Corrosion failures of components in electro-hydraulic control systems can have serious consequences for the operation of an entire subsea oil recovery system, especially in water depths more than 150 metres. One reason for this is that seawater ingress can have a great effect on stainless steel 316L, the most commonly used material for the failed components of the direction control valves, since chloride ions destabilize the passive film. Other materials, claimed to be seawater tolerant, are starting to be used in this system. However, problems can still exist due to the complex factors relating to the corrosion process and how the environmental parameters affect the corrosion mechanisms. In this work, the corrosion behaviour of a Nickel Tungsten Carbide (NTC) cermet is compared with stainless steel 316L, in four different water-glycol hydraulic fluids and 50% fluid/50% seawater solutions using an electrochemical test methodology. Detailed conclusions are then made to summarize the advantages and the disadvantages of NTC to be used in this system. The effects of each factor on the corrosion rates and mechanisms are discussed.
1.1 Industry problem and study objectives Technological advances have allowed operators to venture into deeper and deeper water for oil and gas. However, the serious consequences of corrosion remain a problem in the offshore industry, with its complex and demanding production techniques, and the environmental threat should components fail leading to hydrocarbon leaks. In February 1999, the Schiehallion Oilfield, located 150km West of Shetland was shutdown only seven months after production began . Figure 1  is reproduced from website, showing the Schiehallion layout. The subsea equipment is the link between the hydrocarbon reservoir and the offshore platform. The role of the equipment is to control all aspects of the production including flow rates and the operation of safety shutdown valves. The communication between the platform and this equipment is achieved through an umbilical, containing electrical and hydraulic connections, to a Subsea Control Module. This controls the subsea hydraulic system and receives information from the many monitoring devices. Failure of the module breaks down the ability of the platform to control and monitor the subsea equipment and stops production. Open circuit hydraulic systems have been used to reduce the cost of the projects because hydraulic lines are only required in one direction. This halves the number of lines inside the umbilical as the water based hydraulic fluid is vented to sea after use; hence the deeper the water, the greater the saving. These systems have not shown the reliability of a closed circuit hydraulic system in deep seawater and in the case of the oilfield chosen for the project, contributed to its eventual failure. The failed subsea control system was an E/H multiplexed system of open hydraulic configuration utilising water-glycol hydraulic fluid (Fluid B) and the main parts said to be responsible were the directional control valves (DCVs) fabricated from Stainless Steel 316L . Subsequent investigation concluded that the primary damage was associated with seawater ingress that may degrade the fluid.