Changes have been made to operating permits for Mossmorran chemical plant in a bid to strengthen controls on noise and vibration and prevent significant pollution.
Environment agency Sepa served the permit variations on both ExxonMobil Chemical and Shell UK.
The companies were issued final warnings by the watchdog in April as a result of unplanned flaring in June last year.
A joint investigation has also been announced by Sepa and the Health and Safety Executive.
The alterations require assessment of techniques likely to need investment at Fife Ethylene Plant and Fife NGL Plant, which form part of the Mossmorran complex near Cowdenbeath.
Sepa chief executive Terry A’Hearn said: “Compliance with Scotland’s environmental rules is simply non-negotiable. We were clear that unplanned flaring in June 2017 was preventable and unacceptable and we issued final warnings to both companies in that regard.
“We listened carefully to community calls for a root and branch review and have taken action to begin the joint investigation with the Health and Safety Executive.
“The permit variations we have now served on ExxonMobil Chemical Ltd and Shell UK set out what each operator must do under law.
“They include legally binding conditions requiring them to take all appropriate preventative measures now against noise and vibration emissions through the application of best available techniques, so no significant pollution is caused.
“They also require a refreshed assessment of the best available techniques likely to require investment at both facilities.
“Sepa will be holding review meetings at key stages to ensure progress is made to the timetable.
“These meetings will involve senior executives from SEPA and both companies.”
Further flaring incidents in October, March and May remain under investigation.