* Beams supporting derrick, rig floor collapse overstructure
* Third firefighting vessel en route to scene
* Govt orders well owner to prepare for drilling relief well
By Kristen Hays
HOUSTON, July 24 (Reuters) - A shallow-water Gulf of Mexicodrilling rig has partially collapsed off the coast of Louisianaafter catching fire because of a ruptured natural gas well, U.S.regulators said on Wednesday.
The U.S. Bureau of Safety and Environmental Enforcement said beams supporting the derrick and rig floor on the HerculesOffshore jackup rig had crumpled over the rigstructure.
A third firefighting vessel was en route to the scene,though no sheen was seen on the water's surface duringoverflights conducted on Wednesday morning, the regulator said.
The fire ignited shortly before 11 p.m. CDT on Tuesday (0400GMT). The Walter Oil & Gas-owned well had ruptured hours earlieras Hercules worked to prepare it for production. The wellreleased natural gas, but no oil, according to BSEE.
No one was on board when the rig caught fire, BSEE said. Hercules said 44 people were evacuated after the rupture and noinjuries were reported. The rig is in 154 feet (47 m) of waterabout 55 miles (88.5 km) south of the coast of Louisiana.
Hercules said earlier on Wednesday that the company wasworking to plug the well.
It might drill a relief well, which would intersect theruptured well and provide another avenue for cement to plug it,Hercules said.
BSEE said it directed privately held Walter Oil & Gas toprepare to move a second jackup rig to the scene, possibly todrill the relief well.
Hercules shares have slipped since the accident, erasingabout $83 million from the company's market capitalization.
BSEE has tightened safety regulations for offshore oil andgas operations since the BP Plc 2010 deepwater blowoutand oil spill that spewed more than 4 million barrels of crudeinto the Gulf. It took BP nearly three months to cap thatruptured well, which was ultimately killed and plugged by way ofa relief well.
A jackup rig has legs that can be extended to move the hullabove the surface of the water. Unlike floating rigs in deeperwaters, the legs on jackups reach the sea floor.
Analysts at CapitalOne Southcoast, an investment bank in NewOrleans, noted that the jackup market remains very tight, so "ifthe rig suffers major damage, this should have a positive impacton day rates." They also noted that Hercules has rigs in storagethat could be used to replace the damaged one.
Analysts at Cowen and Company, another investment bank, saidthe fire-damaged rig contributes about 4 percent to thecompany's earnings, and could be a total loss. Hercules has 18of 35 jackups currently active in the Gulf, the analysts said.