This well control incident occurred in connection with cleaning up after a fracturing operation in a production well on the North Sea field.

The clean-up was conducted with the aid of coiled tubing (CT). During circulation, a fracture occurred in the CT on the surface and was followed by further fractures in the vicinity of the reel and injection head.

When CT fracturing also occurred down the well, the blind shear ram on the blowout preventer for the CT operation (CT-BOP) was activated to shut in the well and restore the primary barrier.

Actual consequences

Personnel on deck were exposed to hydrogen sulphide (H2S).

Estimating the degree of exposure is challenging, but it cannot be ruled out that this might have exceeded the ceiling value (the maximum airborne concentration of a chemical to which a worker may be exposed to at any time). Pursuant to the Norwegian regulations, this is 10 parts per million of H2S for a 12-hour shift offshore.

The incident had no consequences for the natural environment.

It resulted in a financial loss through postponed, and possibly lost, production. In addition came losses resulting from extra work as well as costs incurred with damaged equipment.

Potential consequences

Had the string broken with personnel next to the CT reel, energy in the form of released bending moment could have led to them being struck and injured.

Personnel working around the reel on deck could have been exposed to H2S, creating a risk to their health.

Direct causes

The investigation has established that the direct cause of the CT fractures was sulphide stress corrosion (SSC) induced by H2S.

It is almost certain that H2S arose through a chemical reaction between the oxygen scavenger and citric acid being pumped through the CT plus iron from the latter.

The material quality selected for the CT was vulnerable to H2S exposure. Combined with mechanical stresses, this gave rise to the incident.

Underlying causes

The investigation report identifies various underlying causes of the incident:

  • mixing of chemicals, choice of materials and compatibility testing, planning and change management
  • organisation and interfaces
  • capacity and expertise.


The investigation team identified two nonconformities and two conditions it has chosen to categorise as improvement points.

One nonconformity and both improvement points relate to operator company Equinor. The second nonconformity concerns oil service company SLB, one of several contractors involved in the well stimulation activity. It delivered CT services, equipment and chemicals.


Planning the activity (Equinor)

When planning the well intervention, Equinor had failed to ensure that important contributors to risk were kept under control – both individually and collectively.

Issues related health, safety and the environment had not been addressed adequately and from every angle before decisions on the well intervention were taken.

Information and interface (SLB)

SLB had failed to ensure that necessary information related to compatibility and mixing procedures for chemicals used in the well intervention activity had been processed and communicated to relevant users.

Improvement points

Capacity and expertise (Equinor)

Equinor does not appear to have made the resources needed for the well intervention activity available to the project organisation.

Inadequate learning from similar incidents – (Equinor)

Equinor does not appear to have made provision for applying experience acquired by it and others to improvement work.

What happens now?

The investigation is completed. The PSA has asked Equinor and SLB to explain how the nonconformities will be dealt with, and Equinor to provide its assessment of the improvement points observed. The deadline for these responses has been set to 31 December 2023.