No crane vessel available in China could lift a 25,000-tonne converter station. So the engineering team didn't lift it — they floated it, sailed it, and let the ocean do the work a crane couldn't.

The topside is a seven-storey steel superstructure roughly the footprint of a football pitch — 85 by 82 m, 44 m tall — set down onto a 17,000-tonne jacket foundation, eight piles into the seabed, some 100 km off Yangjiang in Guangdong province. It is the world's first 500 kV, 2,000 MW flexible-DC offshore converter station, the platform that decides how far from shore an offshore wind farm can practically go.

≈25,000 t
Topside weight
17,000 t
Jacket foundation
150 mm
Clearance each side
≈7 hrs
Total operation time
Phase 1 — Why AC stops working offshore
The engineering problem the platform exists to solve
Offshore turbines generate alternating current, but a subsea AC cable behaves electrically like a long capacitor — every cycle charges and discharges it, consuming an increasing share of the cable's capacity as distance grows. Past roughly 80–100 km, the cable spends its capacity charging itself rather than delivering power. Direct current has no equivalent penalty, which is why power must be converted offshore, transmitted as DC, and converted back to AC onshore.
Phase 2 — A load too heavy to lift
Choosing float-over over any available crane
At approximately 25,000 tonnes, the topside exceeded the lifting capacity of any domestically available crane vessel. Engineers used float-over installation instead — a technique borrowed from offshore oil and gas — in which a semi-submersible heavy-lift vessel carries the topside on its own deck rather than a crane hook.
Phase 3 — Sailing between the legs, then ballasting down
"The crane is the ocean"
The vessel Xiang Tai Kou sailed the topside directly between the jacket foundation's legs using dynamic positioning and tidal timing, then ballasted down — allowing the sea to transfer the load from the vessel onto the foundation as the vessel sank beneath it, rather than any mechanical lifting system taking the weight.
Phase 4 — Six inches of margin
Rubber fenders and coupling devices guiding final alignment
The operation slotted the 25,000-tonne structure into place with approximately 150 mm of clearance on each side between the topside and the jacket's legs — roughly six inches, on a structure the size of a football pitch. Rubber fenders and coupling devices on the jacket legs absorbed contact loads and guided the final alignment, with the entire operation completed in about seven hours.

The crane is the ocean — a 25,000-tonne structure floated between the legs of its own foundation and lowered into place with a hand's width of clearance on each side.

Once subsea cabling and offshore-onshore testing are complete, the station is expected to carry roughly 6 billion kWh of wind electricity to the mainland grid each year. The China Offshore Converter Station stands as a reference case for float-over installation at the scale where no crane vessel can substitute for it: not a lifting problem solved with more capacity, but one solved by removing the crane from the equation entirely.

For further reading on offshore heavy lift and marine construction, see What is Heavy Lift Engineering?

Related case studies: HVDC Transformer Transport · Vogtle Nuclear Plant · Prelude FLNG

Sources: China Three Gorges Corporation — owner's installation announcement: the approximately 25,000-tonne topside, float-over installation rather than a crane lift, and roughly 150 mm minimum side clearance. Reported installation dates vary slightly between sources; the engineering details are consistent across them.

Frequently Asked Questions

The topside is a seven-storey steel superstructure roughly 85 by 82 m and 44 m tall, weighing approximately 25,000 tonnes, sitting on a 17,000-tonne jacket foundation pinned to the seabed by eight piles. It is described as the world's first 500 kV, 2,000 MW flexible-DC (VSC-HVDC) offshore converter station, and the largest offshore converter platform built to date, located roughly 100 km off Yangjiang in Guangdong province, China.

At approximately 25,000 tonnes, the topside exceeded the lifting capacity of any crane vessel available domestically. Engineers used float-over installation instead — a technique borrowed from offshore oil and gas, in which a semi-submersible heavy-lift vessel carries the topside on its deck, sails it directly between or over the foundation's legs, then ballasts down so the sea transfers the load from the vessel onto the foundation.

The vessel Xiang Tai Kou used dynamic positioning and tidal timing to slot the 25,000-tonne structure into place with approximately 150 mm of clearance on each side between the topside and the jacket's legs — roughly six inches, on a structure with the footprint of a football pitch. Rubber fenders and coupling devices on the jacket legs absorbed contact loads and guided final alignment. The total operation took approximately seven hours. Reported installation dates vary slightly across sources (a 23 July date and a separate 4 June operation-start date both appear); the engineering details are independently corroborated regardless of the exact date.

Offshore turbines generate alternating current, but AC transmission over a subsea cable becomes inefficient past roughly 80–100 km because the cable behaves like a long capacitor, consuming an increasing share of its capacity just charging itself. Direct current has no equivalent penalty, so power is converted to DC offshore, transmitted to shore, and converted back to AC onshore — and the platform housing that offshore conversion equipment is what determines how far from shore an offshore wind project can practically go.