Subsea buoyancy specialist Balmoral Offshore Engineering was tasked by Dana Petroleum with designing and manufacturing key components for a pioneering mooring system to be used on the operator’s Western Isles project in the challenging northern sector of the U.K. North Sea.

This challenging project involves a subsea development consisting of two well clusters tied back to a circular “Sevan-type” FPSO facility. The innovative integrated buoyancy and connecting system will be incorporated into the FPSO vessel’s moorings, which comprise 12 lines in three clusters of four.

The buoyancy element component of the mooring system—which was developed in partnership between Balmoral Offshore Engineering and Global Maritime subsidiary Moorlink—had to be suitable for operations in extreme conditions.

Corrosion and wear are major problems with traditional mooring system connections; these often use a “tri-plate” connector arrangement where the supporting buoyancy floats vertically above the connector. Movement of the connector due to variations in load in the mooring ropes causes significant rotation in the buoyancy connecting shackles, resulting in wear on the mating surfaces. Predicting the speed of wear is difficult, and connections have been known to fail well before their design life expectation.

High-strength materials

The new mooring system design aimed to reduce the number of wearing parts and used high strength materials to minimize the overall weight.

The engineering design teams at Balmoral and Moorlink developed the concept into a final solution and, following discussions with the operator, worked jointly to finalize the design, manufacturing and assembly processes.

On teaming up with Moorlink, Balmoral drew on its extensive experience of the design and manufacture of proprietary mooring rope connectors, known as M-links, for permanent mooring applications. The M-link concept was further developed for this project and adapted to the buoyancy elements by ensuring that the load path from the mooring lines was kept as independent as possible from the uplift generated by the flotation devices.

Design of components in the mooring load path as well as verifying calculations including fatigue and DNV-class approval, manufacturing and proof-load testing was managed by Moorlink. The end deliverable was a complete, tested and class-approved M-link ready to be mated with the buoyancy elements and substructure manufactured by Balmoral.

Buoyancy system

The latter’s design engineering team was tasked with developing a buoyancy system that would be easily assembled, provide stability in water and allow access to the rope connections to be inspected by ROVs. These requirements were to be built into a package that was capable of rapid installation and quick retrieval.

The modular buoyancy system for the project was manufactured using Balmoral’s syntactic foam, with a strong and flexible external skin to protect against impact and abrasion. A key feature of the design allowed standard Balmoral systems to be used, including depth-specific foam-filled polyethylene shells.

The final buoyancy mounting system, which uses elastomeric mountings to reduce any imposed loads on the connectors, is kept separate from the mooring rope line of action to avoid interference with the stress path. This offset mounting also ensures that the buoyancy remains steady in water and stable on the deck of the deployment vessel.

Significantly, there are no welds in the structural load paths, which helps to reduce fatigue failure as well as eliminating the requirement for specialist welding services. Apart from the Moorlink pins, which could be fitted with wear-resistant bushes if required, there are no wearing parts in the system.

Vessel deck considerations

The assembly is dimensioned so that the steelwork contacts the deck before the buoyancy modules; this means that if the assembly is deployed over a stern roller, the load path is directed through the structurally strong steelwork while the buoyancy modules provide stability.

When sitting on the vessel deck, there is only a small gap below the buoyancy modules to stop the assembly from rocking significantly while also allowing the modules to be retracted on the central steel mounting tube.

Demanding proof-load testing of the connection elements was carried out while hydrostatic and uplift tests on the buoyancy proved to be highly successful.

The buoyancy elements are designed to be easily separated, allowing access to the Moorlink pins while still being held on the central steelwork. This performance feature also helps with deck 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.

12 mt of uplift

All fabrication work took place at Balmoral’s plant in Aberdeen, Scotland, and totaled 12 buoyancy and connection sets, providing a net uplift of 12 mt per buoy at water depths of 170 m (558 ft). A design basis service life of 20 years is expected.

Because of the structural design’s simplicity, the buoyancy rating can be altered for different mooring requirements without major structural changes being required.

According to Balmoral’s international business development director, Jim Hamilton, “The successful delivery of this project is a great demonstration of what collaborative engineering design efforts can achieve. A mooring solution that has proved beyond doubt to be exceptionally workable in very demanding conditions has been accomplished.”

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