Gas release from pipeline during diving operations

We have recently learned of a gas release from a pipeline during a diving operation. Gas was released from the open flange face at the end of the 20 inch diameter subsea trunkline. The gas passed a gel slug that had been injected through a ¾ inch valve on the flange during the diving operation to install a pig launcher.

The Incident

On the day prior to the incident a 200 metre (approx) gel slug had been injected into the 20 inch subsea gas export pipeline in order to install a pig launcher. After this was done a request was made to increase the pipeline pressure from 33 psig to 38 psig to ensure a positive pressure in the line before removing the blind flange.

The following day when the diver had removed all but two of the bolts and the flange was lose and a couple of inches away from the end of the pipeline, the diver reported discharge from the open face and that bubbles of gas were clearly visible from the flange.

(L) Diving and ROV spreads (R) approx 6' diameter release area
(L) Diving and ROV spreads (R) approx 6′ diameter release area

A few minutes later after he reported that he had a burning sensation on his skin, the diver was instructed to return to the diving bell. After the bellman reported a chemical smell and a feeling of nausea in the bell, the bell was flushed through and brought to the surface.

During recovery of the bell, the gel plug passed an increasing volume of gas to surface. The escaping gas was clearly visible in the area around the diving bell and remained visible for approximately 40 minutes until the pipeline was depressurised from the shore. It was fortunate that the diving bell was recovered to the surface just moments before the gas escape, as the bell was in close proximity to the open flange face.

The diver was treated for the skin irritation.

After the gas was no longer visible at the surface, an ROV was put into the water to monitor the open flange. After 24 hours and additional risk assessments, the flange was reinstated onto the pipeline.

The client company has reported the following.

What Went Well

  • Calm reaction by the dive supervisor once the decision had been made for the diver to return to the bell.
  • Medical assessment and follow on treatment of the diver
  • Subsequent management support for thorough review of the project execution plans and procedures before continuing with the operation.

What Went Wrong

  • Lack of understanding of the gel mechanism. The gel slug was not designed to withstand pressure differential.
  • The management of change process did not undertake sufficient engineering analysis before being approved.
  • Lack of pressure control and monitoring mechanism during increasing the pipeline pressure, e.g. the scales of the gauges were not appropriate.
  • The risk assessments of the work scope had failed to integrate the onshore aspects.
  • The toolbox talk did not communicate the identified risks from risk assessment exercises to the divers undertaking the work.

Resultant Actions

The following resultant actions were identified by the client company involved:

  1. Establish a completely integrated team with clear roles and responsibilities to execute the work scope. Ensure all parties involved understand the complete scope and the interactions
  2. Reinforce the ‘Stop the Job’ practice.
  3. Ensure that the risk assessment process is clear and effective. Provide additional training, coaching and facilitation if required.
  4. Ensure risk assessments are adequately completed.
  5. Ensure that there is complete integration of the project activities, particularly with combined offshore and onshore operation activities.
  6. Ensure that the project work scope is planned sufficiently in advance to undertake meaning risk assessments.
  7. Review permit to work system.
  8. Ensure full implementation of isolation procedures, and ensure that all personnel assigned are fully competent to use procedures.
  9. The process of a formal management of change should be reinforced at further training carried out for the project team and all contractors involved with the work scope.

Key Message

An integrated, early understanding of the work scope is essential in ensuring effective risk assessments and to help manage future changes during execution.