David Knott
Senior Editor
Hugin survey submersible is controlled with a computer on board the mother vessel, through combined data from the vessel's satellite positioning system and the submersible's acoustic positioning system. The submersible will continuously relay attitude, velocity, heading, and depth data back to the surface and can upload survey data in real time. Photo courtesy of Nutec.
In the future, commercial oil and gas production will require equipment that can be operated at great depths and far from existing infrastructure.
Greater water depths, longer distances to infrastructure, and more stringent legislation call for innovative thinking, according to Stig Gustafson, Subsis Project Manager, ABB Offshore Technology AS, Billingstad, Norway.
As the petroleum industry moves to developing less accessible reservoirs, said Gustafson, ABB is developing new technology that will make it possible to situate operating equipment at depths of 1,000-1,500 m.
The key to deep, remote subsea operations is dealing with produced water. ABB's Subsis (subsea separation/injection system) project is an attempt to meet these challenges by developing a seabed processing system to separate oil and water and reinject water into a reservoir.
Rune Stroemquist, senior R&D manager for oil and gas at ABB Corporate Research, said, "Water in the oil and gas to be produced represents a significant problem in terms of cost.
"By separating the produced water on the seabed and either reinjecting it into the reservoir or dumping it into the sea, it is possible to reduce transportation costs and reduce topsides equipment facility costs and problems associated with hydrates and corrosion. At the same time, it is possible to enhance the utilization of the reservoir."
Gustafson said subsea processing offers a number of advantages. Transportation costs would be cut drastically, as water over time often accounts for nearly 90% of the subsea well stream.
Also, seabed dewatering can release equipment capacity for additional oil production, enhance production from existing fields, and result in less water pollution.
Operational problems associated with the transport of unprocessed well streams generally increase in direct proportion to the horizontal and/or vertical distance between the subsea production stations and the topside facility.
Modules
Gustafson told OGJ that the Subsis system will consist of several main modules mounted on a base with manifolding and an overhead protective structure.
The modules are: a separator; a water-injection module incorporating a centrifugal pump and electric motor; a water-injection christmas tree; a closed-loop control system; and a power distribution package consisting of a cable, transformer, and frequency converter to power the injection module and control motor speed.
The installation and maintenance of seabed facilities puts clear constraints on system layout and design, said Gustafson. For installations in deep water, the high costs of intervention makes reliability a prerequisite.
Intervention must generally be done with a remotely operated vehicle (ROV) controlled from the surface. Consequently, the Subsis design must be simple and accessible with tools adapted to ROVs.
Troll project
Norsk Hydro AS, operator of Troll oil project off Norway, let contract to ABB to build a pilot Subsis system for installation in 340 m of water in Troll field. The unit will be tied back to Troll C platform.
Troll C will be installed to produce a thin oil layer within the giant field's Troll West gas province. First production is anticipated in mid-1999 following a $2.5 billion development (OGJ, Aug. 26, 1996, p. 28).
ABB's contract will include development, engineering, construction, testing, and delivery of a complete Subsis unit. Completion is scheduled for Spring 1999.
The Subsis pilot unit, being built for Norsk Hydro to use in Troll C field, will have capacity to handle about 10,000 b/d of well stream fluids.
A Hydro official said the Subsis unit would be installed on a four-well subsea template about 4 km away from the Troll C platform. It will require as much as 6.6 kw of electric power supplied through a 100 mm cable.
While much of the mechanical equipment to be used in building Subsis is well established, ABB will have to develop a new electrical power system to deliver more power than a subsea unit has ever required before.
The Hydro official said that for distances of as much as 10 km from the host platform, a Subsis unit would require installation of a subsea transformer.
Gustafson said such a unit would require as much as 2 mw of electric power.
Power supply
As part of the overall Subsis project, technical teams in numerous ABB divisions are developing components that will be integrated into Subsis but can also stand on their own as autonomous commercial products, said Gustafson.
For example, at present there are no commercial electrical subsea mateable connectors that can handle voltages of 24-36 kv. ABB is trying to fill this gap by developing a high-voltage subsea mateable connector. The company is also developing an instrument to measure oil particles in produced water.
Subsis will use a distributed monitoring and control system designed according to international field bus standards. Optical fiber transmission between the topsides and subsea equipment will ensure the necessary bandwidth and noise immunity for the control signals.
Monitoring and control of the oil/water level in the separator represents a special challenge, said Gustafson, requiring new types of instrumentation and the implementation of a closed-loop control system. Subsis will be able to utilize an integrated service umbilical or a separate control and power umbilical.
