Material Compatibility in Directional Control Valve Designs
Authors: Turner P.A. 
Published: October 2008

Abstract
Recovery of offshore hydrocarbons in deeper water and the practice of delaying start up for longer periods have bought about an increase of failures of subsea control valves. This being largely due to seawater ingress into the hydraulic system. This has led to the need to assess the failure causes, the limitations of the current materials and the selection of more seawater tolerant materials. The importance and impact that the control fluid has upon valve material selection is discussed and methods of evaluating and testing material compatibility to control fluid as well as to sea water is proposed. Hydraulic system improvements to mitigate against sea water ingress are described and what improvements will feature in future control valves to achieve reliability recovery are discussed.

Control Valves

Types and Usage
Control valves are used in subsea control systems to provide hydraulic power to open and close hydraulically actuated process valves on subsea Christmas trees, manifolds and other similar subsea control equipment. They are electro-hydraulic devices with two stages. Pilot stages, which are small solenoid operated hydraulic valves, provide a hydraulic pilot to operate a main stage. The main stage is a larger hydraulic valve, which diverts the hydraulic pressure to and from the process valve actuator, to open and to close it.
These control valves come in many variations ranging from single pilot electrically held valves for master supply and choke control applications, to dual solenoid, hydraulically latching valves for process valve control applications. Figure 1 shows a typical dual solenoid control valve and figure 2 shows schematics of a typical open to closed operating sequence.
These control valves have been used from the very beginning of subsea control systems. Starting from the experimental Exxon SPS off of the Louisiana coast of North America in 1970 to today's subsea control systems deployed into the wider offshore producing regions throughout the world. An estimate of the numbers of this type of control valve deployed subsea is 10,000. The vast majority of which have been deployed in the last 10 years.

Materials and Control Fluids
The materials of construction for these valves manufactured by Kvaerner Oilfield Products (KOP) are shown in table 1. The table shows that, except for the seals, the pilot stage ball and the solenoid coils and armature, all materials are stainless steel.
The early subsea control systems used a synthesised hydrocarbon control fluid, currently known as Castrol Brayco Micronic, within closed loop hydraulic systems. This control fluid is especially blended to provide low viscosity, anti wear and oxidation stability. In more recent times water based control fluids used in open loop, (vent to sea), hydraulic systems has largely replaced the oil based fluid as the control fluid of choice in subsea control systems.
This combination of control valve materials used initially with hydrocarbon based fluids and latterly with water based fluids has proved a reliable combination in subsea control systems until 1997 when a number of control valve failures were experienced within KOP control systems.