Thunder Horse, the world's largest semi-submersible platform, tilted by more than 20% two weeks ago after Hurricane Dennis passed through the Gulf of Mexico. The staff had been evacuated because of the coming storm and at first it was thought the hurricane was responsible.

But subsequent examinations of the platform found that Dennis was probably not to blame.

BP has since righted the structure. But it was unable to say if there is damage to the platform or what had caused it to tilt.

Thunder Horse is expected to be a significant contributor to North American output when it begins producing. The platform originally was supposed to hit first oil in late 2005.

The giant platform is designed to process up to about 250,000 barrels a day of oil and about 200 million cubic feet a day of gas.

Thunder Horse partners are BP (operator and 75%) and ExxonMobil (25%).