Balmoral Offshore Engineering has designed and manufactured key components for a pioneering mooring system to be used on Dana Petroleum’s Western Isles project in the challenging northern sector of the North Sea.
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The project involves a subsea development with at least five production and four water injection wells tied back to the Sevan-designed FPSO. The innovative integrated buoyancy/connecting system will be incorporated into the FPSO’s moorings which comprise 12 lines in three clusters of four.
The mooring system – developed in partnership with Global Maritime subsidiary, Moorlink – is designed to operate in the harshest conditions.
The buoyancy mounting system, which uses elastomeric mountings to reduce any imposed loads, is therefore 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 that the buoyancy remains stable in the water.
Significantly, there are no welds in the structural load paths offering good levels of fatigue resistance as well as eliminating the requirement for specialist welding. Apart 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 easily separated allowing access to the Moorlink pins whilst held on the central steelwork. This performance feature maintains stability and ensures safety.
The assembly is fitted with a number of lifting pad eyes and crucifixes at each end for deployment and retrieval; simple hydraulic tooling is supplied for offshore assembly of the mooring ropes. Once complete, the entire system is completely stable when sitting on the vessel deck.
In total, twelve buoyancy and connection sets were manufactured providing a net uplift of 12 tonnes per buoy at water depths of 170m. A minimum service life of 12 years is expected.
Balmoral director, Jim Hamilton, said: “The successful delivery of this project is a great demonstration of what our engineering design team can achieve.
“Working closely with Moorlink we designed the system 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.
“Demanding proof-load testing of the connection elements was carried out while hydrostatic and uplift tests on the buoyancy proved to be highly successful and, 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.
“The system has proved beyond doubt to be cost-effective, highly suitable and exceptionally workable in very demanding conditions. We believe it has great potential on a global basis and look forward to taking the system to the wider market.”