How did one of the world’s oldest and largest oil companies solve a tough cleaning problem?

Bitumen Storage tank cleaned with combination jetting unit – March 2009

Removing an accumulated overhead layer of carbonised bitumen that is posing a safety hazard to tank cleaning is not an easy task. One global oil company that found itself in the position has 20 bitumen loading tanks of similar sizes, but holding different grades of bitumen. The tanks are around 10 metres tall, 6 metres in diameter and hold 150-200 tonnes of bitumen, which is heated between 190-220C.

Because Bitumen is stored at different temperatures, some of the tanks suffer more than others. Coke generally accumulates on the heating coils, of which there may be 1-3, depending on the grade of bitumen at the bottom of the tank.

More problematically, however, a coke layer has been found to form on top of the product, rising and falling with the tank level.

This has resulted in the build-up over a period of years of a thick, crusty, carbonised layer – up to 1 metre wide and 1.5 metres deep – around the top of the tank, just below the roof level. Another operational difficulty that the coking problem presents is obstruction of the tank level sensor, which normally floats on top of the bitumen.

The fouling of the tank level sensor’s connecting wire and consequent erroneous tank level readings are usually the first indication that there is a problem. The standard cleaning procedure had been for operators to enter the tank and cut the coke away with pneumatic chisels, but the discovery of the overhead layer of coke meant it was impossible for work to be carried out in the tank safely, because of fears that the coke would be dislodged by vibration from the drills.

A different approach was clearly required. The customer had tried opening more slots in the roof to improve access, with the aim of cutting through the coke, but it remained well adhered to the tank shell. Attempts were also made to lever the coke off in wedges, but they were unsuccessful.

Tube Tech’s MD Mike Watson said: ‘I was convinced this latest problem could be solved using a combination jetting unit, which can convert from 1000 bar to 3000 bar within 15 minutes, and a remote manipulator to direct the cleaning head to all areas inside the tank.

‘Based upon the previous year’s experience in 2005, we returned to tackle the same problem in a second tank in October 2006 and advised the client to carry out some minor tank roof modifications beforehand which made the job much easier.’

The customer agreed: “Tube Tech was able to reach the coke deposits safely via the top of the tank, using safety apparatus and some very impressive Kevlar™ protective ‘Turtle Suits’ which protected them from any stray water jets during the cleaning procedure. The operators cut the coke away from the top in large chunks that then fell to the bottom of the tank for later clean up. By successfully completing the work within the promised two days, Tube Tech was able to ensure we kept the tank clean on schedule and made it possible for the regular cleaning team to work inside safely.’