Dirty pipelines decrease flow, production—pig your line
Allen Pennington, president of Pigs Unlimited International located in Tomball, Texas, shared his industry insights about the challenges related to pipeline cleaning.
Many operators do not understand why pigging—or cleaning—a pipeline is necessary, or the full range of benefits that can be gained from it. They often wait too long between pigging intervals, which may result in lost pumping efficiency and/or increased long term maintenance to the pipeline and downstream equipment. In the worst cases, it may even result in a plugged line. Even before the line becomes completely blocked, it may become extremely difficult to pig, or may even be unpiggable, at least without adding other time consuming and costly measures to the cleaning regimen, such as hot oiling, or heating a crude oil line to melt the buildup of wax.
Maintaining a pipeline pigging program is much like performing the routine maintenance on a car. It is much easier and cost effective to maintain a consistent maintenance schedule, than to wait until the car needs an engine replacement, or a pipeline becomes completely blocked. Much like an automobile, all pipelines are unique and each will require a customized pigging program. Pigs perform myriad services to keep pipeline flow optimized at, or near, 100%.
Consider the analogy of a race car running at 200 mph vs. a passenger car at 60 mph. Standard practice for a family car is to change the tires every 60,000 miles. However, a pit crew working on a race car changes the tires many times during a race that is usually less than 500 miles, based on need. For one track, perhaps, the crew changes tires every 200 laps. On another track of approximately the same length the conditions may be different, and perhaps the tires are now only lasting 100 laps. The frequency of the tire change is based on a specific, observed need. This analogy applies to pipelines-two similar lines may have two different cleaning intervals. If you pig two lines every six months, one line might produce 5 lb of debris and 100 barrels of liquid, while another similar line might produce 10,000 barrels of liquid and 200 lb of debris during the same timeframe. The former is probably being pigged often enough, and the latter probably requires more frequent pigging to maintain optimum flow at the lowest impact to pipeline equipment.
There are more than 1,000 varieties of pigs, many of which perform similar functions, but with subtle differences. Ultimately, there are two main categories: intelligent pigs, also called inline inspection (ILI) tools, and pipeline cleaning pigs, sometimes referred to as utility pigs.
Typically, cleaning pigs must be used before using ILI pigs. It is important that the proper cleaning pigs are run first to ensure passage of the ILI tool, which is at risk of becoming stuck—much more so than any other kind of pig—and to ensure the pipe wall is clean, and that the tool can detect the defects it is looking for.
An ILI pig is equipped with an array of sensors and onboard memory to detect corrosion, cracks, or other pipeline anomalies. The sensors use technology based on magnetic flux leakage, ultrasonics, or other inspection methods that show the operator where-and to what extent-there are defects in the pipe wall. This is done by running the ILI pig in the live-flow pipe, which may contain oil, gas, or other products. The tool collects data throughout its run, which will later be scrutinized by an analysis team and interpretive software to provide a comprehensive report on the physical condition of the line, including any defects that may have been found.
Gauging pigs are equipped with an aluminum gauging plate to inspect pipeline roundness and aperture consistency from start to finish. These are commonly used in new construction lines or before the use of a smart tool to ensure safe passage.
Batching pigs are similar to gauging pigs. The term "batching" refers to those specifically designed to also remove liquids from pipelines, separate different products in the same line, and remove debris, such as wax, sand, or production liquids.
Because there are often overlapping models of pigs that perform the same basic functions despite having a different construction or body type, the operator often has a choice of several pigs that will all accomplish the job. Choice often comes down to preference based on the operator’s experience, line conditions, and the feedback the line gives him. For example, an operator wanting to batch diesel and gasoline in the same pipeline can use a bidirectional disc-model pig, unidirectional multi-cup pig, solid cast urethane cup pig, or even a high-sealing foam pig. The operator may believe that one pig does a better job than another in the particular pipeline configuration he is operating despite having never run other styles, or he may have experience with only one style of pig and is leery of trying anything different. He might also consider risk during the selection process. If the line has never been pigged, or it has been years between piggings, foam is a wise choice. It is often the first choice due to its flexibility and low risk of sticking in poor conditions. Some operators that pig the same line frequently often select re-buildable pigs, which are usually steel mandrel pigs, not because they cannot get a foam pig to do it, but because they can usually reuse the pig multiple times, and rebuild it even after the urethane elements have worn excessively. The shelf life of the urethane cups and discs on steel pigs is generally greater than foam pigs, meaning the pig can be stored until needed.
