The growing trend of energy demand has increased the drilling and exploration of remote offshore oil and gas (O&G) fields. Globally, nearly 1500 oil and gas rigs are located offshore, the largest share of which are in the North Sea and the Gulf of Mexico. Offshore oil and gas deposits are typically much larger than those found on the land. Offshore deep-water O&G production systems need to incorporate high-capacity electric submersible pumps and compressor motors at the seabed, which demand considerable electrical power. Power requirements can be of the order of several hundreds of MW depending on the number of subsea loads. The recent trend in the O&G industry is to install subsea processing loads on the seabed to reduce the required space on the platform or even remove the platform. There are several advantages to installing power converters and equipment close to the loads. 

The installation and operation of subsea electrical systems on the seabed have various challenges [1]. The pressure increases by 10 bars (about 145 PSI) for every 100 m depth in the ocean. If all the systems including power electronics and motors need to be located at a water depth of about 3000 m, they have to withstand 300 bars of pressure. Hence, all electrical systems have to be designed and qualified to withstand high pressure. As seawater acts like a conductor and is corrosive, proper isolation between the electrical equipment and seawater must be provided. Also, as the equipment is located at depths of about 3000 m, post-fault maintenance and repair will be challenging and will not be possible without bringing the equipment to the surface. However, relocation of the equipment to the surface is expensive and results in long production outages. The technology for locating all the electrical equipment including the power electronics is still in the early stages of development by organizations like SINTEF in Norway. Hence, only the subsea pumps, gas compression, boosting, and water injection are generally located on the sea floor, and the power is supplied from a topside platform, including that for the downhole electric submersible pump (ESP) [2].

2. Efficiency Improvement of Platform Power Generation

Gas turbine-based efficiency can be improved by capturing and using the waste heat from the gas turbine using a bottoming steam Rankine cycle ^[3][13]. Power generation using this combined cycle approach can be increased by 30%, and the overall plant efficiency can be as high as 50%. However, the energy loss is still significantly large in a combined-cycle gas turbine-based system, and the processing integration can be challenging, resulting in a sub-optimal emission reduction solution. The average CO₂ emissions from a natural gas-fired, combined-cycle power plant are approximately 0.46 metric tons of CO₂ per MWh of electricity generated. The integration of carbon capture and storage units in subsea processing systems has been investigated, with an incremental improvement in the exergy index ^[4][14]. Emission reduction in platform power generation with the use of renewable energy sources can be achieved using hybrid fuel cell power generation systems. This could be incorporated by combining a high-temperature solid oxide fuel cell (SOFC) with the gas turbine, which would increase the overall cycle efficiency while reducing per-kilowatt emissions. The combined system has far greater efficiency than could be provided by either system operating alone. Combining higher efficiencies combined with low emissions, hybrid systems are a good choice for reducing platform-based emissions due to power generation. A high-level architecture of a SOFC/gas turbine hybrid system is shown in Figure 12. In this hybrid power system, a compressor and heat exchanger are included with the turbine. The ambient air is compressed and supplied to the cathode of the SOFC, and the anode is supplied from the fuel, which is reformed to hydrogen. The exhaust of the SOFC, at a high temperature of about 800 °C to 1000 °C, is supplied to the turbine, which causes the desired heat and pressure difference suitable to drive the turbine. The turbine-driven generator output can be combined with the fuel cell output to supply power to subsea loads. This type of hybrid generation can achieve much higher efficiency, of the order of 65% to 70%, compared to the gas turbine or SOFC operating alone. Also, the emission per kW is reduced significantly. A hybrid fuel cell-gas turbine-based approach has also been explored for CO₂ capture in platform generation ^[5][6][15,16]. If SOFC is used, the fuel cell can directly use the locally generated natural gas with external or internal reforming to generate power. As per the need, the same SOFC can be combined with a gas turbine to increase the overall efficiency as shown in Figure 12. However, a polymer electrolyte membrane (PEM) fuel cell can also be used if the green hydrogen is generated with excess wind power using electrolysis. The reformed hydrogen can also be used for the PEM fuel cell after further purification.

