NTSB: IMPROVE SAFETY IN UNDERGROUND STORAGE

The National Transportation Safety Board has recommended greater safety controls for U.S. underground storage sites that hold natural gas or highly volatile liquids.

NTSB said federal and state regulations governing the underground storage facilities are directed toward environmental hazards, and "do not require an adequate level of safety for the public and employees."

The recommendation, which will be forwarded to federal regulatory agencies, came after NTSB's investigation of an Apr. 7, 1992, accident at Mapco Natural Gas Liquids Inc.'s LPG storage site at Brenham, Tex. (OGJ, Apr. 13, 1992, p. 35.).

The site is on the Seminole Pipeline Co. system.

WHAT HAPPENED

NTSB said the accident probably occurred because Mapco overfilled the Brenham salt dome cavern, and the station's wellhead safety system lacked fail safe features to prevent a release of LPG.

"Contributing to the accident was the lack of federal and state regulations governing design and operation of underground storage systems. Contributing to the severity of the accident was the company's inadequate emergency response procedures."

NTSB said the operator overfilled the storage cavern, and LPG escaped through the brine injection system. The LPG expanded into a heavier than air vapor cloud that covered the station and migrated to surrounding properties. The "mushroom shaped" vapor cloud apparently was ignited by an auto driving near the station, and the explosion produced surface shock wave forces of 3.5-4.0 on the Richter scale.

Three persons died from injuries suffered either from the blast or an ensuing fire, and 21 more were treated for injuries. More than 60 buildings within 3 sq mi of the station were damaged, and many were declared a loss.

NTSB said the LPG escaped because Brenham station's wellhead safety system was not equipped with fail safe features, and it was rendered inoperative because one or two brine sensing lines were closed.

'OPPORTUNITIES FOR FAILURE'

An NTSB investigator told the board the piping and leak detection system at the storage cavern was "a system filled with opportunities for failure."

The agency found a closed valve in the system that could have caused the accident but noted "failure of any piece of the equipment at the time of the overflow would have led to the same accident."

An NTSB investigator said Mapco thought the cavern contained 288,000 bbl, but in fact it held more than 320,000 bbl.

The agency's report said, "Mapco was not aware of the volume of product stored in the cavern because it lacked capability and procedures to balance the cavern storage against station receipts and deliveries, because its procedures and oversight of employee measurement activities were insufficient, and because its measurement procedures did not adequately compensate for the varying specific gravity of the Y-grade product."

It said, "Because the magnitude of the large quantity of highly volatile liquids released at Brenham station remained effectively undetected for an appreciable time period, there was insufficient time to evacuate endangered residents."

The report said Mapco's telemetry system monitor did not display the data from Brenham station, unoccupied at the time, in a format that allowed ready interpretation by dispatchers in Tulsa.

EMERGENCY RESPONSE

NTSB said, "The lack of effective communications among Mapco employees during their response to the emergency increased the risk to area residents and to themselves and resulted in poor emergency response coordination.

"Mapco was not adequately aware of its employees' knowledge of operating and emergency procedures because most of company training did not include formal testing or other methods, such as exercises or drills, that required employees to demonstrate their abilities to perform their duties.

"Mapco's emergency response training and procedures, which are primarily designed for small releases that allow personnel time to investigate the circumstances surrounding a release, were inadequate in this accident.

"Adequate planning between Mapco and Washington County would have improved coordination and initial response actions, including notification of public emergency response agencies and securing the immediate site area, and would have better prepared the responders and the public for the possibility of a large volatile liquids release at Brenham station."

NTSB said not all Mapco employees involved were tested for drugs afterward so it could not rule out the possibility of drug impairment, although there were no indications of that.

"Moreover, although federal regulations were not violated, samples were collected 31 hr after the accident, when results were no longer reliable."

Copyright 1993 Oil & Gas Journal. All Rights Reserved.