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Semi-submersible heavy-lift vessel: ballasts down until the cargo deck submerges below cargo waterline, receives the floating cargo (typically a jack-up drilling rig, semi-submersible rig, warship, or other vessel), then de-ballasts to lift + transport. Owners: Dockwise (now BOKA / Boskalis Heavy Transport), COSCO Heavy Transport. Small fleet of ~15 vessels globally. Recent transports include BOKA Vanguard (2018 delivery, 275m LOA, 117,000 DWT). Rare specialised trade.