Both bottom mounted and floating wind power could be connected to the oil platforms directly, thus enabling renewable electric power instantly to the platform. The oil platforms are customarily located far out at sea in areas where vast wind resources can be harnessed, enabling the potential of a high power production and capacity factor. The size of the wind power plant and operational strategy needs to be carefully selected for securing technically stable and economic operations as the O&G companies only accept little or no production loss. The intermittent power production from wind power thus cause concerns that wind power alone will not be able to supply the platform at all times. For platforms which already have gas turbines or are connected to a cable from shore there is the possibility to run these parallel to the wind power. One example of this, is the “Beatrice wind farm” project where two 5 MW bottom fixed turbines was built at a depth of 40 m and connected to a nearby oil rig, providing about 30% of its total energy demand. The remaining power was supplied from the national grid. [54] Studies have shown that local wind power production matching the offshore power demand will improve both voltage and frequency stability when in parallel operation with a cable from shore. Moreover, it is indicated that offshore reactive power injections or alternative wind power plant control topologies could improve voltage stability offshore. [53] The study also showed that a system with wind power connected to the system was able to restore to normal operation faster after a short circuit on the main offshore AC bus, which is one of the most severe events the offshore grid can experience [53].
Another study made by SINTEF energy research [55] showed that a wind power plant with bottom mounted wind turbines in operation parallel with gas turbines can be an economic and environmentally sound option for supplying electricity to O&G platforms. In this study, logistic simulations show that the wind turbines result in significant fuel and emission reductions, particularly when allowing for start/stop of gas turbines. [55] One of the greatest challenges is to find a good operation strategy that balances the number of start and stops of the gas turbines against dissipating energy and fuel savings. In time, this should also become an increasingly realistic solution for floating wind turbines when the technology has matured. This could be especially useful for platforms located far from shore or on too deep waters, where bottom mounted wind turbines and mainland electrification might be too expensive or too complex to implement.
To have wind power supplying the platforms with energy by themselves is currently not a feasible solution due to the intermittent power production. This would require a significant higher nominal power capacity than the required power of the oil platform to reduce the risk of energy shortage. How much higher depends on the wind characteristics of the site and the specific requirements of the oil platform. However, a nominal power higher than the need of the platform means that there occasionally will be an overproduction and the power output must be reduced, e.g. by pitching the
36 blades, resulting in dissipated wind energy. This would be necessary unless some form of energy storage is introduced. From a technology aspect, it would be possible to support an oil platform with merely wind power together with an energy storage, although this energy storage would have to be significant to secure the supply of energy to the platforms [56]. Even though a large number of methods for storing energy exists today, few of them are considered economic and technically feasible. Even with an extensive expansion in R&D development lately, batteries are still considered too expensive to use for large scale storage. Many other types of energy storage are under development, where different types of pumped hydro storage might come to play an important role in the years to come. The Subhydro concept seen in Figure 24 is one of the alternatives, using one or several large hollow concrete spheres on the bottom of the ocean as an underwater pumped hydro power plant. This concept uses the excess electricity from e.g. wind power to empty the spheres of water. During an energy demand, the valves are opened, allowing the surrounding water back into the spheres through turbines, using the extreme pressure at the bottom of the ocean they claim to achieve a total energy efficiency of about 80 %. [57] There are several similar concepts, but all would require both large investments and R&D before applicable in the offshore industry.
Figure 24. The Subhydro energy storage concept, where seawater is released through a turbine during energy demand and the water pumped out during excess of energy. [57]
Another efficient possibility could be to use a combination of the solutions for a cluster of O&G platforms within reasonable distance from each other [56]. The clusters would have a wind power plant connected to them, using the vast wind resources offshore to produce a large part of the power demand. During low demand of energy, the wind turbines could instead deliver the power to the energy storage alternatively to shore via a HV cable when the storage is full. Together with the cable to shore, a backup gas turbine could be installed at one of the platforms to regulate the flow and further secure the power demand of the cluster.
The solution for clusters or stand-alone platforms would have their own optimized solution based on their specific conditions, locations and opportunities. However, even with the benefits of connecting wind power to the O&G platforms shown in the mentioned studies, it might be unlikely to think that
37 the O&G companies will implement this on their own initiative unless the government demands an electrification of the O&G industries. [56]