Chevron's $US54 billion Gorgon liquefied natural gas project being built off Western Australia is facing a fresh threat of industrial action after unions won official approval to ballot workers on a potential strike just months before exports from the massive venture are finally due to begin. The latest development risks further delaying the start of production at the troubled project, Australia's biggest single resources venture, beyond its second-half 2015 target, which has already slid by more than a year from the original schedule.