March 2012 gas leak

On 25 March 2012 a gas leak occurred at the Elgin Well head platform within well 22/30c-G4 during operations to plug and decommission the well.^[8] No injuries were reported and 219 non-essential personnel were evacuated from the PUQ and the adjacent Rowan Viking jack up drilling rig, which was performing the decommissioning work. The remaining 19 personnel were evacuated later on the same day.^[9]

At the time of the incident the fields were producing 120,000 barrels per day (19,000 m³/d) of oil equivalent, about 7% of the UK's gas production.^[10] Methane gas was released into the environment as was between two and 23 tonnes of condensate which formed a sheen on the sea surface measuring approximately 6 nautical miles (11 km; 6.9 mi) in length. The HM Coastguard declared a 2 nautical miles (3.7 km; 2.3 mi) exclusion zone for ships and a 3 miles (4.8 km) exclusion zone for aircraft. Shell E&P also evacuated non essential staff from the separate Shearwater platform located in block 22/30b 6 km (3.2 nmi) from Elgin.^[11] At its peak, 200,000 cubic metres per day (7,100,000 cu ft/d) of gas was being released, which slowed when a relief well was drilled. The platform flare was alight till 31 March 2012 representing a potential risk of ignition for the leaking gas if wind direction should change. Total announced the flare spontaneously shut down on Saturday 31 March 2012 after exhaustion of the volatile residues within the PUQ processing facilities which it was burning. The leak was stopped following well intervention work on 16 May 2012, which involved pumping mud and cement into the well.^[12]

The cause of the incident was identified as corrosion in the casing of the G4 well, and a sudden release of gas from the Hod formation above the producing reservoir.^[13] Total identified the origin of the gas leak to be an unexploited chalk reservoir layer of the Hod formation located at a depth of 4,500 metres (14,800 ft), above the main reservoir, which was supported by analysis showing the absence of significant concentration of hydrogen sulfide in the gas. The Hod formation had been isolated by steel casing during drilling in 1997. On 25 February 2012 an increase in pressure was observed in the C annulus within the well and remedial operations started on 4 March 2012. Total believe that the C annulus failed and gas was observed leaking from the 30-inch (760 mm) conductor.^[14]

An inspection team comprising eight people from Total and Wild Well Control, a specialist well control company, boarded the Elgin platform on 5 April 2012 to gather information about the state of the platform. The team left safely after four hours.^[15]

During April 2012 a diverter assembly was installed around the G4 well head to divert the leaking gas (estimated then at 200,000 cubic metres per day (7,100,000 cu ft/d)) away from the platform in a controlled manner enabling well control operations to begin.^[16] In May 2012 two drilling rigs were working on repairing the leak. The West Phoenix semi submersible rig was working on the "top kill" operation. This involved pumping weighted drilling mud into the well via the wellhead assembly, a method which was ultimately successful in halting the leak. A relief well, G4-K1 was drilled to "bottom kill" the well by the Sedco 714.

Eleven monitoring overflights of the area by surveillance aircraft from Oil Spill Response Limited were made in early May, and the flow rate from the well was estimated at that time to be 50,000 cubic metres per day (1,800,000 cu ft/d).^[17]

On 16 May Total announced that the leak had been stopped.^[18]

Production restarted on 9 March 2013.

December 2016 Helicopter Incident

On 28 December 2016 a Sikorsky S-92 helicopter (registration G-WNSR, serial number 920250^[19]) operated by CHC Helicopter experienced "unexpected control responses" whilst in mid-air on a routine shuttle flight between two oil installations in the North Sea. The pilots conducted an emergency landing on the West Franklin platform.^[20] The helicopter is reported to have spun on the helideck during the landing, damaging the helicopter wheels and rotor blades.^[21] The helideck also sustained damage during the emergency landing.^[21] There were no reported injuries amongst the nine passengers and two crew during the incident.^[20] The aircraft was transported back to Aberdeen via ship, at which time the Air Accidents Investigation Branch began their investigation into the incident.^[22] Sikorsky issued a notice on 9 January 2017 grounding all S-92 aircraft until the tail rotors have been inspected. The inspection work takes approximately 11 man hours to complete.^[23] The Air Accidents Investigation Branch issued an update on 11 January 2017 which reported that the tail rotor pitch change shaft bearing had seized. The bearing showed signs of severe overheating and significant wear. The failure of the bearing allowed the tail rotor driveshaft to damage the tail rotor servo. The damage to the tail rotor servo is considered to be the cause of tail rotor control. The health and usage monitoring systems had discovered the flaw the day before flight, but the maintenance crew had not.^[24]

Elgin-Franklin and West Franklin

Equity in Elgin and Franklin was fixed by a unitisation agreement between three partner groups, and is shown in the table below.^[2] West Franklin is owned by the same partnership.

Glenelg

The partners in the Glenelg field are shown below.^[5]

Franklin field

The Franklin field is a tilted fault block with little internal faulting. The main reservoir is the Upper Jurassic (Oxfordian Age) Fulmar Sandstone which has porosities in excess of 20%. The Middle Jurassic Pentland formation forms a second reservoir with a separate hydrocarbon pool. The reservoir is at 5,500 metres (18,000 ft) depth. It is at pressures of 14,000 psi (97,000 kPa) and temperatures of 190 °C (374 °F). Originally the recoverable reserves for Franklin were estimated at 820 billion cubic feet (23 billion cubic metres) of natural gas and 120 million barrels (19×10^⁶ m³) of condensate.^[25]

Elgin field

The Elgin structure is a fault bounded high on top of a collapsed Triassic mud "pod". It is heavily faulted and split into several separate fault panels with discrete gas water contacts. The reservoir is the same Fulmar sandstone as Franklin. The Pentland reservoir at Elgin is of poorer quality than that underneath Franklin and has not been developed. The original estimate of recoverable reserves for Elgin was 890 billion cubic feet (25 billion cubic metres) of gas and 245 million barrels (39.0×10^⁶ m³) of condensate.^[25]

West Franklin

The West Franklin structure has a Fulmar formation reservoir.

Glenelg

The Glenelg field is a tilted fault block with a Fulmar formation reservoir.