When the Vesterled gas pipeline to the UK was connected to the Heimdal platform late this summer, all the protective clothing the divers working 120 metres under the sea needed was a boilersuit. They were, albeit, surrounded by a solid habitat, which had been specially developed to repair pipelines running across the seabed.
Well before the Vesterled operation itself took place on the seabed, the divers had trained for their tasks in a vast hangar at Killingøy, just outside Haugesund on the Norwegian west coast. The operation required 15 divers working for a month, and the job was tough.
A 95-meter-long L-shaped intersection had to be attached from the Heimdal platform to the Vesterled pipeline, and a junction between the Frigg pipeline and the Vesterled pipeline had to be installed from the British side. In addition, the new pipeline had to be welded together with the old Frigg pipeline around 60 km from Frigg.
The habitat equipment used during the undersea installation is part of an advanced system for repairing seabed pipelines, called the Pipeline Repair System (PRS). Briefly, the habitat consists of a steel box which is placed over the pipeline. The water is forced out of the box by pumping in a mixture of helium and oxygen. Divers go in to prepare the equipment, and the actual welding is controlled from the surface by the mother ship.
Two divers can work inside the habitat, while a rescue diver is ready for action in a diving bell. Previously the divers themselves had to carry out the welding operation, but now they just install the equipment and do small jobs which it is not worth doing by remote control. The divers work for 14 days, with the next five days being spent in decompression.
Improved emergency response
The habitat solution has been developed in two phases. First in the 1980s in connection with the installation of the Oseberg Transport System (OTS) in the North Sea. Since then the system has been in regular use, including during recent pipeline laying operations in the North Sea. Statoil has used the equipment on all its major pipeline systems.
The habitat system and the PRS are operated by the company, Stolt Haliburton Joint Venture (SHJV). The contract with SHJV is administered by Statoil on behalf of the pipeline owners, Statpipe and OTS. Norsk Hydro and several other oil companies participate in a joint emergency response group which has been established to make use of the equipment in certain circumstances. No less than 8,000 km of pipelines are covered by these agreements.
"The equipment is kept in working order by using it on installations around once a year, and the divers are given the necessary training," says subsea engineer Kjell Andersen from Hydro Technology and Projects.
This latest operation on the Vesterled pipeline, with the hook up to an existing pipeline, has not been quite so routine. However, Andersen believes there will be more and more of this kind of operation in the years ahead, due to new uses being found for the old infrastructure.
"We are expecting more jobs of this kind," he confirms. "We are now working on junctions especially designed for smaller pipelines, with equipment which will make it unnecessary to use divers."
In five or six years it will probably be difficult to get hold of enough divers, since recruitment to the professional is currently minimal. The oil companies are working to improve this situation because they acknowledge that diving is both necessary and cost-effective down to depths of around 240 meters.
Less diver dependent
Hans Olav Knagenhjelm, who led Hydro's participation in the Vesterled PRS operation, was one of those responsible for developing the PRS equipment in 1984-1987. He was also involved when the equipment was further developed by Statoil to be less diver dependent during installation.
"We can now install everything on the seabed without the need for divers. Once it is in place, the divers can go down into the habitat to install the welding equipment and make the welding joints before welding is started and guided from the surface. Every half-hour the welding rods are replaced by the divers," explains Knagenhjelm.
The habitat has been constructed for diving operations down to depths of 360m. The deepest operation so far on the Norwegian continental shelf was at Gullfaks C, when work was carried out at 220m below sea-level. If the system is further developed to avoid the need for divers at all, it could be used at even greater depths. But there are no plans for that just yet. If the system could be used for repairs down to a depth of 360m, it could be used for any repairs needed to the Grane pipeline in the Norwegian trench.
If the PRS equipment were to be purchased today, it would probably cost upwards of NOK 1 billion, estimates Knagenhjelm. Annual maintenance costs alone amount to around NOK 20 million.
Knagenhjelm has personally been involved in PRS installations on a number of Norwegian pipelines, including the Oseberg Transport System (OTS), the Oseberg Gas Transport (OGT), Vesterled, Oseberg II, Multiphase Transportation System (the pipeline between Oseberg A an ad B) and Brage. All the operations have gone extremely well, including the latest work on the Vesterled pipeline.
He does not really see the Vesterled job as being particularly difficult, and is supported in this view by offshore divers Einar Andersen and Harry Kristensen, who took part in the training at Killingøy, and reckoned then that the Vesterled operation would be "quite OK".
"The Vesterled job was just an ordinary welding job. But even so, we are talking here about spot-on accuracy…"
The divers were proved right. The work went ahead according to plan, with no surprises. And the divers did not even get their feet wet - 120m down.
Today the Vesterled pipeline is connected to the Heimdal gas terminal, which Norsk Hydro operates, and ensures that new amounts of Norwegian gas can be offered to UK customers via the Frigg pipeline. A new and important chapter in Norwegian oil and gas history has been written.
| Vesterled in brief
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