Conquering an Unforeseen Problem

Published Dec 12, 2003
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The Tune Field Development

The Tune Field Development, close to Oseberg, will soon start test production. But the road to completion posed a major unforeseen problem to Norsk Hydro and its partners at Tune. Now Tune Manager Rune Rønvik at Norsk Hydro is confident the battle is won, and full production will start as scheduled on 1. October 2002.

Leakage in the flowlines presented a totally unforeseen challenge to the Tune project team last summer. Faced with a problem to which nobody knew the cause Tune Manager Rune Rønvik felt very insecure back then, about whether the Tune development would meet the planned deadline.

‘We had to plan for every possible scenario,’ he admits. While the team worked to find the cause of the leakage they also placed an optional order for brand new flowlines, just in case.

New Material
‘The two approximately 10 km long flowlines are made up of 13 % Cromium martensittic steel (S13% Cr),’ Rønvik explains. ‘That is not new; it has been used among other places at Åsgard (The Åsgard leakages have mostly been related to welding of low alloy hubs). But at Tune we also used matching S13 % Cr welding consumables which was new.’

‘We wanted to take out the potential strength in the 13 % Cr steel, as welded joints with matching consumables are expected to be stronger than the pipeline,’ Rønvik explains. But when the flowline had been laid, test during the summer 2001 proved leakage in three of the fabrication welds.

‘We assembled an internal task force here at Norsk Hydro, with the objective to find the cause for the leakage and to document the integrity of the pipelines once in operation. The pipeline leakages were first attempted repaired by use of hyperbaric welding (i.e. subsea welding), but was terminated in January after two unsuccessful repairs. The S13% Cr has proved very sensitive to hydrogen, and we were not able to maintain sufficient control of the environment in which the hyperbaric welding process took place. Since then we have decided to repair the damaged areas with flanged connections. We cut the flowlines, brought each of the cut ends to the surface where we welded on flanges which we later connected subsea,’ Rønvik continues. In parallel the problems related to hyperbaric welding have been solved, and this repair method serves as a contingency also for 13 % Cr flowlines.

An extensive amount of work has been laid down by the Tune Pipeline Project Team in close cooperation with Det Norske Veritas (DNV) to fully understand the cause of the leakage and to document that the repaired pipeline is fit to be set into operation.

‘In this process we have engaged people at Sintef, The Welding Institute and I guess in one way or another involved every person in the world with some competence on welding 13% Cr steel,’ Rønvik smiles.

He does not hesitate to say that he would use the same material again. ‘We have now learned a lot about how to use this material,’ he says.

‘13 % Cr is a pipeline material for the future. It is planned used in several new developments, and it is important that experiences gained from the Tune project are shared within the industry’ Rønvik says.

The Present and the Future
The present Tune project covers Phase 1 only. The Tune development might be continued with two more phases at a later stage, probably as satellite wells drilled and tied back to the template.

A 4-slot subsea well template with tie-in provisions for two satellite wells has been installed during phase 1. The template has two headers. The production from each well may be routed into either of these headers, which are connected with a pigging loop for round trip pigging. The pigging loop may be removed and the connection points utilized for tie-in of flowlines from an additional future template. The templates are covered by integrated protective structures and are over-trawlable.

The template has been tied in to the Oseberg D platform (OSD) at the Oseberg Field Centre (OFC) by two flowlines with nominal diameter of 12 inches and a service umbilical.

A dedicated tie-in module has been installed on OSD to handle the wellstream from Tune. In this module, gas and liquid will be separated and routed through metering units. Gas and condensate will be further routed to the Oseberg A platform (OSA) for final treatment in the existing facilities.

‘A possible development of Phase 2 and Phase 3 will be decided during 2003, depending on production experience,’ Rønvik says.

‘We will first see whether the wells in A-Main are able to drain gas from A-North and A-South. It might also be that we discover there are not enough reserves in A-North and A-South for further development.’

Should the partnership of licensees decide to develop Phase 2 and 3, the start-up of a well in A-North is scheduled for October 2004, and start-up for Phase 3 of a well in A-South in October 2005.

The Beauty of Tune
The beauty of Tune is that it fits perfectly in with spare gas treatment capacity at Oseberg.

‘The rich Tune gas arriving at OSD will be routed to the M10 compression and injection module on OSA for injection into the Oseberg reservoir,’ Rønvik says.

‘In return, the Tune licensees will receive lean Oseberg gas at sales specifications. By this arrangement Tune may utilize Oserberg’s existing gas treatment facilities in order to deliver the contractual volume of gas.’

‘The capacity of M10 and the reception module exceeds the capacity required to produce the expected gas profile for Phase 1. The surplus capacity provides good flexibility with respect to further phases and it opens the possibility to accelerate liquid production within the capacity of the wells by gas banking at Oseberg,’ Rønvik continues.

This arrangement will be available for Tune until Oseberg starts full rate export of Oseberg gas, scheduled to happen on 1st. October 2010. Then spare gas treatment capacity in no longer available for third parties.

‘We expect this to be time enough to produce all the Tune gas, also Phase 2 and 3 should that be necessary,’ Rønvik points out.

Tune export gas is routed in the Oseberg Gas Transportation (OGT) pipeline to the Heimdal platform and further to the Continent and the UK. Gas transportation to the continent goes through the Statpipe system, while gas to the UK goes through the new Vesterled pipeline. Condensate is exported from OFC in the OTS pipeline to Sture for final treatment.

Tune Geology
Tune consists of a north-south elongated 2.5 km wide and 20 km long structure. Towards the west and east the field is limited by major faults. Several smaller faults cut through the reservoir sections, possibly defining sub-compartments within the field.

The Tune field contains a gas column of approximately 170 m in the Tarbert Formation. A thin oil zone of approximately 12 m thickness is underlying the gas in the Tarbert Formation. Shales and siltstones of the Middle to Upper Jurassic Tarbert, Heather and Draupne Formations provide the top seal of the Tarbert hydrocarbon accumulation.

The Phase 1 development, with four wells in A-main, is based on the reserves in A-Main, and additionally on influx of gas and condensate from A-South. Some influx of gas and condensate from A-North is also assumed. Total rich gas volume from the Phase 1 development is expected to be 23 GSm3.

The Field
The Tune oil, gas and condensate field was proven in 1995. It lies about 10 km west of the Oseberg field centre. The major part of the Tune reserves lies in production license 190, but some extend into production licenses 034 and 053. License interests in 034 and 190 are the same, and the Tune licensees have purchased production rights for the reserves extending into 053.

Conquering an Unforeseen Problem-Link

The field comprises hydrocarbon-bearing reservoirs in two separate sandstone units, the Tarbert Formation of the Brent Group and the Statfjord Formation, respectively Middle Jurassic age and Early Jurassic age. But the resources of the Statfjord Formation are not considered commercial.

Licensees at Tune (after SDFI agreement):
Petoro: 40%
Norsk Hydro: 40%
TotalFinaElf Norge: 20%

Operator is Norsk Hydro

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