Progressive removal of wax and hard scale avoids damage
BG Kershaw Limited
- Dual module magnetic cleaning pig loaded with wax. [30,982 bytes]
- Pin wheel pig. [22,087 bytes]
- Brush wheel pig. [16,258 bytes]
- Affect the operation of the pipeline, such as reducing the flow rate or increasing the operating pressure.
- As a protective layer, preventing injected corrosion inhibitor from reaching the pipe wall in the affected area.
- Seriously affect the proper running of an on-line inspection pig.
The worst cases of wax buildup occur where there is a complete absence of any pigging activity whatsoever, since the scale or wax simply drops to the bottom of the pipe by gravity and stays there. However, when regular pigging runs do take place, the standard cleaning pigs available from the various pig manufacturers are not generally aggressive enough to remove all the scale and wax from the pipeline. In fact, there is an argument for suggesting that some pigs will actually spread the scale or wax in a "buttering" action, thereby assisting with build up on the pipe wall.
Most pipeline operators will not necessarily know that their pipeline is being subjected to a slow buildup of scale and wax until there is either a change in flow rate and/or pumping pressure, or alternatively, they carry out on-line inspection using an intelligent pig designed to identify and record metal loss.
In the case of the former, the running of standard cleaning pigs will often improve the situation by removing some or most of the deposit from the pipe wall, thereby improving the flow and pressure regime in the pipeline. However, it is the second case which presents a much more serious problem since inspection pigs need a clean internal pipe wall for optimum results.
Whatever the reasons for hard scale and wax build up on the internal surface of the pipe wall, it must be dealt with in a way that does not damage the pipe nor must it carry the risk of blocking the pipeline. Ideally the method should enable removal of the deposits to be carried out onstream rather than shutting down the pipeline to carry out a specialist one-off cleaning operation. Also, the entire length of the pipeline needs to be dealt with without the need to sectionalize. Such a method was developed by the authors firm in the mid-1980's using enhanced cleaning pigs.
Enhanced cleaning pigsEnhanced cleaning pigs are specially designed to be more aggressive than standard cleaning pigs, but have the added advantage of being adjustable from an aggressiveness point of view. The reason for this is to avoid over-cleaning during the first and subsequent enhanced cleaning pig runs since this could lead to a stuck pig due to excessive debris ahead of the pig.
Therefore, the first run of an enhanced cleaning pig will be at a level of aggressiveness just above the standard cleaning pig used for operational cleaning by the pipeline operator. Subsequent runs of the enhanced cleaning pig will be at progressively higher levels of aggressiveness, with the next level based on the results of the previous run.
Therefore, if a particular run brings in only a small amount of debris, then the increase in aggressiveness of the pig for the next run will be far greater than it could be if the previous pig had brought in a large amount of debris. The ways in which the aggressiveness can be controlled will be discussed later, but it is important to appreciate just how vital this facility is in relation to enhanced cleaning using highly aggressive pigs.
There are three types of enhanced cleaning pigs. The first is magnetic cleaning pig for dealing with loose debris in both gas and liquid lines. The second is a pin wheel pig for removing hard scale and wax deposits adhering to the pipe wall, usually in liquid lines. The third type is called a brush wheel pig, but this pig is only run after the line has been enhanced cleaned using a magnetic cleaning pig, a pin wheel pig, or a combination of both.
Magnetic cleaning pigAt its lowest aggressiveness, this pig is a single module pig fitted with hard polyurethane support discs (usually a total of two to four), and softer sealing discs (usually a total of four to eight). Attached to the pig body, in between the front and rear disc assembles, are a number of large magnet pods for collecting and holding all types of ferrous debris ranging from fine dust to welding rods.
Many standard cleaning pigs fit this description, but the subtle difference between standard and enhanced cleaning pigs lies in the extremely high powered permanent magnet pods and the towing facility which converts the enhanced pig from a single module pig to a dual module pig. This magnetic cleaning pig in its single module form, will be the first enhanced cleaning pig to be run regardless of what product the pipeline is carrying, and is equally at home in gas or liquids.
