The HSE has said that it is looking into two explosions at the plant caused by pipes over-pressurising and bursting. There were “no flames, just steam”, a spokesperson said.
ExxonMobil temporarily closed the plant on 12 August and has said it will not reopen before November. It is carrying out its own investigation into the explosions.
Stuart Neill, external affairs manager for ExxonMobil, said: “We are concerned that comments have been made publicly on an ongoing regulatory investigation.
“It would be inappropriate for ExxonMobil to comment until the HSE's investigation, and our own, have been completed,” Neill added.
The plant has been a long-standing source of concern because of its reliance on flaring. ExxonMobil announced in September it would invest £140m in the plant after being ordered by the Scottish Environment Protection Agency (SEPA) to make upgrades in order to make flaring “the exception rather than routine”.
The plant uses ethane produced at the neighbouring natural gas liquids facility operated by Shell to produce ethylene, the building block for polyethylene plastic.
In August SEPA handed both plants deadlines to cut flaring, requiring ExxonMobil and Shell to prepare separate reviews of their efforts to minimise the problem.
Its investigation into the complex’s compliance with earlier permit conditions is due to finish by the end of November.