Laying of the 125-kilometre-long control cable to be used in the remote steering of the subsea installations on the Ormen Lange field in the Norwegian Sea has now begun. The cable will be laid in a single length from a gigantic pipeline carousel, on board the multi-purpose offshore support vessel the "Skandi Neptune".

"Since the Ormen Lange field is located in an area where the Storegga submarine landslides took place around 8000 years ago, this installation represents perhaps the biggest technical laying challenge so far on the Norwegian continental shelf," says Sigmund Lunde, Hydro's project manager for the control cable contract.

"The greatest challenge was to find a route where the control cable can lie safely, between large landslide blocks. Some of these are up to 80-100 metres long, and 50-60 metres high. One consequence of this is that the control cable must be laid with an extremely high level of accuracy, typically within 1-2 metres. Having said that, the Ormen Lange project has invested large amounts of money in preparing the cable route, by filling with stone mass and crushed stone and carrying out digging work, so that we can be sure that the result will be successful," says Lunde.

Subsea 7's "Skandi Neptune" started the pull-in on Monday, to the landfall at the processing plant being built at Nyhamna. The cable is then laid out towards the field, over the Storegga slide area and down to sea depths of 850 metres, which puts particular demands on both the product and the installation contractor.

After laying, the cable is buried with the help of three diggers designed by Nexans, belonging to their "Capjet" system. One of them, the Capjet Spider Dredger, is a re-built "Spider" digger.

Custom made giant carousel
"The gigantic pipeline carousel on the "Skandi Neptune" had to be built specially for this task, so that the cable could be installed in a single length. The carousel is 24 metres in diameter and 6 metres tall, and holds the 125-kilometre-long control cable - with its 12 centimetre diameter - in a single length. The carousel has a 3,000 tonne capacity. The equipment belonging to the carousel is sophisticated, in order to ensure gentle installation," says Odd H. Knudsen, Subsea 7's project manager.

The control cable was manufactured at the Nexans factory in Halden, Norway. The electrical and fibre-optic cables inside the control cable were made at Nexans' factory at Rognan, south of Bodø in Northern Norway.

It was in 2004 that Hydro, as operator the Ormen Lange development, awarded Nexans a contract worth around NOK 400 million for the production of control cables, while Subsea 7 was given the installation contract for the control cables, worth around NOK 150 million. The control cables are to be installled by Subsea 7; one this year, and one in 2007.

Umbilical cord

The cables are to supply hydraulic liquid, electricity and fibre-optic signals to be used by the subsea production systems on Ormen Lange. Since the subsea production will be steered from land through these cables, it is critically important that they meet extremely high standards for reliability.

"One of the challenges in the design requirements for the Ormen Lange control cable was meeting the different demands regarding the cable's "wet weight" at different points along the cable route. In several sections of the route it was necessary to increase the weight, due to the prevailing sea current conditions, in order to ensure that the cable remained stable on the sea floor between installation and burial," says Bård Viken from Nexans.