know the water level in order …