3. Integration of Offshore Renewable Energy Sources to Power Offshore Loads

Offshore energy resources can be mainly categorized as offshore wind and ocean renewable energy. Ocean renewable energy (ORE) is broadly defined as all the feasible energy sources harvested from ocean waters. The main categories of ORE are tidal energy, wave energy, and ocean thermal energy conversion (OTEC) ^[7][17]. Offshore wind power is potentially the most feasible option for subsea electrification and subsequent emission reduction. Offshore wind farms have the largest generation potential among all renewable sources. The synergy between offshore wind and offshore O&G is already underway in Europe, where multiple oil, gas, and marine companies are engaged in wind energy development ^[8][9][18,19]. According to the International Energy Agency (IEA), total offshore wind capacity is forecast to be more than triple by 2026, reaching close to 120 GW. Major energy companies are looking to boost offshore wind development for electrification of offshore oil and gas platforms along with green hydrogen production. Equinor is studying possible options for building a floating 1 GW offshore wind farm in the Troll area with a predicted annual production of ~4.3 TWh by 2027. This could provide much of the electricity needed to run the Troll-A offshore fields ^[10][20]. Shell has announced a plan to start building Europe’s largest renewable hydrogen plant in the port of Rotterdam. This hydrogen will use renewable power for the electrolyzer from the offshore wind farm Hollandse Kust (noord), partly owned by Shell ^[11][21]. The 1.32 GW Hornsea 2 (UK) is the largest operating offshore wind farm in the world. It is 462 km² (178 square miles) in size and can power more than 1.3 million homes. China’s MingYang Smart Energy has announced an offshore wind turbine even bigger than GE’s Haliade-X. The MySE 16.0-242 is a 16-megawatt, 242-m-tall (794 ft) behemoth capable of powering 20,000 homes per unit over a 25-year service life. Commercial production is slated to begin in the first half of 2024 ^[12][22]. A case study based on using offshore wind power for powering the platforms located at different places and providing surplus power to an onshore grid is presented in ^[13][7]. In this study, three scenarios were considered: (1) a 20 MW small offshore wind farm with a stand-alone electrical grid on the offshore oil and gas platform; (2) a 100 MW wind farm is connected to five nearby oil and gas platforms by subsea power cables, as shown in Figure 23; (3) a 1000 MW wind farm for supplying wind power both to oil and gas platforms and to an onshore electrical grid. Based on the simulation study, it was observed that these systems can significantly reduce CO₂ emissions. Also, multiple subsea fields can be electrically interconnected as a large microgrid, which then connects to a large offshore wind farm. This technology enables better electrical stability of the offshore grid and reduces the carbon footprint of the entire system. Although wind generation is intermittent, the off-generation period can be compensated for by the integration of other energy sources such as fuel cells ^[14][15][23,24], battery energy storage systems (BESS) ^{[16][17][18][19]}[25,26,27,28], wave energy systems ^[7][20][21][17,29,30], tidal energy systems ^[22][23][31,32], and even platform-based generators. A microgrid based on offshore wind power and the platform diesel generator to power the offshore loads is shown in Figure 3 ^[24]4 [8]. This is an interconnection of clusters of oil platforms in existing oil fields forming a microgrid. The system uses a medium-voltage DC grid for interconnecting the wind generators with the loads using an AC/DC and DC/DC power conversion system. When not enough offshore wind power is available, a local diesel generator is used to compensate for the deficient power from wind. This technology enables immediate power to rigs, as well as potentially supplying the onshore grid with excess renewable energy. However, this system still contributes to the emissions from diesel engines. Wind energy can also be combined with fuel cell-based power generation. Excess wind energy can be used for hydrogen production and storage, to be used by fuel cells as and when required. A modular multiport converter-based offshore grid architecture for integrating renewables is presented in ^[16][25][26][25,33,34] and shown in Figure 45a. In ^[26][34], an offshore grid architecture with a capacity of 135 MW is described to power subsea loads. The grid interconnects various sources, including wind turbines and hybrid storage systems (HSSs) comprising fuel cells, lithium-ion batteries, and a HVDC grid. To facilitate this, the sources are interconnected via a modular multiport DC-DC converter, as an energy router (ER), which is based on modular Quad Active Bridges (QABs). The system has been designed in a way that allows each wind turbine to be separately connected to the ER, without the need for forming a wind farm. Similarly, the