The growing trend of subsea tiebacks in the oil and gas industry has spawned a requirement for more power on the seabed
The oil and gas sector is continuing to stretch the boundaries of economics to extract every last drop of the precious resource from the seabed. The conundrum is that, even with higher oil prices, the cost of extracting the oil is increasing as the remaining resources move into more remote and inhospitable areas.
The main cost of exploration and completion has traditionally been the large and unwieldy topside structure. This has led to oil companies developing a strategy of tiebacks – technology which, at its most basic, allows for remote fields to be piggy-backed onto an existing platform, allowing isolated and otherwise uneconomical fields to be exploited.
The technology for subsea tiebacks has been available for several decades, but advances in flow assurance and multiphase transport – together with new tools for boosting pressure, removing sand and water from the wellstream and the birth of subsea processing – now allow them to be utilised over far greater distances.
One persistent challenge has been how to supply power to locations further and further away from the power hub on the surface. One solution being developed by Siemens is to provide a power grid at the seabed.
'Full subsea processing is unthinkable without an industrial, heavy duty, high-reliability power grid directly on the sea bed.' Tom Blades, CEO of the oil and gas division of Siemens Energy, says. 'The depletion of onshore reserves is forcing oil and gas companies to exploit fields in the remotest places at ever greater depths.'
Addressing power distribution to remote facilities to exploit these reserves is becoming a tremendous technical challenge. The solution encompasses power cables, transformers, switchgear and variable speed drives, to power and control electrical driven pumps or turbo-compressors, separators and other processing equipment. The requisite power is provided by an industrial onshore or topside combined cycle power plant remote to the subsea facility.
'The complete power grid solution provided by Siemens is mounted on a single base frame directly on the seabed,' Blades continues. 'A pressure-compensated design will dramatically lower the weight and increase the robustness of our tailor-made subsea solutions, enabling us to advance subsea processing even further to meet the operators' need for reliable and environmentally friendly production on remote subsea wells.'
'The goal now is to take the responsibility for the grid, including onshore and topside installations with power stations that care for compensations or whatever is necessary there,' Ove Bo, head of R&D at Siemens oil and gas solutions centre, explains.
'We would have a cable going down to the subsea installation and we are currently in the process of developing the four major components for the subsea grid.'
The first of these components is the transformer to step the voltage down from the transmission voltage, normally around 70kV, down to the distribution voltage level which is expected to be 22kV. Next is the switch carrying units that are connected to the transformer.
'The basic design there is four outgoing lines to the consumers and in the middle two circular canisters that contact vacuum breakers, a well-proven and reliable technology,' Bo adds. 'The vacuum breakers are put into the canisters and filled with FS6 for insulation purposes. They have to be designed to withstand immense external pressure at the various depths they operate.'
The foundation of the subsea grid is where the auxiliary equipment is along with the connectors for the incoming jumpers from the transformer and the outgoing connectors to the consumers. This compartment is oil-filled and pressurised and all the components inside are exposed to the same pressure as those in the seawater outside.
'Then we have the variable-speed drives that are of a very well-recognised and well-respected topology for offshore applications,' Bo continues. 'This topology has the advantage of making use of the build-up of individual power cells, which are stacked accordingly to reach the necessary voltage level.
'We also have a built-in redundancy in case of an internal failure in one of those cells so that if needed they can be shorted and completely bypassed, allowing the whole grid to keep on operating with nominal disturbances. It also allows us to add one of more additional power cells to compensate for any redundancy within the unit.'
One unique element of the design is that all the power electronics are kept in the foundation compartment, which is filled with oil and pressurised. The system is designed to work all the way down to 3,000m at which depth the power electronics is exposed to a pressure of 300 bar. 'This is a system that was developed at Siemens and has been in used in other applications since 2004; we do have some operational experience with this technology,' Bo remarks.
The main challenge faced now is to adapt it to the drive technology. One of the advantages to this technology is that as long as the pressure is the same on this inside of the unit as it is on the outside the thickness of the walls can be reduced to that required for handling and transportation.
'All of the heat losses within the power circuit are transferred from the semiconductors to the oil and it gives a natural convection inside the housing,' Bo explains. 'Also, there are very thin walls that allow good thermal conductivity and good connection to the sea water outside of the unit. Any cooling issues that a subsea system may have is a lot better with this system than with pure atmospheric systems.
'Yet another advantage would be that we can make this unit more compact with a smaller footprint; an example unit we have of around 5MVA measures at approximately 30m2, with a weight of approximately 50-60 tonnes.'
With the single cable dropping from the surface structure to the subsea foundation a myriad of smaller cables can spider across the seabed to remote assets. 'The goal is to be able to make a total grid for a large field where we can distribute the power to a large number of templates spread around on the seabed,' Bo adds. 'This would enable us to create a complete grid for a substantial subsea grid with a high number of consumers.
'The advantage of this distribution system, with just a single umbilical rising to the surface, is that when you have a high number of consumers with the technology you consistently see used today you would need one single cable for each consumer going all the way up to the platform.
'There are a lot of challenges regarding cost with a high number of umbilicals and you also have the challenge of slip-ring system on the floaters, which have a limited number of connections possible. All of those barriers are overcome with this grid. In the case that you have a floater you can use one single cable that runs down and then distributes everything on the seabed.'
The biggest challenge is to enhance the reliability and to ensure that it is consistently at a sufficient level. Although ensuring that all the components are suitable for a harsh marine environment has meant a redesign of a lot of separate parts and complete modules.
'Finding the right components that will be able to withstand the pressure is in itself a challenging task,' Bo adds. 'Everything that is going to be used within the system will need to be re-qualified for a marine environment and test very extensively.'