Statfjord subsea

person By Kristin Øye Gjerde, Norwegian Petroleum Museum
A discovery was made five kilometres north-east of the present site of Statfjord C in 1976, and another the following year 22 kilometres to the north. Dubbed the Statfjord satellites, their development represented a departure from the Condeep platforms used on the main field.
— Statfjord Nord subsea installations. Illustration: FMC Kongsberg Subsea
© Norsk Oljemuseum

Although both discoveries were considerably smaller than Statfjord, neither was negligible. Development plans were approved by the Storting (parliament) on 11 December 1990.  The most distant reservoir was named Statfjord North and lay in 250-290 metres of water, while Statfjord East had been found in 150-190 metres.

Satellite solution

Following the oil price slump in 1986, both the oil companies and the government agreed that more needed to done to reduce Norway’s offshore development costs. Production systems had to be simplified and their efficiency enhanced.

Greater attention was paid to investment and total life cycle costs. Questions included the cost of maintenance and whether equipment could be reduced or later removed. The latter consideration also had an environmental aspect.

Developing subsea installation offered an answer to some of these challenges. Existing production platforms, for example, could be supplied through remotely operated seabed systems.  A solution of this kind was first developed for the Norwegian continental shelf (NCS) through the Skuld research project launched in 1980. The partners were Elf, Norsk Hydro, Statoil, the Norwegian Underwater Institute (NUI) and the Sintef research foundation in Trondheim.

Statfjord subsea,
Protective frame for underwater installation on the Gullfaks field is mounted and welded together at Kongsberg Weapon Factory. Photo: Olav Indreberg/Norwegian Petroleum Museum

Through this project, it became possible to operate subsea installations remotely over a distance of more than 20 kilometres. Systems could be installed without diver assistance.  Statoil-operated Gullfaks became the first field on the NCS to adopt the system in 1986. A total of six seabed templates were installed and tied back without divers to the A platform.

Elf also applied experience from the Skuld programme to its development of East Frigg, one of the Frigg satellites. On stream in 1988, this involved three subsea templates and was the first wholly platform-free project in the Norwegian North Sea. That meant the technological foundation had been laid, and a number of discoveries were developed with subsea production systems during the 1990s.  As on Gullfaks, the Statfjord and Sleipner satellites were tied back to fixed platforms. Production floaters with associated seabed wells were chosen for Norne and Åsgard in the Norwegian Sea.

Subsea production facilities were also tied back to Yme in the North Sea, which had been developed with a jack-up rig, and the Heidrun tension-leg platform (TLP) in the Norwegian Sea.

Statfjord North and East

statfjord satelitter, forsidebilde, illustrasjon, Statfjord subsea
Illustration of Statfjord C with adjacent satellites. Illustration: Statoil

These two satellites were tied back to the main Statfjord field. Each was provided with two templates for production and one for water injection – a total of six structures with 18 Xmas trees.

Wellstreams from the producers flow through two pipelines to Statfjord C for processing, storage and onward transport. At the time the two satellites came on stream, they ranked as the world’s largest subsea production system.  This represented an eye-opening quantum leap for technology on the NCS. It was characterised by a close collaboration between Kongsberg Offshore – as the supplier of the subsea installations together with its sub-contractors – and operator Statoil, and between the various licensees. [REMOVE]Fotnote: Gjerde, Kristin Øye and Helge Ryggvik. North Sea divers in Norway , 2009, pp 317-318.

Gunnar Berge, then minister of local government and labour, performed the official inauguration of the Statfjord satellites on the C platform in 1995. His comments included the following:

“The government takes a positive view of the fact that Statoil, as operator for this project, has chosen a long-term form of collaboration with key suppliers. Giving the supplies industry a shared responsibility calls for trust between both sides. Trust can only be created through a long-term customer-supplier relationship because it builds on the security created through experience and communication.

“I regard it as constructive that the basis has been created here for a long-term collaboration over future deliveries and further development, which can yield substantial gains in forthcoming projects on the NCS. This will collectively help to strengthen Norway’s position in an international context, and thereby jobs in Norwegian industry as well.”

Standardised solutions

Berge was also pleased that standardised solutions were being developed:

“Standardisation is an important concept in this context. An increased diversity of technical solutions would represent poor economics if these are to be tailored to each individual case. It is accordingly particularly gratifying when solutions are developed which can not only fulfil the specific job they have been designed for but can also be used a building blocks in other and future field developments. [REMOVE]Fotnote: Gunnar Berge, minister of local government and labour, “Offisiell åpning av Statfjord satellitter på Statfjord C”. 

That was precisely what happened. An advantage of templates for subsea wellheads was that they could be brought on stream quickly once installed. A very solid reduction in development costs per well was seen on the NCS during the decade from 1986.

While each subsea well tied back to Gullfaks A had cost NOK 170 million, that figure was down to NOK 85 million for the Statfjord satellites in 1992. And the price sank further as even simpler and increasingly standardised designs were developed.

The cost per well for Norne came to NOK 45 million in 1994, for example. And the hinge-over subsea template (Host) modules developed achieved an additional reduction to NOK 30 million in 1996. By then, these structures had become so compact that they could be installed through the moonpool on a mobile drilling rig. [REMOVE]Fotnote: Statoil memo: “Viktige beslutninger for UTV-området i Statoil”, 1998.

Choosing the same development solution for a number of fields led to a greater degree of coordination. A tool pool was established, for example, so that installation equipment and spare parts could shared between fields utilising the same technical design. That led to substantial savings. Other licences began to choose the same design for their subsea installations.

Oil was proven in Statfjord’s north flank during 1996. It was decided to develop this area of block 33/9 with two Host modules. FMC in Kongsberg won the contract and fabricated the Xmas trees at Dunfermline in the UK. The electro-hydraulic control system for six trees and a maximum of eight wells was produced as Kongsberg. Production from the north flank was controlled from Statfjord C. [REMOVE]Fotnote: .

Produksjonsstart Sygna, forsidebilde, historie, Statfjord subsea
Sygna field plan. Illustration: Equinor

Sygna was also proven in the north-eastern part of the Statfjord area during 1996. Fifty-five per cent of this oil field lies in Statfjord block 33/9, and 45 per cent in 34/7 (the Snorre block). It was approved for development in 1999, and began producing in 300 metres of water on 1 August 2000.

A development solution based on a subsea template was again chosen for Sygna, with three production wells tied back by a 22-kilometre flowline to Statfjord C. Water is also injected into the reservoir through an extended-reach well from the Statfjord North satellite.

Processed, stored and exported via Statfjord C, output from this field helps to extend the platform’s producing life since it uses the same processing equipment as Statfjord North and East.

From being a technology for the specially interested, subsea solutions have developed into a key component of offshore developments. Few places on Earth have so many subsea wells as the NCS. Statoil was the second largest operator of such solutions in 2005 with 245 installations, surpassed only by Petrobras with 464. [REMOVE]

Fotnote: Gjerde and Ryggvik 2009, p 321.

Published October 29, 2019   •   Updated January 28, 2020
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