Back in the 1960s the first subsea systems were simply waterproofed surface systems placed on the seabed next to a platform in shallow water. These systems were operated remotely using a normal hydraulic system with conventional mineral hydraulic oil. As the oil was returned to the platform via the umbilical hose, the product did not leak into the sea (unless something burst), and therefore the associated environmental impact was minimal.

With time, subsea wells moved gradually farther away from the platform. As conventional hydraulic oils are quite viscous, this increased distance necessitated the development of new water-based fluids as they are much thinner. The advent of water-based products also removed the need for the hydraulic fluids to be returned back to the platform via a circulating system—they could now be discharged directly into the sea within discharge limits agreed upon with environmental regulators and based on environmental impact assessments.

One of the subsea templates being installed on Statoil’s Åsgard Field offshore Norway is shown. As subsea operations further extend into more remote and challenging environments, the need for alternatives to water-based subsea control fluids will increase. (Source: Statoil)

Water-based challenges
Despite certain benefits, these water-based products also brought certain challenges. Inhibitors are required to prevent corrosion, glycol must be added to prevent it from freezing at low temperatures and, as the product is discharged into the sea, not only must it be cheap enough to “throw away,” but the aforementioned environmental legislation also has steadily become more stringent.

The electro-hydraulic multiplex (EH-Mux) subsea control systems that followed were based on avionics control technology and used synthetic fluids such as Castrol’s Brayco Micronic subsea control fluids derived from those used in military aircraft, guided missile systems and even spacecraft. The EH-Mux system concept reduces the impact of fluid viscosity on system response time, allowing control over much longer distances. This also facilitates the use of a closed-loop configuration, where hydraulic fluids are returned back to the platform. At this time, the majority of operators did not transition to this new system, which also meant that they continued to use the standard water-based fluids and discharge them into the sea.

Operators have embraced the transition to EH-Mux subsea control systems. However, there is still a reluctance to return to synthetic fluids to support those systems as this requires a circulating system and return line to the platform, which is considered more expensive in the short term. There is also a perception that today’s fluids are still “oil vs. water” from an environmental perspective, when actually most current environmental legislation refers to individual chemical components and the impact they have on the environment and doesn’t differentiate between oil- or water-based products.

Fundamentally, a fluid is either compliant with the appropriate legislation, or it’s not.

11-million liter global fluid market
The global subsea control fluid market consumes in the order of 11 million liters (2.9 million gallons) annually, and currently this is almost all being discharged into the sea.

From an environmentally sustainable perspective, it is likely that—in the medium to long term—environmental legislation and a desire for continuous improvement will encourage operators to stop discharging products into the sea rather than continually developing less harmful products.

Operations in deep water, remote areas, extreme climates, tieback to shore, HP/HT systems and environmentally sensitive locations each bring unique challenges. As these more challenging operating conditions continue to push the limits of water-based products, an alternative approach should be considered.

Basic chemistry dictates that if a subsea fluid is going to remain stable under extremely adverse conditions, then it must have strong chemical bonds. And a product with strong chemical bonds is unlikely to be biodegraded by marine life when released into the sea.

The industry needs to invest in R&D for a new generation of products that are likely to only be used in a small part of very few systems but are capable of confidently meeting the requirements of next-generation subsea systems. The cost of meeting this challenge effectively is likely to create a product that is too expensive for use in the entire system or to be discharged into the sea.

So shouldn’t the industry instead consider reverting back to a closed system using synthetic fluid technology, which can provide much greater inherent stability under these extreme conditions?

High-performance synthetics
Synthetics have a proven track record, and there are evolving technologies at an advanced stage that can deliver high performance in these extreme conditions. It is possible. For example, the Ormen Lange Field, which is more than 100 km (62 miles) offshore Norway, is tied back to the coastline with the subsea control system running on Castrol’s Brayco Micronic control fluid.

The case for change is not black and white, and there is a need for compromise. There are two relatively distinct areas within the subsea production hydraulic control system, the first being the high-pressure system, which operates in well equipment such as the subsurface safety valve well. This is where the equipment also is subjected to very high temperatures by the raw hydrocarbon flow.

Secondly, the low-pressure system operates valves mounted on the christmas tree. This requires higher volumes of control fluids in a total-loss system but represents a relatively benign environment as the hydraulic actuators are immersed in seawater and so are exposed to lower temperatures.

Simultaneous use of different fluids
At present, a standard water-based hydraulic fluid is generally used throughout the entire system. But could the two nearly separate systems operate autonomously using two different control fluids, each more suited to the operating conditions? This would enable operators to use synthetic products in the high-pressure area and continue to use cheaper water-based products in the low-pressure area.

There are certain technical challenges that will need to be overcome. The differences between the physical characteristics of the two types of fluid need to be taken into consideration when designing the system, and the most frequently quoted argument for maintaining one single total loss system is human error (e.g., if someone puts the wrong fluid in the wrong tank).

But that is something of an unjust appraisal of the ability of subsea operators as their counterparts on surface production facilities are expected to deal with up to 700 lubrication points and about 40 different lubricants.

As subsea production moves into new areas of operations, the industry must be ready with fresh innovations to support successful and reliable operation of such assets in new territories. A collaborative approach between equipment vendors and lubricant manufacturers is essential to ensure the development of new subsea technologies that are reliable and fit-for-purpose.

The technology exists to meet the increasing challenges of the subsea operating environment, but this technology is synthetic rather than water-based, which will require lateral thinking in terms of system design and architecture.

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