OSLO, Norway — Equinor and SLB have until the end of this year to respond to a report by Norway’s Petroleum Safety Authority (PSA) concerning a well control incident on the Gullfaks C platform in the North Sea.
This occurred on Jan. 25 and involved fractured cold tubing. Issues arose in connection with cleaning up, supported by coiled tubing (CT), following a fracturing operation in a Gullfaks production well.
According to the PSA’s investigation, during circulation a fracture occurred in the CT on the surface. Further fractures followed in the vicinity of the reel and injection head.
When CT fracturing also occurred down the well, the blind shear ram on the BOP used for the CT operation (CT-BOP) was activated to shut in the well and restore the primary barrier. Some personnel on deck were exposed to hydrogen sulfide (H2S).
The degree of exposure, although not quantified, may have exceeded the ceiling for airborne concentration of a chemical to which staff may be exposed to at any time. Under Norwegian regulations, this is 10 parts per million of H2S for a 12-hr shift offshore.
While no damage resulted to the natural environment, the incident led to postponed and potentially lost oil production, with further losses arising from the need for extra work and costs incurred with damaged equipment
If the string had broken with staff next to the CT reel, energy from the released bending moment could have led to them being struck and injured, the PSA added.
The probe established that the direct cause of the CT fractures was sulfide stress corrosion (SSC) induced by H2S. It was probably due to a chemical reaction between the oxygen scavenger and citric acid being pumped through the CT, plus iron in the CT.
In addition, the material quality chosen for the CT was vulnerable to H2S exposure, and this, along with mechanical stresses, were the factors that led to the incident.
The PSA’s report identified one nonconformity and two improvement points related to Equinor. Another non-conformity concerns SLB, which supplied CT services, equipment and chemicals for the well stimulation operation.
When planning the well intervention, the PSA added, Equinor had failed to ensure that important contributors to risk were kept under control, individually and collectively, and had not adequately addressed issues related to HSE before taking decisions on the well intervention.
SLB, meanwhile, had not ensured that information related to compatibility and mixing procedures for chemicals used in the well intervention had been processed and communicated to relevant users.
Both Equinor and SLB must explain how they will deal with the nonconformities, and Equinor must also respond to requests for two improvement points.