TANK CLEANING IN 2004

The environmental regulations which prevent the disposal of used drilling fluids to the sea and the unfortunate death of a tank cleaning operative in Peterhead during 2003, have motivated North Sea operators to have a good look at the means of cleaning out the tanks of support vessels without putting people into them. In addition the charges now being raised for the disposal of tank washings to landfill have prompted an interest in any means available to carry out the cleaning operation without producing large quantities of contaminated water.
 

 Carrying mud is a fairly unsophisticated process. And the means by which settlement is reduced has not progressed much since the 1970s. Today most European designed ships have cylindrical mud tanks, often with hopper bottoms and agitators similar to those fitted to the mud pits of rigs. Alternatively circulating systems may be used, sucking mud from the bottom of the tanks and returning it to the top, or in some cases, illogically, returning it to the bottom.
 

 Regardless of the process used, if the mud is in the ship for more than a few days it begins to settle out and when the tanks are emptied quantities of solids are left in the bottom. When the coatings begins to fail it is also left on the sides and it becomes essential to clean out the tanks at intervals, even if the same product is being carried time after time.
 

 Cylindrical tanks have reduced cleaning problems, possibly due to the reduction of bottom area and possibly due to the fact that the agitators cover more area than those fitted in rectangular tanks. Many modern ships are also fitted with tank cleaning systems but until 2003 few had ever been used in earnest.
 

 Today a number of vessels in the UK and Norway are being required by their charterers to use their tank cleaning systems, and the results seem to be dependent on the specification of the original installation. It has to date been more or less up to the shipyard building the ship to purchase and install tank cleaning equipment which apparently fits the building specification, and the results have been spectacularly different. Many of the Vik Sandvik designs from the 1990s are provided with machines manufactured by Dasic Marine and supplied by Marex Marine Services of Aberdeen. The intent of these systems is that the tanks will be cleaned with the product in a similar manner to crude oil washing, returning the settled out solids to suspension and therefore ensuring that tank cleaning to a higher standard than “mud clean” will only be necessary on a change of product.

 Other systems supplied by Gunclean-Toftejorg, now a division of Alfa Lavel, and Scanjet are designed to clean the tanks with hot water and detergent. The builder’s specification for these systems generally requires that the machines be constructed from stainless steel and be capable of cycling dirty water so that a single charge can be used more than once. Their Achilles heel appears to be the recycling of the water charge, since even after cleaning a single tank the liquid is now a combination of water, mud and solids, and the solids sometimes include drill cuttings.
 

 Indeed, apart from the systems and procedures used to ensure the cleanliness of the ship’s tanks, the rest of the carriage and storage process seems to be fairly casual. The products are loaded into the ships and discharged onto the rigs via hoses which are used for all the drilling fluids. On the rigs the mud pits are fitted with primitive mixing systems and are usually alarmingly rusty. Cleaning of the pits used to be achieved by the simple process of opening a valve in the bottom and sending a guy in with a hose, but of course environmental legislation in the UK now prevents this, and so the guy may still be sent in with the hose, but the resulting liquid is pumped into an attendant vessel which must then transport the stuff back to the beach and by some means end up with clean tanks.
 

 Even the cleaning systems designed to cycle the product are challenged by this requirement which necessitates that they are run for extended periods. Harrisons (Clyde) whose Vik Sandvik designed VS483s, Inverforth and Inverclyde, have been using their tank cleaning systems on a regular basis for the last eighteen months, are finding that all the components of the system are beginning to experience wear which may be the result of being required to cycle drill cuttings in addition to the product. They have therefore decided to upgrade the system by replacing all components with stainless steel equipment and by installing dedicated stripping lines.
 

 Their new tank cleaning machines have been supplied by Marex and are a direct replacement for the old Dasic gunmetal machines. Marex claim that they are unique in that they are the only stainless steel machines in production with enclosed gearboxes and a drive system which ensures that the machine will still be able to cycle mud and its attendant impurities, for extended periods.
 

 One would think that the place to install tank cleaning systems would be on board the rigs, which is where the drilling fluid ends up, and at risk of boring those who don’t know what the stuff is for - and if you aren’t a driller why should you - here is a short explanation. Mud, oil based, water based or synthetic is pumped down the centre of the drill string, picks up the drill cuttings at the bottom of the hole and is returned to the mud pits of the oil rig, via the shakers, which remove most of the bits of rock, shingle and shale. Its specific gravity is raised by mixing the liquid with a solid, usually the mineral baryte, which until the oil industry discovered it was used as a lining for ponds. The baryte is introduced to add weight to a point where the well designers believe that any pressure in the substrata will be controlled by the mud column. Hence any fallout during transportation is bad news – because more baryte must be added on the rig. 
 

 The baryte is a suspension in the liquid not a solution, hence the sediment on the bottom of the tanks, the pits and any additional storage on the rig. Many operators are now hiring specialists to descend on the rig with portable tank cleaning equipment, at one bound replacing the man with the hose and putting the oil industry somewhere close to the position the shipping industry was in the 1950s. The guys with the tank cleaning machines, pumps and hoses are doing a job, although they are probably producing a lot more contaminated water than they say. Some are using Toftejorg machines and some Marex. No-one is using Scanjets. One company has retitled their machines “wizzie heads” and has claimed the name as their intellectual property. This of course does not prevent oil rig personnel from calling all tank cleaning machines “wizzie heads”.

 To help those wishing to clean out rig mud pits Marex introduced a 180 degrees down machine at Offshore Europe, which overcomes the one disadvantages of the dual nozzle machine, the fact that it jets all of the surfaces of the tank. Most mud pits are either open or at best have many holes in the top and the last thing you want is water, or even worse mud, all over the pit room.
 

 There are those who believe that mud tank and pit cleaning is, for the oil industry in Europe, the next big thing. And Europe is certainly the place where the environmental pressure is on. Eventually the rest of the planet will catch up, if for no other reason than, regardless of any regulatory or environmental requirements it is better to put machines in the tanks than people. Firstly its safer, and secondly and possibly only a little less importantly, it’s a great deal cheaper.

ASSOCIATED PICTURES

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Vic Gibson 2004.