Converter platform for long-distance export

How does Converter platform for long-distance export work, step by step?

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   Step 1: Establish the transmission case and the interface

   The decision to convert offshore follows from the distance to shore, the capacity to be transmitted and the losses and cost of the alternatives, and it is taken jointly with whoever is responsible for the connection to the network. Once taken, the interface between the wind farm and the transmission asset is defined formally: what each party supplies, where the boundary sits physically and contractually, what the technical requirements are on each side, and the dates each party is committing to. That interface document governs the relationship for the whole project and is kept current rather than filed.

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   Step 2: Design the topside around very heavy equipment

   The conversion equipment, its transformers, its cooling and its control systems are large, heavy and demanding of their environment, and the topside is arranged around them rather than the other way round. Multiple decks are needed, and the layout has to allow the heaviest items to be brought in during construction and removed for replacement during operation. Cooling capacity is a major design driver and it shapes the external form of the platform. Weight and centre of gravity are tracked obsessively, because the installation method depends on them and there is very little margin at this scale.

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   Step 3: Choose the installation method early

   Whether the topside is lifted or floated over is decided at the start, not at the end, because it changes the design of both the topside and the substructure. A lifted topside needs lift points and a structure that can take the rigging loads. A float-over topside needs legs and mating units designed for the transfer, a substructure with the clearance for the barge to enter and a hull that can be ballasted precisely. The choice follows the weight, the availability of vessels, the water depth and the weather climate at the site, and the installation contractor is involved in the design from an early stage.

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   Step 4: Fabricate in a yard capable of the scale

   Very few yards in the world can build a converter topside, and securing a slot is one of the earliest commercial actions on the project. The structure is built as a steel frame with multiple decks, equipment is set as the build progresses because much of it cannot be brought in later, and the fit-out proceeds around it. Cabling, piping, cooling, fire protection, ventilation and accommodation are all installed and tested on land. Fabrication commonly runs for years and its progress is monitored closely, because there is no way to recover a late topside.

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   Step 5: Commission as much as possible on land

   At this scale the argument for finishing work in the yard is overwhelming. Systems are tested and handed over progressively, and equipment is commissioned as far as it can be without the platform being connected to anything. The project aims to leave the quay with a defined, short and well-understood offshore scope, because offshore work on a converter platform is slow, needs a large workforce living on or beside the platform, and is exposed to weather for months at a time. Every hour of testing avoided offshore is worth many in the yard.

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   Step 6: Install the substructure and confirm the interface

   The jacket or substructure is installed and piled first, then levelled and surveyed to a tolerance appropriate to what will be set on top of it. For a float-over the tolerances are particularly tight, because the mating units have to receive the topside legs with very little margin as the barge ballasts down. Any deviation is measured and reported before the topside sails, so that the installation contractor knows exactly what the topside will be landing on. Scour protection and access arrangements are completed while the substructure is on its own.

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   Step 7: Lift or float over the topside

   For a lifted installation, the largest heavy-lift vessel available takes the topside from its barge and sets it on the substructure in a single operation, planned in detail and independently reviewed. For a float-over, the barge carrying the topside is manoeuvred into position between the substructure legs, held there, and then ballasted down until the substructure takes the load and the barge is withdrawn. Both are single-shot operations requiring a good weather window and a great deal of preparation, and both are among the largest offshore installation operations undertaken anywhere.

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   Step 8: Hook up, terminate and hand over to the transmission owner

   Offshore hook-up on a converter platform is a campaign rather than a visit, with a substantial workforce accommodated on or beside the platform for an extended period completing connections, finishing systems and closing out the fit-out. Array cables from the wind farm and the direct current export cables are pulled in and terminated by specialist crews, and every cable is tested. The platform is then handed over to the transmission owner and its commissioning proceeds under their regime. Energisation itself is a controlled step carried out with the network operator under a formal authorisation regime.