- Accident date
- North Sea
- Accident area
- 240 km east of Aberdeen (Scotland)
- Spill area
- Cause of spill
- Nature of pollutant
- Gas and condensate
- Ship / structure type
- Oil rig (Jack-up oil rig)
- Built date
- Oil field discovered in 1991; production started in 2001
On 25 March 2012, a gas and condensate leak started from the plugged G4 production well at the Elgin wellhead platform in the North Sea.
The well at which the leak occurred was not in operation and had been isolated for a year. Initially, a chalky formation, as yet unexploited, began to unexpectedly leak gas, from approximately 1000 metres above the operated reservoir. Then G4 well tubing ruptured at a pressure 30% lower than that which it should normally withstand. This failure was due to unforeseeable corrosion.
Due to the fire and explosion risks, all workers (238 people) were evacuated and an exclusion zone was set up. Another nearby platform was also evacuated. All power was shut down but the flare continued to burn until 30 March.
Elgin and Franklin gas fields
Elgin and Franklin are two high pressure/high temperature gas and condensate fields in which production started in 2001. They are located in the North Sea, around 240 km east of Aberdeen, Scotland. The Elgin field was discovered in 1991 and the Franklin field in 1986.
The Elgin drill rig/wellhead platform is connected to a Production/Utilities/Quarters (PUQ) platform by a 90 m bridge. The gas and condensate produced are processed and sent via a pipeline to shore from the PUQ platform. The jack-up rig is in waters 93 m deep and is mounted on piles fixed to the seafloor. It is an original rig in that it is one of the few to work at high pressure (1100 bars) and high pressure (190°C).
The products: Gas/Condensate
Once at atmospheric pressure at the base of the rig where the leak occurred, part of the substance expanded in the form of a gas into the atmosphere while part of it liquefied to form a liquid condensate comparable to gasoline.
The condensate spread out at the water surface forming a very extensive and very thin slick (a few microns in thickness), with a sheen-like appearance, the majority of which evaporated within a few hours.
This condensate is very light, has very low solubility and does not form an emulsion. Given the distillation data, the maximum quantity expected to evaporate at sea can be estimated at around 75%.
This product is mainly composed of linear alkanes, with around 25% of molecules containing 10 to 38 carbon atoms. The paraffin content (molecules containing over 20 carbon atoms) is around 8%.
The polycyclic aromatic hydrocarbon (PAH) content is relatively low for a petroleum product. If we consider the 16 compounds on the US EPA list, the content is approximately 0.1%, half of which is made up of naphthalene, the most volatile molecule. These concentrations are not likely to have a significant impact on the marine environment.
In late March and during April, the operator (Total) used satellite, aerial and naval equipment to observe the situation and to establish, in cooperation with international experts, the most appropriate response strategies to control the leak.
In collaboration with the authorities, the operator currently worked on the two main actions to be implemented to regain control of the leaking well:
- pumping of heavy mud into the wellhead (Top Kill operation) from a semi-submersible drilling rig. On the 26th April, a device called a "diverter" was fitted to the G4 wellhead in order to carry the flow of leaking gas away from the wellhead and the platform via 4 flexible hoses. This device reinforced the safety of the well intervention operations and helped alleviate restrictions on helicopter landings on the platform. The drilling rig West Phoenix was mobilised to pump the heavy mud into the well.
- drilling of two relief wells. Two drilling rigs were mobilised: Sedco 714 and Rowan Gorilla V (RGV). On 18th April, around 2 km east of Elgin, once the necessary approvals had been granted by the UK authorities, Sedco 714 began drilling the first relief well in waters 90 m deep. The Rowan Gorilla V (RGV) was on standby to drill a second relief well.
On 15th May, intervention teams pumped water-based mud into the G4 well, in compliance with one of the two plans developed with the authorities. After 12 hours of pumping, the methane leak was stopped. The G4 well was closely monitored over the following days to ensure that the operation had been successful.
Securing the well
On 21st May 2012, it was confirmed that the leak had been completely stopped. The Rowan Viking drilling rig was restarted to set cement plugs into the well. Once the first plug had been put in place, the drilling of the relief well by the Sedco 714 was stopped. Furthermore, it was decided that the drilling of a second relief well by the Rowan Gorilla V was no longer necessary and was therefore cancelled. Over a five-month period, the well was plugged with 5 cement plugs, forming a 1000-metre thick safety barrier. The last of these plugs was set in place on 22nd October 2012.
In two months, the well had released up to 200,000 m3 of natural gas per day. The gas leak rate, initially estimated at 2 kg/second, gradually diminished to 0.5 kg/second.
Following an in-depth review and the implementation of new technical criteria and revised safety-related operating procedures, the Health and Safety Executive granted approval on the 6th March 2013 for production to resume at the platform, which was restarted on 9th March.
Production resumed gradually: 70,000 BOE per day (50% of its production potential). The aim was to reach its pre-leak production rate (120,000 BOE per day) by 2015. Following this incident, it was decided that certain wells would be abandoned, requiring new wells to be drilled on Elgin and Franklin.
No lives were lost in the incident. The 500 million BOE (barrel of oil equivalent) capacity was not affected by the incident. However the disaster cost several hundred million euros (estimation of daily losses: approximately £1 million, i.e. €1.15 million).
IThere have been many cases of crude oil spills caused by blowouts, such as the Deepwater Horizon spill in the Gulf of Mexico in April 2010. Cases of gas and condensate releases are however very rare. Reference can be made to an incident off the eastern coast of Canada in 1984. The uncontrolled eruption of the Vinland drilling rig caused a gas and condensate leak that lasted 13 days. For more information see the Environment Canada website.
Sea & Shore Technical Newsletter, 2012, n°35
- Gas leak incident at Elgin platform in the North Sea: Website launched by Total for this incident.
- Total E&P UK PLC: Elgin/Franklin: Total brochure on the platforms produced in 2004.
- Elgin/Franklin: Description and activities of the platform.
- Hydrocarbon entrapment: Explanations about oil and gas on the educational website "Planète Energies".