MARGINAL FIELDS Statoil converting subsea template design for marginal developments

Simpler and lighter subsea structures
less expensive to fabricate and easier to install

This graph depicts Statoil's reduction in cost for each subsea well over time, expressed in NOK (Norwegian currency).

Around 80 small fields have been discovered on the Norwegian continental shelf, each containing around 30 million bbl of oil. Lower-cost subsea technology will play a key role in pushing these developments ahead.

At the forefront of this initiative is Statoil, which has been pursuing standardization subsea through re-use of designs and equipment developed for the Statfjord Satellite Project (SSP). Statfjord was Statoil's third subsea development following Gullfaks (1982) and Tommeliten (1988).

The SSP concept was developed to handle oil and gas production and water injection in diverless water depths using basically a repeat template, well control system and flowline/umbilical design. The four-slot template represents a combination of the design approaches taken on the two previous subsea projects.

SSP in its first guise was actually applied to two separate subsea developments: the Loke and Sleipner East Fields connected to the Sleipner A facility, as well as the Statfjord North and East satellite fields tied back to the Statfjord C platform. The former were gasfields in 80 meters of water, which came onstream in summer 1993; the latter are both newly flowing oil producers, in 280 meters and 180 meters water depths.

Statoil grouped deliveries for the two developments under a common EPC contract. This comprised two production templates with three christmas trees and a four-tree water injection template for each for the Statfjord satellites; two templates and four trees for Loke and Sleipner East; workover, intervention and control systems, and a total of 75 km of umbilicals.

Template advantages

The template is designed to be operated without divers, with an emphasis on reliability. It has an over-trawlable protection structure, a retrievable manifold unit and insert-type manifold isolation valves. One subsea control pod has been installed with each tree, with redundant supplies of hydraulics, electrical power and signal transmission to maintain system availability as high as is practical.

Conoco also chose to standardize on the SSP subsea template design for the Heidrun Field, with resultant cost and lead time reductions. Now Statoil is applying SSP to two new Gullfaks satellites, Rimfaks and Gullfaks South. These are oil/gas condensate fields being developed in parallel for eventual tie-back to the Gullfaks A platform. In fact Statoil aims to reduce equipment costs on this project by 25% and installation costs by 40% compared with the original Statfjord Satellite Project.

To attain this goal, Statoil entered an agreement with Kongsberg Offshore last year to develop new subsea building blocks beyond the original SSP concept. The aim is to shrink the present template size, keeping all modules within a 20-tonne limit with a minimum size of 6.5 meters by 5.5 meters. This would allow most operations to be performed from a drilling rig, eliminating the need for heavy lift vessels normally required to install SSP templates.

The hexagonal template design envisaged would handle up to five wells: any one of the slots could be installed over an existing wellhead, incorporating it within the template structure. The manifold would be launched on top of the template with a separate protective cover. For future well tie-ins, a four-slot hexagonal template design is likely, allowing for two pull-in areas.

Norne technology

SSP technology has also been transferred to the Norne floater-based development off Mid-Norway, with five standardized templates ordered here from Kongsberg. Re-use of design, test equipment and handling tools and minimal extra documentation has helped bring costs down on Norne from over NKr100 million to NKr40 million per well.

As operators in Norway move into deeper waters such as the Voering region, CAPEX will climb again, leading to new initiatives for controlling costs.

Statoil believes that one way forward is continued development of simpler and lighter subsea structures that are less expensive to fabricate and easier to install than some of today's typically used structures.

One way to do this might be to eliminate the overtrawlable design criteria for subsea projects in water depths beyond the reach of trawlboards - and maybe to apply the same practice for trenching and burial of flowlines.

Statoil claims that subsea production hardware is increasingly proving its reliability in the North Sea, as evidenced by the recovery of two Gullfaks trees last year: all seals and valves on these units were shown to be still operational after a decade under water. And standardization is paying off: the NKr40 million figure for subsea equipment per Norne well is one third of what Statoil was paying out just three years ago.

REFERENCE
Endresen, E., "Cost Effective Satellite Field Developments," technical proceedings, OMC, Ravenna, Italy, 1995.