Even today, a lot of people laugh when you talk about putting a pig in a pipeline. And it takes a good 15 minutes to explain what they do and what they look like!
There are an estimated 18-20 manufactures of pipeline pigs, most residing in the United States, with an estimated annual global market size of $100-120 million. Pigs come in a variety of configurations, each with a specific purpose.
Pigs are primarily used for:
Cleaning pipelines: To improve or increase flow and reduce or minimize internal corrosion. Increasing the flow or production can quickly offset the cost of pigging. And some state that if you pig your pipeline routinely with the right type of cleaning pig, you can forget spending millions of dollars by injecting corrosion inhibitor in your pipeline to mitigate internal corrosion.
Displacing applications: Displacement pigs displace something with something else. Example: When a pipeline is built it is full of air; the first thing we need to do is displace the air with water for hydro-testing purposes. Once the line has been successfully hydro-tested, we need to displace the water with air, and eventually displace the air with the product (natural gas, refined products, crude oil) for which it was built.
Batching Applications: This is a term used for moving different products through a common pipeline in batches. One method used to keep these different products from mixing or becoming contaminated is to place a batching pig in between the two different products. One might place a batching pig between gasoline and diesel fuel.
Inspection: Most high-pressure, cross-country transmission lines must be inspected every five to seven years, depending on whether it is gas or liquid, to prove the pipeline’s integrity and that it is safe to operate. This is accomplished using a “smart pig,” a tool that carries instrumentation that can detect and report flaws within the steel pipe wall.
These inline inspection (ILI) tools are becoming smarter, processing multiple data sets of information, to determine pipe flaw characteristics.
Now that you know why we pig a pipeline, let us look at the choice of pigs available, and how to select the right one for a given application. A lot of time and money is wasted on selecting the wrong pig for the right application.
To select the right pig, there are certain factors we need to know:
Pipeline size – Pigs are normally size-specific; however, some pipeline are dual-diameter, containing different sizes of pipe. Many years ago, we saw this occur in the Middle East when it was found practical to buy pipe in two different sizes, shipping the smaller inside the larger to save shipping costs.
Pipeline product – What is in the line? Some pigs are not compatible with certain products. Most pigs are made from polyurethane due to the great wear characteristics and compatibility with hydrocarbons. However, urethane is not compatible with ammonia or high temperatures.
Pipeline length – We do not want the pig to wear out before it gets to the end, or it might not come out! Usually, the longer the pipeline, the more cups and discs it may carry.
Pipeline wall thicknesses – Normally not an issue with standard or close to standard wall pipe, but sometimes offshore risers are extremely heavy wall and may pull the cups off the pig before reaching the line pipe.
Bend radius – How sharp are the bends or the bend radius? Sharp bends require short pigs; normally, the longer the pig, the better, because then it can be loaded up with more cups, discs and cleaning elements. However, there are limits in the bends that pigs can traverse. Also, the often-forgotten question is “What is the degree of bend?” Most assume 90° bends but rarely are 90° bends used because of flow-loss characteristics and possibly pipe-wall erosion.
Valve type – Determine what kind of valves this pig must go through. Pig suppliers like full-opening, full-bore valves with little or no cavity drop within the valve. Check valves, wedge gate valves and reduced port valves. These are places that usually stop or tear up pigs.
Pig launcher and receiver – Pig traps, a term used to describe a launcher and receiver, must be designed correctly and certainly must be long enough to hold the pig or pigs. The primary purpose of a pig launcher and receiver is to allow the operator to launch or receive a pig into an operating pipeline without interrupting flow. Imagine, for a moment, having to shut down a pipeline to launch or receive a pig.
Line piggability – Has the line ever been pigged before, and if so, with what type of pig and when? Often, the answer will be something similar to, “Yes, when it was built five years ago.” A lot of material can accumulate over five years, so if you move too quickly with an aggressive cleaning pig, the pig might plug the line with debris.
The purposes of pigging – Cleaning, batching, displacement and inspection. Oftentimes, gauging and cleaning pigs are used prior to performing an ILI. The cleaner the line, the better the internal inspection results.
Having determined the pigging application and having gathered information on the pipeline characteristics, we can start our pig selection process. At this point, we pick up a supplier’s catalog and might see 15 different types of pigs offered in 40 different configurations. It then comes down to the process of elimination.
When we select the size, say 12-inch, we most likely eliminated half the options, because not all types are offered in that size. If we selected cleaning, that eliminated probably another half of the options, because all the batching/displacement tools have been eliminated.
Then, if we are cleaning a natural gas line from hard internal deposits such as scale, black powder or corrosive materials, this tells us we need a brush-type pig. This eliminates pigs with plow blades, which work well at removing soft deposits.
Then we determine it must traverse 1.5D – 45° bends, which shortens the usable pigs or requires that they be equipped with universal joints. This eliminates the pigs restricted to 3-D bends.
At this point we are probably looking at three or four choices. Now it comes down to personal preference. Do we want wear-compensating brushes or fixed-OD brushes? Do we want a throw-away non-rebuildable pig or a rebuildable pig? Certainly cost and delivery also become part of our selection process.
I have probably over-simplified this process, having spent a lifetime designing, developing and testing pigs. I still find applications that require a special pig. But, hopefully, you have a better understanding of what pigs do and which pigs do it well.
By Larry D. Payne, LP Services LLC, and Jesse Green, Pipeline Equipment, Inc.