During this run, and all subsequent runs, accurate monitoring and recording of the running pressure behind the pig will be carried out to indicate how hard the pig is having to work from a cleaning point of view. For instance, if the running pressure progressively rises throughout the run, it probably indicates that the pig is collecting and pushing more and more debris.
If, on the other hand, the pressure remains constant or rises only marginally, then it means that the pig is having a relatively easy ride. Once the pig has arrived at the receiver, both the condition of the pig and the amount of debris immediately ahead of the pig and adhering to the magnets, will give a good indication as to whether the run has been an easy one or a hard one.
It should be noted, however, that the amount of debris received immediately ahead of the pig is only residual since most of the debris will have been carried with the flow of product into downstream facilities. It is necessary to note down in a log the full details of the above for a comparison with subsequent pigging runs.
Unless the pipeline is exceptionally dirty, this first run using a single module magnet cleaning pig will not need to be repeated; in which case, the second run will be more aggressive.
This is achieved by introducing a second module which is connected to the first module using a universal joint thereby producing a dual module magnetic cleaning pig which is more aggressive than the single module pig for obvious reasons. For the first run of the dual module pig, the second module will be identical to the first module.
Unless the pipeline is exceptionally dirty, a maximum of two or three dual module runs will be all that is required, and enhanced cleaning operations will have been completed. However, some pipelines are exceptionally dirty, in which case an increased level of aggressiveness is required.
In the case of liquid lines, and in particular waxy crude oil lines, a different type of enhanced cleaning pig will be used. This is described in the next section under pin wheel pig. In the case of gas pipelines, the same dual module magnetic cleaning pig is utilized except that heavy duty brushes are fitted to the front module.
These brushes are not the cantilever or spring loaded brushes normally associated with standard cleaning pigs, but rather the rigidly mounted brushes seen on in-line inspection pigs operating on the magnetic flux leakage principle. This is the main reason why in-line inspection pigs are the best cleaning pigs in the business. However, to fit a full set of brushes to the front module at this stage would produce an overly-aggressive pig, and this is where real control of aggressiveness comes into play.
By fitting only 25%, 30%, or 50% of the brushes, the aggressiveness of the pig will be substantially reduced compared to fitting all the brushes. The actual percentage of brushes fitted will depend on the results of the various enhanced cleaning runs to date. If indications are that the line is exceptionally dirty, then only 25% will be fitted as opposed to 5O% if the line is thought to be less dirty.
The next run of the pig will generally have more brushes fitted, and following the run with all the brushes fitted will be the option to start fitting brushes to the rear module. This results in an extremely aggressive enhanced cleaning pig to rival, if not exceed, the aggressiveness of an in-line inspection pig. As far as gas lines are concerned, the dual module magnetic cleaning pig fitted with brushes will continue to be run until the line is considered to be clean and ready for inspection.
Pin wheel pigAs mentioned earlier, this pig is generally only used for the enhanced cleaning of liquid lines, although it has been effective in the removal of hard scale and other deposits from gas pipelines on the rare occasions this has occurred.
The pin wheel pig is used once the dual module magnetic cleaning pig (without brushes fitted) has removed all the loose debris from a liquid line and all that remains is hard wax and scale adhering to the pipe wall. One might argue that the dual module magnetic cleaning pig fitted with brushes should be effective at removing hard wax and scale. The fact is that they are, but it is impossible to control aggressiveness using brushes. This could lead to an over-cleaning situation which in turn might lead to a blockage.
The pin wheel pig consists of a cylindrical steel body very similar to any standard cleaning pig. However, instead of being fitted with the usual polyurethane support and sealing discs, it is fitted with four or six specially deigned and manufactured pin wheel discs. These discs are manufactured from medium hardness polyurethane and are 2 in. thick with an outside diameter of around 4 in. less than the nominal inside diameter of the pipeline.
