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On Friday 1 September 1983, the jack-up rig Key Biscayne was under tow off Lancelin, Western Australia, by two offshore supply vessels, Lady Sonia and Atlas Van Dieman, bound for Cockburn Sound from a drilling location off Darwin. Gale force winds were causing rough seas and a heavy swell.

At 0644, the tow line to Lady Sonia parted and attempts to pass a new tow line were unsuccessful.

During the day, Key Biscayne settled by the stern and developed a starboard list, so that seas started breaking over the deck. Therefore, all 52 persons on board the rig were evacuated by helicopter, this being accomplished by 1620.

At about 1840, the tow line to Atlas Van Dieman also parted, shortly after which Key Biscayne sank in 41 m of water, in a position 10 miles off Ledge Point.


The absence of "insurance" towing equipment and a marine crew aboard Key Biscayne resulted in the failure to connect a new tow line to Lady Sonia.

The break in the tow line resulted in the loss of directional control over the rig, allowing the rig to wallow in the heavy seas and swell.

Sea water entered the after section of the rig by way of the shale shaker return line, flooding the mud pits and pump room, and later other compartments as the rig settled lower in the water, causing a loss of stability and eventual capsize.

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