On June 28, No 4 LP Holder was inflated to allow cup seal cleaning and, at the end of the operation, it was left partially inflated. On July 4, a recorded check was made on the holder. It included the check of the cup seals, too. The gas holder was deflated on July 9 to the top cup seal level to allow visual inspection of the cup below the water line. This inspection involved reduction of water in the seal by pumping water from the cup. The examination was completed and water pumped back into the seal to the original level. The holder was then inflated normally overnight to allow physical measurements in preparation for fitting low cup water alarms. Soon after full inflation, at approximately 7:00 am on July 10, the top seal failed and gas escaped. The holder rapidly lost height until the top cup seal entered the holder watertank. The gas cloud had dispersed without igniting. The last operator intervention was on the afternoon of July 9 and the seal failure occurred at approximately 6:55 am on July 10.

The release of natural gas occurred from a low pressure 4 lift water sealed holder. The capacity of the gas holder was about 41,000 kg. The gas holder was about 27 metres high with a diameter of 47 metres. The location of the storage plant is shown on two maps attached to the Original Report.

The safety systems were not activated during the accident and therefore the operators did not intervene. A member of public alerted British Gas but no emergency measures were necessary, neither on-site nor offsite, because the gas holder re-sealed itself after the lift descended into the tank.

– Natural Gas [mainly Methane] (C.A.S. CODE: 74-82-8, E.E.C. CODE: 601-001-00-4): amount involved = 37000 kg (37 tonnes).
The total establishment and the potential directly involved inventories of natural gas refer to the capacity of tha gas holder. Methane is the main component of the natural gas.

Due to an operator error (the seal of a gas holder was not refilled correctly after an inspection), a large amount of natural gas was released into the atmosphere. The gas cloud dispersed without igniting. The accident occurred because, after the visual inspection of the cup below the water line, the seal was not refilled according to the required recommendations. Besides, the process analysis was inadequate because the system to load the water into the seals did not operate automatically.