A BP spokesman said the listing could be caused by excess water in ballast tanks due to Hurricane Dennis and not a result of major structural damage, according to a Reuters report. He could not say if the listing would delay inaugural production.

BP had planned to begin production of 250,000 barrels per day of oil and 200 million cubic feet of natural gas from the platform late this year.

The company said workers were monitoring the platform and had determined yesterday that the listing was not getting any worse and there had been no known release of fuel or hazardous substances. Thunder Horse evacuated on Friday ahead of the hurricane.

Built at a cost of $5 billion, the Thunder Horse rig is 50% larger than the current largest semi-submersible rigs and will be capable of producing 250,000 barrels of oil or 200 billion cubic feet of natural gas per day. Peak production is scheduled for 2011.

Designed to tap into deep-water reserves and to run on the gas it taps, Thunder Horse is larger than four aircraft carriers. Its cranes, gangplanks, and derricks rise 130m above the water.

Thunder Horse facilities include a production-drilling-quarters semi-submersible. The semi-submersible is moored by 16 chain, wire and suction pile anchors with a weighing about 82,543 tonnes. The 185 man quarters are built into the hull of the semi-submersible.

Thunder Horse floats 1.8km above a 140 square-kilometre oil field of the same name about 240km south-east of New Orleans.

The field is estimated to contain about 1 billion barrels of oil equivalent, making it the largest discovery in the Gulf of Mexico to date.

Up to 20 subsea wells will be located beneath the semi-submersible, with additional subsea wells connected from locations nearby.

The production-drilling-quarters unit was built in Louisiana. Fabrication of the hull and rig was done in Okpo, South Korea, one of only two yards in the world capable of building the giant rig (the other was in Indonesia).

No vessel was large enough to tow the structure, so BP had to spend $30 million expanding Dockwise Shipping's Blue Marlin, the largest ship it could find.

BP has a 75% stake in the Thunder Horse development; Exxon Mobil holds the balance.