Common base frame for subsea applications with ROV interconnections. (Credit: PRBX/Shutterstock/Vismar UK)

 

By Patrick Le Fèvre

Subsea power applications range from offshore oil and gas fields to intercontinental communications to deep-sea earthquake detection, to name just a few. Equipment can be installed on the seabed at depths of 4,000 m (13,000 ft.) and at pressures of 400 bar (5.816 psi), requiring safe and reliable power sources. In such highly demanding applications, what factors must an electronics engineer consider when designing subsea power solutions?

Subsea Power Architecture

Typically, seafloor installations are powered from the shore or a platform at sea via long-distance, high-voltage transmission cables to floating or submerged transformation stations. There, the voltage is stepped down locally to AC or DC within the range of 300 to 900 VAC or 400 to 1,500 VDC to power local equipment and, ultimately, lower DC voltages for the final application. This standard approach is like the one for power grids and electronics equipment in industrial applications on terra firma. However, when the equipment is located on the seabed and is sensitive, the level of quality and safety requirements are significantly higher.

The subsea power grid contains components such as switchgear, step-down transformers, energy distribution, and monitoring and control equipment (for pumps, compressors, water-injection systems and safety controllers). For efficiency and safety, the power components can be installed on a common base frame on the seabed and interconnected through application-specific connectors that can handle low voltage for signal use and high voltage for power supplies, and can sustain hydrostatic pressure and underwater operation. For safety reasons, and to guarantee uninterrupted operation, the local grid is secured by uninterruptible power supplies (UPS), which could also be located on the seabed.

All components within the grid are retrievable and are designed and constructed for normal and incidental operation. But because they are often deployed at depths below which humans can operate, they usually require ROV intervention, which is often rolled into the design.

 

Subsea power supply designed to fit into pressurized cylinder. (Credit: PRBX/VB120-5)

 

Subsea Power Supply Categories

Subsea power supplies are classified into two major categories. One is the standalone container power supply unit (SC-PSU). SC-PSUs are designed to sustain very high pressures. They feature deepwater connectors and built-in removable chassis for ROV handling. SC-PSUs require very high levels of expertise in deepwater equipment design and are often designed and manufactured by the same companies who provide subsea transformers and substations.

The other category is the embedded power supply unit (E-PSU). E-PSUs are integrated within the final pressurized equipment and not exposed to high pressure or in contact with seawater. E-PSUs are closer to industrial power supplies but with extremely high reliability and dedicated functionalities for subsea operators’ requirements.

Design Guidelines

Designers of subsea power supplies often describe their job as designing a highly rugged power supply with extreme reliability and intelligence for one of the most hostile environments on Earth where failure is not an option. This is a good summary of the challenges faced by designers when developing power solutions that will be operated in the deep sea in locations that are mostly inaccessible to humans, which is where ROVs come in.

The range of subsea applications is large, and depending on which segment the power supply will address, different standards and best practices apply, but for all of them there is a common approach to perform risk analysis properly for operational safety.

In cooperation with the equipment manufacturer, the electrical load calculation is one of the earliest tasks during the power system design. Engineers should estimate the required electrical load of all the subsea elements so that they can select an adequate power supply. Each local load may be classified into several different categories, for example: vital, essential and nonessential.

Vital: Will the loss of power jeopardize the safety of personnel or cause serious damage within the platform/vessel/seabed equipment?

Essential: Will the loss of power cause degradation or loss of the oil/gas production or, in the case of transcontinental cable, communication?

Nonessential: Does the loss have no effect on safety or production?

Depending on the final equipment and the level of risk, different technologies might be considered, such as a redundant power solution, automatic load balancing or an emergency power resource switching to UPS. In all cases, the power supplies must be able to communicate with the central monitoring system, where, using the latest evolution of digital control and predictive algorithms, high levels of operational safety can be achieved.

As certain parts of subsea oil and gas equipment migrate from full hydraulic operation to hybrid—for example, motorized valves that electronically monitor and control—power supplies are required to be integrated within the pressurized cylinder. Also, when designing power supplies for transmission cables, e.g., signal repeaters, because space is critical, power designers must consider the volume available for fit into the final application.

Due to the demands for compactness and high level of integration, another important point to consider when designing a subsea power supply is the electromagnetic compatibility within the embedded system. Extensive interoperability tests are performed during the design of the final equipment, and sometimes this could require the adoption of a different topology, e.g., multiphase with active phase-shifting to reduce electromagnetic interference.

In subsea applications, reliability and longevity are very important. Power supplies must be designed with a high safety margin and with the lowest possible amount of stress—electrical and thermal—on every component. The selection of components is key to the design because the choices could influence the topology and building practices. (One example is the choice of the preferred switching transistors with a baseplate to facilitate conduction cooling.)

Designers for subsea oil and gas applications can look to the American Petroleum Institute’s (API) Standard for Subsea Production and Processing Control Systems (API 17F) for guidance. This standard includes specific tests and communication protocols.

 

Mocean Energy Blue X wave energy converter. (Credit: PRBX/Mocean Energy/Colin Keldie/EMEC)

 

Sustainable Power

Most subsea applications are powered from shore through long-distance power cables, or a platform at sea via high-voltage transmission cables often 10 to 100 km, but there are several applications that require remote power solutions. Conventionally, a vessel or floating platform with traditional power generators can be located above the field, but when considering the environment and sustainability, this is not optimal. Accordingly, the subsea engineering community has begun exploring alternatives.

The Renewables for Subsea Power (RSP) project is one of the most promising. It seeks to answer the question: How can green technologies be combined to provide reliable and continuous low-carbon power and communications to subsea equipment, offering a cost-effective alternative to umbilical cables?

Mocean Energy in Scotland, which developed Blue X, a 20-m-long, 38-t, 10-kW wave energy converter machine, and partners in subsea energy storage/management have made a business case combining wave power and solar with energy storage to power subsea equipment for oil and gas projects.

RSP’s test and evaluation is now complete, and the Blue Star ocean energy floating device, which developed from Blue X, is ready for commercial deployment. This is an excellent demonstration that wave energy combined with solar power and battery technologies can offer a reliable and cost-effective alternative to expensive umbilical cables for subsea applications.

 

Blue X provides renewable power for a range of subsea applications. (Credit: PRBX/Mocean Energy)

 

Conclusion

Developing power solutions for the deep sea is a great opportunity to learn about a very interesting range of applications that require advanced technology, extreme reliability and innovation. With the new ideas in initiatives such as Renewables for Subsea Power, we can push the limits of underwater electrical design.

Patrick Le Fèvre is the chief marketing and communications officer at Powerbox (www.prbx.com).