SUBSEA PIPELINES UK-Continent interconnector inching towards payback

Gas exports through the Interconnector must take several "detours" before arriving in Zeebrugge.

Although water depths between Bacton and Zeebrugge are hardly colossal, there are some unusual seabed hurdles to clear (by North Sea standards) at the approaches to Belgium. Planning and schedules at both ends have also been constricted by tourism and environmental issues.

Britain's Department of Trade and Industry initiated a feasibility study for the Interconnector in early 1992. By 1994 a nine-strong ownership team had been assembled comprising UK North Sea gas developers, a UK power utility and three Continental gas distributors. Stakes in the project reflect the percentages of the total capacity (20 bcm/yr throughput) booked by the nine participants. British Gas, for instance, having reserved 8 bcm annually, leads with 40%.

Transportation contracts between individual shippers and the operating company, Interconnector UK, cover a fixed capacity level over 20 years. Whether they have sales agreements in place across the Channel or not, shippers must pay capacity charges. Until recently, when tumbling UK gas prices led to suspension of some new field developments, these arrangements drew scorn from non-Interconnector participants.

But the laughter is dying down. BP has just announced a 15-year agreement with Ruhrgas for 1 bcm annually through the Interconnector; supplies will continue on to Aachen via a new trans-Belgium pipeline financed by Distrigaz. This follows deals totaling 3 bcm/year clinched by British Gas and Conoco with Wingas.

Conoco is now reactivating deferred gas developments in its southern sector V-fields area. Elf's investment in the new line should pay off once its Elgin/Franklin fields come onstream in the Central North Sea in the next decade. Meanwhile, Statoil and Mobil are now thought to be interested in participating in the new line as third party shippers. These positive developments are emerging at a time when Norway has secured yet more long-term gas supply deals, this time with Ireland, Italy and Ruhrgas.

Significantly, Ruhrgas vice-chairman Dr Burckhard Bergmann commented that the contracted imports, including those via the Interconnector, were "an important precaution against the presumed development away from the present oversupply to a tighter supply situation expected at least in the long run".

Offshore schedule

Offshore engineering work for the UKP50 million pipeline project is handled by an alliance between Interconnector (UK), Brown and Root, European Marine Contractors (EMC) and the Dredging International/Jan de Nul joint venture Baczee. EMC's barge Semac-1 is laying most of the offshore section of the 150-mile, 40-in. diameter line.

Preliminary work on the landfalls at either end started last fall. Caravanners and residents at the UK end succeeded in diverting the approach to the new gas reception terminal in Bacton through an industrial site owned by British Gas. Land & Marine Engineering is managing the pull ashore of the pipeline and a 30-in. test water discharge pipe into a shaft and tunnel, through a cliff face, that lead to the termina; work should be completed late this year.

Around 130,000 tons of line pipe are going into the new line, including 12,000 joints from Nippon Steel - for tax reasons, these are being laid outside territorial approaches. The X65 carbon steel line is being coated internally by Universal Pipe Coaters using asphalt enamel to enhance flow and to protect against corrosion. Externally, a high density three-layer coat is applied comprising polypropylene/fusion-bonded epoxy and concrete. Unusually for the North Sea there will be varying wall thickness - 65, 85 and 105mm - for different sections of the line.

Ideally, a straight line direct to Zeebrugge would have been furrowed, but there were numerous obstructions en route. Heading south-east from Bacton, Semac-1 must first bypass dredging areas and communications cables to the Continent. Near the median line with Dutch and Belgian waters, a military training area has to be avoided. Then the Interconnector must veer south through the Belgian sector due to an aquatorial dispute with The Netherlands. Finally, intersection with the new NorFra 42-in. diameter gas line from Norway will also have to be negotiated: this will be laid later in the season to Dunkerque via Zeebrugge.

Seabed preparation includes pre-sweep dredging - 30-40 km ahead of the main pipelay vessels - along the entire route. Shifting sandwave peaks have to be shaved to avoid over-stressing of the pipeline. In fact, the route had to be re-surveyed, partly to determine the optimum way of going in and out of the sandwaves (via S curves). Substantial sections of the pipeline will be unsupported, and special instrumentation will be deployed to ensure accuracy of line placement between the sand troughs.

Further dredging will be necessary across the Scheur channel, the main route for ships to the major Benelux ports. And yet more dredging will be needed over the shallow water approaches to Zeebrugge. Here, pipelaying will be handled by the flat-bottom barge Castoro 2, starting next month. Sixteen kilometers from the landfall, in 15 meters of water, this mini-section of the line will be welded to the main pipeline by the Semac-1 at a tie-in point just above the water surface.

Following completion of the line, a plough will be used to trench it flush with the seabed. Semac-1 will inject water into the sand to create a fluidized bed, allowing the high density coated pipe to sink in. Afterwards there will be notional backfill, with rockdumping to correct any identified spans or pipe exposures. The pipeline will then be fully hydrotested.

The alliance team will share the first #25 million savings or cost overrun. A separate alliance is building the onshore terminals at Bacton and Zeebrugge, due to be completed end-June 1998. Depending on how long UK gas supplies last, the Interconnector can also be configured to import 10 bcm/yr to Britain from the Continent, via turbines at the Bacton end working in suction mode.