Newsgroups: sci.environment
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Re: oil seepage from tanks
Date: Tue, 08 Oct 1996 14:58:53 GMT

charliew@hal-pc.org (charliew) wrote:

>Nobody in their right mind would hazard a guess regarding
>leaks to the water table.  Big oil tanks are in service for
>many years between cleanings, and there is no way to know if
>they contain pinhole leaks on their floors or not.  Since it
>costs a large amount of money to clean and inspect one of
>these tanks, and there are big risks to personnel involved in
>this operation (hydrogen sulfide in sour crude, scale
>deposits that burn on exposure to air, etc.), this procedure
>is deliberately done as infrequently as practical.

Charlie, what is the US practice?.
Here in NZ, all the above ground storage tanks are on massive
concrete bases, and most of the larger ones are also slightly
conical to run down to the central water trap and drainpipe.
Thus the complete metal structure is above ground. When a
large tank is empty it's often common to see a gap of a few cm
open around the edges due to the thermal expansion of the
steel and the absence of product to hold the tank down.

Pinhole leaks on tank bottoms seldom occur here, and if they did
they would be seen exiting around the water drain line ( which
is also above ground ). I suppose the bases could be because
of our tendency for occassional earthquakes, but from memory
they were also present in UK installations. Given the weight of
the product a decent foundation would be necessary to prevent
settling and associated stresses. Most corrosion occurs
at the water/oil interface and in the headspace above the oil
and draining usually keeps the interface in the robust water trap.

>There are different types of corrosion, so an assumption will
>not be all that good.  The best you could hope for would be
>cathodic protection.  I'm not an expert on this form of
>protection, and I don't know if this is normally done or not.

I doubt it is used for above ground tanks that are connected
through conductive pipes and are usually well earthed ( lightning? :-) ).
Here it is used mainly on buried pipes passing through soils
that have a history of rapidly corroding pipes.

>By keeping tank downtime to a minimum, you are reducing risk
>potential to humans, which is an area that OSHA is highly
>involved in.  Thus, another regulatory agency has established
>rules that may make it more difficult to act in this
>instance.

Interesting, most large companies here have maximum inspection
periods for storage tanks, based on historical evidence of
corrosion rates. For safety reasons, tanks have to be internally
inspected every 2-20 years to ensure no unusual corrosion or
( more importantly ) cracking or other failure mode is developing.
the actual frequency depends on the age, previous condition
of the tank, and product being stored - a tank storing solid
fuel oil at ambient temperatures may seldom be inspected
because the energy cost of emptying and cleaning is significant,
hence it is well protected prior to use for that product.

Whilst making a tank safe for inspection is significant downtime
( can take weeks ), inspection is usually performed by independant
contractors and is considered part of the normal maintenance
schedule at most operations here, and the problem of unscheduled
downtime if a leak is discovered acts in favour of cautious
preventative maintenance schedules. I can't actually recall a
leak in any hydrocarbon storage tank, but I do recall seeing some
pretty amazing corrosion in a water storage tank that hadn't been
properly protected.

Almost all the hydrocarbon contamination of ground that I saw in
tank farms had arisen from historical practices of water draining
and maintenance, rather than as a result of leaks. Although I
have several times seen fountains of product from tanks that
weren't as large as the pumpman thought. One impressive
thing I've seen is an 8" spherical rubber pig come out of a 3 km
pipeline with 100psi of nitrogen continuously accelerating it.
The noise it made as it exited, and then  bounced between nearby
storage tanks at 4am was pretty awesome, as was the dent in
the first tank it hit - rubber hitting steel doesn't alway mean the
rubber comes off worst :-).

                      Bruce Hamilton