Electro-Hydraulic Control Systems for Subsea Applications
Authors: Stivers G.S. 
Published: December 2007

This paper describes a versatile family of control systems for subsea BOP'S, wellheads, and valves. It also presents results of extensive shop and field tests including one serie conducted eight hundred feet below the water surface.

After describing in some detail the various subsystems used in making up such systems, the paper will discuss several applications for some of the systems and review operating conditions and their effect on choosing systems.

Because the primary power output of the systems is hydraulic, they operate valves and blowout preventers without requiring modification or intermediate equipment. However, considerations of economy and reliability frequently favor electrical signal transmission. Test results reported show the success of composite electro-hydraulic systems including several unique design features.

Introduction

A versatile family of control systems for subsea BOP'S, wellheads, and valve manifolds has been designed and extensively tested in the shop and in the field. These systems are electro-hydraulic in that the communication subsystems are electric and the primary power trains are hydraulic, so that the systems are easily used with existing equipment such as BOP stacks, wellhead valves, and flowline valves.

This family of controls offers several important advantages over the all-hydraulic systems used almost exclusively today in subsea applications. The first is speed of response. Electric communications are almost instanteous, whereas all-hydraulic systems require several seconds to several minutes for the pressure signal to propagate down the hose or pipe. The pressure signal to propagate down the hose or pipe. The greater the separation between the control panel and the operating equipment, the greater the time saving. A second advantage is that information on the status of the subsea equipment and the control system itself is easily obtained at little additional cost. This feature can be an important factor in safety, pollution control, and timely maintenance of the equipment. A third feature, particularly important in BOP Controls, is that armored electro-hydraulic umbilical or electric cable can be used in place of the unarmored hose bundles used today, thus providing a strong control link that is less likely to be damaged and can be used to raise and lower a retrievable control system, eliminating separate running lines and winches. A fourth feature, of considerable importance in deep water operations, is the capability of these systems to incorporate an acoustic communications link to operate the subsea equipment in the event the primary control line is lost. Finally, these systems offer a very flexible package of subsystems that can be selected to provide the most economical control system for whatever combination of performance features the application requires. performance features the application requires. As an illustration of this flexibility, consider the various communications and power subsystems that are available. Any control system can be visualized as consisting of two subsystems, a power train and a communication subsystem, which interface in some type of switching component, such as a solenoid operated hydraulic pilot valve. The communications subsystems available are 1) a one-wire-per-function system, in which the control panel push button is connected directly to a solenoid-operated valve; 2) a multiplex communications system, in which a pair of wires carries a coded message, much as a telegraph does; and 3) an acoustic communications system which can be used independently or as a back-up to either of the other systems. In fact, the acoustic system and the multiplex system can use the same receiving and transmitting electronics to reduce cost if the primary aim is to guard against cable failure. The one-wire-per-function system is more economical for shorter separation distances and smaller numbers of controlled functions, whereas the multiplex system is more economical for longer separation distances and larger numbers of controlled functions.