Published: 12/14/2021
Published: 12/14/2021
The increasing pressure on offshore operators to reduce costs and produce hydrocarbons more sustainably is driving significant change in the oil and gas industry. For many operators, this means rethinking conventional processes and technologies to remain competitive today and throughout the energy transition. One of the most beneficial things offshore operators can do to enhance their operational efficiency and minimize carbon emissions is to reevaluate electrohydraulic subsea production technologies for both their existing and new installations.
We sat down with Schlumberger’s Technology Manager for Production Systems Hebert Heidenreich to discuss how replacing hydraulic with electric systems can enable performance-based returns, offer better reliability, help operators re-imagine their subsea field architecture, and reduce CO2 and environmental footprint.
Subsea electrification brings considerable simplification of the subsea templates and associated topside infrastructures. Operators can remove all hydraulic elements, simplify subsea distribution units, and eliminate hundreds of kilometers of hydraulic control lines without adding major electrical requirements to the system. This enables the creation of simplified, optimal system architecture, which enables new frontiers with deeper and longer step-out wells. It also addresses all the health, safety, and environmental (HSE) issues associated with pressurized fluids.
Reducing the size and weight of the subsea structures naturally translates into lower costs. This also applies to the umbilical interfaces as they are much smaller in diameter. Because subsea electric systems are simpler in terms of their design, they can be delivered and installed within shorter lead times and with fewer vessel trips, making project economics more favorable.
Subsea electrification comes with built-in sensing and monitoring that enable distributing the system’s intelligence differently and decentralizing the decision points closer to the physical actuators. Increased well connectivity with smart electric completions also reduces subsea structures, enhances recovery, and reduces production deferment. The possibility of daisy-chaining electrical actuators instead of the usual point-to-point connection of hydraulic actuators opens new opportunities for system architecture optimizations.
Furthermore, subsea electrification enables accessing stranded reserves at deeper water depths and with long step-outs that tie back to existing infrastructures. The all-electric system can be operated with conventional topside systems, which means all-electric trees will be able to co-exist with conventional trees for brownfield applications. Adding more remote wells without having to deploy specific topside infrastructures will both contribute to the cost benefits and lower carbon impact.
Besides the drawback of hydraulic control fluid disposal to sea on normal valve closure, another inherent issue of hydraulic systems is fluid leaks. While measures can be taken to reduce or mitigate leaks from hydraulic systems, it’s not cost efficient from a maintenance standpoint. Aside from the contamination risks to the offshore environment, there is also an emissions impact from continuous vessel trips that are required to properly maintain the system. On top of this, typical fluid replenishment costs on aging fields can range between $500K to $2M per year. Interventions to repair hydraulic leaks and facilitate fault finding also contribute to higher costs and deferred production from unplanned downtime. All-electric systems don’t require hydraulic fluid and they require less overall maintenance and interventions over the field’s lifetime. Subsea electrification also increases the number of process variables that can be monitored and enhances diagnostic coverage, with direct measurement of previously inferred conditions, enabling lower downtimes and shorter fault-localization time with reduced intervention infrastructure.
Another thing to consider is the overall size and complexity of hydraulic versus electric systems. As I mentioned, all-electric subsea production systems are simpler—and lighter—regardless of the environmental conditions or application. By contrast, hydraulic systems equipment must get bulkier and thicker to meet requirements of high pressure, deepwater environments, and long tieback scenarios. They require more steel, more hydraulic controls, and larger hydraulic power units (HPUs), which translates to higher costs and carbon intensity to not only manufacture the hydraulic system, but also more vessel trips for transport and installation.
All-electric systems have more sensors, which provides intrinsic information about position (motor resolvers), torque (consumed current), and many other internal parameters that can be used for prognostic health monitoring (PHM). This will support superior reliability and availability of the system to minimize intervention, and it is also a key enabler of not permanently attended installations (NPAIs).
Electric subsea production systems are not new, and they have a proven track record of reliability. For example, our initial designs of all-electric trees have been deployed by TotalEnergies in the North Sea and have operated without failure related to electric actuation since 2008. In a recently published technical paper from Equinor and TotalEnergies on subsea all-electric technology, the authors note subsea production system (SPS) suppliers have delivered several hundred electric actuators on multiple subsea development projects and have collectively accumulated over 10 million fault-free operating hours.
One concern with all-electric production systems is power. For our downhole systems we use very little power supply from surface. Our actuators for the valves downhole draw around 25W during actuation. This is the same for the subsea production systems where the aim is to provide highly efficient actuation systems.
Currently, advanced applications for subsea electrification leverage trickle-charged battery-powered systems. Such systems use a more cost-effective umbilical compared to traditional electrohydraulic systems or spring fail-safe battery-less all-electric systems. Most of the time the system is in monitoring mode consuming very little power. However, when the valves need to occasionally be opened and closed, instantaneous demand for higher amounts of electrical energy are required for a couple of minutes. Battery-powered systems enable the most efficient usage of the existing infrastructure because they can kick in only when they are needed to instantaneously provide sufficient power for valve movements. Battery-powered systems also offer a much better alternative to standard systems for future integration with renewable power sources due to their local energy storage capabilities. This mitigates issues with non-steady power availability from renewable energy sources.
Another concern is contingency for failure. Subsea electric actuators offer increased levels of contingency when compared to hydraulic systems. Mechanical overrides are possible for fail as is or closed as is within hydraulic systems, incorporating redundancy and retrievability.
Many existing subsea control system failure modes are related to hydraulics. Elimination of these potential failures coupled with simplification of components result in a more reliable system. This is also demonstrated in other industries, such as aerospace, automotive and medical sectors, where electric actuators are increasingly used for safety-critical applications.
