Lower Cost Control Systems for Marginal Fields
Authors: Loth W.D. 
Published: October 2008

A number of alternatives have been proposed for reducing the cost of subsea control. Components under development to facilitate these evolving systems are described and a number of possible configurations outlined. Cost comparisons are presented which indicate that the most effective lower cost approach is a minimum umbilical system in which remote power supply and seawater hydraulics are connected to the surface with fibre optic and chemical supply links.

INTRODUCTION

Current electrohydraulic subsea control systems function well and have few technical restrictions as step out distance or water depth increases. The' incentive for considering alternatives is largely cost reduction. Although some reduction in functional capability may be accepted if cost savings are of sufficient magnitude, most alternatives will be judged on their ability to meet established performance standards.

The largest single cost component of most subsea control systems is the umbilical. Substantial work has been done on autonomous systems to eliminate the umbilical. While perhaps a most worthy target, it is also a very ambitious one. While the entirely autonomous system may be technically feasible for offsets up to about 15 km, available technology limits the cost effective application of autonomous systems to small developments not requiring significant chemicals for treatment of producedfluids. In the very long run, these limitations may be overcome but for the near future somewhat lessambitious systems will provide substantial cost reduction while incurring much less technical risk.

This paper examines the problems which must be addressed with lower cost subsea control systems and evolving solutions to those problems.

POWER SUPPLY

More efficient electronics and electro-mechanical components, and improved subsea mateable electrical connectors have facilitated a long term decrease in the power which must be delivered by subsea systems. In an electrohydraulic system, power to actuate components is conveyed hydraulically and, with resupply from the surface, only a very modest amount of power need be stored subsea. The power requirements for a lower cost system could vary widely depending on the approach selected. On one end of the spectrum is a closed loop, acoustically linked, control system powered by a passive thermoelectric device. This approach is attractive in that the hydraulic portion of the system can be conventional, requiring only attention to avoid hydraulic fluid losses. The power supply, while not without limitations, is a mechanically passive, and hence inherently reliable, component. The limiting factor in this approach is the geometry of the power supply. A continuous power supply of 100 W seems reasonable but does require the balance of the system to be efficient. It may also be necessary to store more energy for restart purposes since the charge rate is low.

At the other end of the spectrum are power supplies driven by produced or injected fluids. Figure 1 illustrates a gas expander unit, designed by Hayward Tyler 1for Mobil, and currently being tested by British Gas. This is a 15 kW supply. With substantial power available, the control system designer has additional options. Open hydraulic systems operating on raw seawater can beconsidered.