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Repair of the Siri platform by Subsea7

Thursday, June 11, 2015

Subsea7 came up with a new way to reduce the swaying / vibration of an offshore platform

Subsea 7 were recently contracted for a project that was part of a major repair of the substructure for the 'Siri' platform operated by Danish operator DONG E&P A/S, in block 5604 / 20 in the Danish sector of the North Sea, about 220km from the coast of Denmark.

The subsea solution, using cable stay technology, is understood to be the first use of the onshore technology being used in an offshore context.

Subsea 7 project manager Alan Cassie said it was "the most interesting project I have ever been involved with'

Inspection of the platform revealed cracking to the subsea structure, impacting its capability to carry full design loads.

To reinstate the platform to its design specification a novel platform reinforcement solution, using cable stay technology, was devised. Analysis showed that if the swaying / vibration of the platform could be reduced (from a 6 second period to 3 seconds), only a relatively simple support of the damaged area would be sufficient to reinstate the structural safety of the installation. Without this stiffening of the structure, such a support would not be technically feasible.

The platform is located in the Danish sector of the North Sea, approximately 220km from the coast in 60-65m deep water.

Braces

The chosen method was to fix large friction clamps onto the legs of the platform, and tie them together by high tension steel cables, similar to the stays that are used to brace bridges. This would all 'stiffen' the platform structure, thereby reducing its natural period.

Each friction clamp is 6.4m high and weighed 150 tonnes. There are 56 bolts on each clamp joining the front and rear shells together around the leg tubular. Each of the 56 bolts is 1.9m long, and has a mass of around 300kg.

The stay cables between the clamps each contains 169 steel strands, making it thicker than most cables used on bridges. Subsea 7 needed to develop a special system for manufacturing the cables. Also the duct (covering the strands of metal in the cable) needed to be watertight.

The clamp design was handled separately to the overall platform strengthening design, with a different design house, which added to the complexity.

Everything needed to be constructed and installed with a 0.15 degree tolerance. "That's a Swiss watch tolerance," Mr Cassie said.

Installing the system required a lot of co-ordination between teams on the ship and teams on the platform.

Surprisingly, the dry clamps (above the water line) proved to be more complex to install than the wet clamps.

The cables were tensioned to 1250 tonnes.

Ultimately the project was successful, with the platform's period reduced to 3 seconds, with a corresponding reduction in the size of the motion. The entire design and installation has been certified by a third party and the platform is now fully repaired. The complex project was executed with a very good safety performance.

Lessons

One lesson from the project was "don't underestimate the complexity and challenge associated with delivering an innovative solution," Mr Cassie said.

But "don't underestimate the ability of good engineers to overcome these challenges."

'The novel design had to fulfil specific safety, technical and schedule objectives as well as cost pressures," Mr Cassie said.

"Adapting the methods and practices developed in onshore civil engineering to an offshore project was challenging, but the successful conclusion has advocated the approach.

'As offshore assets continue to age the oil and gas industry will witness a greater demand for reliable long-term solutions that are also cost-effective.

"Those solutions do not necessarily need to be completely new. Often reflecting on smart engineering previously used in other industries offers a better alternative for both contractor and client, which was the case for this project.'

Associated Companies
» Subsea7

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