Well integrity/control

Report Recounts the Missed Signals Leading to a Blowout that Killed Five

Regulators say the blowout that killed five workers on a Patterson-UTI rig in Oklahoma was the product of a slow-moving series of missed signals, misleading testing, and miscalculations that failed to control a natural gas influx.

The aftermath of the Pryor Trust gas well blowout and fire in Pittsburg County, Okla. Source: US Chemical Safety and Hazard Investigation Board

The missed signals leading up to a blowout with the most fatalities since Macondo resembles that offshore disaster.

The investigations by the US Chemical Safety and Hazard Investigation Board (CSB) of the blowout on the Pryor Trust 1-H 9 well and Macondo concluded that both were the result of a failure to maintain the barriers needed to prevent an influx of gas into a well.

In both cases, the board found a series of missed signals, misleading tests, and miscalculations that allowed gas to build up in a well unnoticed, leading to a blowout that could not be stopped by a blowout preventer (BOP). Workers died in both cases—11 at Macondo and five at Pryor Trust.

An obvious difference is in the level of notoriety. Macondo is still remembered for the destruction of the Deepwater Horizon and an offshore oil spill that lasted for months.

The Pryor Trust well was an onshore gas well in rural Oklahoma where a blowout destroyed a rig in the morning and the fire was put out that afternoon. There was little media coverage. The environmental impact was minimal, and the companies involved, Patterson-UTI Drilling and Red Mountain Energy, are not well known.

Still, it provided a stark warning of the risks associated with some widely use drilling practices.

“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,” said Kristen Kulinowski, interim executive at the CSB, a federal agency that investigates major accidents, in a statement on the report.

“When the blowout mud and gas ignited, it created a massive fire on the rig floor. All five of the workers inside the driller’s cabin were effectively trapped because fire blocked the driller’s cabin’s two exit doors,” said Lauren Grim, a CSB investigator who noted there is no guidance to ensure that an emergency evacuation option is present.

“The investigation revealed that there are no regulations specifically developed for onshore oil and gas well drilling,” the statement said.

It explained that while the Occupational Safety and Health Administration (OSHA) does monitor oil and gas operations under a clause that “protects workers from serious and recognized workplace hazards,” onshore exploration and production (E&P) is exempt from the OSHA process safety management standard. Offshore oil and gas process safety is regulated by the US Bureau of Safety, Environment, and Enforcement. The report said onshore E&P operations should be subject to OSHA’s process safety standard to “addresses the hazards unique to the onshore drilling industry.” The CSB asked the American Petroleum Institute (API) to provide recommendations to fill these gaps.

“As onshore oil and gas extraction grows, it is imperative that the industry is using proven and reliable safety standards and practices. If some of these safety practices had been in place, this tragedy could have been averted,” Kulinowski said.

API said it is reviewing the report. Erik Milito, API’s vice president of upstream and industry, said, “We will review the report and consider its recommendations. Our thoughts are with the loved ones of the workers who lost their lives in this incident. When it comes to safety, API has developed dozens of standards under the American National Standards Institute’s accredited process that further safety in operations.”

A statement released by Patterson-UTI said the company cooperated with the investigation and offered its sympathies to those who died. It said that while it “does not agree with all of the finding in the report,” it is evaluating policies, procedures, and training that address issues raised by it. The company could not be reached for further comment.

Koray Bakir, chief financial officer of Red Mountain Energy, declined to comment because of pending litigation.

Failed Barriers

The full CSB report provided an hour-by-hour account leading up to the blowout, which began on Sunday, 21 January, 2018.

A tall flare was evidence that drilling was in progress. The Patterson-UTI rig was equipped to safely manage gas produced during drilling, using a rotating control device and an orbit valve to divert mud with gas to a mud/gas separator and on to a flare that ranged from 20 ft high, to as high as 50 ft, the report said.

Low-mud-weight fluid can allow faster drilling and limit formation damage. But after drilling stopped at 3:30 p.m., the rig crew needed to add enough mud weight to stop the gas influx. The calculation of how much mud-weighted fluid to add was complicated by the fact that they were tripping more than 13,000 ft of drillpipe out of the hole.

The report said two Red Mountain drilling supervisors (“company men”) decided to increase the mud weight in installments as the pipe was tripped, using the calculated fill method. The calculated fill method was described in the report as “stopping all mud flow from the well by closing the flow line isolation valve with the pressure-containing rotating head installed. A calculated volume of mud was periodically pumped into the well with the intent to replace the calculated volume of the drillpipe removed by turning on and off the trip tank pumps.”

While the drilling engineer said that method was safer, the report said calculated fill tripping was a first for the night crew.

