MUD TANK CLEANING. LOSE LOSE OR WIN WIN

 The History

 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.

Often the improvement consisted of plating in the internal framing to present smooth sides and bottom, but even this process did not com 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.

The 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".

If 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 nozzles.

The 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 com pared with a full tank.

Of course tank cleaning became more frequent on the theory that no sediment meant no blockages.

Eventually 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.

Improvements 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 com plete discharge.

Alternatives were tried, and hopper suctions usually worked though they were com plex 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 com pleted, and then solidifying.

It 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 clear.

Many 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.

The 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 suctions.

Recent Developments

During 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 com plete cargoes.

These 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.

The agitator replaces the circulating system.

It 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.

The 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.

The 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 contaminated water.

 The 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.

Even 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.

Tank Cleaning on Oil Rigs

Only a 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.

Drilling 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 design.

Mariners 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.

In 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.

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 be com e 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.

In addition to the unwanted costs to the rig owners, their clients are now be com ing 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 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.

In an effort to reduce the levels of residues some Operators in the North Sea 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 1950s.

Attempts 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.

Therefore – 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.      

  Vic Gibson