October 2009 Vol. 236 No. 10
Features
Pipeline Pigging And Cleaning: What Do We Really Know About It? Part 2
In a previous P&GJ issue Part 1 discussed: Paradigms, Polyurethanes, Durometer, All Cleaning Pig Types, How to Get Started Pigging, Types of Cleaning, What is Clean, and False Paradigms. This conclusion will discuss: “Cleaning a Pipeline, Types of Cleaners, and Pipeline Efficiency, an added benefit in preparation for running an ILI tool.
Cleaning A Pipeline
There are two types of pipeline cleaning programs available. One type is referred to as mechanical cleaning (the running of mechanical pigs dry) and is the answer most mentioned when asked, “Do you clean your pipelines?” Most debris in a pipeline is in the 4 to 7 o’clock position due to gravity. In most cases, if a pipeline is mechanically dry pigged, this type of cleaning will only displace the debris from the 6 o’clock position to 360 degrees around the pipe wall. Even if more mechanical pigs are run, the solids (i.e., black powder), iron compounds, and other organic and inorganic compounds can be broken-down to sub-micro particles causing down stream nuisances. For example: plugged meters, fouled turbine/compressor filter elements, customer’s treating equipment, etc. Again, the more one mechanically dry pigs, the smaller the particles can become. If the solids are iron compounds (iron sulfides, iron carbonate, iron oxides, etc.) these particles and sub micro-particles will be pressed by the pig’s disc and cups, at pipeline pressure, into any pipe wall anomalies and/or pitting which may interfere with the magna-flux readability of actual pipe wall metal loss areas especially if these compounds are magnetite.
Liquid Cleaning With Surfactant Base Cleaners
Achieving greater solids removal from pipelines with fewer pig runs requires liquid cleaning, referred more commonly to as chemical cleaning. This type is becoming more popular in the industry . Liquid cleaning in tandem with mechanical pigs will remove a greater volume of debris with fewer runs. Liquid cleaning by definition means the use of liquid cleaners mixed in a diluent (water, diesel, methanol, IPA, MEK, etc.), to form a cleaning solution, pushed through a pipeline using mechanical pigs. Most cleaning companies will use a calculated volume (2 to 3-mils) of liquid solution to coat the interior walls based on a given diameter and length of pipeline, using 10 to 20% of that volume as pipeline cleaner. A minimum of 10% by volume of pipeline cleaner is suggested to not only allow the cleaner to penetrate and permeate the solids but also enough percent to be able to carry the solids out of the pipeline. The cleaner mixed with a diluent should be enough in volume to form a froth to keep the solids in suspension so as not to allow them to settle before being expelled from the pipeline.
There are various manufacturers of cleaners. However, a careful choice of designed pipeline cleaners should be based upon the following characteristics:
- Neutral pH,
- Permeating and penetrating capabilities,
- Original design parameters of the cleaner, and
- Case histories.
Other Liquid Cleaner Types
Pipeline acid cleaners that dissolve solids could form harmful gases and be also detrimental to pipe-wall metals. Other liquid cleaners that dissolve iron sulfide MUST have water in them in order to facilitate the reaction. A popular product, aside from acids, is a product with an active ingredient called THPS (tetrakishydroxymethylphosphonium Sulphanate), formulated with an ammonium ion or organic phosphonate to speedup the dissolution of iron compounds. 1 THPS is well known as a highly effective biocide for a variety of water treatment applications including oilfield down hole treating. However, in those applications THPS was used in ppmv quantities and only lately is being used in percentages to dissolution FeS+. THPS dissolves certain iron sulfides by chelation avoiding the production of any insoluble byproducts and any significant hydrogen sulfide. Rhodia Incorporated continues to state in their laboratory findings “that they are not sure of the fate of the sulfide component of scale but one theory is that it might be incorporated in the complex, possibly attached to the iron atom.” Test have shown that 100 ml of 20% concentrate of formulated THPS solution will dissolution approximately 120 grams of iron sulfides (two types tested: trolite [FeS] and pyrite [FeS2]).2 In other words, one gallon of 20% of neat THPS dissolves approximately one pound of iron sulfide. For reference see in Part 1 Figure A “ Mass Balance Calculations” converting 1.0 ppm of H2S to FeS in a 10 MMSCFD gas flow requiring 847 gallons of the aforementioned solution to dissolve the iron sulfides. THPS is environmentally friendly and is completely, rapidly deactivated in the presence of free oxygen and/or contact with high pH products; i.e. corrosion inhibitors. There is some concern when using THPS that downstream neutralizing of odorants of ethyl or methyl-mercaptans may occur. In some cases THPS is suspected in causing the formation of arsine gas when arsenic is present in the pipeline.
Another liquid cleaner is the gel type. Gels are very good carriers but rely on mechanical pigs to disassociate solids from the pipe walls due to their viscosity. Temperature, concentration, and pH can affect the stability of gels.3
Liquid cleaning basically consists of two types of cleaning products, liquid and gel with distinct physical property advantages/disadvantages between both. We use the term liquid cleaning because the term chemical cleaning conjures up the impression of acids and caustics. Even though inhibited acids may be required at special conditions, most liquid cleaners are surfactant base and gel’s viscosity base. Both types require different cleaning procedures, number of personnel, amount of equipment for the project, and the disposal/recycle cost concerns of spent cleaners and solids. Disposal and recycling, considered off-pipeline cost, is an issue that must be discussed as an overall total package. Off-pipeline cost consists of cleaning of contractor’s equipment, cleaning of frac tanks, disposal of urethane pig parts, and any third party confined-space service charges to complete the aforementioned.
Pipeline Efficiency
One of the great side benefits of liquid cleaning your pipeline is the possible increase in operation efficiency (Figure C). Actual field tests done before and after cleaning have shown that liquid cleaning increased gas capacity with less horsepower due to less pipeline pressure required.4 Increased gas throughput with less horsepower required can result in increased revenues both with more gas being sold, less horsepower means less fuel (less gas shrinkage) which also reduces maintenance on compression units. Another stated benefit is that gas control may start to see their computer modeling balancing out.
With the stated benefits, who can not afford to properly clean their pipelines? Liquid cleaning should more than pay for itself.
Conclusion
In conclusion, the pigging process consists of the aforementioned cleaning applicability, running a gauging plate pig, a caliper pig, pipeline construction corrections if required, the ILI dummy tool run, (optional with approval of ILI company), and finally the ILI tool run itself.
The overall objective is to retrieve data faithfully showing the integrity conditions of your pipeline. To maximize truer conditional data results we believe improving the internal condition by liquid cleaning the pipeline before trying to mechanically/electronically retrieving data, greatly increases the chances of a more reliable and truer ILI tool performance. Cleaner pipelines yield truer data from your ILI tool run.
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