The U.S. Chemical Safety Board released its final investigation report into a January 2018 blowout that fatally injured five workers at the Pryor Trust gas well in Pittsburg County, Oklahoma.

The report identifies a lack of regulations governing onshore drilling safety as well as shortcomings in safety management systems and industry standards as contributing to the accident. The report calls on regulators, industry groups, the state of Oklahoma and companies to address such gaps.

CSB investigators determined that the cause of the blowout and rig fire was the failure of two preventive barriers that were intended to be in place to stop a blowout. The primary was hydrostatic pressure in the well produced by drilling mud, and the secondary barrier was human detection of gas flowing into or expanding in the well and activation of the rig’s blowout preventer (BOP). The presence of these preventative barriers is considered an industry best practice.

The report said that unplanned, underbalanced drilling and tripping operations allowed a large quantity of gas to enter the well. Safety-critical operations called “flow checks,” used to determine if gas is in the well, were not performed.

CSB Interim Executive Kristen Kulinowski said, “Our investigation found significant lapses in good safety practices at this site. For over 14 hours, there was a dangerous condition building at this well. The lack of effective safety management at this well resulted in a needless catastrophe.”

Contributing factors

The final report outlines several factors contributing to the loss of barriers, including a lack of planning, training, equipment, skills and procedures. Because oil and gas well drilling is exempted from the OSHA’s Process Safety Management standard, which governs safety for chemical processing facilities, OSHA has been utilizing the general duty clause, which “protects workers from serious and recognized workplace hazards,” but does not address safety hazards associated with drilling for oil and gas. As part of its report, the CSB urged OSHA to develop "effective oversight that addresses the hazards unique to the onshore drilling industry."

The blowout-caused fire lasted for several hours after the initial explosion. Credit: CSBThe blowout-caused fire lasted for several hours after the initial explosion. Credit: CSBThe CSB also found that the drilling contractor failed to maintain an effective alarm system. Likely due to excessive “nuisance” or unnecessary alarms, the entire alarm system was disabled by rig personnel, the report said. Ultimately, the lack of critical alarms contributed to workers' lack of awareness that flammable gas was entering the well during operations before the incident.

At the time of the blowout, three workers were in the driller’s cabin. Two other workers who were on the rig floor ran into the driller’s cabin during the blowout and fire. All five of these workers were killed.

Accident timeline

On Jan. 22, 2018, a blowout and rig fire occurred at Pryor Trust 0718 gas well number 1H-9, located in Pittsburg County. The fire killed five workers who were inside the driller’s cabin on the rig floor. They died from thermal burn injuries and smoke and soot inhalation. The blowout occurred about three-and-a-half hours after removing drill pipe (“tripping”) out of the well.

A day earlier, on January 21 at 6:30 a.m., while drilling at about 13,000 ft in the horizontal section of the well, gas began entering the wellbore. At 7:30 a.m., gas started to reach the surface of the well, apparently showing as elevated gas units in the mud. The gas in the mud caused mud to spit out and cover a portion of the rig, the mud pits and other equipment.

After 11:00 a.m., the crew aligned the mud piping so that the mud was routed to the mud gas separator, and by 11:11 a.m. the flare activated. Witnesses reported the flame from the flare at times was 20 to 30 ft high and at one time may have been 40 to 50 ft high.

At 8:35 a.m. the next morning, the floorhand was standing over the open well hole on the rig floor when he again saw mud flowing out of the open rotating head bowl. He communicated that the mud from the wellbore was shooting up steadily, getting closer to the rig floor.

Around one minute later, mud started spraying out of the well and covering the windows of the driller’s cabin. Quickly after the mud started spraying out of the well, the diesel oil-based mud and gas escaping the well ignited.

The floorhand and the rig manager made their way to the stairs on the rig floor opposite the driller’s cabin, where they were able to escape to the ground. The derrickhand was high in the mast at the time of the blowout and tried to get down by hooking himself onto the slow descent line (an escape line like a “zip line”). When that proved unsuccessful, he threw his legs around the descent cable and shimmied down the cable toward the ground. He fell off the slow descent line near its bottom from a height of about 10 to 12 ft.

The floorhand observed fire reaching 20 to 30 ft above the rig crown. The rig manager called his superintendent to alert him to the blowout. The rig manager then told the floorhand to wake up everyone in the crew house and tell them to move to a safe location.

Accident cause

The CSB said the cause of the blowout and rig fire was the failure of both the primary and secondary barrier, which were intended to be in place to prevent a blowout. Contributing to the loss of barriers were many factors, including:

  • Underbalanced drilling was performed without needed planning, equipment, skills or procedures, thus nullifying the planned primary barrier to prevent gas influx.
  • Tripping was performed out of the underbalanced well, which allowed a large amount of gas to enter the well.
  • The driller was not effectively trained in using a new electronic trip sheet, which is used to help monitor for gas influx.Wrecked equipment after the fire was extinguished. Credit: CSBWrecked equipment after the fire was extinguished. Credit: CSB
  • Equipment was aligned differently than normal during the tripping operation, leading to confusion in interpreting the well data, which caused rig workers to miss indications of the gas influx.
  • Surface pressure was not identified two separate times before opening the BOP during operations before the blowout, when there was evidently pressure at the surface of the well. This non-identification of surface pressure contributed to the gas influx not being identified.
  • A weighted pill intended to overbalance the well was apparently miscalculated. After pill placement, the well was still underbalanced.
  • Both the day and night driller chose to turn off the entire alarm system, contributing to both drillers missing critical indications of the gas influx and imminent blowout. The alarm system also was not effectively designed to alert personnel to hazardous conditions during different operating states (e.g., drilling, tripping, circulating and surface operations) and would have sounded excessive, non-critical alarms during the 14 hours leading to the blowout, which likely led to the drillers choosing to turn off the alarm system.
  • Key flow checks to determine if the well was flowing were not performed before the incident. Drilling rig workers performed few of the company-required flow checks during the drilling of well 1H-9 and the previous well. The drilling contractor did not effectively monitor the implementation rate of its flow check policy.
  • The drilling contractor did not test its drillers’ abilities in detecting indications of gas influx through, for example, simulated pit gains. The absence of testing drillers’ influx detection skills — a safety-critical aspect of well control — might have contributed to both drillers not detecting the significant gas influx leading to the blowout.
  • The operating company did not specify the barriers required during operations, or how to respond if a barrier was lost. This contributed to the performance of underbalanced operations that the drilling rig and its crew were not equipped or trained to perform.
  • The safety management system in place was not effective for managing safe rig operations. There is also no drilling-specific regulatory standard governing onshore drilling safety.

Investigators said that when the blowout mud and gas ignited, it created a fire on the rig floor. All five of the workers inside the driller’s cabin were trapped because fire blocked the two exit doors. The investigation found that no guidance exists to ensure that an emergency evacuation option is present on board similar rigs or can protect workers in the driller’s cabin from fire hazards.

The CSB called on the American Petroleum Institute to address design improvements needed to protect driller’s cabin occupants from blowout and fire hazards. The report also recommends to API to create guidance on alarm management for the drilling industry, to help ensure alarm systems are effective in alerting drilling crews to unsafe conditions.