OVERFILLING OF CAVERN BLAMED FOR LPG BLASTS

Three explosions and a fire Apr. 7 at an LPG salt dome storage cavern near Brenham, Tex., were triggered when the cavern was overfilled, the Texas Railroad Commission (TRC) has reported.

A TRC investigation found that LPG escaped to the surface at the Brenham site through brine injection tubing after excessive fill from an LPG line forced the cavern's water level below the brine tubing's bottom. At the surface, LPG was released into a brine storage pit where it turned into a dense, explosive vapor.

At 7:08 a.m., the vapor was ignited by an unknown source. The resulting blast killed three persons and injured 19 and brought operations at the site to a halt (OGJ, Apr. 13, p. 35).

TRC Chairman Lena Guerrero said commission investigation and testing of the cavern and injection well will continue for several months. Results of National Transportation Safety Board laboratory tests on several pieces of equipment from the accident site will not be available for several months.

The Brenham LPG storage site is owned and operated by Seminole Pipeline Co., Tulsa, a majority owned subsidiary of Mapco Inc.

Because of the accident, Guerrero said, TRC is surveying operating, monitoring, warning, and safety equipment at more than 400 underground hydrocarbon storage sites in Texas. The commission also is reviewing state rules regulating underground hydrocarbon storage to determine whether changes are needed.

"We will make sure storage facility operators do the same and make changes in their operations where necessary," Guerrero said.

CHAIN OF EVENTS

Gurrero and Railroad Commissioner Jim Nugent recreated this sequence of events for newsmen at TRC headquarters in Austin:

In the early morning hours of Apr. 7, LPG was pumped into the cavern through an 8 in. Seminole pipeline, displacing brine and forcing it to the surface through a brine injection/extraction line to two storage pits.
5:43 a.m.-The level of brine in the cavern apparently dropped below the bottom of the brine injection/extraction tubing, allowing LPG to enter the tubing and move up to the brine pit, where it was released to atmosphere and turned into a dense vapor.
6:09 a.m.-Storage site sensors detected the vapor and activated an alarm at Seminole control headquarters in Tulsa. Pumping into the cavern was automatically shut down. The dispatcher on duty in Tulsa notified a Seminole technician living near Brenham of the situation.
6:45 a.m.-The Seminole technician arrived at the Brenham storage site, saw the LPG vapor cloud moving east away from the site, ran to a nearby house and warned the residents, then called the Tulsa dispatcher.
6:59 a.m.-A resident of a mobile home next to the site telephoned the emergency number, 911, and reported smelling gas and hearing a hissing sound.
7:00 a.m.-Two more Seminole employees arriving at the storage site for their normal shift also reported seeing the vapor cloud and smelling gas. They and another employee moved into position to stop traffic on a road near the site entrance.
7:08 a.m.-A large blast erupted, followed immediately by two smaller ones.

"This was an extremely unfortunate chain of circumstances which the commission believes can be prevented in the future," Nugent said.

Underground storage facilities can be operated in a safe manner, he said.

NO LEAKS FOUND

No investigator has reported finding evidence of leaks or breaks prior to the accident in any of three pipelines that serve the Brenham site.

At the time of the explosions, product was flowing through a Seminole 14 in., 120,000-130,000 b/d LPG main line. But a valve connecting the pipeline to the storage unit was closed, and Seminole detected no pressure drop in the line before the blasts.

That line was shut down after the explosions, but TRC has allowed it back in service after required damage and pressure checks.

A 6 in. pipeline belonging to Coastline Gas Pipeline Co., a subsidiary of United Texas Transmission Co., was inactive and holding pressure before the explosions. At least one of the blasts damaged an aboveground loop on the line, causing the release of LPG, which burned as it escaped.

Workers isolated the line Apr. 9 to allow the remaining LPG to burn off, and the line was repaired. TRC in mid-June authorized the line to restart after hydrostatic testing was completed and results verified.

Prior to the explosions, Seminole was pumping LPG into the Brenham storage cavern through the 8 in. Bryan lateral, the third pipeline serving the site, from three gas processing plants in the Bryan-College Station, Tex., area. The Bryan lateral can transfer LPG into the Brenham storage site, Seminole's 14 in. main line, or Coastal's 6 in. pipeline.

The Bryan lateral was returned to service May 1 after testing and TRC approval.

TRC commissioners said the storage cavern will remain idle until the commission approves its return to service.