Troll Pilot Control System: Advanced Control System for a Subsea Separator

ABSTRACT This paper describes the design of a subsea control system for control and monitoring of a subsea water separator, for use by Norsk Hydro in the Troll field, offshore Norway. The separator uses a gravity separator to extract water from produced fluids. The water is then re-injected into a disposal well, via a subsea variable speed pump. The control system comprises topside computers and fibre optic communication, together with a subsea control module for control of separator valves and monitoring of pressure, temperature and separator water level sensors. The system operates in a closed loop, with water level measurements resulting in control of the water injection pump speed. Key feature is fibre-optic communication at 1.5 MHz, primarily for immunity from interference from the pump electrical supply, operating in half duplex mode on a single fibre. Full redundancy is provided for all electrical and optical equipment including sensors with 4 redundant communication channels. The level sensors use a combination of technologies to give accurate and reliable, long-term measurements in the hostile separator environment. A redundant high-speed general-purpose Fieldbus (DC power and communication) is provided subsea for connection of complex sensors (level sensor) plus others as required in the future. Topside computers, which are fully integrated with the Troll C facility network, provide analysis of the data from dual level sensors, to detect the Interfaces between water, oil and gas, in the presence of emulsions and foam. This is used to control the pump speed in order to maintain a constant water level in the separator. The equipment began integration and land trials in the last quarter of 1998, and was installed subsea in September 1999. INTRODUCTION The Troll field in block 31/2 and 31/5, 100 km west of Bergen, contains large quantities of oil and gas, together with significant amounts of water. It was decided therefore to install a subsea separator in order to avoid installing additional topside processing equipment and to maximise useful flowline capacity. Figure 1 shows the location of the installation and a diagram of the subsea arrangement, together with some basic field details. The separator is a self-contained seabed unit, based on ABB Offshore Systems' SUBSIS technology and is located between two conventional production manifolds and the Troll C platform. PROCESS SYSTEM A simplified P&ID of the separation process is shown in figure 2. Essentially water, oil and gas flow into a large gravity separator tank, where the water sinks to the bottom and the oil floats on top, with gas filling the void at the top. The oil and gas flows over a weir plate and flows on to the Troll C platform under the driving pressure of the wells. The water is drawn out of the bottom of the tank and is injected into a disposal well by a large 1.6 M Watt subsea pump. Figures 3 and 4 show the pump and overall separator assembly.