Protruding radially from the circumferential edge of each disc are a number of steel pins which are screwed into threaded housings anchored into the disc. The length of the pins is such that the diameter across any two opposite pins is greater than the inside diameter of the pipeline by up to 0.75 in., depending on line size.
This means that when the pig is travelling through the line, the pins are bent back at
a slight angle, which both assists in the cleaning action, but more importantly compensates for any changes in internal diameter. The tips of the pins have hardened inserts to reduce wear to the pins to a minimum and the inserts are radiused to prevent damage to the pipe wall.
The pin wheel pig is always towed behind a single module magnetic cleaning pig using a universal joint to couple both pigs together. Each pin wheel disc is appropriately orientated to ensure that the cleaning pins on each disc are suitably offset from one another; this offset ensures that virtually the entire surface area is cleaned.
The facility for removable pins enable many aggressiveness options for wax/scale removal, and cleaning to be adopted, and on completion of each run, any worn or damaged pins can be simply replaced with new ones. By increasing the hardness of the polyurethane pin wheel discs, much increased aggressiveness is achieved.
When the enhanced cleaning of a pipeline is undertaken using the pin wheel pig, it is necessary to adopt a progressive approach in relation to aggressiveness in order to reduce the risk of a blockage which might occur if too much material is removed from the pipe wall.
To achieve this, it is preferred that during the initial cleaning runs, less than the entire internal surface of the pipe will be cleaned, as it is better to remove wax/scale progressively during a number of cleaning runs rather than trying to remove it all during one run.
This is achieved by running the pig with some of the pins (say 50% to 75%) removed for initial runs, and then fitting additional pins for each subsequent run until all the pins are fitted. Beyond this, if more aggressiveness is required, then harder pin wheel discs are fitted as previously mentioned.
The design of the pig is such that none of the wax/scale removed from the pipe wall will actually be pushed forward by the pin wheel pig itself; it will be deliberately left behind in the line. For actual removal of this debris from the line a dual module magnetic cleaning pig (with no brushes fitted), generally referred to as a bulldozer pig, is used. The sequence of running the pin wheel pig followed by the bulldozer pig, will continue until no further debris is evident.
The number of runs required will depend on how dirty the pipe wall is. With light debris it might take three or four runs of both the pinwheel and bulldozer pigs; in heavily contaminated lines, which have never been pigged before, it might take as many as twenty runs of each type of pig. The pig wheel pig is mainly used in liquid lines with crude oil lines having the biggest requirement.
Brush wheel pigThe part of the pipe wall which neither the magnetic cleaning pig nor the pin wheel pig will effectively clean are the sides and bottom of relatively large corrosion pits. With the magnetic cleaning pig, the brushes fitted to one or both modules fail to enter the corrosion pits, but rather bridge across the pits.
The pin wheel pig is better in that individual pins will enter the pits, but the cleaning action of a pin is not aggressive enough to properly clean the pits. This is where the brush wheel pig comes into its own since it is designed to clean the larger corrosion pits. This pig is virtually identical to the pin wheel pig except that all the pins are replaced by small circular brushes having a diameter of around O.5 in.
The pig has been developed from extensive pull-through trials in various pipe sizes, and has proven to be highly effective in successfully cleaning corrosion pits in operational pipelines.
The brush wheel pig is towed behind a single module magnetic cleaning pig, but in the case of heavily contaminated lines, two brush wheel modules are used, thereby making a triple module pig, which reduces the number of cleaning runs required.
Enhanced cleaning is an effective way of dealing with hard scale and wax removal from production pipelines. Because enhanced cleaning is carried out onstream, there is little or no interference with normal production and it can be carried out at any time.
It does not require the use of hazardous chemicals and there is no danger of the pipeline being damaged in any way. Because the enhanced cleaning pigs can be set up for various levels of aggressiveness ranging from very low to extremely high, the cleaning can be carried out by implementing controlled and gradual increases in aggressiveness to prevent over-cleaning occurring which might lead to a blocked pipeline.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.