Rolf Olavesen
Offshore Resources Group AS
Judy Maksoud
International Editor
Subsidence can wreak havoc on subsea installations. Subsea structure displacement and strain on flow lines can compromise recovery or halt production. Stopgap fixes can temporarily forestall that eventuality, but the situation requires a permanent solution. No operator wants to be saddled with a field that cannot be reliably produced.
This is a situation Statoil found itself addressing on the Vigdis and Tordis fields, which lie due east of Statfjord in the Norwegian sector of the North Sea. Statoil assumed operatorship of the blocks in January 2003 and, in the process, inherited the subsidence problems that had plagued the fields for years.
Setting the scene
Though the Vigdis and Tordis fields shared a problem, the subsea production systems installed on the fields are not identical.
The Vigdis oil field was developed with three subsea templates in roughly 280 m of water and has been in production since January 1997. Vigdis' subsea templates support eight production wells and three water injectors. Seawater is injected to maintain pressure in the reservoir. The Snorre A TLP, 7 km away, remotely controls production at Vigdis and processes recovered oil before outputting it via pipeline to Statoil's Gullfaks A platform for storage and export. Production from Vigdis is 41,000 b/d.
The seabed installations at Tordis, an oil and gas field, comprise a central manifold unit at 200-m water depth, two templates, and seven satellite wells. There are four wells on each of Tordis' two templates, and the manifold is also the tie-in point for two other developments in the same license – on Tordis East and Borge. Oil and gas from Tordis are piped over 10 km to the Gullfaks C platform for processing and export.
The Snorre A TLP remotely controls production at the Vigdis field, which lies 7 km away (Photo: Kjetil Alsvik).
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The problem
The seabed in the Tampen area, where the Vigdis and Tordis fields lie, is made up of soft clays. Subsidence has been a chronic problem. The generally accepted explanation for the problem is that subsidence is caused by a combined effect of seabed displacement resulting from washout from drilling operations and the low density of the clays.
In the case of the two Statoil-operated fields, one of the primary consequences of the subsidence was the displacement of the installations' primary base structures, which rest on the seabed. The christmas trees maintained their position because they are mounted on the wellheads that are in turn supported by 30-in. casings that extend into the more stable substrata below the seabed. Subsidence caused flow lines between the christmas trees and the base structures to be strained, and the base structures were sinking. The problem was temporarily solved using airbags and shims to lift the flowlines, but Statoil needed a permanent solution if the fields were going to be able to continue long-term production.
Creative solution
The project to find a permanent solution got off the ground in 2001, when Norsk Hydro was operator. Norsk Hydro engaged Stavanger-based Offshore Resource Group AS to resolve the subsidence problems. When operatorship transferred to Statoil in 2003, the project continued under Statoil's operatorship.
ORG based its solution on a unique, original concept that locks the base structures to the 30-in. casings, thereby impeding the effect of subsidence on the base structures. ABB performed additional structural analysis to ensure that the solution was compatible with the existing subsea structures.
The principle idea behind ORG's solution is based on an ROV-operated system and specially developed hydraulic power jacks. Each power jack is made up of two toothed steel plates that contain an expandable steel diaphragm between them. With the introduction of hydraulic pressure, the diaphragm expands, pressing the toothed steel plates into the two structures that need to be locked together.
When the desired effect has been achieved, a locking mechanism made up of mechanically actuated wedges is activated, and any movement between the two structures is stopped permanently. In association with more traditional methods and tooling systems, this total solution proved to be fast, effective, and low-cost. And most significantly of all, the installation could be carried out without interrupting production.
Norsk Hydro chartered the support vessel Statoil used for subsidence rectification. The Far Saga is a DP2 vessel with a moonpool and work ROV spread. Guidelines were used to lower the intervention panels, and ROVs installed the power jacks.
The project moved forward rapidly once the vessel was on site. It took only a few hours to install jacks around a single slot and only two days to complete a template. Both Vigdis B and Tordis were secured using ORG's solution. Since the installation, there has been no evidence of subsidence.