Despite the mud added, a series of charts with estimates prepared by the CSB showed steadily rising fluid level in the trip tank. Near the end of tripping, fluid flowing into the trip tank was twice the volume of the mud added, which the report said “should have been a key indication gas was in the well.”

Standards and Rules

The report from the US Chemical Safety and Hazard Investigation Board called for expanded regulation of the unique risks posed by drilling. Following the lead of offshore regulations, it is seeking guidance in standards from the American Petroleum Institute (API). The board asked API to expand its recommended practices for onshore drilling to promote:

  • Design improvements to protect those in the driller’s cabin from blowout and fire hazards.

  • Guidance on safely tripping drill pipe during a variety of well operations, such as underbalanced drilling, covering the equipment required, controlling fluid influxes, and accurately accounting for displacement.

  • Alarm management for the drilling industry based on guidance from a standard used in process industries (ANSI/ISA 18.2).

  • A widely used method to bring a well to a safe state using an automatic instrumented system. This would include the BOP.

Accounting Shortcomings

The report said the crew did not react to the fluid displaced by the gas influx in the well.

“Both the day and night tour drillers missed or did not respond to pit gains leading to the incident. Patterson did not regularly test whether drillers would quickly identify and respond to simulated pit gains,” the report said.

The report said an added complication was “the driller was not effectively trained in using a new electronic trip sheet” to calculate the mud needed for well control.

Rather than using the software to do the calculation, the report said the overnight driller made “a mental note” of stands of pipe pulled and determined the mud volume added per stand based on “what he recalled the calculated volume would be.” Based on the CSB calculations, the result was significantly short of the amount needed to stop the influx.

Flow testing, which is supposed to directly measure pressure buildup in the well due to a gas influx, failed to do so. The CSB report said the high-pressure gauge used may have been unable to detect a low-pressure change, or failed to display it clearly.

Flow test data was limited. On that well and a well nearby, the crew performed “only two out of a required 27 flow checks” required by the drilling contractor’s policy, the report said.

Rising fluid levels in the tanks did not trigger alerts because the alarm system was off. The CSB report explained that a high number of false alarms “likely led to the drillers choosing to turn off the alarm system.”

Alerts were sent when the volume of fluid changed by a set amount—for Patterson-UTI the set point was ±5 bbl. Routine activities while drilling, such as tripping pipe, could regularly exceed that limit, triggering a false alarm.

“With the alarm system off, the safety of the operation solely relied on workers to either visually identify signs of the gas influx or calculate volume differences that could indicate gas influx—and in this case, neither method was effective, and workers were unaware of the very large gas influx into the well before the incident,” Grim said.

The bottomhole assembly (BHA) was finally pulled out of the well at 6:08 a.m., and the BOP’s blind rams were closed a couple minutes later to seal it. Then the driller “transferred mud from the nearly full trip tank to the mud pits. The mud pits were nearly full as well.

At 7:57 a.m., the rams were opened and the bottomhole assembly was lowered into the well for testing. Although an annular test had just shown no pressure, the report said that over the next 38 minutes, 107 bbl of mud flowed into the tanks. Rising fluid levels “would have displayed as a number on a monitor” in front of the driller, the report said.

A worker on the drilling floor told the CSB he observed mud bubbling, which the report said was “evidence of gas in the mud.” It said the worker did not know if the driller was told.

At 8:35 am, the BHA was removed from the well after testing so they could attach the drill bit. A floor hand told the driller he saw mud flowing out of the open rotating head bowl and then saw that “mud was shooting up steadily, getting closer to the rig floor,” the report said.

The driller announced he was going to close the blind rams and was seen walking to the BOP controls 5 ft from his chair. The floor hand told the CSB that he does not believe the driller ever made it to the BOP controls.

Mud was spraying from the well and the floor hand saw the motor hand and a floor hand run into the driller’s cabin joining the driller, company man, and directional driller.

The mix of gas and diesel-oil-based drilling fluid from the well ignited and the report said the “fire was massive and intense, engulfing much of the rig floor and the driller’s cabin. It is unknown how long the victims remained alive inside the driller’s cabin, but they had no viable escape routes.”

The floor hand and the rig manager escaped down the stairs to the ground. A derrick hand in the mast “threw his legs around the descent cable and shimmied down the cable toward the ground.”

The rig manager and the company man from the previous shift tried to activate the BOP at the accumulator, which provides the high-pressure stream needed to power the device. The report said they each pulled a series of control handles, but were unable to activate any part of the BOP.

“The CSB determined the BOP did not seal the well likely because the control hoses that supplied hydraulic fluid to the BOP to function the rams had burned and leaked the hydraulic control fluid,” the report said.

The fire was put out at 4 p.m. when Boot & Coots and Red Mountain representatives manually closed the blind rams. The report said that “one side of the blind rams was fully closed, and the other side was halfway closed.”