The incident occurred in connection with plugging a well on the Troll field, while cutting a 13-3/8" casing at a depth of about 510 metres. A leak of gas and fluid spread to the drill floor and the shaker room , where rocks and cuttings are removed from the drilling fluid before the fluid is returned to the well.
One person had trouble evacuating from the room due to the differential pressure that occurred. This person suffered minor injuries and received first aid treatment aboard after using force to get out. The gas and fluid column from the leak also damaged the ventilation system in the ceiling of the 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," says Rune Nedregaard, Equinor's Senior Vice President for Drilling and Well.
Automatic gas detection triggered the rig's safety systems and potential ignition sources were disconnected. The crew on Deepsea Bollsta activated the rig's blow-out preventer (BOP) and diverter system in line with relevant procedures. The diverter system routed gas, fluid and pressure from the well away from the rig before the blow-out preventer closed.
The blow-out preventer closed after 71 seconds, thus stopping the flow of gas. The situation was normalised in 30 minutes. Equinor has initiated multiple measures following the incident. A new requirement was immediately implemented to close the BOP in the event of shallow cuts and pulling the casing, regardless of activation time.
The investigation's calculations show that about 930 kilos of gas leaked out in a short period of time. Based on the size, this is classified as a red 1 incident, the most serious category in Equinor's management system. 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 cause of the incident was that the BOP was in the open position when the casing was cut, while at the same time, confined gas was present behind the casing. The annulus behind the casing was logged before the cutting, but the equipment was not calibrated correctly. This meant that the gas was not identified in advance. The incident did not have the potential for an uncontrolled blowout. The gas was from a limited volume confined behind the casing, and was not in contact with the reservoir. The barriers against the reservoir were 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," Nedregaard says.
