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This paper will discuss the feasibility of transmitting grid-supplied power to offshore platforms at long step-outs using high-voltage AC (HVAC) transmission systems. In recent years, in an effort to reduce their environmental footprint, an increasing number of operators have considered supplying offshore installations with power from mainland electrical grids using high-voltage subsea cables. This is particularly the case in places like The North Sea, where there is a widespread push for O&G producers to ditch on-platform generators in favor of cleaner sources of electricity (hydro, wind, etc). On account of their low electrical losses and high transmission capacity over long distances, subsea DC transmission lines have traditionally been utilized to supply power to offshore platforms — especially for structures at long step-outs from shore (>50 km). However, because these systems require the installation of a DC to AC power converter on the platform (along with other ancillary equipment), they pose significant challenges with regards to space, weight, and cost. This paper will examine how flexible AC transmission systems can be used to eliminate this problem by supplying offshore installations long distances from shore with reliable and eco-friendly grid-supplied power. It will outline the advantages of deploying an AC transmission system over conventional on-platform gas turbine generators – some of which include reduced CO2 and NOx emissions, increased availability, and less maintenance. A case study will be presented on Total's Martin Linge platform in the North Sea, which currently employs the longest subsea AC power link in the world at 163 km (Power Martin Linge, 2015). The AC transmission system helps reduce Martin Linge's CO2 emissions by two million metric tons through elimination of on-platform generators. The paper will focus on the technologies and methodologies that were used on Martin Linge and will discuss the role that power grid simulation and modeling played in lowering the cost for critical power infrastructure, ensuring onshore grid stability, and minimizing overall project risk. The paper will conclude by discussing how producers can use flexible AC transmission systems in other regions of the world to reduce the environmental impact of their offshore installations by capitalizing on clean, reliable grid-supplied power.

It becomes evident today's Oil&Gas projects in average have higher electrical power demand than years back. In most cases technical decisions are to simply increase current to compensate power needs. Design ratings for operating and short-circuit currents of medium-voltage switchgear on generator voltage level are limiting grid design. This is the case especially for power islands. Stepping up generator voltage can be a perfect solution in particular for power grids feeding extended oil fields. Installing step-up transformers for each generator unit and working with a network voltage up to 33 kV or higher sometimes creates disposition to believe that this is a more expensive solution. A load-flow and short-circuit calculation for the main substation is required to properly size the switchgear and the other distribution equipment derived from planned grid arrangement and oil field process specific operation modes. It has also to be considered expected power supply quality, reliability and availability. A cost comparison will be based on total cost of ownership between the solution with main substation on generator voltage level of 11 kV and the solutions with step-up transformers up to 22 or up to 33 kV. This comparison will also include the additional heat losses of overhead lines or cables to and between the wellpads for a year of operation. When using higher voltages, there should be no limitation with respect to grid design and grid operation. Generally, the voltage level has to be adequate for the supply purpose. A network should be designed to avoid use of current limiters. With proper voltage level selection the bus sectionalizers can remain in NC position. It is possible that generator units are operated that loss of one set can be compensated to avoid any interruption of power supply. Power generation can be increased when feeding via transformers to higher voltage levels of switchgear. The Power Plant Switchgear will require only a reduced short-circuit level and lower design currents for busbars and feeders to achieve optimized grid design. Unit transformers between generators and switchgear will prevent any negative influence of ground faults from the grid to the generators. Also with respect to heat losses, maintenance, grid availability and reliability as well as aging the advantages are clearly on the higher voltage level. The required power grid will be assessed based on different voltage levels. The optimized solution for the oil field will be discussed in detail. Solution approach with higher voltage levels and optimized grid design will have reserves to deliver additional electrical power for extensions and also for operation in depletion mode. There are now oil fields which do not allow bridging distances between wellpads by means of overhead lines but by underground cabling because of environmental conditions. Considering this aspect in cost comparison between different grid designs and voltage levels the advantage for higher voltage levels with optimized grid design will be even clearer.

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