Equinor has released a summary of its investigation into a well control incident classified as the company’s highest severity category.
The event occurred on 23 September 2025 aboard the Deepsea Bollsta offshore drilling rig during a plugging operation on a well in the Troll field, which lies in the Norwegian side of the North Sea. The incident took place while a section of casing was being cut at a depth of approximately 510 m.
Equinor reported that gas and fluid reached the drill floor and the mud shaker room. One offshore worker was unable to immediately evacuate the shaker room because of differential pressure but sustained only minor injuries after using force to exit. The individual was treated onboard. The gas and fluid column also damaged the room’s ventilation system.
“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 senior vice president for drilling and well, in a statement issued 3 March.
According to the report summary, automatic gas detection triggered the rig’s safety systems and potential ignition sources were disconnected. The crew activated the blowout preventer (BOP) and diverter system in accordance with procedures. The diverter routed gas, fluid, and pressure away from the rig before the BOP closed.
Equinor said the BOP closed within 71 seconds, stopping gas flow to the surface. The situation was stabilized within 30 minutes.
Following the incident, the company implemented several measures, including a requirement to close the BOP during shallow cuts and casing pulling operations regardless of anticipated activation time.
The investigation estimated that approximately 930 kg of gas was released over a short period. Based on the volume, the event was classified as a “Red 1 Incident,” Equinor’s most serious safety category.
“There was combustible gas on the drill floor and in the shaker room for a brief period of time, but potential ignition sources were disconnected. The investigation has determined that the circumstances would have to be different for the gas to ignite,” the report summary stated.
Equinor determined that the BOP was in the open position while the casing was cut, leading to the well control event. A confined volume of gas was present behind the casing. Although the annulus was logged before cutting, the gas detection equipment was not correctly calibrated, and the gas was not identified in advance.
Despite the event’s severity classification, Equinor stated that it did not present the potential for an uncontrolled blowout because the gas volume behind the casing was limited and not in contact with the reservoir. Barriers isolating the reservoir remained intact throughout the incident.
“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,” Nedregaard added.
Odfjell Drilling acquired the semisubmersible Deepsea Bollsta from Northern Ocean in December 2025 and plans to rename the rig Deepsea Bergen. The sixth-generation harsh-environment rig has been operated by Odfjell since 2022.
