Tracking down bugs in the system

Diesel supply is critical for offshore platforms: diverse systems from power generation to safety items like fire pumps and lifeboats all rely on it, so if it’s contaminated, “significant” problems ensue, says Graham Hill of ECHA Microbiology.

“A major operator had to completely shut down a platform, demanding a complete evacuation on two occasions because of fuel contamination – so diesel fuel quality has a very high priority in operator’s minds,” he points out.

Mr Hill adds that there is a significant ‘finger of blame’ issue. “When a microbial contamination, commonly known as diesel bug, is found, the first question is always going to be, ‘Was the bug grown at home or was it delivered?’ so there’s a need to check supply vessels on a regular basis, as well as the storage facility.”

However, although so-called diesel bugs have always been around, the worthy principle of using low-sulphur fuels has put the cat amongst the pigeons, because many low-sulphur fuel sources have a percentage of biodiesel in them.

Unfortunately, biodiesel and the marine environment do not sit well together for a number of reasons and biodiesel tends to be less stable in its makeup than its petrochemical cousin. “Even completely petrochemical fuel is a natural product; gums and particulates form when it degrades, but this usually takes place over a very long time, maybe 20 years depending on the fuel chemistry,” explains Mr Hill.

Further, the worrying thing is that very small quantities of biodiesel, which is primarily made up of Fatty Acid Methyl Esters (FAME), can act as a trigger, a stepping stone for the microbial growth to get a foothold in other fuels. “For example, you can get explosive microbial growth with just 2% biodiesel in the system,” says Mr Hill.

On top of this, there are some unique factors associated with rig supply, explains Mr Hill: “At the platform end, the fuel that gets loaded might often sit there for a long time as a back-up as the rig will normally might well be burning flu fuel gas instead of diesel.” The problem here is that any diesel bug entrained in the fuel will have ample time to proliferate and get a good run at taking over.

On the PSV side, you often have a lot of fairly small tanks so keeping those clean can be difficult. “You could say, dose everything with biocides – but its not as simple as that, you have to find out if the biocide in question is approved for use on the platform,” says Mr Hill.

In addition, virtually all vessels have a habit of getting water into their tanks – if only via condensation from the air. Even very small amounts of water are sufficient to initiate microbial growth.

However, non-visible free water possibly presents a much worse threat than the visible kind. Studies show that biodiesels can hold micro-droplets in suspension which are much harder to detect using conventional tank dipping techniques and much of the fuel in the tank may still look clear and bright, explains Mr Hill.

Consequently, whereas microbial growth in mineral diesels tended to be restricted to pockets of accumulated water on the tank floor, or films of condensate water on tank and pipeline surfaces, microbial growth can occur in biodiesels all the way throughout the fuel phase. What is worse, because this water is held in suspension in the fuel it is not as easy to remove it from tanks by routine draining from sumps.

There is reason to believe the problem is getting more prevalent. “People send in samples when they suspect problems and we have had an increase in the number of samples we are receiving,” says Mr Hill. He adds, “It really seems no one thought the consequences through in a scientific manner when they legislated for 7% allowable FAME in fuel, not understanding that it might trigger an exponential rise in microbial infections.”

Further, although an operator might be able to find a supply of non-FAME diesel – not necessarily an easy task – it is increasingly difficult to keep FAME out of the fuel as there’s often shared tankerage and common pipelines: even the aviation industry is having issues. However, the air industry has had a set practice to deal with this for some time and stipulates a microbiological check of aircraft fuel tanks each year – more if it’s operating in the tropics.

“While this kind of contamination doesn’t cause quite as significant a problem in the marine environment as it does in aircraft, it shouldn’t be underestimated – you still risk life if your engine shuts down while at sea,” points out Mr Hill.

It’s worth understanding that normal filters onboard simply won’t take out single cells, and these can grow, very quickly, into a problem. “You have to go down to micron filters – and this just isn’t practical in most cases,” says Mr Hill.

So keeping on top of it all with a good monitoring and control regime is the only sensible way through. If you act early when there is only a trace of microbial growth present, you can treat it with a biocide. If you wait till it actually shows visibly, it’s too late. “You are looking at using centrifuges, emptying and degassing tanks, sending pigs down pipelines and often shovelling feet – not just inches – of sludge out, and even then you are still left with a quantity of fuel that needs to be treated with a biocide,” explains Mr Hill.

“If a PSV or OSV doesn’t monitor its tanks, it can’t prove where contamination came from. At least if it does, it can say, ‘I had a clean tank on such a date, filled up from you, and subsequent testing showed…’ which makes for a good case.”

By Stevie Knight