The cutting of a casing on the Troll field on 23 September 2025 resulted in an uncontrolled flow of gas and fluid onto the drill floor. The incident had major accident potential. We have produced a video presenting the sequence of events and the key learning points from the investigation.
Published:
18 May 2026
Well integrity
Barrier management
In the video above, we review the sequence of events and the key learning points from the investigation.
The incident occurred on the drilling unit Deepsea Bollsta (now Deepsea Bergen) during work to permanently plug the Q21 well. Shortly after the casing was cut at a depth of 510 metres, gas and drilling fluid flowed up through the casing, via the rotary table and onto the drill floor. The flow continued into the shaker room, where one person sustained minor injuries.
Several gas detectors were activated, triggering a general alarm, emergency shutdown and automatic isolation of ignition sources. In total, approximately 930 kg of gas were released. With only minor changes to the course of events, the incident could have developed into a major accident.
What happened?
Prior to the cutting operation, logging was carried out to verify cement quality and to assess whether gas was present in the annulus behind the casing. The logging tool was incorrectly calibrated, and the measurements were interpreted as indicating that no gas was present.
On this basis, it was decided to carry out the cutting operation without closing the annulus valve on the BOP.
When the casing was cut, gas that had been trapped behind the casing flowed rapidly up to the drill floor.
Learning points
Learning from incidents is an important part of continuous improvement, particularly in high‑risk industries such as the petroleum industry.
This incident provides several important learning points related to well control and barrier management:
Barriers must be assessed based on their actual function in the given situation, not on assumed function or interpreted data alone.
Uncertainty requires conservative operational choices, particularly in situations with short response time, such as operations close to surface.
In shallow cutting operations, the need to close the BOP must be carefully considered. In this case, the gas reached the drill floor faster than it took to close the valve manually.
Critical measurements must be quality assured. Miscalibration of the logging tool had a direct impact on the decision to proceed with the operation.
The integrity of cement barriers may change over time. Gas migration can challenge established barriers and must be taken into account in assessments.
