CLEANING. LOSE LOSE OR WIN WIN
When oil based mud was first developed in the 1970s, the supply vessels of
the day were not fitted with purpose built tanks, and initially the product was
carried in converted fuel tanks. The result was that little of the product
actually reaching the oil rig, the majority being removed by the tank cleaners.
Tank cleaning took days and cost a fortune. Hence it was not long before the
tanks were improved.
the improvement consisted of plating in the internal framing to present smooth
sides and bottom, but even this process did not
pletely cure the problem. If the mud was left in the ship for
more than a couple of days the baryte began to drift out of the oil and settle
on the bottom where it would remain until manually removed.
oil industry is never at a loss for a knee-jerk response, known by some as
"opinioneering", and some ships were fitted with jetting nozzles not
unlike those fitted to the mud pits of oil rigs. They are known on the rigs as
mud guns were successful on oil rigs the success certainly was not repeated on
the support vessels. The mud was of course drawn from the suctions in the bottom
of the tanks, using the discharge pumps or possibly specially installed
circulating pumps, and then re-injected into the bottom of the tanks through
baryte continued to drop out of suspension so that the liquid at the bottom of
the tanks became thicker and thicker. This was circulated by the pumps until
they finally gave up or the nozzles blocked up. Even worse, the suctions from
the tanks frequently blocked so that ships returned to port with a full cargo
and were faced with the problem of how to discharge it. In the 1970s contractors
in Lerwick were often employed to transfer mud from one tank to another through
the deck manholes using portable pumps. So
a further development understandably was the multi-level suction. Many ships
were fitted with a lower and an upper suction and some even had three levels.
The theory was that even if you had to leave a couple of
feet of the product in there, it was nothing
with a full tank.
course tank cleaning became more frequent on the theory that no sediment meant
the circulating systems became more practical, taking the liquid from the bottom
of the tank and returning it to the top although designers remained obsessed
with small diameter nozzles. The nozzles were usually removed by the ship's
staff as soon as the vessel entered services on the basis that the possibility
of blockages would be reduced.
in formulation of the product and the provision of agitators, together with the
acceptance that frequent tank cleaning must take place more or less removed the
requirement for the upper suction. However the main suctions were generally
placed well above the tank bottoms and were of large diameter, both factors
which reduced the possibility of a
were tried, and hopper suctions usually worked though they were
to install. Suctions in hatboxes were generally less successful due to the
tendency for the mud residues to drop into the hatbox after discharge had been
and then solidifying.
appears to be purely an accident of design that many tanks were, and probably
still are, filled through the suctions and this ensures that they will be kept
early systems used conventional centrifugal pumps, and indeed some are still so
fitted where economy is considered to be of greater importance than efficiency.
Centrifugal pumps suffer from a tendency to lose suction if the pump is higher
than the suction or if the lines to them are at any point higher than the pump.
These inherent deficiencies may well result in high levels of residues remaining
in the tanks.
increasing use of constant displacement pumps has considerably reduced this
problem, and hydraulically powered pumps which offer variable pumping rates may
well provide the means of reducing residues by avoiding cavitation at the
the 1990s the major Norwegian designers addressed the whole problem of the
carriage of mud, and almost universally adopted cylindrical tanks with hopper
bottoms. The redesign of the tank on its own has vastly improved the ability of
the vessels to discharge
cylindrical tanks are also now often fitted with a single tank cleaning machine
somewhere in the upper part and an agitator in the lower part.
agitator replaces the circulating system.
is certain that the tank designs and coating has reduced the level of residues
remaining in the tanks and that the tanks may now be cleaned to what ever
standard is required using tank cleaning machines. Here there is a difference in
view as to the manner in which the tank cleaning should take place between the
British machine suppliers and the Scandinavian suppliers.
British, represented by Marex and Dasic believe that the tanks should be cleaned
with the product during discharge, reducing and maintaining the residues to a
minimal level, and that very occasionally a water wash should be undertaken. The
Scandinavians represented by Gunclean Toftejorg and Scanjet think that a water
wash should be carried out every time.
