Alba Phase II, North Sea Northern, United Kingdom

Alba is located in block 16/26, 130 miles north-east of Aberdeen. The field lies in a water depth of approximately 138m. Alba was discovered by Chevron in 1984 and it came on-stream ten years later, in January 1994. The field contains estimated total oil-in-place reserves of approximately 1 billion barrels (bbl) and has recoverable reserves of around 400 million bbl.

Field development: phase I

Field developments were carried out in two phases. The first involved the installation of a fixed steel platform with integrated production, drilling and living quarters. This, the Alba Northern Platform (ANP), was used to develop the northern sector of the field. Oil is exported via the Alba floating storage unit (FSU) – the first of such to be purpose-built for the UK sector of the North Sea.

Field development: phase II

The second stage of Chevron’s Alba development plan envisaged using a second Alba Southern Platform to develop the south of the field. At that time, the technological limit to the extended reach of a well was 9,500ft.

However, as extended reach drilling (ERD) techniques improved, it became clear that the increasing 'step-out distances' were becoming sufficient to exploit reserves at the southern end of the reservoir, using the existing Alba Northern Platform area.

Phase IIA

Phase II was divided into two more stages. The first, Phase IIA, increased production capacity from a design rate of 75,000bopd to 100,000bopd. This was completed in October 1996 and since then, the platform has handled consistent production rates of around 100,000bopd.

Phase IIB

This phase was intended to increase gross fluid handing capacity from 240,000bopd to a maximum of 390,000bopd. As part of this phase, a subsea water-injection facility was installed 6km away, to increase pressure for the planned ERD wells. This was drilled by the J W McLean rig, which was connected to the platform with a new 6km pipeline.

Three new modules were attached to the existing platform to hold the necessary processing equipment. The process module at the north of the west face contains the new first-stage separator, which operates in parallel with the existing separators; a new hydrocyclone package with high-efficiency liners to process fluids from the new separator; and a produced water pump to recover heat from the produced water stream for injection water feed. It has a new polishing/degassing vessel, to provide improved produced water overboard fluids to a target of 30 parts per million (ppm) oil in water and two new water-injection pumps, each providing 100,000 barrels of water per day (bwpd) at a 2,500psi discharge pressure.

The power-generation module at the south of the east face includes a new supplementary main generator, rated at 10Mw, with associated control-cabin and air-inlet duct work. It also houses the new waste heat recovery unit, exhaust stack and diesel day tank, which serves both the new generator and the existing main generator.

A water-injection treatment module, located next to the base of the existing deaerator tower on the platform west side, accommodates an injection water preheater, additional vacuum packages, a new seawater coarse filtration unit and new de-aerator residence vessel.

Subsea

The drill centre manifold is located approximately 6km south-west of the Alba Northern Platform, connected by a flowline and an electrohydraulic control umbilical. The water-injection well produces through an ABB Vetco Gray horizontal 6,3/8in throughbore tree, with choke control and flow measurement on the platform.

This is linked to a drill centre manifold which has provision for up to two subsea water-injection wells. The lightweight structure (50t) has the provision to expand for future field requirements or house a pig receiver. The 12in piping header has two 8in branch offtakes and ROV operable isolation valves. It links to the wellheads via flexible jumpers.