Water-glycol solutions (HFC) [1] are one of the most commonly used hydraulic fluids in applications where fire hazard is a concern such as in the offshore oil industry. However they may cause corrosion failures of components in electro-hydraulic control systems that can have serious consequences for the operation of an entire subsea oil recovery system. The principal objective of this study is to assess the corrosion behaviour of the main material of construction used for the components in such systems (Stainless Steel 316L) in a range of commercial water-glycol hydraulic fluids (Oceanic HW443, HW525, HW540, HT, EE1), and to find out the probable factors that may affect the corrosivity of each fluid.

The corrosivity of each fluid was ranked according to the breakdown potential obtained from cyclic potentiodynamic polarization (CPP) tests [2]. Gas chromatography-mass spectroscopy (GC-MS) analysis was carried out to assess the components of the different organic additives in each fluid and the water percentage of each fluid was determined by Karl Fischer titration. A comparison of relative ethylene glycol content has indicated that the higher the concentration of ethylene glycol the more corrosive the fluid is to stainless steel 316L if the additive package is the same. However, additive packages dominate the corrosivity.

Industry problem and background

Technological advancements in the offshore oil industry have allowed people to venture into deeper and deeper water for oil and gas. However, more and more significant cost is generated from corrosion problems. From BP Amoco's subsea experience, which operates more than 100 subsea wells today, the direction control valves (DCVs) are the most susceptible components to corrosion in a open loop hydraulic system that controlling subsea operations, in particular the pilot-stage valve spindles and the balls that allowed the flow of hydraulic fluid when it was displaced from its seat. Figure 1 shows the directional control valve and the key components blamed for failure are enlarged.

Subsequent investigation of the failure of the Foinaven Project indicated that the unusually long-term static presence of seawater from ingress during the initial installation operation was the prime cause of both the corrosion and the biodegradation of the HW540 hydraulic control fluid. The investigation highlighted the fact that at water depths below 150 metres, the external hydrostatic water pressure was sufficient to overcome the thrust of the hydraulic coupler poppets and allow seawater to enter the hydraulic system [3]. Lessons from the Foinaven experience were transferred and incorporated into the Schiehallon Project and the project team from Kvaerner had made significant efforts to minimize the risk of seawater ingress during initial equipment installation. However, in February 1999, a complete loss of HP hydraulic fluid pressure was experienced which shut in production from the Central Manifold, though it did not appear to suffer initially in July 1998 [3]. Schiehallon experience found that in addition to seawater, the hydraulic fluid itself may also cause corrosion to the direction control valves.

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