Union members have protested outside Parliament House in Perth saying floating LNG projects will rob locals of jobs.

Australian Manufacturing Workers' Union members gathered on Tuesday to voice their concerns about the vessel-based liquefied natural gas projects.

Energy giant Royal Dutch Shell is using floating LNG technology for its Prelude gas project off the WA coast.

Woodside is leaning the same way, having abandoned plans for an onshore processing plant for Browse Basin gas.

Shell's local chief Ann Pickard said earlier this year that Prelude would be "full of Australians", but unions are not convinced, with the AMWU labelling floating LNG "economic vandalism".

"It cuts out any opportunity for Western Australian workers," the union's WA state secretary Steve McCartney said.

"We are continuing to see contractions in local manufacturing, we are seeing the hollowing out and de-skilling of our economy as our jobs and future are allowed to be sent offshore.

"The gas could and should be brought onshore."

The union said the WA Government had not done enough to protect local jobs in the state's gas sector.

Premier Colin Barnett wants the Browse project to be based onshore, but recently conceded floating LNG could still bring "some" benefits to the state.