Integrated subsea pumping/ power distribution in prospect

The subsea frequency converter developed by Framo for Mobil's MEPS project.

The development of a subsea power distribution system is a subject close to the heart of Framo Engineering. Originally established to develop subsea pumping technology for the offshore industry, the Bergen-based company has now been drawn into directly tackling the problems of distributing electrical power on the seabed for running multiple items of subsea or downhole equipment. By about the turn of the century, it aims to have a prototype system ready to test.

The company has participated in a number of projects which constitute technological milestones along the path to subsea power distribution. An early step was the development of a prototype subsea frequency converter with a 500 kva rating for Mobil's multiphase electrical pump system, MEPS. This was followed by the ELSMUBS - electrical subsea multiphase booster station - system, which was itself based on the hydraulically driven SMUBS system developed by Framo for Norske Shell's Draugen Field in Norway.

In 1997, Framo supplied five mudline booster pumps to Statoil's Lufeng 22-1 project in China. The 400 kW units, which are required to provide artificial lift, are capable of pumping 35,000 b/d of liquids each. For this project, the company had responsibility for the total drive system: frequency converters, transformers, umbilicals, connectors and motor drives. Most of the power equipment is located topsides on the FPSO.

This year the company is delivering two subsea multiphase booster pumps to Mobil's Topacio project in Equatorial Guinea. Though a small satellite development, Topacio is an important project for Framo, representing, in power distribution terms, a further step towards a subsea system. A step-up transformer on the host FPSO converts the power to 12 kV - a high voltage is required to reduce transmission losses over the 9 km tie-back distance. At the site of the Topacio facilities, a subsea transformer reduces the voltage to the motor operating voltage.

Framo has also worked closely with ABB on electrical power systems, a collaboration which, according to Steine, has subsea electrical power distribution as its primary aim. "Since we are not primarily an electrical manufacturer, we involved ABB," says Jens Gams Steine, section manager, electrical and instrument. "The cooperation has since evolved further into the Subsea Electrical Power Distribution (Sepdis) project."

Sepdis is a joint industry project between ABB Offshore Technology and Framo which is backed by Statoil, Norske Shell, Saga, Norsk Hydro and Mobil. It is closely linked with Subsis, the subsea separation and reinjection system to be supplied to Norsk Hydro's Troll Oil II development, for which Framo is supplying the 2 MW subsea reinjection pump module.

For tie-back distances of 6-7 km, both topside and subsea distribution systems are cost-effective, Steine says. But for longer distances, the subsea system provides a lower-cost solution. This reflects the fact that with distribution taking place topside, the power has to be delivered from the surface to the subsea facilities through individual power lines.

For a full Sepdis system, however, only the step-up transformer is located topside. The power is transported to the subsea facilities through a single common cable. On arrival, it passes through a step-down transformer and is then split into individual lines. These run a relatively short distance to the power consumer, on the way passing through a subsea frequency converter. Hence the cable requirements for the subsea system are significantly reduced compared with topside distribution.

Framo's project takes account of three main types of subsea pump: single phase pumps, multiphase pumps and water injection or reinjection pumps. Motor power will be up to 2.4 MW.

The Sepdis base case is for five users of up to 2.2 MW. These may be placed on a template, manifold or separately located structure with a footprint of 8 meters by 8 meters. However, the project will make available a set of building blocks, thus accommodating a wide range of configurations of a subsea distribution system.

Two ratings, 1,500 kva and 3,000 kva, are under consideration for the frequency converters. Installed power on the template would typically be 10 MVA. The Sepdis base case system is intended to be applicable to depths down to 2,000 meters, but the system will be developed for application in greater depths.

The first phase, involving concept evaluation, was carried out in 1997. Here the system was put together from a broad perspective, with feasibility being assessed from both a technical and an economic point of view. "We put a lot of effort into looking at the operational aspects, such as maintenance and installation," says Steine. Practical issues such as market considerations and potential applications also ranked high.

The second phase is divided into two parts. The first, which was completed this mid-year, focused on systems design, bringing the various systems together at a reasonable degree of detail. Currently detailed design of these systems is being performed, and this is due to be completed before year-end.

Finally, beginning in late 1998, comes phase three - the manufacture of parts. This will take about a year, after which the parts will be tested individually, then brought together and tested as a system at Framo's test facility outside Bergen.

Marinization of the equipment is one of the key issues. Although all the electrical parts are available for topside usage, a lot of engineering and re-engineering is required in order to deploy them subsea, Steine says. The equipment for each user will be placed in a cylindrical atmospheric pressure vessel, the thickness of which will vary according to the depth.

Power will be brought through a high-voltage cable. This will link to the equipment on the seabed by means of a wet mateable connector. This is another key element which is being developed - for the Sepdis project, it will be based on the ABB ME-connector. Control will be by fibre-optics.

Another key issue is installation. To a certain depth - 1,200 meters in calm tropical waters, but only 500 meters in the more turbulent waters of the North Sea - guide wires can be used. Beyond this depth, guidelineless techniques are required. The layout of the equipment on the template is also strongly influenced by water depth, as is the ROV tooling and access.

As the end of the engineering phase approaches, Steine is confident that early in the new century Framo will be in a position to supply a total subsea pumping system, including subsea distributed power to meet the increasing demand for large subsea electric pump motor drives.