North Sea pipelay sets reeled lay record

May 28, 2001
Installation of what the contractor claims to be the largest and heaviest reeled pipe was completed in recent weeks for the Blake field, located in 95-m water about 9 km north of the Ross field in the Moray Firth.
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Installation of what the contractor claims to be the largest and heaviest reeled pipe was completed in recent weeks for the Blake field, located in 95-m water about 9 km north of the Ross field in the Moray Firth (northern North Sea; Fig. 1).

Production from Blake will begin next month into the existing Ross floating production, storage, and offloading (FPSO) vessel Bleo Holm.

Coflexip Stena Offshore Ltd. (CSOL) is performing the work under an engineering, procurement, installation, and construction contract from BG International, which operates the field for partners Talisman UK and Paladin Resources.

Ross came on stream in April 1999 and the subsea infrastructure was designed, manufactured, and installed by CSOL. Ross field production will continue at 25,000 b/d; Blake plateau production will add 44,000 b/d.

Blake development

The base case development plan for Blake is six production wells clustered around a manifold and two satellite water-injection wells (Fig. 2). Each well is some 3 km on either side of the main well cluster and is to maintain aquifer pressure and hence production rates.

The manifold distributes lift gas, injection water, control functions, and chemicals to the wells and routes individual well production to either or both the 10 in. and 12-in. production-test pipelines to Ross.

The manifold has been connected to the Ross FPSO by a 10-in. production line, a 12-in. production line, a 6-in. gas-lift, a 12-in. water-injection line, and a dynamic-static umbilical carrying power, communications, and chemicals.

In turn, each satellite well is served by a 6-in. water-injection flow line and umbilical. A new, dedicated control system is being installed on the FPSO.

Critical to the Blake Development, says CSOL, is flow assurance; the production pipelines must be capable of carrying the hydrocarbons during the critical early production period and turndown conditions.

With the flowing wellhead temperature at close to the wax-appearance temperature, high levels of insulation are required on the production pipelines to prevent wax formation that would restrict flow.

CSOL's technical solution involved use of a 10-in. in 16-in. pipe-in-pipe to maintain the FPSO criterion for arrival temperature.

Use of this type of system resulted from of a development program undertaken by the group's offshore engineering division in Aberdeen, says CSOL. It is this pipe-in-pipe system that the company believes to be the largest and heaviest reeled system installed and operating in the world.

It is intended that the Blake gross production, at a maximum of 100,000 b/d, will be routed to the production separator and the lower Ross production to the test separator. This scheme prevents well testing in the normal manner, and downhole gauges and multiphase flow meters in the Blake manifold are required to gather well and reservoir information to optimize field performance.

Because it is imperative to maintain reservoir pressure, injection water must be provided at a maximum flow rate of 120,000 b/d with a maximum of 70,000 b/d to an individual well.

At the Ross FPSO, existing risers are being reconfigured to accept separately Ross and Blake production fluids with water injection and lift gas being tied into an existing distribution manifold within the FPSO swing circle.

The new dedicated control system requires installation of the new topsides control and power units to provide power, electric, and hydraulic and to communicate with the subsea system, issuing commands and receiving data.

In turn, says CSOL, each subsea tree will have a dedicated control module to operate the valves and to receive and transmit data back to the FPSO. New electro hydraulic umbilicals will be provided to support the control system.

The manifold (about 200 tonnes) is a gravity-based design, housing all the pipeline headers and diversion valves and protecting them from fishing activity.

FPSO refurbishment

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The existing Ross FPSO could not provide the entire Blake requirements and required extensive modifications, more than could be carried out with the FPSO on location.

Therefore, in early April, the FPSO was disconnected (risers and mooring chains) and towed to the McNulty Fabricators conversion yard (a wholly owned CSO subsidiary as of January 2001) where upgrading nears completion before the vessel is towed back to the field and reconnected.

Management of this phase of the works led CSOL to have up to six vessels in the field at any one time to support the disconnection and reconnection of the FPSO.

During the period that the FPSO has been off location, the Blake field infrastructure was installed in a carefully sequenced operation, utilizing the CSO Apache and a number of other CSO support vessels.

The infrastructure installed consists of the 200-tonne production-water injection-gas lift-multiphase flow metering manifold installed in the center of the six-well cluster, one 10-in./16-in. pipe-in-pipe production pipeline (9 km), one 12-in. insulated production pipeline (9 km), 15 km of water injection and gas lift flexible pipelines, 15 km of control umbilicals, all trenched and backfilled, and one new umbilical riser at the FPSO for Blake.

After successful installation and commissioning, the Ross and Blake fields will be brought on stream.