A subsea control system operates and controls the valves and chokes on the subsea trees, manifolds, templates, and pipelines in the subsea production system. The state-of-the-art in controlling and operating subsea production systems are by electro-hydraulic systems. Such systems need to transfer both electric and hydraulic power from the host to the subsea components. The electric power is needed for powering the subsea electronics and sensors, whereas hydraulic power is used to control subsea valves and chokes. The electric power system consists of a topside electric power unit, power cables, and subsea electronics. The hydraulic control system consists of a topside hydraulic power unit, hydraulic lines, accumulators, and hydraulic actuators. Compared to the electric power supply, the hydraulic power supply requires a large and comprehensive infrastructure. In an all-electric subsea production system, electric solutions are used to replace the hydraulic control system. This means eliminating the hydraulic power supply units and hydraulic control system units such as hydraulic lines and accumulators. Hydraulic actuators are replaced by electric actuators.
All-electric subsea systems are believed to be one of key technologies for reducing cost in subsea oil & gas field developments1 and the benefits of subsea all-electric control systems are many. All electric control systems are expected to be simpler compared to a conventional electro-hydraulic control system. For instance, it reduces the topsides footprint and cost through removal of the topsides high pressure unit system, through smaller and lighter subsea modules reducing the cost of hardware and installation, and through smaller and less complex umbilicals. All-electric hardware and software will give large benefits both in terms of CAPEX and OPEX in addition to improved HSE (safety and environment in particular), improved reliability, flexibility as well as increased functionality. All-electric technology is favorable to use when developing marginal fields at great distances from a processing facility because of the lower cost of umbilicals. It also provides solutions to problems associated with high-pressure and high-temperature wells because there is no need for hydraulic fluid. It also improves HSE and simplified testing through removal of pressurized equipment during testing, handling, and operation. Further, it provides a higher degree of flexibility when expanding an existing system and when introducing new equipment into the system. Finally, the removal of the hydraulic system omits environmental and economic problems related to the leakage of hydraulic control fluids and the complexity of working with hydraulics2.Opportunities and market impacts
The industry has installed and operated electric systems (electric actuation of chokes and manifold valves) for more than 15 years. For instance, development of subsea electric valve actuators was started in 1999 and installed in 2001 for the non-safety critical production valves and chokes3. Since then 200+ electric actuators for choke and manifold valve actuation have been installed in subsea applications4. For example, at the Åsgard subsea compression station, Equinor have installed 78 electric actuators for choke and control valve actuation 6. Since the development and installation of electric actuation of production valves, the focus has been on developing and qualifying electric systems for safety-critical valves. Equinor installed electric actuators with a "fail-safe close" functionality (both not formally SIL certified) as a pilot system at the Norne field in 20055. Total installed the first all-electric systems at a Xmas tree at the K5F field outside the Netherlands in 2008, whereas Total installed the world first all-electric downhole safety valve in 20166.
Even though the development of all electric safety-critical systems has been ‘around the corner’ for about 20 years, it is only now we are seeing real effort to solve the safety and business related questions. There is now a general industry trend to replace hardware-based control with lighter, but more complex, electric and SW-based (‘all electric’) solutions. ‘Subsea All-electric’ for operation of subsea well barrier elements are planned by operators to be implemented in projects in 2020 and be operating in 2022. Unlike today’s electro-hydraulic systems, ‘all electric’ represents a step-change in digital barrier monitoring and opens for dynamic barrier management and digital assurance. DNV GL is a part in this transition, both in assisting the industry to solve the safety aspects as well as the general trend of digitalization7. All electric is in fact now ‘just around the corner’, and it is here to stay. Putting this in the greater perspective, beyond being favourable for long step outs and deeper waters, ‘all electric’ can be a game changer also to the unmanned offshore field. Operators and suppliers within Oil & Gas are developing ‘all electric’ subsea control systems, driven by the need for reduced cost, remote operation and access to deeper waters. All electric can be an enabler for unmanned platforms and performance-based business models. Operators plan to operationalize all-electric in frontrunner projects from 2020. Given that implementation challenges are solved, all-electric will be implemented in greenfield developments in sensitive areas and in deeper waters towards 2030, before becoming the mainstream option.
While the short-term benefits for implementing all-electric are clear, the long-term benefits of all-electric are believed to be the future opportunities that lies in smarter and more efficient operation, such as online risk-management, predictive maintenance, etc., and the alignment with future electrification of subsea oil & gas production and utilization of power supply from renewable energy sources. Electric actuation, sensing, integrated power management and control offer the promise of smaller, lighter, and more cost-effective subsea production systems. They also offer the potential for more precise measurement, control- and maintenance monitoring, with significant benefits in terms of reduced OPEX8. Subsea all-electric will therefore act as an important building block for future low carbon oil & gas production.Risks and uncertainties
The actuation of the safety critical valves is today made by hydraulic power. The safety functions are provided by return springs, which can be rather heavy and strong. The force needed to compress the spring can be more than 50 metric tonnes. At the same time are we seeing that very large complex systems being designed, exemplified with subsea compression, which are totally all electric controlled and actuated. So why are the well barriers not all electric?
The main reason for this is that all-electric and digital technologies are challenging existing safety philosophies. While regulations encourage innovation, existing safety standards used by the industry restrict the envelope of new solutions and may impose excessive cost.
Existing industry standards and guidelines, derived from best practices for traditional technologies (such as electro-hydraulic actuation and safety functions provided by spring return) and operational concepts, may not provide relevant support for demonstrating safety of new all-electric technology. Consequently, traditional or “hybrid” solutions are often chosen to ensure safety demonstration within the timeframe of the project. Furthermore, without a common approach (work-processes and methods), the means for sharing of experience and improving best practices within the industry is lacking.
Key principles in the Norwegian regulations are fail-safe principles, independence of process control and safety control systems, and independence of different safety barriers. Ensuring independence can be difficult – and knowing just how independent individual safety barriers are, may be an even greater challenge: While independence of different safety functions is often assumed on a high level, more subtle dependencies may still exist, arising from physical dependencies (e.g. stemming from communication and information exchange), logical dependencies (e.g. in software), location dependencies (e.g. common external events), etc. Thus, the actual level of independence of safety functions is not fully known today, and increased digitalization may lead to even more dependencies.
How can the industry take advantage of these new and upcoming subsea all-electric technology opportunities without imposing excessive costs for demonstrating safety? Work done in the initial phase of the joint industry project Safety 4.0 led by DNV GL7 showed that the industry needs a common and simplified approach that enables a faster safety demonstration and enables a cost-effective implementation of new subsea technologies. The objective of Safety 4.0 is to enable and accelerate up-take of novel subsea solutions by developing a new risk-qualification framework for standardized demonstration of safety.
Main author: Tore Myhrvold
Editor: Peter Lovegrove
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