9 October 2002
Cause of fatal pipeline break: Human error
Excavation damage and inadequate inspection were some of the factors that led to a 1999 gasoline pipeline rupture in Washington state that left three dead, the National Transportation Safety Board (NTSB) said.
On 10 June 1999, a 16-inch-diameter steel pipeline owned by Olympic Pipe Line Co. ruptured and released about 237,000 gallons of gasoline into a creek that flowed through Whatcom Falls Park in Bellingham, Wash. About 90 minutes after the rupture, the gasoline ignited and burned approximately 1.5 miles along the creek.
Two 10-year-old boys and an 18-year-old male died as a result of the accident. In addition, eight injuries occurred, while a single-family residence and the city of Bellingham's water treatment plant suffered severe damage.
Prior to the accident, the controller operating the pipeline involved in the accident rerouted product flow from one facility into another, according to the report. As the delivery points switched, pressure in the accident pipeline began to build back upstream. As a consequence of this rising pressure in the pipeline, a block valve at Bayview (about 23 miles downstream from the accident site) closed, completely blocking the flow of product through the pipeline.
That, in turn, caused a pressure increase upstream toward Bellingham, where the pipeline ruptured, the NTSB said. Simultaneous to that event, the supervisory control and data acquisition (SCADA) system controllers used to operate the pipeline became unresponsive, preventing the controller from starting additional pumps needed to alleviate the pressure backup. Also, because of the SCADA system problems, the controller who restarted the pipeline did not promptly recognize the rupture.
The probable cause of accident was damage by an IMCO construction crew while it conducted modifications to a water treatment plant and Olympic Pipe Line's inadequate inspection of work during the construction project, the NTSB determined.
The NTSB said investigators found that had the pipeline not been weakened by external damage, it likely would have been able to withstand the increased pressure that occurred on the day of the rupture, and the accident would not have happened.
In addition, Olympic Pipe Line's inaccurate evaluation of in-line pipeline inspection results led to the company's decision not to excavate and examine the damaged section of pipe, the NTSB's report said.
Other NTSB findings of probable cause include Olympic Pipe Line's failure to test all safety devices associated with the Bayview products facility before activating the facility; its failure to investigate and correct the conditions leading to the repeated unintended closing of the Bayview inlet block valve; and the company's practice of performing other duties on the SCADA system while it was being used to operate the pipeline.
The last element led to the system's becoming nonresponsive at a critical time during pipeline operations, the NTSB said.
As part of its investigation, NTSB investigators said they reviewed the SCADA system that operates and controls the pipeline. It consists of field sensors and actuators, remote terminal units (RTUs), a communications link, and the main SCADA computer. Field sensors and actuators include pumps, valves, temperature monitors, flowmeters, and other devices to measure field data and the signal I/O to those devices. RTUs collect signals from the field hardware and convert them to digital signals for transmission to the control center.
In findings cited in the report, the NTSB noted that if the SCADA system computers had remained responsive to the commands of the Olympic Pipe Line controllers, the controller operating the accident pipeline probably would have been able to initiate actions to prevent the pressure increase that ruptured the pipeline.
The board also noted that had the SCADA database revisions performed prior to the accident been adequately performed and tested, errors resulting from those revisions might have been identified and repaired before having an effect on the pipeline's operation. Overall, the development, implementation, and protection of Olympic Pipe Line's SCADA system was not adequately managed, the report said.
As a result of its investigation, the board made the following recommendations to the Research and Special Programs Administration:
- Issue an advisory bulletin to all pipeline operators who use SCADA systems, advising them to implement an offline workstation that can modify their SCADA system database or perform developmental and testing work independent of their online systems. Advise operators to use the offline system before implementing any modifications to ensure that those modifications are error free and that they create no ancillary problems for controllers responsible for operating the pipeline.
- Develop and issue guidance to pipeline operators on specific testing procedures that can 1) approximate actual operations during the commissioning of a new pumping station or installation of a new relief valve and 2) determine, during annual tests, whether a relief valve is functioning properly.
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