Equinor investigates drillfloor gas/fluid ingress during Troll well plugging

Equinor has concluded an investigation of a well control incident that occurred last September on Odfjell Drilling’s semisubmersible Deepsea Bollsta rig.

The event, which Equinor classified as in its highest category of severity, occurred while the rig was being used to plug a well on the Troll field in the Norwegian Sea.

While the team was cutting a 13-3/8-inch casing at a depth of about 510 m, there was a leak of gas and fluid which spread to the drill floor and the shaker room, where rocks and cuttings are removed from the drilling fluid prior to returning the fluid to the well.

One individual had problems attempting to evacuate from the room due to the differential pressure that occurred, and had to use force to get out. This person suffered minor injuries and was given first aid treatment aboard.

In addition, the gas and fluid column from the leak caused damage to the ventilation system in the ceiling of the room.

Rune Nedregaard, Equinor's Senior Vice President for Drilling and Well, said: “There were considerable forces at play and gas came aboard…At the same time, all safety barriers functioned as intended and the crew handled the situation well, thus preventing any escalation.”

The incident caused the automatic gas detection to trigger the rig’s safety systems, with potential ignition sources disconnected. The crew followed procedures by activating the rig’s BOP and diverter system, which routed gas, fluid and pressure from the well away from the rig before the BOP closed after 71 seconds, halting the flow of gas.

Within 30 minutes, the situation was normalized.

Equinor has since initiated various measures, including a new requirement (immediately implemented) to close the BOP in the event of shallow cuts, and pulling the casing, regardless of the activation time.

The investigation revealed that around 930 kg of gas leaked during a short period, representing a red 1 incident, the most serious category in the company’s management system.

Although combustible gas was briefly on the drill floor and in the shaker room, the potential ignition sources were disconnected: circumstances would have had to be different for the gas to ignite, the company added.

What caused the incident was the BOP being in the `open’ position when the casing was cut, with confined gas present behind the casing.

Although the annulus behind the casing was logged prior to cutting, the equipment was not calibrated correctly, so the gas was not identified in advance.

Equinor also determined that the incident would not have escalated into an uncontrolled blowout. The gas, from a limited volume confined behind the casing, was not in contact with the reservoir and the barriers against the reservoir were intact.

The investigation report will form the basis for lessons learned from the incident, Nedregaard said. Equinor will share the findings and implemented measures with its suppliers and will also act on any recommendations from the Norwegian Ocean Industry Authority's own investigation.