Decentralised mini-grids based on renewable energy

 - Reducing emissions of offshore oil&gas platforms by installing wind power based mini-grids

PhD stud. Valerie-Marie Kumer, Geophysical Institute, University of Bergen, PhD stud. Pål Preede Revheim, Department of Engineering, University of Agder, PhD stud. Til Kristian Vrana, Department of Electric Power Engineering, The Norwegian University of Science and Technology (NTNU)

Introduction

Gas turbines are used for electricity production on offshore oil&gas platforms all over the world. These gas turbines account for a significant share of Norway's total CO2 emissions. The nice 'fact' of Norwegian electricity production being 98% hydro power, which is cited everywhere and all the time, is simply not true, as all those electricity generating offshore gas turbines are located in the Norwegian part of the North Sea and therefore in Norway. Due to these facts has the oil&gas industry been obliged by the Norwegian government to reduce CO2 emissions.

A first 'solution' to the problem has been a cable connection to shore, to avoid the need for the offshore gas turbines. From a local point of view, this is a solution as there is no more emissions directly from the platform. Global warming is however not a local problem. From a global point of view, this 'solution' is mostly a hoax, as the consumed electricity has to be produced somewhere else onshore. To supply the platform by clean and green Norwegian hydro power is a myth. The additional onshore electricity demand is covered by the marginal production units, which at the moment mostly are hard coal and gas fired power plants. Therefore can the grid connection of offshore oil&gas platforms not lead to a reduction of CO2 emissions. It only leads to a relocation, which has no significance.

A real solution would be to install renewable generation units in proximity of the oil&gas platform, that can partly cover the electricity demand and therefore lead to a reduction in fuel consumption of the gas turbines. The combined system of the oil&gas platform and the renewable generation units would form a so-called decentralised mini-grid.

To realise such a mini-grid, there are several renewable electricity generation technologies available, but most of them are not suitable for the implementation offshore. Photovoltaic systems need a large area. The supply of an oil&gas platform needs two orders of magnitude more electricity, than a photovoltaic system on the platform could generate. There is also little experience with the application of solar panels in hard salty offshore conditions. Wave and tidal electricity generation would of course be a nice option offshore, but those technologies are still in the early phase of development and not available for industrial application. This leaves wind power to be the only realistic solution for this purpose. We therefore propose a wind power based mini-grid as power supply for future oil&gas platforms.

Proposal

As an illustrative example we use the oil&gas platform Valhall in the North Sea, which has a maximum electricity demand of 78 MW. For power supply, the platform is connected to the onshore grid with a power cable of 292 km length. The expenses of such a cable are very high, and it does not help to reduce the global CO2 emissions.

For future oil&gas platforms, a wind power based mini-grid should be considered instead. To reduce the fuel consumption of the offshore gas turbines, we would construct an offshore wind park of 15 turbines with 5MW capacity each. To choose a suitable wind turbine design, layout and location, we analysed wind data from the Heimdal platform. We do not have wind data from Valhall, but the regional differences within the North Sea are limited, and therefore the Heimdal data should be to some extend representative for Valhall as well.

Figure 1. Mean wind speeds and max. gust at the Heimdal platform

As the wind power production is not constant, the gas turbines will produce electricity in the range of 3 to 78 MW. Depending on the weather condition, some gas turbines could be switched off or run on reduced power. Switching off all gas turbines is not a viable solution, as they are needed for power regulation and to provide the platform with e.g. heating. This implies the installation of an intelligent power regulation system, to account for wind fluctuations.

Figure 2. Cumulative expected wind power production as share of the installed capacity

The daily maintenance of the wind power plant could be performed by the technical staff at the oil&gas platform if trained appropriately.

Realising this approach would cover 44% of the over all electricity demand. This would lead to a significant reduction of GHG emissions, which would be a huge and attractive step in reaching future emission goals.

Pushing the boundaries

We could even go further than 44% electricity demand by increasing wind power capacity. If we would install 165MW we would achieve an average production of 78 MW. However, this would lead to overproduction and therefore energy loss, in strong wind cases for 45% of the time. Therefore, an introducing of innovative energy storage facilities would be of great profit.

To increase the share of RES without risking wasting electricity we would need to include some sort of storage solution. As the electricity consumption of the system is high and the periods with over- or underproduction of wind power must be expected to last for periods of multiple days at a time, the storage capacity would need to be relatively large.

The most mature large capacity storage solution is at the moment pump-hydro. This however requires some height difference between an upper and a lower reservoir, which is difficult to achieve in the middle of the North-Sea. Even though some very innovative solutions with inflatable reservoirs creating a few meters of height difference have been proposed, we consider pump-hydro as out of the question as a storage solution for off-shore wind power.

Battery storage is another rather mature technology, but this however has the disadvantage of being heavy and space-demanding. One solution will be to install batteries in the base of the towers of the wind turbines. This will solve the space and weight problem, but it will create challenges with regards to maintenance and durability as the design lifetime of the wind turbines by far exceed that of the batteries and replacement of large batteries encapsulated in the towers will be impossible. All in all we consider batteries to be an unrealistic storage solution for this purpose.

Hydrogen storage would also be an option, and it might have the advantage that the hydrogen could be used in the turbines already on the platform, thus reducing the need for new electricity generating equipment. Operational experience have however shown that hydrogen storage solutions have had a low efficiency and high maintenance requirements,  which is a serious disadvantage for this purpose.

Compressed air storage is a relatively new technology for large scale energy storage, but it has a high potential where one has some natural reservoir for the compressed air. At an oil&gas platform we will have two options for air storage, either the legs of the platform or empty oil&gas reservoirs. To supplement the compressed air storage we propose to include flywheels in the base of the wind turbine towers. These will help smooth out high frequency fluctuations making the system more reliable and help save maintenance. Compared to batteries, flywheels has the advantage that they are less subject of load-unload cycle tear and wear.

Conclusion

There are large plans for the installation of offshore wind power plants in the North Sea, with long cables to transfer the produced electricity to shore. At the same time long cables are installed to supply power from shore to oil&gas platforms. It would be highly beneficial to integrate offshore wind power plants and oil&gas platforms together to form offshore mini-grids instead of sending power over long distances to shore and back. This would lead to less long distance electric power transmission (fewer expensive cables, less electric  losses) and to CO2 emission reductions of the offshore oil&gas platforms.

Without storage, up to 44% of the power supply could be implemented with offshore wind power plants. To increase the renewable share beyond that will call for more complex system design including electric energy storage.