There is one area of subsea technology that perhaps holds the greatest potential for establishing a step change in its implementation globally. That area is subsea processing, and things are moving on fast.

Put simply, by removing most of the water from the wellstream on the seabed, much of the hydrate problem is taken care of.
The significant reduction in fluids, with a minimum of water, results in smaller diameter flowlines and risers with fewer requirements for thermal properties.
It's not very economical generally to carry a lot of water through a flowline from the wellhead perhaps 2,000m or more up through a riser to a surface installation - just to pump it back down to the same location for reinjection.
Jan Ingar Knudsen, chief engineer for Norsk Hydro and formerly Saga Petroleum, has more than 25 years of experience in the offshore business, mainly within pipelines, subsea installations and marine operations. He in particular sees the way things are going related to deepwater developments. He commented recently at IIR's DeepTech conference in Aberdeen: "Deepwater separation will require systems different from the traditional gravity vessels we see on topsides and which Norsk Hydro now has installed subsea on the Troll field, the so-called Troll Pilot. So far it is concluded that the separator vessel will have to be designed for the hydrostatic overpressure. The requirement to the wall thickness makes the gravity separator impractical in deepwater, hence cyclone or centrifugal separators need to be developed."
The separator is only one part of a process system, of course. There will also be the need for multiphase booster pumps, water-injection pumps, multiphase meters, oil-in-water meters, control systems, power distribution systems and sand removal and disposal systems. In cases where it may be desirable to go for three phase separation, there may be a need for gas dehydration systems and wet gas compressors (the latter for reinjection or export purposes).
Knudsen added: "All these elements are also needed for longdistance wellstream transfer in shallow water to make marginal fields economical by tying them back to existing infrastructure. Development work is therefore ongoing and is in some areas already well advanced. The future deepwater developments will inevitably include subsea processing equipment.
"The subsea process technology will also have a positive impact on the topside facilities, less equipment and weight, which again reduces the requirements to the substructure."
Of course, there is still one step further - downhole processing. Knudsen commented: "The ultimate processing system for deepwater fields is without doubt located down in the hole. Downhole separation systems are under development and will in my opinion be the next step of the evolution."

Seabed separation
Providing the current step in subsea evolution appears to be ABB. Industry eyes will be focused on the contractor's Subsis (Subsea Separation and Water Injection System) concept when it finally gets underway on Norsk Hydro's Troll C field.
This is the first commercial unit to be installed by ABB, which knows the importance of making it work. Many believe it will spark a new era in the subsea exploitation of oil and gas reserves. But operators are wise to all the hype that accompanies new innovations, and are waiting to see what its operational performance will be like.
The system will be the first to attempt to separate water from the oil and reinject it at seabed level. It was installed in October last year when Troll C came onstream but is still waiting to start operations as the specific well it will be used on is not yet ready. (It is expected to come onstream around May).
Water was expected to appear right from the start of the field's production, although it is probably likely to be 3 or 4 years before the amount of water in the wellstream makes the system pay off. But between now and then there will be valuable testing time.
The Troll SUBSIS unit itself is specified to be able to work at a water depth of up to 800m (2,609ft). But it should be able to go deeper with changed dimensions.
Development of the system started in 1996 as a joint venture between ABB and Framo Engineering. The passive gravity separation process used by the unit also serves the additional purpose of separating gas and sand, resulting in a four-phase separation process.
Gravity separation takes place in a large pressure tank about 10m long and 3m in diameter, containing a system of specially designed internals. The tank consists of a main cylindrical part with two hemispheric end covers and penetrations for the instrumentation and piping. In principle, the separator is similar to the first stage of a traditional topsides process train, but moved down to the seabed.
ABB is eventually planning to develop a complete subsea processing plant. Its Siors (Subsea Increased Oil Recovery Station) project is the next-generation subsea processing plant. This would be a modularized, self-contained system that, besides actively separating produced water, would include subsystems for water cooling and injection, active gas liquid and sand separation, sand disposal, liquid fraction metering, wet gas compression, gas cooling, gas reinjection or transportation, gas lifting, closed-loop adaptive control and high-efficiency
power supply.

Seabed boosting
Seabed boosting, meanwhile, is a technology significantly further advanced than seabed separation.
Work by pioneers such as Framo Engineering, also involved in the above project, has seen the technology proven effective - the contractor has gained 2 years of operating experience for four electrically driven seabed booster pumps on Statoil's Lufeng field offshore China, in addition to previous operating experience gained on Shell's Draugen field offshore Norway.
The contractor is currently waiting to have two booster pump systems installed for BP Amoco's ETAP field development offshore the UK, and Mobil's Topacio field offshore Angola.
Subsea booster pumps simply add pressure energy to unprocessed well streams, and are now beginning to show that even when operators are in control of the production from a field, most of those fields can increase their production considerably by use of boosting - with the payback period of the investments often less than 6 months.
On Lufeng, five of Framo's subsea booster pumps have been integrated with the trees on the subsea template at a water depth of 330m. The pumps have been in successful operation since January 1998, before which low wellhead pressure of about 100 psig at the seabed had dictated that artificial lift was needed from the initial phase of the field to maintain pressure to the inlet of the topside processing facility.
Each pump is capable of delivering 20,000 b/d of liquid with a pressure increase of up to 35 bar (500 psi), more or less equal to the hydrostatic head.
One well was producing mainly water and was shut down after a few weeks, but the four others have been operating successfully for more than a year, according to Framo, with 100% availability.
Subsea booster pumps themselves are designed to provide a safe, reliable and economic solution for boosting the pressure of the production from each individual well. The retrievable pump and driver insert is installed in a receiver barrel flanged directly to the Xmas tree. The integral pump and oil-filled motor design, combined with a barrier fluid for cooling and lubrication, gives a low weight and compact design.
Each pump has its own dedicated electrical power and barrier fluid supply through a combined six-line power and barrier fluid umbilical. It is driven by a frequency controlled 400kw motor, which receives its power from a power generation system on board the surface FPSO.

Contracts
Confidence in the pumps resulted in Framo clinching contracts with BP Amoco for its ETAP project, and also Mobil's Topacio field. The ETAP contract will see two multiphase subsea booster pumps supplied to boost unprocessed production from the Machar field through a 35.2km pipeline to the Marnock processing facility. The two pumps will increase output from Machar by at least 4,000 b/d.
On Topacio, the pump system will consist of two 1mw booster pumps installed in 500m (1,630ft) of water. This will boost field production back through a 9km pipeline to the Zafiro field's FPSO. The pumps are each capable of delivering 500 cu m/hour at 75% gas volume fraction at a differential pressure of 35 bar. The system is designed as a combined pump and pigging module to be integrated into the subsea manifold system.
The system supplied by Framo Engineering includes two Framo Elsmubs pumps, pigging module, high-power jumpers with wet mateable connectors, subsea transformers, subsea cable and umbilical, topside control module and intervention tools.