Duke Energy Gas Transmission (DEGT) told the Texas Railroad Commission (RRC) on Thursday that a series of unusual events, some with unknown causes, led to the out-of-control gas leak and fire at cavern No. 1 at its Moss Bluff salt cavern storage facility in Liberty County, TX, on Aug. 19. The blaze lasted six-and-a-half days until all 6 Bcf of gas in the cavern was burned (see Daily GPI, Aug. 20, Aug. 24, Aug. 27). The other two caverns were undamaged.

Duke told the Railroad Commission, which has jurisdiction over the facility, that prior to the Aug. 19 incident, storage cavern No. 1 was operating in normal “de-brining” mode where brine is withdrawn from the cavern as gas is injected. Brine is brought to the surface through an 8 5/8-inch well string and transported to an above-ground holding pond. Up to the time of the incident, no unusual events occurred in the de-brining process and operator reports indicate that the system was performing normally.

The company said the “initiating event” was a “separation” on the 8 5/8-inch brine disposal pipe at a depth of about 3,724 feet that allowed gas to seep into the pipe, flow into an above ground 8-inch diameter pipe and then trigger an emergency shut down (ESD). “The resulting mechanical forces or ‘water hammer’ produced by the sudden surge of flow caused the 8-inch piping between the wellhead and the ESD valve to breach” causing the fire. The heat of the flame eventually melted the wellhead assembly, which separated allowing gas to escape from the 20-inch diameter production casing.

The reason for the breach of the 8 5/8-inch brine disposal pipe is unknown and the affected materials are not recoverable from the cavern, Duke said. “The evidence indicates that, as recently as 10 days prior to the incident, the 8-5/8-inch well string showed no signs of a separation that would ultimately have resulted in gas entering the above-ground brine piping during de-brining operations.”

Another unexpected factor was the weakness in the 8-inch pipe. Duke found internal corrosion on it despite the fact that it was installed in 2000. “Past experience had not indicated such corrosion as a problem,” Duke told the RRC.

Unlike many gas industry accidents, this one apparently was not caused by human error or third-party damage. “Operating procedures were reviewed and found to be adequate and appropriately followed,” Duke said. “Valve positions were confirmed and found to be correct. A thorough review of operator logs and employee interviews did not reveal evidence that procedural or human error contributed to the event.

“The Moss Bluff facility is manned by trained personnel around the clock, and no unusual circumstances or trespass activities were noticed on the night of the event or during the days leading up to it,” the company said.

Duke still has not determined the cost of the accident. A spokeswoman said the company has normal liability and property insurance. The cost of the lost gas alone would be about $42 million in today’s market. Meanwhile, the other two caverns have been shut in ever since the accident. They are still holding about 8.7 Bcf of gas and storage customers have been unable to use the facilities to store or cycle since Aug. 19. The RRC will decide when the facility can be brought back into operation.

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