Floating substructures and moorings

How does Floating substructures and moorings work, step by step?

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   Step 1: Select the hull concept against site and port

   The designer weighs water depth, wave climate, seabed conditions and the ports available. Draught is often decisive: a shallow-draught semi-submersible can be assembled and launched from ordinary quaysides, while a deep spar needs deep sheltered water for upending. Turbine size, the motions the machine will tolerate and the mooring layout that the seabed will support all feed the same decision. Because the concept determines the fabrication route, the port requirement and the installation method, it is settled very early and is expensive to revisit.

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   Step 2: Fabricate the hull

   Hulls are built in steel, in concrete, or as hybrids, and the material choice follows the yards available as much as the engineering. Steel hulls suit shipyards and heavy fabricators; concrete hulls suit marine civils contractors and can use local supply chains. Either way the objective is a series of near-identical units produced at rate, because a floating project needs many of them and serial production is what makes the cost work. Compartmentation, ballast systems, pumps, valves, fairleads for the mooring lines and the cable hang-off arrangement are all built in during fabrication.

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   Step 3: Survey the seabed and design the mooring system

   The mooring design needs a geotechnical picture across the whole mooring footprint, not just under the platform, because anchors are spread out around it. The designer sets the mooring configuration, the line composition and the anchor type from the ground, the water depth and the loads. Anchor types in use include drag-embedded anchors, driven or suction piles and gravity anchors, and different positions on one project can need different solutions. The layout also has to be routed clear of the array cables, of other platforms' moorings and of any other seabed user.

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   Step 4: Pre-install the moorings and anchors

   The mooring system is installed ahead of the platforms, in a separate campaign using anchor handling vessels. Anchors are set, lines are laid out and each leg is tested and left with a recoverable pick-up arrangement floating or buoyed off ready for hook-up. Pre-installing decouples the slow, weather-tolerant seabed work from the short, weather-critical hook-up operation, and it means the platform arrives at a position where everything is already waiting. Positions, tensions and line lengths are recorded as the campaign proceeds.

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   Step 5: Assemble the turbine at the quayside

   The substructure is brought alongside and the tower, nacelle and rotor are erected on it using land-based cranes, in sheltered water with the crew standing on a quay. This is the whole economic point of floating: the most difficult lifts happen in a controlled environment rather than on a vessel in open water. The unit is commissioned as far as it can be before it leaves, and the more that is done at the quay the less has to be done offshore later. Quay strength, water depth alongside and crane reach set what is possible.

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   Step 6: Tow to position

   The complete unit is towed to its position by tugs, at a speed and in conditions set by a tow plan that the installation contractor prepares and that is independently reviewed. Stability during tow, the ballast condition, the tow route, the availability of shelter and the go and no-go points are all defined in advance. Towing a fully erected turbine is a slow operation and it is planned against a forecast with limits agreed beforehand, with a defined point after which the tow continues rather than turns back.

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   Step 7: Hook up the moorings and connect the dynamic cable

   On arrival the pre-laid mooring lines are recovered and connected to the platform one at a time, and the system is tensioned to bring the unit into its design position and orientation. Tensions are measured and adjusted until the whole system matches the design, and the position is confirmed by survey. The dynamic array cable is then pulled in and hung off, with its buoyancy modules and bend restrictors set so that the cable takes the shape the designer intended through every motion of the platform. Both operations are short, weather-sensitive and heavily proceduralised.

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   Step 8: Commission, monitor and plan for tow-back

   The unit is commissioned and brought into service, and monitoring begins on the things that are new: platform motions, mooring line tensions, and the condition and fatigue exposure of the dynamic cable. Mooring and cable inspection are recurring operations for the life of the project. The operations strategy also has to answer the question fixed-bottom projects never face - whether a major component change is done in place or by disconnecting the unit and towing it back to port - and the answer shapes the port arrangements the project keeps on contract.