West Mira is a sixth-generation semi-submersible drilling facility built to the Moss Maritime CS60 design. Operated by Seadrill Europe Management, it received an acknowledgement of compliance (AoC) from the PSA in 2019.

The incident

The unintentional disconnection of the LMRP occurred during a drilling operation on the Maria field in the Norwegian Sea, where Wintershall Dea is the operator.

After the facility lost position under the impact of dynamic weather forces, the automatic disconnection system (ADS) was activated. That in turn initiated separation of the LMRP.

Drilling was not under way in hydrocarbon-bearing layers of the well, which meant no danger existed of discharges from the reservoir to the natural environment. The well was isolated by the shear ram in the blowout preventer (BOP). Since the weight of the drilling mud included a riser margin for the section, the barriers remained intact.

Most of the oil-based drilling fluid in the riser was drained to the sea.

Actual and potential consequences

The incident caused the discharge of 49.9m3 of drilling fluid from the riser to the sea/natural environment. The drill string was cut and had to fished out of the well. The drilling operation was delayed for about three days.

No personal injuries were caused by the incident.

The PSA has concluded that the probability of losing well control while drilling in the reservoir section was low. Plans called for the well to be drilled with a fluid able to balance pressure in the formation if the mud column in the riser was lost. In the event of an unintentional disconnection, the BOP would cut the string and shut in the well.

The incident could have caused material damage to the facility and its equipment.

Direct and underlying causes

The direct cause of the incident is a combination of several factors.

It occurred when the facility lost position because of dynamic weather forces. The drilling contractor concluded that the drift-off activated the ADS. A combination of sub-optimal mooring lines and extensive use of thruster capacity to maintain position meant thruster power was insufficient to withstand the dynamic forces at the time of the incident. The disconnection signal was given after the ADS exceeded the five-degree activation point, which in turn automatically cut the string and shut in the well.

The investigation has identified several underlying causes for the West Mira incident. These relate primarily to:

  • reduced situational awareness and assessment of risk
  • procedures and compliance with these
  • use of analyses
  • management of change
  • design of the facility
  • cost cuts and concentration on efficiency.

Regulatory breaches

The investigation has identified four nonconformities with the regulations. These relate to

  • procedures and compliance with procedures
  • risk understanding and prioritisation of risk-reduction solutions
  • learning lessons from incidents and preventing recurrences
  • deficiencies in meeting the operator’s see-to-it responsibility.

The last of these concerns operator company Wintershall Dea, which has been given until 17 September to explain how this nonconformity with be dealt with.

The three others relate to Seadrill. They coincide to a great extent with identified nonconformities following an earlier incident on West Mira in January 2020. Seadrill has thereby failed to ensure that management of HSE is corrected, followed up and improved to take account of lessons learnt from the earlier incident on West Mira


On that basis, Seadrill has now been given the following order.

Pursuant to section 69 of the framework regulations on administrative decisions, see sections 6 and 21 of the management regulations on management of health, safety and the environment and on follow-up respectively, Seadrill is ordered to:

- identify and initiate measures which ensure that similar incidents do not recur on West Mira, and present a plan for ensuring that the necessary measures will be implemented and are effective on all relevant facilities covered by an acknowledgement of compliance (AoC) within Seadrill’s area of responsibility in Norway.

The deadline for compliance with the order is set to 15 October 2020.