This blog post examines the fundamentals of maintaining and keeping your pipeline safe throughout its lifecycle. Hear from our own industry expert, Dr. Mike Kirkwood, on the valuable information inside. Enjoy!
The question “Why pig a pipeline” is an interesting one. Essentially there are two reasons: 1) Maintain continuous and optimal operations and 2) Ensure the integrity of the pipe. But before we do a deeper dive, let’s start with a quick, but interesting, history lesson on pigging.
Often for the pipeline novice the first question is, “What is a pig?”
There are a few explanations as to how the term came about. According to “De Architectura,” written by Marcus Vitruvius Pollio, the Romans used inflated pig bladders to clean pipes and aqueducts. Another explanation suggests that, in early U.S. oil pipelines, straw bales, wrapped with barbed wire, were used to clean wax and deposits from pipelines. As a result, when the bale passed by in the pipe, it squealed like a pig.
These details are referenced in countless documents, lectures, training events, etc., but the original source is unknown, and images of these original pigs are extremely obscure. In addition, Wikipedia cites the straw and barbed wire explanation but also states the etymology of the word “pig” as follows:
“PIG" is sometimes claimed as an acronym or backronym derived from the initial letters of the term "Pipeline Inspection Gauge" or "Pipeline Intervention Gadget.”
Here are a few examples of some earlier pipeline pigs that came along much later.
Typical Early Cleaning Pigs
Do Pigs Have Class?
There is also a distinction in the “class” of pig. Sometimes referred to as a “dumb pig,” this class has no “intelligence” and is generally a class that does not inspect the condition of the pipeline, which is commonly referred to as an “intelligent pig” or “inline inspection (ILI)” tool. In certain regions, the word pig is replaced by “scraper,” “diablo” or “go devil.” One thing that is common to them all is they use a pressure drop,(also known as differential pressure), from front to back, to generate motive force. Hence they flow with the product in the line.
Why Pig a Pipeline?
Back to the initial question, “Why pig a pipeline?” Well there are various reasons for doing so and they really fall into two main applications that are outlined below.1. Maintain continuous and optimum operations:
- Cleaning the pipeline – to remove wax, debris, scale, other unwanted products and hence prevent corrosion or buildup reducing flow – removing wax from crude oil pipelines.
- Batching products in liquid pipelines – to separate diesel fuel from aero jet fuel.
- Displacing product from the pipeline – to remove hydrocarbons before decommissioning.
- Treatment of the internal surface – deployment of a corrosion inhibitor.
- Gauging the internal bore of a pipeline – to assess if there are any restrictions in flow or bore passage issues often before a more complex tool is used.
- To stop flow or isolate a section of pipeline – to replace a valve or section of pipe.
2. Ensure the integrity of the pipeline:
- Inspection of the pipeline (using an ILI tool) to assess its integrity – assessing the pipeline for damage, such as dents or corrosion.
What Kind of Pig do you Have?
Pigs are like cars. There are many manufacturers that produce different versions and even different colors, although the color does not impact performance. That said, it’s important to select the right pig for the job.
The simplest of pig is probably the sphere, as it’s an inflated ball that can be used to displace liquids. Up from the sphere, is the foam pig – a cylindrical-shaped piece of foam that is useful for assessing if more complex pigs can pass. Foam pigs can also be used to locate obstructions by intentionally getting the pigs stuck with an onboard locator or transmitter. More and more complicated varieties are available in the market from single bodied, single material to multi-bodied, steel brushed with magnets to pick up magnetic material. Some pigs have small pins, hammers or chisel-like fingers for specialty cleaning, and some have speed control to allow them to run at the optimum velocity. A useful reference in pig designs is provided in this Guide to Pigging.
A Selection of Pig Types
Often to get a very dirty pipeline clean, it is necessary to use a sequence of pigs called a “pig train" or progressive pigging. The selection of the pigs, the sequence and the number of pigs is often determined from experience, but this can change in the field, as the debris comes in from each run. Also, there is a class of pigging that does not use mechanical pigs but such things as gels, liquid batches and even ice.
When it comes to intelligent pigs, again there are different types, depending on what the operator wants to detect. Here is a list of ILI tools and their primary purpose:
- Caliper (Kaliper) - this is a low resolution tool mainly used to detect diameter changes.
- Deformation - a much higher resolution tool used to detect and accurately size geometric anomalies.
- Magnetic Flux Leakage (MFL) - detects volumetric metal loss, mill anomalies, and extra metal.
- Low field axial Magnetic Flux Leakage (LFM) - identifies changes in the steel microstructure in the pipe material.
- Circumferential Magnetic Flux Leakage (CMFL) - provides inspection of longitudinal pipe axis.
- Spiral Magnetic Flux Leakage (SMFL) - provides inspection of longitudinal pipe axis.
- Ultrasonic Wall Measurement (USWM) - detects volumetric metal loss and mill anomalies.
- Ultrasonic Crack Detection (USCD) - locates anomalies of zero width (cracks) in liquid lines.
- Electro-Magnetic Acoustic Transducer (EMAT) - locates anomalies of zero width (cracks) in liquid and gas lines.
- XYZ mapping - enables high-resolution mapping of the pipeline centerline to sub-meter accuracy when tied to above-ground coordinates.
- Combination tools - multiple sensors on one inspection platform providing enhanced characterization and alignment of anomalies.
Combination (Multiple Dataset) Tool
Another class of pigs are those used for isolation. These generally comprise of:
- High-friction pigs for temporary flow stopping of lower pressure pipelines.
- Activated sealing pigs which are again used for isolation of lower-pressure pipelines.
- Double block isolation pigs, which are used where safety is paramount and leakage cannot be tolerated.
Double Block Inline Isolation Tool
When Should you Pig a Line?
The next obvious question related to pigging is “when should you pig a pipeline and how often?” There are a variety of circumstances that determine when you should pig. They include:
- Construction. During construction to get the pipeline ready for operations, pigging is done only once.
- Normal Operations. During normal operations to ensure flow with a frequency dependent on need but can be as much as once a day to once a year.
- Inspection. For inspection purposes, to assess integrity usually set by regulatory requirements, is typically every 5-7 years, or per company policy.
- Maintenance and Repair. For general maintenance and repair, for example, to isolate and replace pipe , when needed.
- Rehabilitation. During rehabilitation or re-use, to bring a pipeline back to service.
- Decommissioning. For decommissioning to remove a pipeline from service, which is done only once.