From service stations to exploration and productionPotential for offshore wind power in Japan

Potential major accident on Songa Endurance

A well control incident on the Songa Endurance drilling rig during 15 October 2016 was the worst event of its kind on the Norwegian continental shelf (NCS) since 2004. Although nobody was physically injured when gas and seawater spurted up in the incident on Troll, it could easily have become a major accident. Had Statoil had been too lax over safety and was supervision by the Petroleum Safety Authority Norway (PSA) good enough?
By Björn Lindberg, Norwegian Petroleum Museum
- Songa Endurance. Photo: Songa Offshore

The PSA had the following to say in the introduction to its report on the incident:

At 09.33, the top drive with the completion string was raised six metres at the same time as large quantities of fluid and gas flowed out of control up through the rotary table. This flow lifted the 2.5-tonne PS-21 hydraulic slips and threw some two tonnes of bushings several metres across the drill floor. The fluid column reached right to the top of the derrick. Activation of a number of gas detectors led to local equipment shutdowns. Nobody suffered physical injury during the incident.[REMOVE]Fotnote:

The incident on the semi-submersible rig occurred during a slot recovery operation, which meant that the original well path was to be permanently plugged and abandoned (P&A) before drilling a new sidetrack to the reservoir and completing it for production. That would allow existing infrastructure in the well to be reused.[REMOVE]Fotnote: Ibid.

“Easy” field and time pressure

According to rig owner Songa Offshore, a long period without incidents on Troll had allowed an attitude to develop in Equinor that drilling the field was easy.[REMOVE]Fotnote: 107. With pressure on time to save money, this created a fateful combination.

The relevant well was equipped rather differently than most other Troll producers. While the majority of the wells on the field feature horizontal Xmas trees, a small number have been fitted with a vertical type delivered by GE Vetco Gray. That included the G-4 well being drilled by Songa Endurance.[REMOVE]Fotnote: An Xmas tree is an assembly of valves mounted on the wellhead. The main difference between horizontal and vertical types is that the tubing hanger in the latter is locked to the wellhead before the tree is installed, while in a horizontal model it is installed and locked to the actual tree. Pressure under the hanger can also be monitored in wells with the horizontal type, but not the vertical kind. The design variation between critical components can be said to present a substantial safety risk in this case, and horizontal trees were an innovation which took two decades to be rejected.[REMOVE]Fotnote: Gjerde, Kristin Øye and Nergaard, Arnfinn, 2019, Getting down to it. 50 years of subsea success in Norway: 260-262.

Training was provided ahead of the incident, as Statoil/Equinor always does, but this was inadequate for the type of well system represented by G-4 or for the special kind of operation conducted on it. Nor were the crew sufficiently informed about Troll-specific conditions. This was noted in the investigation reports from both the PSA and Statoil.[REMOVE]Fotnote: 105.

Part of the P&A job on G-4 comprised pulling out the tubing hanger, which was still connected to the wellhead. This called for barriers – such as cement or mechanical plugs or a combination of these – to be established so that gas and liquid cannot flow out from the reservoir. Equinor’s drilling organisation responsible for Troll drew up a concept selection report in May 2016 which described the choice of barriers to be put in place before production tubing was removed. This report defined a deep-set plug as the primary barrier, which was also the recommended primary barrier in Equinor’s Troll main activity programme (TMAP) for wells with the type of valves and piping used on G-4.

Amended plans

Using a deep-set plug is time-consuming, and plans for the operation were amended to cut costs and speed up the work by 12 hours. The change involved replacing the plug with the flow control and gas lift valves (FCV/GCV) on the production tubing as the primary barrier. It emerges from Statoil’s investigation of the incident that this solution conflicted both with the diagram in the concept selection report and the recommendation in the TMAP.

Statoil held internal meetings on 15 February and 28 June 2016 where key risks related to P&A on G-4 were discussed. Representatives from suppliers of key equipment components – GE Vetco Gray for the tree and Baker Hughes for the valves – were not invited to attend. Nor were subcontractors who had installed the valves in G-4 in 2012 involved in the decision to use these units for a job they were not intended to do.[REMOVE]Fotnote: Ibid: 106.


