FAWLEY refinery has revealed that a fractured pipe was responsible for an ear-splitting bang that caused widespread alarm across the New Forest.
ExxonMobil says the fracture - the result of a chemical reaction - led to air escaping at high speed in an "incredibly rare" event.
The bang occurred on the evening of March 3, causing homes to shake and windows to rattle.
In a statement issued at the time ExxonMobil dismissed suggestions of an explosion, saying the noise stemmed from the sudden release of air from a pressurised line.
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The petro-chemical company has now released more details following an in-depth investigation.
In a bulletin to people living near the refinery it said: "We recognise that this was a loud and unique sound and again apologise for any concern caused.
"The noise was caused by the sudden release of high-pressure air from a fracture in a metal pipe.
"We had taken one of our units offline to conduct routine annual maintenance which requires pipework to be flushed of hydrocarbons before atmospheric air is pushed through at pressure.
"The bang occurred when the pipe fractured and there was a sudden and concentrated release of air travelling at very high speed.
"The fracture was caused by a chemical reaction between the air and a very small amount of residual hydrocarbons that had unexpectedly remained in the line following the purging process.
"This caused a small, very localised combustion point leading to a break in the pipe and the sudden release of energy.
"The event lasted about one second. Nobody was injured, there were no subsequent impacts and the community was not at risk.
"We subsequently completed the maintenance work and the unit was safely returned to normal service.
"While this was an incredibly rare occurrence that we could not have foreseen or planned for, we have now revised our process for this routine work to remove the risk of a repeat in the future."
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It was the latest in a series of mishaps at the refinery.
Last November the Health and Safety Executive launched an investigation after an incident involving the collapse of a 100-tonne steel platform and a leak of liquid petroleum gas.
In February about 100 litres of residual gas oil leaked from a pipe and contaminated part of Southampton Water.
In recent months the repeated use of Fawley's flares, the refinery's safety valve, has also caused concern.
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