Royal Dutch Shell has introduced gas to its 490 meter (1,600 ft) long Prelude floating liquefied natural gas (FLNG) unit as part of the cooling process before start-up, a spokeswoman told Reuters on Wednesday.

Gas was fed to the unit from liquefied natural gas (LNG) carrier Gallina, she said. Thomson Reuters Eikon ship tracking data shows that the vessel left Prelude FLNG port on June 7.

"This will cool down the tanks and process equipment and pipework with gas and is an opportunity to test processes and systems before the subsea wells are opened at start-up," she said.

Shell and Inpex Corp, Japan's biggest oil and gas producer, are vying for first gas from two overlapping fields from offshore northern Australia, where both have spent billions of dollars building the world's biggest maritime vessels to grab a slice of Asia's booming LNG market.

The Shell spokeswoman declined to comment on the estimated start-up date but reiterated that the oil major expects to get cash flow from Prelude in 2018.

(Reporting by Jessica Jaganathan; editing by Richard Pullin)