HSS units are separately connected to each QAB module shown in Figure 45b, resulting in a more flexible system with fewer maintenance requirements. In the system in Figure 45b, the four ports of the energy router are used to integrate wind energy, backup energy storage, an on-shore DC grid, and the subsea load. The energy router provides galvanic isolation, matches the voltage level between the distributed energy sources and the load, and manages power flow from the sources to the load. Such solid-state transformer-based multiport active bridge converters are used for integrating renewable energy sources to power the subsea loads. The main advantage of these converters is that a single unit of a solid-state transformer (SST) can be used to interconnect a number of sources, enabling an increase in power density as compared to using separate converters. SSTs have the features of instantaneous voltage compensation, power outage compensation, fault isolation, bi-directional power flow, controllability, etc. SSTs can be optimally designed and integrated into the microgrid system to have the best performance both during transient and steady-state conditions. Apart from wind generation, a significant portion of the energy requirement for subsea electrification can be obtained through ocean renewable energy (ORE) ^[7][17]. As per ^[22][31], the theoretical potential of tidal power (including tidal range and tidal currents) is 26,280 TWh/year. Wave energy and OTEC showed an even higher energy potential of 32,000 TWh/year and 44,000 TWh/year, respectively. Despite the huge potential of ocean energy, the actual energy harvesting is limited by the low technology readiness level (TRL). Ocean thermal and salinity gradient technologies have only been explored in a few demonstration projects. Contrarily, tidal energy technology is more established. Tidal generation requires a minimum operating depth of 15–40 m, which is suitable for the electrification of platforms nearer to the shore ^[27][9]. Tidal barrage plants have been in grid-connected operation both in Europe and Asia, with installed capacities of around 250 MW ^[22][31]. Tidal current harvesting has been successfully proven in several test sites to synergize with wind turbine technology ^[23][32]. Another type of ORE, wave energy, has also been identified for its potential utilization in offshore microgrids to drive subsea electrification. A wave energy conversion (WEC) system typically consists of a floating buoy mechanically coupled with an electrical generator. The wave-induced movement of the buoy is transformed into a rotational movement to drive the generator and produce electricity. Ref. ^[18][27] explores the vast potential of ORE in South America with a 25,000 km coastline, with a significant share from WEC. The first full-scale prototype of a WEC with 100 kW capacity was installed in Pecem Port, Brazil in 2011. Similar medium TRL projects have been ongoing in other South American countries like Argentina, Uruguay, Chile, etc. WEC is also proven to be useful when combined with floating wind turbines ^[20][21][29,30]. A floating wind–wave-combined power generation system named ‘DBSC’ is presented in ^[20][29], which shows high power generation capacity and excellent stability. As per the techno-economic analysis of a combined wind and wave generation system, the levelized cost of energy (LCOE) is reduced by 10% ^[21][30]. As the all-electric and electro-hydraulic production systems in subsea processing have momentary load demand in several kWs, the deployment of offshore energy storage systems (ESS) has gained momentum. The offshore ESS is also a key enabler of offshore renewable interconnection to mitigate the intermittency of offshore renewables. Offshore ESS also facilitates the DC collection grid for ORE, which manifests lower transmission loss and a smaller footprint ^[28][35]. As offshore ESS can be deployed close to the power generation sources, the power processing cost is reduced. However, the harsh sea environment makes the design of long-life ESS quite challenging. Nonetheless, industries have explored offshore battery systems along with fuel cells in recent years ^[19][29][28,36]. A multitude of ESS options such as batteries (lead-acid/Ni-Cd/Li-ion), supercapacitors, flywheels, compressed air energy storage (CAES), and hydrogen energy storage have been reviewed in ^[17][26]. It has been observed that the best demand response is provided by Li-ion batteries and CAES. The potential of green hydrogen in offshore microgrids is also a state-of-the-art topic for research ^[30][31][37,38]. A techno-economic assessment of a green hydrogen-based microgrid in a remote island of Northeastern Australia is presented in ^[30][37]. The article demonstrates that the LCOE for electricity generation is reduced, along with 20,000 kg lesser CO₂e emission through this microgrid. However, the proliferation of green hydrogen-based microgrids is currently limited due to environmental regulation and compliance policies.