Power requirements are not always huge, nor are companies – and sometimes smaller can be better. Elaine Maslin reports on work to qualify lower power subsea variable speed drives.
The 45kW subsea variable speed drive. Photo from Nebb Engineering.
Norway’s Nebb has its eye on a market the larger players are overlooking – subsea variable speed drives (VSD) for power users requiring smaller amounts of power than the likes of compressors.
To date, subsea VSDs, used to regulate the speed and rotational force of an electric motor, have only been deployed once, in a pilot at Shell’s Nyhamna plant in Norway. These were designed for the large Ormen Lange subsea compression system, which Shell put on hold in 2014.
Yet now, Nebb Engineering, based in Asker, near Oslo, has qualified a 45kW subsea VSD, for operations down to 3000m, and is now working on VSDs for the 4kW and 350-1500kW ranges.
While 45kW VSDs wouldn’t be quite big enough for the likes of a subsea compression project, they could be used to power actuators or small pumps. Using electric power transmission, instead of hydraulic, for subsea power users, removes the need for hydraulic power units and topside VSDs, in cases where electric power is supplied direct from the topside to seafloor users.
Nebb’s 45kW, 400V VSD was in part developed for a project with Kongsberg Oil & Gas and Fuglesangs, to develop an active subsea cooler, to cool production fluid for export or compression, and which could be controlled to prevent hydrate formation. This project was due to complete full qualification this summer. The system comprises a heat exchanger, a pump, from Fuglesangs, which incorporates an oil-filled motor isolated from the pump by a magnetic couple, and a control system, all for use up to 3000m water depth.
Nebb’s part of the project was developing the control system and communications including the 45kW subsea VSD, which is contained in a 1053mm high, 356mm-diameter canister. This system could have been used on Shell’s Ormen Lange project, until it was put on hold in 2014.
A 20-45kW VSD could also be used to power medium sized pumps. Meanwhile 300-1500kW VSDs, could be used for electrical submersible pumps (ESPs), water injection or condensate pumps. This size VSD Nebb hopes to have ready in 2017. The 4kW VSD, due to be qualified this year, contained in a 450mm-high, 210mm diameter canister, could be for use on small pumps (for grease, hydraulics etc.) and valve operation (from remotely operated vehicles). They could also be used in brownfield applications by incorporating a battery pack, that Nebb also has in development.
“This is an enabler for all electric subsea control,” says Alexander Risøy, managing director at Nebb Engineering.
One of the main challenges is to get rid of the heat, he says. “On a 45kW VSD you need to get rid of 1.5kW heat and keep it below 35°C to extend the life time.” This is Nebb’s secret and they’re not willing to say how they do it. “It is difficult, but possible to do,” says Wolfgang Trötscher, chief engineer. “It is even more difficult with 500kW unit, which produces 5kW heat.”
Nebb’s goal is a 1MW VSD, for which there would be a larger market than say a 500kW VSD. For this, the firm will be looking to work with oil majors.
The company was founded in 1996, and has about 40 staff, with an office in Macedonia. It also provides automation control and safety systems, SCADA and information management, as well as subsea control modules.