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Qualifying Lead-Acid Batteries For Use in Subsea Applications
Testing of Absorbed Glass Mat Variant Shows Charging Under Pressure Is Feasible for Supplying Power at 3,000 Meters Depth

By Steven Pridie
Dr. Andreas Huster
Adrian Woodroffe
Senior Electrical Engineers
OceanWorks International
Burnaby, Canada

The implementation of seafloor networks is increasing in many fields, including scientific observation and oil and gas exploration. Seafloor networks, such as the Victoria Experimental Network Under the Sea (VENUS) and Tsunami Warning and Early Response Cyprus (TWERC), provide a power and data infrastructure for connecting subsea instruments and devices.

Seafloor networks in the science community were originally intended to connect low-power devices to the network, allowing for real-time, continuous data monitoring. These early monitoring devices were typically designed to run on batteries and optimized for low power consumption. Now that seafloor networks have become common, more instruments are being designed to make use of the high data throughput and power available. Examples of these new technologies include vertical profiling systems, seafloor crawlers and AUVs with docking stations. Assurance of power delivery to these new critical loads can be a significant issue. For example, ensuring a vertical profiler is not left deployed in the event of power failure could be an instrument-saving feature.

Measured gas volume for each battery bank per cycle. The device under test generates similar gas volumes as compared to stock control bank.

The peak power demands of these types of instruments can easily consume all available system power, requiring operational coordination of these devices. The practical limitation of increasing the power capacity of a seafloor network is the resistance in the power cable, which is constrained by the length of the cable and the available conductor cross-sectional area. As the cable resistance increases, more power is dissipated in the cable, requiring higher transmission voltages. The higher the transmission voltage, the higher the costs for equipment, particularly for DC power systems.

Power delivery less than 10 kilowatts is relatively easy for cable lengths less than 50 kilometers, but the system cost escalates rapidly for power requirements beyond 10 kilowatts and transmission distances in the hundreds of kilometers.

To solve the power capacity issue, OceanWorks International (Burnaby, Canada) is developing a large-scale subsea uninterruptable power supply (UPS) system to support critical and high-power devices on seafloor networks by deploying an energy storage and delivery platform.

The UPS is designed to guarantee supply of power to critical systems, such as magnetic bearings, and provide load leveling for high-peak-demand instruments, such as in-situ ROVs and crawlers. The UPS can be recharged at times of lower power demand.

Designing the UPS
The most important element to the UPS design is the selection of the electrical battery. Many battery chemistries have a history of subsea deployment, including lithium, silver-zinc and lead-acid. This application is unique in that the UPS will be a stationary structure and does not require high energy density to achieve high capacity. A system with a capacity of 100 kilowatt-hours can be housed within a manageable 20- foot ISO container. The selection of battery chemistry for this application was driven by the need for low risk, low cost, high reliability and long operational life. The target environment for the batteries is oil-compensated.

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Steven Pridie is a senior electrical engineer at OceanWorks International. He specializes in electronics for subsea deployment and has developed a variety of systems for cabled seafloor networks and the HS1200 Quantum atmospheric diving system.

Dr. Andreas Huster is the lead electrical engineer at OceanWorks International. He is a graduate of Stanford University, with a Ph.D. in electrical engineering.

Adrian Woodroffe is a senior electrical engineer at OceanWorks International. He is a graduate of the University of Surrey, England, with a master's in satellite engineering.

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