Tanks for the memory - empty giants fuel senses

So vast that echoes from a shout lasted seven seconds - entering one of Gull's truly cavernous petroleum tanks at Mount Maunganui was a memorable experience.

Nothing really prepares you for the colossal steel interior, despite the evidence of your eyes walking around the huge exterior of the tank.

For sheer scale, it seemed to even dwarf the Waitomo Caves. Peering into the gigantic space, through the gap in the aluminium blanket that normally floats on top of the fuel, it resembled a mind-bendingly large circus big top.

The once-in-a-lifetime opportunity to experience the inside of a fuel tank was offered by Gull which had just finished the 10-yearly inspection and upgrade.

The refurbishment was a massive task that took six months and hundreds of thousands of dollars, a job that would be repeated for the other six tanks on the tank farm.

Terminal manager Ren Keepa put a whole new perspective on running a tank farm which, despite its indomitable appearance from the road, concealed a never-ending list of compliance tasks involving regular checks, maintenance and environmental protection.

"There is always something going on."

Mr Keepa's guided tour of the empty tank took place a week before it was filled with enough petrol to fill seven olympic pools. Or put another way, the tank could run 12,500 family cars for a year.

The opportunity to go behind fences usually off-limits to the public was a real eye-opener, particularly the extent of work needed for tanks to get a new warrant of fitness.

Even technical details were interesting, like how the earthquake-proof tanks sat on a clay liner, enclosed by a bund designed to capture all the fuel in a catastrophic event.

Fire was the obvious hazard at a terminal. The answer was cooling rings that sprinkled water down the outside of the tanks if there was an accident at a neighbouring tank, with another ring that poured foam around the rim of the tank.

The tank farm is fed by a pipeline from the port, discharging the two grades of petrol and diesel at the gargantuan rate of 1.3 million litres per hour. The pipeline will be pressure tested to twice its normal pressure for 24 hours.

Mr Keepa was always surprised at how fast the fleet of road tankers drew down the fuel. "There's weeks of storage rather than months."

The critical part of emptying a tank for a refurbishment was drawing out the vapours, a job that needed to be done as fast and as safely as possible. It involved a keen weather watch because a lightning strike on a tank empty of liquid but still full of vapours could be disastrous.

A key part of the WOF was scanning the steel rings to determine correct thickness was being maintained and to detect corrosion. Sandblasting the floor of the tank produced 24 tonnes of garnet - the material given off from the blasting.

The floating blankets that moved up and down with the level of the fuel were needed to compress the vapour and stop evaporation.