The Petroleum Safety Authority Norway (PSA) has completed its investigation of an incident with a stuck drill string and blocked BOP on the Deepsea Yantai mobile facility on Gjoa on 20 September 2020.
This incident occurred during production drilling on the North Sea field, where Neptune Energy (Neptune) is the operator. Deepsea Yantai is operated by Odfjell Drilling (Odfjell).
The PSA’s investigation has been directed at both the field operator and the drilling contractor.
Neptune was drilling the 35/9-G-4 H production well in the P1 segment of Gjoa when the incident occurred.
At that time, the well had been drilled to total depth (TD) in the 12 ¼-inch section, a few metres into the top of the reservoir.
The drill string was being pulled out when it became stuck in the formation at a depth of roughly 2 582 metres. Efforts to free the string caused it to part and a 150-metre section of drill pipe fell into the well alongside the remaining lower part of the string.
This left two drill-pipe sections penetrating through the blowout preventer (BOP) and blocking its safety valves.
As a consequence, the function of primary and secondary barriers was unclear for a long period, until the well was secured through cementing 30 days after the incident occurred.
Direct and underlying causes
The direct causes of the incident cover the following conditions:
- the mud weight was lower than the formation’s estimated collapse pressure
- the drill string above the BOP composed of joints with different torques
- the torque on joints was exceeded.
The following underlying causes have been identified:
- experience transfer from earlier wells on Gjoa
- well design
- pressure of time.
- Nonconformities and improvement points
The investigation has identified four breaches of the regulations. These are:
- insufficient use of change management (Neptune)
- lack of robustness in well planning (Neptune)
- inadequate processes for experience transfer (Neptune and Odfjell)
- the continuous circulation system (CCS) was not qualified in accordance with applicable requirements (Neptune and Odfjell).
What happens now?
With its investigation completed, the PSA has asked Neptune and Odfjell to explain how the nonconformities will be dealt with.