Statoil completes investigations of serious incidentsStaff Writer | January 23, 2017
On Saturday, October 15, a serious well control incident occurred during well plugging by the mobile rig Songa Endurance at the Troll field.
Oil exploration The Mongstad processing complex
Nobody was injured in the incidents. Statoil’s internal investigation unit has studied the incidents thoroughly, in addition to the Petroleum Safety Authority Norway.
The well control incident led to a gas leak that pushed seawater more than 30 metres up the derrick, before the well was closed by the annular preventer inside the blowout preventer (BOP) about one minute later.
Statoil’s internal investigation defines the incident to have a high degree of seriousness, and concludes that at worst it could have led to loss of life if the safety equipment had failed to function as intended, or if the gas had been ignited.
The BOP was quickly activated and stopped the gas leak, and five gas detectors automatically turned off equipment that could have produced sparks.
The investigation report concludes that two main findings have weakened the barriers and helped gas reach the drill floor.
The first finding revealed that existing downhole valves were used as barriers against the reservoir and were unintentionally opened.
The other main finding is related to the annular preventer inside the BOP that should have been closed before the operation was started, because it was not possible to measure the pressure below the wellhead sealing.
After the incident some immediate actions were taken to ensure that the downhole valves are not used as barriers, and a deeply set plug was reintroduced as a barrier during use of vertical Christmas trees.
During pipe inspection in connection with surface maintenance in the isomerization plant at Mongstad, a portable gas detector was triggered close to a valve. When an attempt was made to close the valve, the pipe socket broke and high-pressure hydrogen-rich gas was released.
Two people were in the vicinity when the incident occurred. The evacuation alarm was activated immediately, followed by evacuation of employees, shutdown of the facility involved and pressure relief to flare. After about one hour the situation was clarified.
The investigation points to external corrosion as the triggering cause. Wrong prioritisation of maintenance as a result of insufficient risk understanding was identified as root causes. The report concludes that the incident was serious and at worst it could have led to loss of life.
It has been decided to intensify the programme surface maintenance over the next two years.
Safety management will furthermore be improved by increased management presence at the plant.
New detection technology will be considered to improve our ability to detect leaks. ■