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November 2013 Issue

Subsea Processing Powered by Modular Switchgear
Greg Peregrym

Building upon 25 years of experience in subsea production, the oil and gas industry has recently performed several technological leaps in subsea processing. Particularly, subsea compression systems will soon be operating at power levels above 10 megawatts, a fourfold increase beyond existing plants.

Subsea pumping and compression have many advantages over traditional offshore production methodologies. They accelerate production and increase recovery rates, and, thus, life of field. Furthermore, they eliminate the need for a local surface production facility, which is especially attractive in geographic regions with challenging conditions. Processing subsea allows the system to exist in a more stable environment, which could open up oil and gas exploration in, for example, the Arctic, where seasonal ice makes a traditional facility impractical.

High-voltage subsea switchgear could help meet increasing underwater power demands. Subsea power distribution must be reliable and modular to adapt to changing production requirements. Care must be applied when considering how to modularize and integrate a subsea switchgear system to maximize the benefits of the final architecture.

Providing highly reliable subsea switchgear presents unique challenges. Existing switchgear is designed for terrestrial applications, in environments where conducting repair and regular service has minimal impact on power availability. Subsea switchgear, however, cannot be repaired underwater, which is an obstacle to maintaining power availability. A modular system is required for elements to be retrieved for repair or replacement, while allowing power to be supplied to the subsea loads without interruption. The key to modularity is the positive localization of electrical faults, thus providing operators with precise information to retrieve only the modules requiring maintenance.

Seafloor networks in deployment have grown recently in quantity, size, depth and capability, proving their reliability. For example, the Victoria Experimental Network Under the Sea (VENUS), located off the coast of Vancouver Island, has been operational since February 2006. With such existing seafloor networks in mind, a viable concept to enable modularity for subsea processing is to utilize well-established seafloor network control and diagnostic technology in combination with switchgear technology to create a subsea switchgear module that meets the power availability requirements for subsea processing.

The proposed concept involves splitting a terrestrial-style switchgear system into several subsea modules, whereby each module can be recovered separately. For dividing the switchgear system into subsea modules, the recommended solution is to separate the larger, more reliable, high-voltage circuit breakers from the much smaller and more complex peripheral protection, control and diagnostic components. These electronic control and diagnostic modules would be housed in separate subsea replaceable modules networked together by the communications and power distribution technology used in existing seafloor networks. Peripheral component single points of failure could be mitigated using different proven techniques, e.g., adding redundancy. Should a failure occur, replacement of the faulty module during the next available maintenance opportunity would return the system to its original availability target. In the meantime, the subsea power distribution system would continue to supply power to all the loads. This is essential for maintaining subsea production in areas where ice cover or seasonal sea conditions limit access to subsea equipment.

Because the peripheral components are housed in smaller modules, they would be easier to replace after a first failure has occurred or for feature upgrades. Switchgear diagnostic and control modules would be powered from a source independent from the main power flowing through the subsea switchgear. Well-established cabled shore power systems for seafloor networks and telecommunication can be combined with a main power umbilical for a subsea processing plant. Separate control power would allow the switchgear control and diagnostics to be verified before energizing the main plant power. This black start sequence would allow switching circuit breakers to the appropriate position prior to energizing main plant power. Alternately, a subsea uninterruptible power supply can be used to enable control and diagnostics during the power-up sequence.

With the main technological features of a modular system already proven, the realization of a working system is mostly a matter of design and system integration. A complete subsea switchgear module could be developed in as little as 18 months. Seafloor networks are well-established, with proven reliability; therefore, their components have a high degree of technical readiness. Switchgear systems are also well-established, proven components that have been used in onshore and offshore facilities for decades. High-voltage subsea switchgear would use the same power components, with only packaging differences, to meet the constraints of the subsea environment.

The modular command and control architecture need not be limited to switchgear modules. With Ethernet-based communications, the diagnostics and control system would be expandable to other subsystems in a subsea processing plant. Nodes within the system could act as repeaters and allow connection of additional modules, allowing plant expansion as requirements change over time.

As oil and gas high-power subsea pumping and compression technology transitions from development to reality, demand for robust and reliable high-capacity subsea power distribution will grow. A strategic, modular approach would meet requirements for high-voltage, reliable and expandable power.

Greg Peregrym is a senior subsea electrical engineer at OceanWorks International (Houston, Texas), a subsea technology company. He develops custom subsea solutions for the oil and gas industry, including large-capacity battery systems, hydraulic controls and subsea switchgear. OceanWorks, together with engineers from Schneider Electric (Rueil Malmaison, France), has developed high-power subsea switchgear solutions.


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