MULTIPHASE by-pass for Troll

Seabed separation is one of a number of technologies whose aim is to reduce the cost of production from subsea satellite wells. The basic idea is to avoid multiphase flow problems in the production pipeline by separating the different fractions close to the wellhead. Separation would, for instance, allow conventional oil pumps to be used for flow boosting.

Seabed separation does, however, present an enormous challenge because the complexity is an order of magnitude higher than subsea systems currently in use. The process would need to be remotely controlled and intervention would need to be carried out by divers or by remotely operated vehicles.

The feasibility of seabed separation was demonstrated towards the end of the 1980s when a company called British Offshore Engineering Technology built a prototype for short-term trial in the Argyll Field. The project was partly funded by the European Union and the Offshore Supplies Office in the UK. The unit was designed to handle well fluids with a water cut of 8 per cent and to produce dead crude suitable for tanker transportation without further processing. The prototype subsea unit consisted of three horizontal separator vessels enabling the water and dead crude to be pumped independently to a floating production.

The Argyll project was followed by a number of high profile R&D projects. One of these was a two stage subsea separator system developed by Goodfellows Associates known as the GA-SP system. A GA-SP prototype was tested in a drydock in Middlesborough during 1991. Essex-based Alpha Thames has more recently developed AlphaPRIME, which is described as a `second generation' system. This all-electric, modular system can process oil and gas from subsea fields without diver intervention. The equipment will be manufactured and tested by Kockums in Malmo.

An important aspect of this R&D was to develop multi-ported and valved connectors to enable process equipment to be retrieved to the surface for maintenance and repair. These connectors could find further uses within the process industries. Considerable effort was also devoted to improving the subsea reliability of components, such as electrical connectors and valve actuators, which could also have spin-off benefits.

The original incentive for developing seabed separation systems was to allow the use of conventional crude oil pumps to boost flow from subsea wells. But multiphase pumps that are capable of boosting wellstreams in their natural state became available during the 1990s, and this has dampened enthusiasm for this particular application.

The use of seabed separation in the Troll Field has, however, opened up a new range of opportunities for this technology. The Troll subsea separation and injection system will handle production from up to eight subsea wells. The oil field operator Norsk Hydro says that by separating water on the seabed, the fluid processing capacity of the floating production platform could be reduced.

Reduced processing capacity has resulted in a lower topside weight for the Troll C floating production unit. This 24,000 tonne semi-submersible is due to be completed at the Umoe's Haugesund yard during the summer and is to be installed as part of the latest phase of the Troll development.

Norsk Hydro has also pointed to environmental benefits from this scheme as separated water from the production wells will be pumped directly back into the reservoir.

ABB Offshore Technology was awarded a NOK 170 million order for the system by Norsk Hydro. The overall subsea unit measures 16 metres square and weighs 250 tonnes. The separator vessel is the largest single component, and weighs 80 tonnes. The 9m long vessel is thermally insulated to eliminate hydrate formation.

A water injection module, supplied by Framo Engineering, has been integrated into the subsea unit. This has a centrifugal pump driven by a 2MW variable speed electric motor. Electric power (6.6kV) is transmitted at variable frequency from the floating production unit to the seabed unit by means of a 3.6km umbilical cable. A frequency converter controls the pump to regulate water level in the separator.

The distributed monitoring and control system uses fieldbus solutions for subsea connections and a fibre-optic data transmission system to the host facility. Two different types of transmitters are provided for separator level measurement. One of these is a non-intrusive nucleonic device which can be retrieved to the surface whilst the separator is fully pressurised.

The unit will be installed in 340m of water, which is too deep for commercial divers so all intervention will be carried out by remotely operated vehicles. As the separator vessel is too large for retrieval during routine operations, components are designed for retrieval whilst the separator vessel remains on the seafloor. The high oil density and tight emulsion characteristics of the Troll crude dictated the relatively large size of the separator vessel.

The process design is similar to that used elsewhere in the Troll Field. The performance of the separator was assessed using a transparent scale-model to visualise flow conditions. Tests were also carried out at Norsk Hydro's Porsgrunn multiphase flow loop using Troll crude at realistic operating conditions. Computer simulation models were also used to study the dynamic stability of the system. PE

Jeff Crook is a chartered marine engineer and technical author specialising in offshore technology.