Code red part 2. A spark away from catastrophe

Planning for well C-06 ATS began in 2008, when Statoil decided to drill a sidetrack from an older well from 1991.[REMOVE]Fotnote: Skoland, Kathrine; Nygaard, Gerhard; Nesheim, Torstein; Mykland, Solfrid; Kjestveit, Kari; Iversen, Fionn; Hansen, Kåre; Gressgård, Leif Jarle; Engen, Ole Andreas; Cayeux, Eric og Austnes-Underhaug, Randi. (2011). Læring av hendelser i Statoil: en studie av bakenforliggende årsaker til hendelsen på Gullfaks C og av Statoils læringsevne (Vol. 2011/156, pp. X, 123, 45). IRIS. S. 18-21.That well had been shut in for production the same year and was to be plugged before drilling the sidetrack. The purpose of the sidetrack was to investigate the Meldefly project on the field’s northeastern flank. Despite known geological complexity, in January 2009 the decision was made to use a conventional drilling method.[REMOVE]Fotnote: Conventional drilling is overbalanced drilling, where the drilling mud is weighted with a densifying agent to ensure constant overpressure that prevents fluid from flowing in. (Teknisk Ukeblad. (2010.14.mai). Ptil kritisk til Statoils teknologivalg: Valgte billigste boremetode).

During spring 2009, the well began sending worrying signals. Pressure was detected in the annulus between two casings, indicating a weakened well barrier. .[REMOVE]Fotnote: The annulus is the space between the production tubing and the casing. By circulating heavy kill fluid into the production tubing via the annulus, a well can be “killed.” Gas can also be injected down the annulus when gas lift is required. When gas lift is used, the annulus is equipped with a safety valve designed to prevent gas in the annulus from flowing up to the rig in an undesired situation.

Regulations require two independent barriers to prevent uncontrolled flow of oil or gas. With one barrier compromised, the operation was temporarily halted and a new sidetrack higher up the well was planned.

Challenging geology

Geological conditions at Gullfaks made operations particularly demanding. After decades of production, reservoir pressure in the Brent Formation had declined significantly, while the overlying Shetland Group still held high pressure. Water injection had further increased pressure in the Shetland formation, leaving a very narrow pressure window. That meant only a small margin between the mud’s pressure and the point at which the formation would start taking losses.

After 20 years of production the formation was more fragile, requiring less pressure to fracture. Narrow margins often occur in depleted reservoirs like Gullfaks. Sudden mud losses can give oil or gas a free path upwards and cause a well kick. If heavy mud is not added quickly enough, or the blowout preventer (BOP) is not closed, the risk of a blowout increases markedly.

Drilling of the new sidetrack began on 15 December 2009. Eight days later, at 2,665 metres, a gas kick occurred and well pressure dropped. Attempts to circulate out the kick failed and the BOP had to be activated. However, a central element of the BOP — the annular preventer, a rubber element meant to seal around the drillstring — was defective. Fortunately, the remaining safety systems worked, and the crew managed to regain control.[REMOVE]

Fotnote: Helgesen, Ole K. (2010, 14. mai). Ptil kritisk til Statoils teknologivalg: Valgte billigste boremetode. Teknisk Ukeblad.

After three months of persistent well problems, Statoil decided to switch from the conventional method to Managed Pressure Drilling (MPD), a more advanced technique offering tighter control of well pressure in narrow-margin formations.[REMOVE]Fotnote: Managed Pressure Drilling (MPD) involves applying additional pressure to light drilling mud, making it possible to regulate wellbore pressure at all times. This is done to ensure that the pressure is high enough to counteract the reservoir pore pressure, but still lower than the pressure the formations can withstand. In managed pressure drilling, surface-applied pressure is used in addition to the hydrostatic pressure from the mud column in the well to control the pressure against the formations. This makes it possible to adjust wellbore pressure more quickly than in conventional operations.The company had MPD equipment available on Gullfaks C and prior experience with the method, but initially chose not to use it — a decision later criticized.[REMOVE]Fotnote: Helgesen, Ole K. (2010, 14. mai). Ptil kritisk til Statoils teknologivalg: Valgte billigste boremetode. Teknisk Ukeblad.

MPD adds complexity, requires specialised equipment and extensive training, and cost considerations may also have influenced the early choice of a conventional approach.

We try again…

Drilling with MPD resumed on 13 April 2010, but problems persisted. A sealing element turned out to be leaking, and on 30 April a new kick occurred while pulling out of hole. Between 10 and 18 May, well pressure increased gradually, but the changes were not captured. When drilling was completed on 19 May, the well lost mud to the formation through a casing leak, causing a pressure drop and subsequent gas influx from the Shetland formation.[REMOVE]Fotnote: Skoland, Kathrine; Nygaard, Gerhard; Nesheim, Torstein; Mykland, Solfrid; Kjestveit, Kari; Iversen, Fionn; Hansen, Kåre; Gressgård, Leif Jarle; Engen, Ole Andreas; Cayeux, Eric og Austnes-Underhaug, Randi. (2011). Læring av hendelser i Statoil: en studie av bakenforliggende årsaker til hendelsen på Gullfaks C og av Statoils læringsevne (Vol. 2011/156, pp. X, 123, 45). IRIS. s. 20.

