Finding Hod and Valhall was more closely linked than one might suppose. Amoco had caught whiffs of both fields before Phillips Petroleum made the nearby Ekofisk discovery, but it made its breakthrough only came after the latter had begun production. And that was after something pretty close to espionage by Amoco.

The Valhall field centre was completely modernised during the 2000s. This partly represented a response to seabed subsidence on the field, with new platforms installed on its flanks. The installations were connected to the electricity grid on land, making them much more environment-friendly. And a permanent seismic array was installed on the seabed to improve reservoir surveillance. Last but not least, a control room established at BP’s offices in Forus outside Stavanger made it possible to monitor all aspects of production and processing much more effectively than before.

Seabed subsidence on Ekofisk was first observed in the autumn of 1984, when its installations were clearly lower in the water. The same phenomenon was also found on Valhall, and action had to be taken to counteract its effects. The problem was most obvious with the Ekofisk tank. It was easy to see that more of the rows of holes in its concrete breakwater had become submerged than when it was first installed.

Hod was discovered in 1974 with the second well in block 2/11, which found both oil and gas. The Storting (parliament) gave the green light to bring it on stream in 1977, at the same time as the Valhall development was approved. Hod nevertheless first began production in 1990. The question is why it took so long to develop this relatively small field.

The first application from the AmocoNoco group for official consent to develop Valhall and Hod was submitted on 4 October 1976, and welcomed with enthusiasm by the Ministry of Industry.

Only one well had been completed when Valhall began producing on Friday 1 October 1982. Production from the field initially failed to go as planned – an experience which was to be repeated.

BP installed a permanent seismic survey network on the seabed in 2003 in order to monitor the Valhall reservoir and thereby improve recovery from the field. This “life of field seismic” (LoFS) installation represented the first full-scale facility for tracking changes in a sub-surface formation, and offers several benefits.

Installations on the outer edges of the field were included in the plans for Valhall from as early as the 1960. Although the initial proposals were dropped, two flank platforms arrived in 2003-04. White Paper no 92 (1976-1977) on the Valhall development included plans for a third development phase from 1982, with a platform on the south-western part of the field. A fourth stage scheduled for 1983 envisaged another installation to the south-east. But these plans were dropped in favour of a focus on improving recovery from the field centre.

The operations department for Valhall had pressed over many years for increased water injection in order to improve the field’s recovery factor. Even before production began in 1982, a study had looked at whether this could boost recovery for the whole field.

The Valhall WP wellhead platform was installed as part of the field centre in 1996 in order to increase the number of well slots while permitting deviated drilling towards the flanks. Also called extended reach drilling (ERD), this technique involves deviating the well path away from a vertical line at an angle which can be steadily raised as drilling proceeds. But ERD is difficult to implement on Valhall because of its reservoir structure, where the thickness of the formation rocks varies considerably.

The Hod platform was an unstaffed facility with only the most essential equipment aboard. So a mobile rig had to be chartered for drilling work.

The Hod field was discovered in 1974, with the first development plan submitted in 1980 and a subsea template installed in 1982. After the first well on Valhall proved disappointing and the Storting (parliament) adopted new tax rules, however, the initial proposals were shelved.

Using the existing transport system to carry Valhall oil and gas to market called for new pipelines running from the field to the deck of the Ekofisk tank via an additional riser platform. This solution was agreed after long and difficult negotiations with the Ekofisk licensees over access to their infrastructure for carrying oil to Britain and gas to Germany.

The original quarters platform (QP) on Valhall was built in 1979-80 and taken into use in July 1981. It originally provided 170 berths over four stories. However, seabed subsidence, aging of the platform and extending the field’s producing life meant that a long-term solution for secure accommodation eventually had to be found. The decision was accordingly taken in 2006 to replace the QP with a new combined process and hotel (PH) platform. This was placed on the field in 2012, and the QP is being phased out.

The process and compression platform (Valhall PCP) was installed on the field in 1981 but only came on stream the following year after experiencing several storms – both physical and political – on its way from paper plans to completed structure. Until the new production and hotel (PH) platform was installed in 2012, all oil and gas production from Valhall and Hod were processed and readied for pipeline export on the PCP.

The Valhall drilling platform (DP) forms part of the original 1981 development and stands between the quarters (QP) and production/compression (PCP) installations. Plans call for a gradual shutdown of this facility and the plugging of its wells over a number of years.