Wednesday, July 13, 2005 10:31:54 PM

BP Plc. is breathing a bit easier Wednesday, two days after its giant Thunder Horse oil rig was found listing precariously in the wake of Hurricane Dennis. While BP said it is too early to say how long it will take to get the $1 billion rig back on an even keel, but reported it at least appears to have stabilized "There has been no worsening of the situation for the last two days" a BP spokesman at BP's Houston office.

An initial survey of the rig's pontoons showed no damage, an indication it probably was not damaged by debris torn adrift by the storm. But how a rig designed to withstand Gulf of Mexico hurricanes came to list so severely remains a mystery. BP said it is still sifting through data collected by sensors onboard to figure out what went wrong.

The rig was evacuated as a precaution ahead of the weekend hurricane A salvage team was put on the platform Tuesday, restarting some of its generators to begin pumping excess seawater out of the huge pontoons that keep it afloat Initial surveillance photos of the crippled rig raised concerns that righting it would push the Thunder Horse project past its scheduled start-up in late 2005. "We have no estimate yet on how long it will take to get it back upright," the BP spokesman said. "It's too early to say what impact of this event will be on project schedule."

There was also speculation among offshore industry engineers that the rig might sink, a fate that has befallen other semi-submersible rigs over the years, most recently Petrobras' huge P-36 production platform that went down off Brazil in 2001 following an onboard explosion BP is operator of the Thunder Horse deepwater development projct with a 75% stake, with Exxon Mobil Corp. holding the remaining 25%. Production from the field is expected to reach about 250,000 barrels of oil and 200 million cubic feet of natural gas by 2007 The giant semi-submersible production platform is the centrepiece of the technologically cutting-edge project.

Semi-submersibles have been used for several decades, primarily for drilling exploration wells as the industry pushes into ever deeper waters in the search for oil and gas. What sets the Thunder Horse platform apart from predecessors is its huge size. Its main work deck is the equivalent of three football fields. The rig supports two drilling derricks and has full accommodations for a crew of 229 that is ferried by helicopter.

Once on location, the giant rig's pontoons are deliberately flooded, partially sinking it to provide a stable drilling platform and production platform while 16 anchors embedded in the seabed 6,000 feet below hold it in place.

The Thunder Horse platform was designed by GVA of Sweden and built at South Korea's Daewoo Shipbuilding and Marine Engineering Okpo yard To keep it all the lights, pumps and ventilation systems running, the platform has a 100 megawatt power station on board, enough electricity to supply a town of 80,000 homes. BP, in its corporate literature, calls the Thunder Horse semi-submersible the "platform that for the next 25 years or more will be the oil and gas production hub for the largest hydrocarbon discovery to date in the Gulf of Mexico."

This story was supplied by MarketWatch.