The Alwyn North Development in Block 3/9a of the United Kingdom Continental Shelf came on stream in November 1987 The field, owned by Elf UK plc (2/3) and Total Oil Marine plc (1/3), is operated by Total Oil Marine plc. The initial development plan called for two subsea satellite wells, formed into a subproject called North Alwyn Underwater (NAU) These were installed and started up in mid-1988. This paper describes the experience Total Oil Marine has had with these subsea wells as follows introduction to NAU, integration testing, offshore installation, start-up and operations
The North Alwyn Field in block 3/9a of the UK Continental Shelf includes several separate reservoirs, of which four were included in the initial development plan (see Fig. 1) Gas and condensate are produced from the Statfjord reservoir, oil from the Brent Two platforms standing in 130 m of water support all the production facilities and are linked by a 70 m footbridge. NAA carries the drilling and accommodation modules, NAB carries the oil and gas separation modules, including the power generation and export facilities. Oil is exported through a 12 in line to the Ninian field and then on to the Sullom Voe oil terminal. Gas flows through a 24 in. pipe to the Frigg field and then on to the St Fergus gas terminal. Current daily production levels are 100 000 barrels oil and condensate, and 8.0 million cubic metres of gas.
The development plan called for two subsea satellite wells-an oil producer from the Brent North reservoir (PN1) and a water injector into the southern end of the Brent East reservoir (IE3), the reservoir targets for these two wells being outside the drilling reach of the NAA platform.
The production well is linked to the platform via a 5.4-km 6-in. rigid oil flowline that is protected by gravel along the whole of its length The injection well is fed from the platform via a 4 6-km 6-in. flexible water flowline, and has no special protection.(Fig. 1 is available in full paper)
The tree is of a standard dual bore design. The system is stacked up on the permanent drilling guidebase as shown in Fig. 2. The subsea control module (SCM) is electrohydraulically operated (two independent channels) with a hydraulically piloted back-up system.
On the tree are pressure and temperature transmitters, and a permanent downhole pressure and temperature gauge has been installed in the oil well completion. All the installation work and any subsequent operations on the trees are diver-assisted.
The hydraulic circuits for the workover control system and the production control system, via the SCM, are independent This is achieved by having dual-function actuators. Production control operates the main valve actuator, the workover control operate as valve override that is permanently in stalled on the actuator. These actuators extend outside the guideposts; hence an enlarged protective frame is required for the Xmas tree.