At this point, two of the well’s barriers — the drilling fluid and the casing — were compromised. All attempts to regain control failed, and large volumes of gas flowed onto the platform. The evacuation alarm was triggered; 89 people were evacuated while 140 remained to try to stabilise the situation. The work continued for nearly two months before the well was finally declared secure.[REMOVE]Fotnote: Camilla Hetlevik og Elin Holta. (2014). Læring i det norske petroleumsregimet – hvilken rolle har Petroleumstilsynet? Masteroppgave i samfunnssikkerhet. UiS. S. 17.

Assessment and consequences

The Petroleum Safety Authority concluded that small changes in the course of events could have led to a major accident, with potential for blowout and explosion.[REMOVE]Fotnote: Ptil. 2011. Granskingsrapport. Gasslekkasje på Gullfaks B 4.12.2010.

Avoiding that outcome was largely a matter of chance — weather conditions, technical characteristics of the well, and the crew’s efforts onboard.

Following the incident, production on Gullfaks C was shut down for almost two months, which Statoil estimated resulted in a total production loss of NOK 1,084 million.[REMOVE]Fotnote: Bjørn Haugan, Ingunn Andersen og Vidar Enerstedt. (2010. 19. november). – Bare tilfeldigheter at det ikke utviklet seg til en storulykke. VG.

Investigation

The well was drilled to a depth of 4,800 meters using managed pressure drilling. A hole in the casing led to drilling mud leaking into the formation, and gas flowed into the well. Although loose sediments and cuttings temporarily sealed the well and limited further inflow, gas releases, weakened barriers, and a damaged reputation were unavoidable.

Statoil launched an internal investigation of the incident, and the report was published on November 6, 2010. It concluded that under “slightly different circumstances” the incident could have developed into an underground blowout — a “hidden” well blowout occurring within the formations, where fluids are transferred from one underground zone to another, without necessarily being visible at the surface — a potential catastrophe.

The investigation revealed that both the platform crew and the onshore organization struggled greatly to understand and manage the complex situation during the first 24 hours. Normalization work took almost two months, and the well barriers had to be re-established before safe operations could resume.

Code Red

The internal investigation report that Statoil prepared following the incident on Gullfaks C in May 2010, when the company lost control of well 34/10-C-6, uncovered serious weaknesses in planning, risk assessment, and operational follow-up. .[REMOVE]

Fotnote: Statoil. (2010.4. november) Granskningsrapport CAO INV Intern granskningsrapport. Brønnhendels på Gullfaks.

The report classified the incident as “Code Red level 1” — the highest level of severity in Statoil’s internal categorization.

When it came to the weakening or loss of safety functions and barriers, the damage category was assessed as “potentially red level 2,” but for parameters such as reputation, financial losses, and material damage, it was set at level 1. This was mainly due to the extensive production loss that followed when the platform had to be shut down for an extended period.[REMOVE]Fotnote: Statoil. (2010.4. november) Granskningsrapport CAO INV Intern granskningsrapport. Brønnhendels på Gullfaks: 29.

The investigation concluded that the triggering cause of the gas leak was a hole in the casing. Gas inflow developed gradually as the pressure in the annulus outside the casing increased over time without being detected or addressed. Furthermore, gas was encountered earlier than expected, but the crew failed to identify the source, which contributed to the escalation of the situation.

A central task in investigating serious offshore incidents is to identify not only the immediate causes but also the underlying ones. Statoil’s internal investigation commission pointed to several systemic weaknesses that contributed to the incident. Among these were inadequate risk assessment, insufficient planning, weak compliance with internal requirements, low competence in Managed Pressure Drilling, and lack of involvement of relevant professional environments in decision-making processes.

The investigation also uncovered significant collaboration problems between the Gullfaks offshore organization and the company’s onshore drilling expertise. Communication between drilling personnel on the platform and experts onshore was poor, further worsened by a strained working climate. The report described it this way: “The investigation group has been told that the working climate between Statoil’s technical community and the B&B organization at Gullfaks has not been good.”[REMOVE]

Fotnote: Statoil. (2010.4. november) Granskningsrapport CAO INV Intern granskningsrapport. Brønnhendels på Gullfaks: 37.

One concrete example of communication failure was that production personnel carried out pressure measurements around the well without informing the drilling team of their findings. In addition, the report pointed out that the platform manager and the emergency response leader onshore had such different understandings of the situation that a joint mobilization was not initiated when the crisis occurred. It also emerged that Statoil did not have an emergency plan for drilling a relief well should a serious blowout situation arise.

Despite the serious findings, Statoil believed that the handling of the actual crisis situation, once it occurred, was carried out in a responsible manner. The company also assessed that the risk of the incident developing into a major accident was relatively low. The Petroleum Safety Authority’s investigation, however, reached a different conclusion, emphasizing that with small changes in circumstances, the outcome could have been catastrophic.