The May 2, 2017 incident sparked a full-scale emergency response as 17,000 tonnes of flammable gas leaked from the 10-inch corroded pipe.
Roads surrounding the area were closed and the area evacuated as on-site firefighters deployed special hoses to create a wall of water to prevent the ethylene gas from reaching the plant’s furnaces and exploding.
Despite the size of the leak, which was equivalent to 26 Olympic-sized swimming pools, no gas was detected outside the perimeter of the plant itself and all 97 personnel were ushered to safety.
The site’s owners, Ineos Chemicals, admitted safety inspections failed to identify corrosion of the pipe during a hearing at Falkirk Sheriff Court.
The court heard that ethylene cracker at Grangemouth had been re-commissioned in 2016 to process methane from the United States for use in the production of plastics.
The plant contained more than 17,500 lines of pipe, and the section that failed had been wrongly identified as not high risk and therefore not subjected to insulation strips as part of a regular inspection programme.
This allowed a process called "chloride-induced stress load corrosion cracking", which may have been caused by sea salt.
Investigators from the Health and Safety Executive and the Crown Office and Procurator Fiscal Service(COPFS) later learned that the alarm was raised by an employee who heard what he believed to be safety valves being triggered automatically.
Ineos (Grangemouth) Ltd pleaded guilty to failing to take steps to prevent the incident, contrary to the 2015 Control of Major Accident Hazards Regulations.
The leak was first detected at 11.50 am and the cloud eventually dispersed and the incident was finally declared over at about 9.45 pm the same day.
The worker who heard safety valves lifting made his way to an emergency operating valve and attempted to close it manually but was told that multiple gas detectors were sensing the lead at ground level and the building was being evacuated.
It was agreed that the company's emergency response to the leak had minimised the risk of explosion. The leak was blamed on a change in design which resulted in the pipe working at higher than normal operating temperatures which led to stress corrosion cracking. Following the incident Ineos has spend millions of pounds replacing the pipework.
Sheriff Wyllie Robertson reduced the fine from £500,000 to £400,000 for submitting an early guilty plea and gave the company 28 days to pay.
Following the case, Alistair Duncan, head of the Health and Safety Investigation Unit of COPFS, said: "Ineos Chemicals Grangemouth Ltd accepted liability and the Crown accepted their guilty plea to the contraventions of the Control of Major Accident Hazards Regulations 2015 and the Health and Safety at Work etc Act 1974.
“This was the leak of a large cloud of gas that risked exposing people to toxic vapours and the risk of fire and explosion, though the effectiveness of the company’s emergency response prevented injury or damage to the site.
“The risks at high hazard sites such as this cannot be understated and the failings on the part of the company to maintain an effective maintenance regime underlines that duty holders should take nothing for granted.
“Hopefully this prosecution and the sentence will remind other duty holders that failure to fulfil their obligations can have serious consequences and that they will be held to account for their failings."