The Valhall WP wellhead platform was installed as part of the central installations in 1996 to increase the number of well slots and to permit deviated drilling towards the field’s flanks.[REMOVE]Fotnote: This method involves gradually increasing the angle of the well path away from the vertical. However, experience with this technique indicated that it would be difficult to reach the flanks because the wells needed to be very long and the reservoir rock was so porous. It was accordingly decided to launch the Valhall flank development (VFD). A plan for development and operation (PDO) of this project was approved by the government on 9 November 2001.
The aim was to improve recovery from the field by gaining better access to the resources in the outer sections of the reservoir. Heerema Tønsberg AS was appointed main contractor for the two wellhead platforms, with fabrication of their jackets subcontracted to Heerema’s Vlissingen yard in the Netherlands.[REMOVE]Fotnote: http://www.offshore.no/sak/1679_plan_for_utbygging_og_drift_av_valhall-flanker_godkjent At that time, recoverable oil reserves in Valhall’s northern and southern flanks were estimated to total around 110 million barrels.[REMOVE]Fotnote: Stavanger Aftenblad, 13 January 2004, “Olje fra Valhalls nordflanke”.
The Valhall flank south (VFS) wellhead platform is unstaffed and stands six kilometres from the field centre. On stream from 8 May 2003, it provides 16 slots for production wells. A copy of the VFS, the Valhall flank north (VFN) installation is located 16 kilometres from the field centre and became operational on 7 January 2004.
Laying a fibreoptic cable from land to Valhall permits the flank platforms to be operated remotely. They are supplied with power via an 11kV, 1.6MW high-voltage line from the field centre. Compared with the alternative of diesel-powered generators, this reduces emissions to the air and the risk of acute diesel oil discharges. Both the platforms and their pipelines are fabricated in materials which minimise the need for chemicals in the production phase.[REMOVE]Fotnote: BP: Utslippsrapport for Valhallfeltet og Hod, 2012, 4.
Output from the flanks is carried in three-phase pipelines, laid in 2002 by Coflexip Stena Offshore, to the field centre for processing and export.[REMOVE]Fotnote: Teknisk Ukeblad , March 2003, “Sikrer inntekter”.
Drilling on the flanks presented the biggest technical challenge in the VFD project. Covering 23 wells, this programme began in October 2002. West Epsilon was chartered to carry out the work, which had originally been scheduled to start during the summer. However, a delay occurred in completing the platform. Since this was expected to make the project three-four months late, the rig was sent for upgrading. It arrived on the northern flank during August.
The original plan called for five wells to be drilled on VFS, followed by eight on VFN, eight more on VFS and finally two on VFN. But the delays meant that this sequence had to be changed. Øystein Eide, the experienced drilling superintendent, nevertheless aimed to use lessons learnt from the first wells on Valhall in the early 1980s. His goal was to reduce the time needed to drill the wells by 40 per cent from the average for the field. Drilling the 23 wells averaged 22 days. And 10 in a row were completed even more quickly, compared with the accepted figure set in relation to the technical constraints with a well.[REMOVE]Fotnote: Rasen, Bjørn (2007): LF6A. Valhall at 25 … and it’s only the beginning , 118–120.
When the wells were brought on stream, the results far exceeded expectations, with an overall output of 30 000 barrels per day – roughly double the design capacity of the pipelines. That was hardly a problem.
Gas lift on the flanks
Work began in the autumn of 2008 on a gas lift project for the Valhall flank platforms to boost production. Modules for this purpose, fabricated by Aker Solutions in Egersund, were installed in September 2009 and became operational in 2011.[REMOVE]Fotnote: BP’s web site.
Both flank platforms stand in about 75 metres of water and are supported on welded steel assemblies known as jackets, which each weigh 3 000 tonnes including piles. Weighing 1 600 tonnes, the topsides for each platform have a surface area of 25 x 32 metres and are 10 metres high. They also support a helideck measuring 23 metres square, standing about 10 metres above the top deck, and projecting just under 15 metres from the southern edge of each topsides.