It is thought one of the four legs on the huge JB104 barge, resting on the floor of the Haven, suddenly gave way, tipping it over at a sharp 45 ° angle.
The jack up rig, a self-contained combination drilling rig and floating barge, fitted with long support legs that can be raised or lowered independently of each other, was being used to support a crane at the time of collapse.
The terminal is part of a £6bn project to import LNG from Qatar to Milford Haven and then through South Wales to a distribution centre at Gloucester. It will eventually supply 20% of the UK’s energy needs.
Salvage experts within the Haven arrived at the scene on Monday to assess a recovery operation which involves the safe retrieval of a 2,000 litre diesel tank which has fallen below the water line.