Pioneering work
Subsis involved pioneering work in several fields, not least in the development of systems and components that can tolerate extreme water pressures at great depths.
ABB is combining innovative new concepts with tried-and-true solutions for subsea facilities.
Before long, implementation of subsea processing facilities will require a supply of electrical energy in the megawatt range. Electricity will be required to drive the pumps needed for the transportation of oil and gas, boost the well stream, reinject separated water, and compress gas.
Cost-efficient solutions for long tie-in distances and deepwater installations call for new subsea components such as marinized transformers, high voltage connectors, and frequency converters.
ABB's starting point has been the group's own land-based technology, which has been adapted to the special operating environment found subsea.
In the Subsis project, ABB will develop technologies for processes, power, injection systems, subsea integration, monitoring and structural/mechanical systems.
Framo Engineering AS, Bergen, Norway, has been ABB's main partner in Subsis, responsible for development of pumps and boosters for the injection systems.
The project has also been supported by Norwegian offshore operators Norsk Hydro AS, Norske Shell AS, Den norske stats oljeselskap AS (Statoil), and Saga Petroleum AS, in addition to the Research Council of Norway.
Boosting
ABB says another challenge for subsea production is of a geological nature: Reservoir pressure may drop over the productive life of an oil field, and such pressure reduction can lead to flow reduction.
As the water content increases, the need for pressure as a drive mechanism also increases. Removal or reduction of the water content will improve the situation, but it may also be a good idea to boost the pressure to further improve the flow.
Markus Bayegan, managing director of ABB Corporate Research, said, "Boosting is sometimes more easily achieved in combination with phase separation than without, although both approaches have benefits and are being pursued by Subsis.
"Subsea separation and injection will also help solve yet another challenge facing modern subsea oil and gas production: environmental protection.
"The transport of unprocessed well streams often requires the injection of corrosion inhibitors into the flow lines. Some inhibitors can also have adverse effects on the environment, so oil companies try hard to minimize their use.
"Reducing the water content could lead to a proportional reduction in the use of corrosion inhibitors. A system for separating water at the production facility requires a system for water disposal.
"There are two options available. One is to clean the water and dump it into the surrounding waters. The other is to inject it into a reservoir, either the production reservoir or another one.
"The former approach requires process and instrument capabilities that are not yet available. The latter requires an ample supply of electric power to the subsea facility. ABB is currently developing capabilities in both fields."
All-Electric Control System (ACE) is an ABB project aimed at developing technologies to reduce costs and increase the capabilities of subsea oil and gas production in terms of longer operational distances and greater water depths.
The goal is to reduce the number and size of subsea control pods and lower maintenance and installation costs. The new control system will be based exclusively on a distributed control architecture, repair by ROVs, and a communications system based on emerging industrial field bus standards.
The system will also be able to control future subsea devices and systems, including subsea pumps, boosters, and electrically operated processing equipment
Norway develops free-swimming survey submersible
Norwegian Underwater Technology Centre AS (Nutec), Bergen, is leader of a group that has developed a free-swimming submersible for seabed mapping and surveying of subsea installations.
Called Hugin, the submersible can work in water depths to 600 m and is being developed to operate in as much as 2,000 m of water. Norway's Den norske stats oljeselskap AS (Statoil) is a project sponsor.
Hugin is driven by a newly developed battery that enables it to travel about 150 nautical miles on one charge, said Nutec, or to work for 36 hr at sea.
The vehicle is controlled by acoustic signals transmitted from the mother vessel and has an operating range of 2 km. Hugin is 4.8 m long, weighs 700 kg, and travels as fast as 4 knots.
The vehicle is fitted with a multibeam echo sounder for seabed surveys and can carry a range of other instruments, said Nutec. It can store data or transmit it in real time to the surface.
Anticipated tasks for Hugin include surveys of potential pipeline routes, topographic surveys, sonar inspections of pipelines and subsea installations, and marine environmental and oceanographic measurements.
Nutec said, "In a remotely operated vehicle, the advantage of virtually unlimited power via the umbilical is offset by the cable's own drag as operating depth increases.
"An autonomous underwater vehicle like Hugin, on the other hand, can maintain the same cruising speed at any depth, cutting the costs of surveys by a factor of three or four.
"As offshore exploration and development projects move to depths of around 1,000 m, this advantage becomes critical. Redundancy is built into critical systems, while individual components are easily replaced at sea, cutting downtime due to system failure to a minimum."
Statoil was the main supporter of the 35 million kroner ($5 million) Hugin development program. Nutec intends to make the first Hugin vehicle available to offshore survey contractors by late 1997.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.
