Equinor has completed its investigation into a well control incident on Odfjell Drilling's Deepsea Bollsta drilling rig in September 2025, classifying it in the company’s highest category of severity.
The incident occurred during plugging operations on the Troll field while a 13-3/8-inch casing was being cut at a depth of about 510 meters.
Gas and fluid leaked onto the drill floor and into the shaker room, where rocks and cuttings are removed from drilling fluid before it is returned to the well.
One person experienced difficulty evacuating due to differential pressure and sustained minor injuries after using force to exit the room, receiving first aid treatment onboard. The gas and fluid column also damaged the ventilation system in the ceiling of the shaker room.
"We're taking the well control incident that led to a gas emission on Deepsea Bollsta very seriously. There were considerable forces at play and gas came aboard. This was a dramatic event for those who were at work. At the same time, all safety barriers functioned as intended and the crew handled the situation well, thus preventing any escalation," said Rune Nedregaard, Equinor's Senior Vice President for Drilling and Well.
Automatic gas detection triggered the rig’s safety systems and disconnected potential ignition sources. The crew activated the blow-out preventer (BOP) and diverter system in line with procedures, with the diverter routing gas, fluid and pressure away from the rig before the BOP closed.
The BOP closed after 71 seconds, stopping the gas flow, and the situation was normalized within 30 minutes. Equinor said it has initiated multiple measures following the incident, including a new requirement to close the BOP during shallow cuts and casing pulling, regardless of activation time.
Investigation calculations showed that about 930 kilograms of gas leaked in a short period. Based on the size, the event was classified as a red 1 incident, the most serious category in Equinor’s management system.
Combustible gas was present briefly on the drill floor and in the shaker room, but ignition sources were disconnected. The investigation found that circumstances would have had to be different for the gas to ignite.
The cause was identified as the BOP being in the open position when the casing was cut while confined gas was present behind the casing. Although the annulus behind the casing was logged before cutting, the equipment was not calibrated correctly, meaning the gas was not identified in advance.
The incident did not have the potential for an uncontrolled blowout, as the gas came from a limited volume confined behind the casing and was not in contact with the reservoir. Barriers against the reservoir remained intact.
"We've received a thorough investigation report that will form the basis for lessons learned. Among other things, the investigation points to technical factors that can prevent such incidents. Several measures were initiated immediately following the incident.
“Equinor will share the investigation findings and implemented measures with our suppliers. The Norwegian Ocean Industry Authority's investigation will also be important for Equinor's follow-up moving forward," added Nedregaard.
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