If your whole 15-year experience is from LNG or oil and gas, the offered salary is definitely not up to your qualification. But if you're coming from other industries that's still in the range.

Is 18K basic salary?
If you will work in Ras Laffan, you should ask for location, commuting and transport allowance (that may add 5K to your 18K).

18K is okay for family of 4 given you're provided with housing and school.