Development Of A Valve Actuator Powered With Untreated Seawater
Authors: Loth W.D. , Thorne B.C. , Loth K.W. 
Published: October 2008

Seawater hydraulics leads to lower cost subsea control systems with reduced environmental impact. The first development phase of a raw sea water powered, failsafe, gate valve actuator is complete, The prototypes, manufactured largely from plastic composites, have performed well during life-cycle testing. Development continues with testing at hyperbaric conditions.

INTRODUCTION

While the use of seawater for offshore, and especially subsea, operation may seem obvious in view of its' unlimited supply and negligible cost, the incentive for developing equipment powered and controlled with untreated seawater is far from a simple replacement of existing control fluids. The cost of these fluids is not a major factor in offshore operations, their performance is generally excellent, and spillage does not present a serious environmental threat. Still, the major incentives for development of seawater hydraulics are capital and operating cost reductions and reduced environmental impact.

Capital cost savings depend on the application but in all events can be significant. The most obvious, and largest, potential gain is elimination of the control umbilical in autonomous subsea well systems. The savings achievable are projected in Figure 1. Seawater hydraulics may be only one of several technologies which need to be advanced to achieve these savings but it can be readily seen that as the number of wells and, more significantly, step-out distance increase cost savings achieved by elimination of the control umbilical are significant.

Actuator costs can be reduced, The design discussed in this paper is executed in composite materials. While material unit costs may be higher, simplified manufacturing techniques 1ead to a lower cost. Furthermore, the cost and complexity of hydraulic system operation is reduced by eliminating the need for clean hydraulic fluid. When compared to other system costs, the actual cost of control fluid is minimal in closed 1Oop systems and hardly significant in competent (non-leaking) total loss systems. The cost of cleaning and assembling components to high cleanliness standards can be significant if a number of components are involved.

Lower cost satellite water injection wells are a further possibility with direct powering of activated components with injection water. For developments with a mix of producing and injection wells, very straightforward contro1 systems can be devised taking all power from the water injection stream.

A further incentive for pursuing sea water hydraulics is the environmental benefits Most of this advantage will be achieved with the umbilical-less system. Environmental aspects have been studied using a life-cycle analysis technique This illustrates that fabrication, installation, and abandonment of the umbilical are far more significant than any impact of use or spillage of control fluids.

DEVELOPMENT OF AN ACTUATOR FOR USE WITH RAW SEA WATER

The following discussion summarizes the overa11 development programme, describes the actuator design concept, material testing, component fabrication, testing arrangements, and test results. Discussion closes with a synopsis of the approach used for, and results of, environmental impact analysis and an outline of ongoing work.