TANK CLEANING MACHINES, OIL RIGS AND THE ENVIRONMENT
It is amazing to those of us who have experience of the use of tank
cleaning machines, firstly in oil tankers and latterly in the tanks of support vessels,
that there are not more used on oil rigs.
Drilling fluids are difficult liquids to deal with and almost all of
them contain solids which will settle out if the liquid is left unstirred for any length of
time. Traditionally, in order to prevent the settlement of the solids to take place in the
mud pits they have been provided with agitators which are small propellers and mud guns
which are small nozzles which jet the mud in the bottom of the pits.
Mariners who visit oil rigs are usually amazed that these floating
objects are provided with mud pits at all. Virtually non of the pit systems are totally
enclosed. The best are decked in but still have holes through which pipes may pass and
manhole coamings with no hatches. The worst are decked out only with gratings. They are
therefore not tanks in any sense of the word except in the way that an ordinary domestic
bath is a tank. In addition the agitators and the mud guns are not designed in any
scientific way and as a result quite a lot of solids settle out in the bottom of the pits.
In the past this was not much of a problem. When the tanks were empty
some-one would open what is known as a "Dump valve" or to continue with the bath
tub analogy pull out the plug, and then some minions would climb into the tank and
wash it out with water. The result would then drain out through the dump valve into the sea.
Traditionally the only place which held mud was the mud pit area and
the total quantity held was about 2000 barrels in oilfield figures or about 300 tonnes.
Now with the great increase in the expected water depths at which
mobile rigs will operate, and the extra depth in the substrata to which they are expected
to drill it, has become impractical to hold all the mud in the pits and so older rigs are
modified so that some of the mud can be held in tanks. New rigs are provided with purpose
built tanks. Of course those in the oil industry, remaining oblivious to the technology
available in the marine industry, have also fitted these new tanks with agitators and mud
guns and the lack of effectiveness is only apparent when the rigs return to port for
maintenance and the tanks are found to be half full of solids. These solids can cost a five figure sum to remove.
In addition to these unwanted costs to the rig owners, their clients
are now becoming more interested in the environmental effects of the drilling process.
Today drill cuttings are being transported back to the shore for processing solely
because they are contaminated with drilling fluid. However the residues within the mud pits and
the mud tanks remain a problem.
Even knowing the extent of the problem does not seem to provide the
industry experts with a solution. The manifestation of their difficulty is the depth of
solids in the bottom of the tank and so the approach up to today has been to attempt to
find a means of cleaning up this residue without creating an unacceptable level of
contaminated water. This either has to be transported ashore or disposed of in some other
way such as by passing it though an oily water separator and then sending the collected solids ashore.
We in Marex have been rather coy over the years and have not generally
revealed the principles behind the tank cleaning process to any but those who have
been trusting enough to purchase machines. One exception was our wholehearted collaboration
with a Norwegian shipbuilder who then passed the principles on to a competitor.
So, in the interests of environmental protection here is the first rule
of tank cleaning:
PREVENT THE ACCUMULATION OF SOLIDS IN THE FIRST PLACE.
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