The world’s largest pipe laying vessel will arrive in Shetland on Sunday to start laying the offshore section of the pipeline that will go from Total’s new gas plant to the network which runs to St Fergus, near Peterhead.
The Solitaire will anchor 900 metres from shore near Mossbank and preparations are well underway for her arrival. The ship will complete the subsea section of the Laggan-Tormore export pipeline, currently being laid across the 5.5 kilometres from the SGP, adjacent to the Sullom Voe terminal, to the beach at Firths Voe.
The pipeline, known at this stage of its journey as the SIRGE system, has spare capacity to carry future production from west of Shetland, possibly from fields such as Edradour. It will eventually join the FUKA (Frigg UK) pipeline, carrying compressed natural gas which will go for further processing at St Fergus and then join the national grid.
The 30-inch export pipeline is made of steel with a heavy concrete coating. The section on land has been laid in an excavated trench and covered with netting for security before having the spoil from the excavation laid on top. Once in the sea the concrete coating will ensure the pipeline is heavy enough to sink.
However for its first 300 metres underwater the pipeline will lie in a two-and-a-half-metre deep trench that has been specially created from a stone causeway extending from the beach at Firths Voe. Recently the backhoe dredger Abercoserver with the tug Baloo have been working on this excavation. Sophisticated sonar equipment was used to show the ships where to dig as the Brent pipeline also enters the sea from this beach.
For the length of the trench the pipeline will be buried under sand and rock to protect it from hazards such as boats’ anchors. The covering will be flush with the sea bed and will not be visible at low tide. After that the 243 km pipeline will lie on the sea bed.
The 300-metre Solitaire, which has a 92,396 gross tonnage, carries the necessary pipeline on board and welding is done on the vessel. The pipe will be winched ashore from the beach where a 600 tonne winch, secured with a ship’s anchor, is in place. The 126-mm steel wire from the winch has a 1,200 tonne breaking strain, more than is actually required. Buoys will be attached to the pull wire to absorb the force, and to keep it just below the surface of the water in the centre of the trench. The pull is expected to last two days.
All the pipeline work east of Shetland is being done by Allseas: the company also owns the Solitaire. The pipeline to the west of Shetland was done by Allseas and Subsea 7. Onshore work has been done by Van Oord, Sicim and Roadbridge.
Eventually the beach at Firths Voe will be reinstated and the only visible sign of work will be a structure to house an isolation valve and small control room. However this will be built to blend in with the landscape.
Throughout the operation care has been taken to consider marine mammals, and if any are spotted operations are stopped for 20 minutes.
The 18-inch import pipeline bringing gas and condensate from the Laggan and Tormore fields north-west of Shetland, comes ashore at Orka Voe and was finished last year. Once in operation, the condensate will be removed from the gas and taken from Sullom Voe in tankers.