Flow on Songa Endurance on 15 October 2016 seen from the top of the derrick. Source: Petroleum Safety Authority Norway’s investigation report

The blowout occurred at 09.33. Large quantities of uncontrolled liquid and gas flowed up through the drill floor, and seawater was thrown more than 50 metres up the derrick. Activation of gas detectors caused local shutdown of equipment on the rig. The well was shut in after about a minute by the annular preventer in the blowout preventer (BOP). The blind shear ram which formed the next barrier was then activated.

About 20 of the 107 people on the rig were transferred to another platform when the incident occurred.

A Statoil press release that evening reported:

On Saturday 15 October a well control incident occurred on the drilling rig Songa Endurance. The well has now been secured with the blowout preventer around the drill string on the seabed and with a valve on the rig. Work is now being carried out to stabilise the well.[REMOVE]Fotnote:, accessed 28 June 2022.

The following day, a “kill” operation was begun to balance the pressure in the well. This was completed 10 days later after a lengthy and challenging period of stabilisation work.[REMOVE]Fotnote: 108.

Aftermath and investigation

The PSA described the well control incident as one of the most serious on the NCS since Statoil’s Snorre A event in 2004. Nobody was hurt but, in slightly different circumstances, a major accident could have occurred with loss of life, substantial material damage and emissions/discharges to the natural environment.[REMOVE]Fotnote: Ibid, accessed 28 June 2022.

A number of barriers which failed to function in the planning and execution of the job were identified by the PSA, which noted a series of improvement points in its investigation report:

  • control of personnel on board (POB) failed to meet the drilling contractor’s own performance requirement during this incident
  • deficiencies in the training system for the well securing team
  • inadequate provision for ensuring that detailed operation procedure (DOP) documents are used as intended
  • compliance with the drilling contractor’s pressure control manual was insufficient
  • the TMAP document did not refer to the latest blowout and kill simulations conducted in

Statoil’s internal investigation also reached clear conclusions and noted that allowing financial considerations to override safety concerns contributed to the incident.[REMOVE]Fotnote: Ibid.

According to the company’s press release:

Statoil’s internal investigation defines the incident to have a high degree of seriousness, and concludes that at worst it could have led to loss of life if the safety equipment had failed to function as intended, or if the gas had been ignited. The BOP was quickly activated and stopped the gas leak, and five gas detectors automatically turned off equipment that could have produced sparks.[REMOVE]Fotnote: Dagens Næringsliv, “Brønn ute av kontroll,” 20 January 2017,, accessed 30 June 2022.

“This is a very serious well control incident,” said Margareth Øvrum, executive vice president for technology, projects and drilling in Statoil. “The actions taken will improve our ability to assess risk, both before and during operations. We will share our experience from this incident with the rest of the industry.”[REMOVE]Fotnote: Ibid.

Auditor General’s criticism of the PSA

In 2018-19, the Auditor General’s office investigated the PSA’s follow-up of health, safety and the environment (HSE) in the petroleum industry. This reviewed a number of serious incidents and their follow-up – including the Songa Endurance event.

In-depth studies revealed that the PSA had repeatedly ordered Statoil – by now Equinor – to ensure that lessons were learnt from serious incidents where the company was the operator. It transpired that these orders had not been sufficiently effective, and the PSA was criticised for failing to follow them up.

A number of the incidents covered had more or less the same underlying causes. The Auditor General’s assessment was that the PSA had not adequately checked that Equinor was actually learning from earlier serious incidents, and it criticised the authority for this.[REMOVE]Fotnote:

According to the Auditor General’s report:

The in-depth study of the Songa Endurance incident shows that orders issued after serious incidents where Equinor has been the operator have failed to have sufficient effect. Five serious incidents during the 2004-16 period have many of the same underlying causes. These causes have been identified by the PSA in earlier audits and investigations, and Equinor has been ordered to do something about them. In orders following incidents on Gullfaks C in 2010, Heimdal in 2012 and Songa Endurance in 2016, the PSA has told Equinor to assess why measures initiated after earlier incidents have not had an effect.[REMOVE]Fotnote: Ibid.

Safety and sustainability officer (SSU) learning package 1: Songa Endurance. In Norwegian only. Video: Equinor/Upright Music


    close Close

    Leave a Reply

    Your email address will not be published. Required fields are marked *