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Dunbar Phase II, United Kingdom




In late 1999, a Poseidon multiphase pump was installed on the Dunbar field, as the final part of Dunbar Phase 2. This will increase recoverability and extend field life.

Two Dunbar wells and one Ellon well were brought into production at the platform start-up. For the remaining wells, drilling was carried out from the platform using the Sedco 706, which was moored alongside it in a tender-assisted mode. Linked by a telescopic gangway to the Dunbar platform, the tender support vessel (TSV) provided high-pressure mud and cement through catenary hoses. It also supplied air for the platform-drilling facilities.

JACKET

The four-legged jacket is 167m high; 20mx26m at the top and 42mx45m at the base. The weight of the 9,500t jacket was reduced using diamond bracing, the extensive use of high-strength steels, special anti-corrosive paint on the jacket and pre-installed pile guides (which eliminated the use of mud mats).

Fabrication was undertaken at Methil, on Scotland's east coast. After transportation to the field, the jacket was lifted onto pre-installed pile guides, so as to provide stability during piling. This was necessary because of the poor soil conditions. The steel piles - four at each corner - measure 8ft in diameter and weigh 450t - totalling a gross pile weight of 8,000t.

TOPSIDES

The topsides process facilities design were simplified, to maximise the available infrastructure of Alwyn North.

The deck was built in stages, one level at a time. Then finally, when the helideck, crane, drill module and derrick were all in place, the structure was loaded out onto a barge moored in the Clyde. This was achieved by skidding it along specially-constructed teflon-coated rails, using hydraulic rams. The completed structure, including lifting appliances and weighing 9,100t, was transported to the Dunbar site for a single offshore lift.

The topsides have a dry weight of 8,277t and an operating weight of 1,2246t. The deck is 50m long, 30m wide and 22m high. It can accommodate 26 beds. The production facilities include 16 production wells and six water injectors. There are six spare slots and the platform has a simultaneous workover and drilling capability.

Dunbar contains high- and low-pressure manifolds, a water-injection manifold, a test separator, chemical-injection facilities and a flare. Alwyn North provides all the necessary power via two 20kV cables, together with pressurised water for the injection wells. Power is fed to Dunbar through submarine cables, which incorporate fibre-optic cores carrying voice, data and control signals. The Ellon subsea installation is linked via two 6in flowlines and control umbilicals.

The platform is equipped with two freefall lifeboats, each able to hold 45 people, four fire pumps (two diesel, two electric) each pumping at 1,200m³/hr. The platform is equipped with two freefall lifeboats, each holding up to 45 people, access to which is enclosed.

In November 1999, TotalFina started using the newly-installed Poseidon rotodynamic multiphase pumps on the Dunbar. These were designed to increase production by enabling the simultaneous liquids pumping and gas compression of the commingled wellhead effluents. This type of pump was installed so as to simplify production facilities, by reducing the amount of equipment required in a conventional arrangement. The installation consisted of two electrically driven multiphase pumps, each 4.5MW, to deliver a flow of 40,000 barrels of liquids and 3 million m³ of gas per day.

EXPORT

Hydrocarbons flow in diphasic mode from Dunbar to Alwyn North through an innovative double-wall insulated pipeline, known as Interpipe. The 16in inner pipe is clad with high-performance insulation and surrounded by a 20in external pipe. This effectively maintains the temperature of the oil and gas, to prevent hydrate formation that would otherwise necessitate further process facilities on the platform.

Fabrication of the Dunbar jacket.
The LB200 laybarge was used for the Ellon-Dunbar lines.
The double-wall interpipe, running between Dubar and North Alwyn.
The subsea choke manifold on the Ellon.
The Sedco 706, used for field drilling.
Topsides of the Dunbar platform.
The Dunbar platform, moored to the Sedco 706 tender-support vessel.