Nick Terdre
Contributing Editor
Gas from Norsk Hydro's Ormen Lange field in the Norwegian Sea will have a long way to go to reach the market – first a 120-km stretch from the field to the onshore processing terminal, then 600 km to the Sleipner field center in the North Sea, and finally another 600 km to the UK or 700 km or more to continental Europe.
When it starts up in 2007, the gas flow from Ormen Lange's 400 bcm of reserves is due to keep going for a long time – up to 50 years. Plateau production of 15-20 bcm a year of gas, equivalent to up to 55 MMcm/d, is expected to continue for 15-20 years. Pipeline planning is therefore one of the central issues for the project team.
The toughest pipeline challenges lie in transporting the mixture of gas, condensate, and water from the wells to shore. The first part of the journey is the worst: a 22-km climb up the Storegga slide over which the water depth decreases from 850 m to 250 m. The slide, thought to have been triggered by an earthquake about 8,000 years ago, has left the seabed in a chaotic state, pockmarked by craters and rocky outcrops.
The remaining stretch to the shore is relatively straightforward, though some careful planning is required to make landfall across Norway's skerry-strewn coastline.
New export line
Once the gas has been processed to export specification, it will be transported south to Sleipner in a 42-in. pipeline. The trickiest part of the pipelay will probably be negotiating the narrow Bjørnsundet sound outside Nyhamna. The new export line to the UK, for which a new gas transport treaty between the two countries is required, will be either 42 in. or 44 in. in diameter. To reach the continent, the gas can be sent down a number of existing routes in Norway's extensive North Sea pipeline network. Additional compression may be needed at Sleipner.
Two 30-in. production pipelines have been specified for the field to shore section, partly because of the high volume of production and partly to ensure that production can continue even if problems such as hydrate formation put one of the pipelines out of action. With sea bottom temperatures of -1° C to -2° C at the field, flow assurance is clearly a major challenge, according to Asbjørn Wilhelmsen, pipeline lead engineer for the Ormen Lange field project. During the exploration phase, one of the wellheads completely froze due to the action of hydrates.
Two contracts will be awarded for the installation of these lines. One is for the stretch from the processing terminal at Nyhamna to the top of the Storegga slide (for which the conventional S-lay method will be used) and the other for the Storegga slide section.
The Storegga stretch is probably unique in the challenges it poses for underwater pipelay. Precise positioning of the lines is required. They will have to be placed in a corridor about 5 m wide on a route that will snake around various obstacles. The J-lay (steep lay) method will be used for this section. Because it involves laying the pipe almost vertically, it also allows more accurate placement on the seabed than S-lay. The changeover is planned to take place 94 km from the shore.
Pipelay vessels
A limited number of pipelay vessels are fitted with J-lay capability, among them Saipem's S7000 and Heerema's Balder – both semisubmersible barges – and Technip-Coflexip's DeepBlue and Allseas' Solitaire, which are both monohull vessels.
All of these contractors have carried out J-lay studies for Ormen Lange. During the laying operation, the pipe sections will be welded together vertically. It is also intended to include structures such as manifolds and tie-in tees on the line as it is being laid to avoid having to perform subsea tie-ins.
The Ormen Lange gas field lies at the bottom of the Storegga slide in the Norwegian Sea.
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The pipe sections, which will be of carbon steel, will be relatively heavy. An unusually high corrosion allowance of 10 mm has been made, so that the total wall thickness will vary between 30 and 35 mm. There will also be a concrete coating of 50-80 mm down to 550 m water depth. From there and going into deeper waters, the pipelines will have polypropylene coating only.
Each pipeline will run by both of the two eight-slot subsea templates that will be installed on the field 3 km apart. Each template will be connected to both pipelines by a 20-in. spool, so that production can be routed to either line. To enable the lines to be pigged, either a pigging loop or a subsea pig launcher will be installed at the end of the pipelines.
Injection lines
In addition to the production pipelines, there will be two MEG-injection lines of 6 5/8-in. diameter, and two 4-in. control umbilicals using fiber optics. One MEG line and one umbilical will run to each template. The templates also will be joined by an interconnecting MEG line and umbilical line. The umbilicals, which will each be supplied in a single length of around 120 km, will be among the longest yet installed offshore.
Continuous MEG injection at the wellheads is part of the strategy to keep the pipeline contents flowing – the same strategy as is used on the Troll Oseberg Gas Injection project, under which gas is transported 48 km from the Troll field to Oseberg, Wilhelmsen says. Corrosion inhibitors transported through the umbilicals will be injected into the production pipelines. The cocktail of inhibitors to be applied is currently being studied at Hydro's Porsgrunn research facility in southern Norway.
The pipeline route has yet to be fixed. Seabed surveying is under way this summer using the Hugin autonomous underwater vehicle. Up to 1,000 km of potential routes have been surveyed in the slide area, and 7,000 km in total. Fishing activity is high between the field and the coast, so it is important to determine a route, particularly in the shallower waters, that minimizes conflicts with the fishing community. While the production lines will be capable of withstanding trawl-board impacts, the same is not true of the MEG lines and umbilicals, and these will be trenched.
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Each template will be connected to each pipeline by a 20-in. spool.
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Currents and free-spans
Understanding the currents in the area is of great importance, not only for the laying operation but also for the behavior of free spans as great as 80-100 m. Extensive research has been performed on free spans, Wilhelmsen says, and new guidelines are being drawn up by DNV.
Currents are the result of a number of forces, including the Gulf Stream, which give rise to complex patterns including the rotation of currents through 360°. A reliable prediction of currents is required, and the University of Bergen has been assigned the task of modelling current profiles. Measuring devices will be installed along the pipeline route to enable currents and other relevant parameters to be monitored both before and after pipelay.
Statoil plans a lot of pre- and post-lay work to minimize problems with the pipelines. The company intends to flatten the highest peaks on the route by dredging. Statoil is soliciting contractors for ideas on how best to perform this task. Rock supports for the pipelines will be built up to reduce the length of excessive free spans. The production lines will be hot, so it will also be necessary to allow room for thermal expansion.
Several types of remediation strategy for dealing with hydrate formation are also under consideration. On a conventional pipeline, a hydrate plug would normally be removed by depressuring the line, and this would be the first remedy for the Ormen Lange pipelines. In such deep waters, however, it is not guaranteed that plugs can be melted in this way.
Hydrate removal vehicle
Another means of tackling them would be the hydrate plug removal vehicle developed by Statoil. This method involves injecting methanol into the back of the plug through coiled tubing carried through the pipeline by a pipeline tractor. Another possible remedy – perhaps the last resort – is replacing the blocked section of pipe.
Contractors submitted bids for laying of the 94-km stretch of the 30-in. pipelines and the export pipeline in 2Q 2003, and contract award is expected in September. A contract for the Storegga slide pipelay is scheduled for 1Q 2004. Statoil is procuring line-pipe and was expected to place an order in July.
These contracts will be provisional pending government sanction for the development and pipelines, due to be issued next March. Pipelay operations are scheduled for 2005 and 2006. Reinertsen Engineering is providing pipeline engineering for all the pipelines for the Ormen Lange field project and for the first 33 km of the export pipeline. Snamprogetti has been awarded the pipeline engineering for the remaining part of the export pipeline.
