Novel mooring arrangement strengthens stability of Western Isles floater

Balmoral Offshore Engineering and Global Maritime subsidiary Moorlink have delivered components for the mooring system for Dana Petroleum’s Western Isles project in the northern UK North Sea.

Mar 19th, 2015
Buoyancy mounting system
Buoyancy mounting system

Offshore staff

ABERDEEN, UK – Balmoral Offshore Engineering and Global Maritime subsidiary Moorlink have delivered components for the mooring system for Dana Petroleum’s Western Isles project in the northern UK North Sea.

The development involves the subsea tieback of at least five production and four water injection wells to the circular, Sevan-designed FPSO. The buoyancy/connecting system will be incorporated into the vessel’s moorings which comprise 12 lines in three clusters of four.

According to Balmoral, the buoyancy mounting system, which employs elastomeric mountings to reduce any imposed loads, is kept separate from the mooring line to avoid interference with the stress path. The mounting is also offset vertically from the mooring rope to ensure the buoyancy remains stable in the water.

There are no welds in the structural load paths, an arrangement that is said to lead to good levels of fatigue resistance as well as eliminating the requirement for specialist welding. Aside from the Moorlink pins, which could be fitted with wear-resistant bushes if required, there are no wearing parts in the system.

The buoyancy modules are designed to be separated easily, allowing access to the Moorlink pins while held on the central steelwork. This feature, Balmoral says, maintains stability and ensures safety.

Various lifting pad eyes and crucifixes are fitted at each end of the assembly for deployment and retrieval, with conventional hydraulic tooling supplied for offshore assembly of the mooring ropes. Once complete, the entire system is said to be completely stable while sitting on the vessel deck.

In total, the partners manufactured 12 buoyancy and connection sets providing a net uplift of 12 metric tons (13.22 tons) per buoy in a water depth of 170 m (558 ft). They anticipate a minimum service life of 12 years.

Balmoral director Jim Hamilton said the system is designed to be easily assembled and rotationally stable in water while the rope connections are accessible for ROV inspection. These requirements had to be built into a system that was capable of rapid installation and quick retrieval.

Other features of the program included demanding proof-load testing of the connection elements, he added, and hydrostatic and uplift tests on the buoyancy. “Due to the simplicity of the structural design, the buoyancy rating can easily be uprated for different mooring requirements without major structural changes.”

Moorlink managing director Björn Palmquist said: “This is a new concept and both partners worked very closely with the end client to provide a new and exciting mooring solution...We believe it has great potential on a global basis…”

03/19/2015

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