The pipeline and its operation determine the pig
Because of the aforementioned pipeline uniqueness, and the innovations of various pigging manufacturers and users over the past 60 years, there are few regulations or specifications that apply to pigs. However, it cannot be assumed that what works in one line works in all lines, or that one style of pig is always the most effective kind for a particular cleaning application, because line conditions may rule out the use of such a pig.
For example, consider pipeline A and pipeline B, both designated for crude oil, There is one set of pigs for pipeline A, and these pigs can’t be used for pipeline B because it may be producing more sand or wax than pipeline A. Obviously, the operator on B can’t use the pigs intended for A. The pipeline and its operation determines the pig. There is no credo that dictates that for all batching pigs, a specific type of pig must be used; for all wax situations, another particular pig; or for all scale situations, etc. It simply doesn’t work that way. This presents a conundrum for operators because pigging is not well understood. Pigging is part art and part science, and because of this dichotomy, it makes it a hard art to master and experience is the best teacher. Consult a pig manufacturer to best specify the right type of pig to use and a proper pigging schedule.
Keep corrosion out and pipelines clear
Pigs are good at mitigating—but not eliminating—corrosion. This is due in part to keeping the inside of a pipeline stirred up. When the pipeline’s environment is dynamic, it is hard for bacteria to take hold and form colonies, or allow areas of corrosive water to form.
The differential pressure required to move a pig is often misunderstood. The surface area of the back side of the pig must be taken into account when considering the amount of pressure required. The larger the area of the pig, the more square inches the pushing medium has to create force to move the pig. The area of the back of a 42" diameter pig can easily create tons of force to move it with little pressure, in fact a 48" diameter pig might require as little as 10 psi ΔP, whereas an 8" diameter model might require 100 psi or more. This results in about 8 tons of force in the case of the 42" diameter and more than 2 tons of force for the 8" diameter pipe. If there are debris, undersized valves, or protrusions that reduce the identification of the line or change the surface tension between the pig and the pipe, more force may be required. A line pigged with a gas containing sand requires more force than a crude oil line that has only waxy deposits. A pig with brushes may also create more tension. The operator must "listen to" the pipeline to know the type of pig to run and the pressure and flow required. This is the art of perceiving what the pipeline is "saying," and understanding the science of what the pig is doing.
Knowing what type of pig to use in a particular pipeline and in differing situations requires experience and often trial and error. Many engineers, working from generalized guidelines and industry standard rules-of-thumb, specify to run a pig at set intervals for all of their pipelines usually based on the category of the line (production, transfer, condensate, and the fluid medium in the line). These guidelines are not applicable to all pipelines though, and the operator should be willing to be flexible and adjust pigging regiments based on the specific needs of each line. An example of a challenge that an engineer working on an offshore platform may encounter is as follows.
An engineer in the Gulf of Mexico consulted a coworker on another rig for a recommendation for pigs. Based on the information he received, the first engineer ordered the specified pig used by the other pipeline operator. After receiving it, he launched it into the pipeline, and consequently plugged it. The pipe remained stopped for nearly 2 years.
There is not enough emphasis placed on pigging. While schools teach corrosion mitigation techniques, there is not enough training about pigging. The process was often regarded as something simple to do and understand, and only mildly beneficial. However, owners and operators are beginning to see how subtle, intricate, and vital to their operation a pigging program can be. Much more thought is given to pig selection and pigging procedures now than 20 years ago. As interest in pigging increases, so will understanding and knowledge.
– Eric R. Eissler is the editor-in-chief of Oil & Gas Engineering. He can be reached at firstname.lastname@example.org.
Pipelines are not pigged often enough. This decreases flowrates, efficiency, and may lead to a stoppage.
Pig pipelines as needed.
Actions to take
- Learn about the pipeline; know how much sand or wax is produced to make a pigging maintenance plan.
- Reach out to someone who knows about pigging. This could be a senior operator in your company.
- Seek out a mentor with industry experience. Again, senior engineers can help younger engineers learn the best practices and know more about the pipelines on the rig.
- Remember that all pipelines are unique. No one pig nor maintenance plan works for all pipelines.
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Learn as much as you can about the art and science of pigging. In your search for knowledge, consider the following tips:
- Don’t make the mistake thinking you know everything.
- If you are not sure, ask someone who has pigging experience.
- Use your knowledge to figure out flow rates, design pressures, pipelines, and how to repair them.
- Get a mentor and season your knowledge with your experience and advice from others.