Features: - Tanker Operator

TECHNOLOGY - TANK CLEANING
The Science of Cargo
Tank Cleaning
If you ask any commercial or operational management company of a chemical
or petroleum product tanker, they will probably say that tank cleaning
is the Achilles Heel of the entire operation*
Certainly it is the rate determining
step and very often it is the main
reason why a vessel may or may
not load its nominated cargo.
But contrary to what you may hear, tank
cleaning is not difficult; it is sometimes
confusing and very often frustrating, but there
is always a reason why something has
happened and identifying this reason will very
often resolve a problem. The importance of
experience can of course never be underestimated,
but as a great deal of experience has
already retired, or is in the process of retiring, it
has to be said that experience is not critical.
It is actually noticeable that the days of the
little 'black book' containing tank cleaning
secrets are clearly becoming numbered, not
just because the number of individuals with
the experience is decreasing but also because
many of the most effective tank cleaning
secrets are pre-MARPOL regulations and
based on using tank cleaning materials that are
now prohibited.
The use and availability of tank cleaning
materials today is far more strictly controlled
in terms of marine pollution, but as a direct
consequence, the potency of many of tank
cleaning materials has diminished, which does
have an impact on the tank cleaning.
Safety is also a far more significant concern
and the tendency is to only use tank cleaning
materials that are non hazardous, non toxic
and non flammable. This too also affects the
efficiency of the tank cleaning materials and
does have a bearing on the tank cleaning,
particularly when coated cargo tank surfaces
are involved.
When one considers that most tank cleaning
materials are detergent based and detergents
contain surface active ingredients, it is clear
that the primary role of such products is to
'clean the surface of the cargo tanks'. If the
cargo tanks are lined/plated/clad with stainless
steel, these products are completely
satisfactory, but if the cargo tanks are lined
with organic or inorganic based paints that
allow previous cargo residues to become
A large cargo tank with a deepwell pump, drop line and a cleaning machine.
trapped inside the matrix of the coating, surface
active cleaning materials are largely ineffective.
This is mainly common sense, but knowing
and understanding this is sometimes difficult
to comprehend when there are apparently no
other solutions available.
What is tank cleaning?
All tank cleaning procedures are a logical
sequence of events that will ultimately lead to
the objective of loading the next nominated
cargo. The precise nature of the cleaning
process is specifically determined by the
chemical and physical properties of the cargo
being cleaned from, the type of lining inside
the cargo tanks, the size and dimension of the
cargo tanks and the pre-loading specifications
of the next nominated cargo.
The key to any successful cleaning
operation is very simply knowing how far to
clean and determining whether each step of
the cleaning has been effective.
In practice most tank cleaning procedures are
very similar, because there are not that many
different variables available to the vessels:
i) Fixed tank cleaning machines or portables
(or both).
ii) Water or solvent for the pre-wash?
iii) Reaction of the previous cargo(s) with the
cargo tank coating.
iv) Cold water or hot water?
v) Tank cleaning materials or not?
Determining the correct plan is essential, but
this can usually be gained from the many and
varied tank cleaning guides available on the
market. Of far greater significance and
importance is the monitoring of each step of
the plan, in order to make sure that it has
actually been carried out.
Tank cleaning guides are useful but they can
also be extremely misleading for the simple
reason that inexperienced operators will tend to
use the guides as a definitive method for any
particular tank cleaning. This is a mistake which
can and does cause problems.
Still today in legal disputes on whether a
vessel is deemed to have cleaned (or not as
the case may be) with "due diligence" in the
preparation of the cargo tanks prior to loading
the nominated cargo, legal reference is drawn
August/September 2008 � TANKEROperator 57