Foinaven Oil Field, United Kingdom

Foinaven is located in blocks 204/19 and 204/24a, which are operated by BP Exploration. Shell UK Exploration and Production is the co-venturer. These blocks lie some 190km west of the Shetland Islands, in a water depth of between 400 and 600m. Recoverable reserves are estimated to be in the region of 250 to 600 million barrels of oil.

The project is being carried out as a phased development. The first of these is based on the recovery of 200 millions barrels within the Foinaven field.

These developments centre on subsea wells completed on the seabed. They produce oil, via a manifold, which passes through rigid flowlines and then flexible risers into a floating production, storage and offloading system (FPSO), which is permanently stationed in the field. Shuttle tankers then export the crude oil.

TECHNOLOGY/DRILLING

Horizontal drilling is a technology which has undergone radical developments since the early 1990s. Both BP and Shell have successfully used this type of drilling in other projects. Horizontal wells expose a larger section of the reservoir to the well bore, resulting in increased flow rates. For Foinaven, the low rock strength of the oil-bearing sands necessitates the installation of pre-packed screens. Horizontal sections in excess of 1,500m are possible. The selection of horizontal wells for Foinaven has reduced the number of production wells required from 28 to 14. Water- and gas-injection wells are also required.

Drilling wells in 500m of water with changeable currents can create problems, as the water depths are beyond those in which divers can operate. Therefore, the costs associated with the development of the subsea gathering scheme are high.

For the first time in a BP-operated offshore environment, some of the producing wells are multi-lateral - that is, two wells or more share the same well bore, with oil flowing through each one.

FPSO VESSEL - PETROJARL IV

This five-year-old vessel, built in Finland, underwent a major rebuilding programme. The vessel's bow and stern were removed, upgraded and joined to a new mid-section of double-skinned hull, increasing its length to 240m.

The process facilities have the capacity to process up to 95,000 barrels of oil per day, and to inject 165,000 barrels of water per day into the reservoir. Gas lift and re-injection were available from first oil.

The vessel's ten anchors enable it to maintain stationary, even in the most severe weather conditions. The anchor wires, and up to 15 flexible risers and control umbilicals reach the surface inside a turret. The vessel 'weathervanes' around this turret, under the influence of the wind, waves and currents, with thruster assistance to aid station-keeping. Shuttle tankers visit approximately every three days during peak production periods and storage for some 300,000 barrels of produced oil are available onboard the FPSO vessel.

SUBSEA

The subsea layout comprises of two drilling centres, each based upon a manifold and well cluster arrangement; with rigid steel flowlines connecting to the riser-base locations, flexible risers connecting the riser bases to the FPSO vessel, as well as the associated control system umbilicals.

Innovative diverless connectors (DMaC) were used for the subsea installation and hook-up.

At Drilling Centre 1 (DC1) there are eight production wells, four water-injection wells and up to two gas-injection wells. At Drilling Centre 2 (DC2) there are six production wells, with three remote water-injection wells. These drilling centres are located approximately 3.5km from the FPSO vessel, one to the north, the other to the west. All drilling facilities are performed by semi-submersible drilling rigs.

PIPELINES

The common flowlines running from each drilling centre to the riser-base locations are:

• Two 10in production flowlines
• Two 8in production/test flowlines
• One 8in gas-injection/gas-lift flowline

A single 10in water-injection line services the DC1, with 8in and 10in water-injection lines serving the remote water-injection sites.

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