A Review of Engineering and Safety Considerations for Hybrid-Power (Lithium-Ion) Systems in Offshore Applications
From prior experience in the automotive sector, and now the mari- time sector, hybridization of power systems is known to increase en- ergy efficiency and reduce emissions, with lower fuel consumption. With impending emissions-control areas in the US continental shelf, and nitrogen oxide enforcement mechanisms in the North Sea, emissions reduction in oil and gas exploration-and-production operations is increasingly relevant. Hybrid-power systems can address some of these issues with batteries to offset peak loads, thereby reducing size requirements for the total system. The challenge that the oil and gas industry faces is to decide when and where hybrid-power systems provide the most value for operations, how they should be implemented, what technologies are acceptable, what safety considerations there may be, and how these technologies can improve the bottom line. There is a wealth of information on lithium-ion batteries, though it is not all consistent—cost data are unclear, lifetime and energy density considerations vary under different conditions, and ruggedness and application to harsh environments constitute a large uncertainty. A review of these technologies is provided to serve as a selection guide.
The offshore oil and gas industry faces two challenges in the near future: increased regulation on emissions and increasing cost of operation. The following discussion outlines the way in which hybridpower systems—primarily those enabled with lithium-ion (Li-ion) battery technologies—can address both of these challenges, though not without some safety and technology qualification needs. US emissions-control-area regulations will require nitrogen oxide (NOx) reductions by 80% by 2020 (DNV 2012b). In preparation for this, engines use urea injection to reduce emissions, which adds cost to the power system. These systems reduce particulates, NOx, and sulfur oxides, yet hybrid systems can be used to level out transient loads, which will also lead to more-efficient urea management. In addition, the push for deepwater exploration and production implies increased costs in transmission of power to the seafloor, and greater distribution of subsea equipment in deep water. Also, because of stringent safety requriements, subsea equipment will require increased reliability by means of backup power and self-sustaining power systems. When connected, this equipment will require transmission lines that must be sized for maximum capacity, even if that capacity is met infrequently, unless hybridization can reduce the transmission requirement. Battery-based energy storage on the seafloor can be used to discharge during peak loads, thereby reducing capacity requirements for the overall power system. The smaller diameter cables or reduced capacity of umbilicals translates to reduced capital cost and less losses in the transmission system. Thus, topside and subsea systems enjoy the same benefits (increased efficiency and reduced costs), yet for different reasons. Oil and gas stakeholders must determine when and where hybrid-power systems provide the most value for operations, how they should be implemented, what technologies are acceptable, what safety considerations there may be, system suitability for extreme environments, and how these technologies can improve the bottom line. There is a wealth of information on Li-ion batteries, though it is not all consistent—cost data are unclear, lifetime and energy density considerations vary under different conditions, and ruggedness and application to harsh environments constitute a large uncertainty. In the following sections, we will address these issues to help provide clarification for the oil and gas operator.
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