Brits are of the view that their continental rivals have taken this stance
because of limitations in the ability of their machine to circulate mud.
The Dasic manufactured machine can successfully circulate mud with its
attendant impurities which gives it a number of advantages. On the other hand
the continental manufacturers are always looking to reduce throughput in order
to limit the amount of water which is required to be heated and the volume of
residue produced. In the tank cleaning business this aim can be counter
productive. If too little volume is applied to the surface to be cleaned then
more cleaning cycles have to be carried out. Result - same amount of
British suppliers also claim that their process of cleaning the tanks with the
product is more beneficial for retro-fitting to vessels constructed before the
latest designs ensured easy-to-clean tanks.
where there is substantial internal framing, the installation of one or two
machines can reduce residues to a minimal level when the tanks are cleaned with
the product and no heaters or chemical injectors are required. The result will
be a substantial reduction in or the removal altogether of tank cleaning costs,
and most importantly today, a vast reduction in the volumes of contaminated
water which have to be disposed of somewhere.
Cleaning on Oil Rigs
few oil rigs and platforms have been fitted with tank cleaning systems. Tofjorg
have supplied some platforms in the Norwegian sector and Marex has supplied a
full set of machines to the semi-submersible Stena Don, and a number of
individual machines to other rig owners.
fluids are as difficult to deal with on board rigs as they are on support
vessels. The solids still settle out even in pits which have been provided with
mud guns and agitators. This is probably because the mud guns were never going
to work anyway and the agitators are mainly too small and of unscientific
who visit oil rigs are usually surprised that these floating objects are
provided with mud pits at all. Virtually none of the pit systems are totally
enclosed. The best are decked in but still have holes through which pipes may
pass. Some have manhole coamings but 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.
the past this was not much of a problem. When the pits 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 pit
and wash it out with water. The result would then drain out through the dump
valve into the sea.
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.
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 be
impractical to hold all the mud in the pits and so older rigs are modified so
that some can be held in tanks in the legs or pontoons. New rigs are provided
with purpose built tanks. Of course offshore 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. The solids can cost a five figure sum to remove.
addition to the unwanted costs to the rig owners, their clients are now be
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.
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 sediment
in the bottom of the pit and so the approach up to today has been to clean out
the pits with water and then to send the resulting contaminated fluid ashore in
the tanks of supply vessels. This process transfers both the sediment and the
problem to the support vessel and still necessitates the processing of the
contaminated water. When the water is discharged, all the sediment from the pits
is left in the bottom of the supply vessel tanks which then have to be cleaned.
This produces even more contaminated water which has to be sent for processing.
It is a total lose, lose, situation.
an effort to reduce the levels of residues some Operators in the
have employed contractors who use tank cleaning machines, detergent and water,
and then suck out the fluid for transport to the shore in one way or another.
This technique appears to have cut down the level of residues, although claims
of a maximum of 60 bbls (For those who deal in cubic metres there are 6.3 bbls
to a cubic metre) per tank seem to be a trifle optimistic. Retired mariners may
remember that Dasic and other tank cleaning machine manufacturers actually
provided experts who carried out exactly the same task on oil tankers in the
to install fixed nozzle mud cleaning machines in the mud pits of rigs have been
frustrated by the tendency for the mud spray all over the pit room through holes
in the top of the tank, and programmable single nozzle machines so far have been
unable to cycle the product. However Marex are now marketing an ingeniously
modified fixed nozzle machine which only jets downwards, and which can therefore
be installed in open mud pits. Hence the operators of rigs can now enjoy the
same advantage as the operators of ships. They can clean the pits with the
product, removing the sediment without producing any residues for disposal.
– by cleaning the pits with the product every time they are emptied there will
be a minimum of sediment left, and by making a special effort during the return
of the mud to the supply ship there should be no sediment left. Similarly if the
supply vessels clean the tanks with the product at every discharge the residues
will also be minimised. Production of contaminated water is negligible.
Operators and ship-owners can conform to their environmental policies and avoid
needless expenditure. This is surely a win, win situation.
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