June 2019, Vol. 246, No. 6
Features
Preparing for Unpiggable Pipelines
By Larry D. Payne, President, LP Services
The simple truth is some pipelines may be piggable for certain applications and non-piggable for others.
A must for all pigging applications is having the ability to load and receive the pig. The pig launcher and receiver must be designed long enough to allow for loading and unloading the pig and be equipped with nozzles, nozzle location, and valves to allow this operation to occur.
For example, cleaning, batching and displacement pigs are normally no longer than twice the pipeline diameter. A 12-inch pig would be about 24-inches long. Whereas, a 12-inch ILI tool could be 10 to 16 feet long depending on the type of ILI tool. Today, a multi-data set ILI tool may carry four or five different inline inspection technologies which can make them extremely long.
The bends within a pipeline must be enough to allow the pig to traverse it. Most pigs, including ILI tools, will negotiate a 3-D bend, which means the radius is three times the pipe diameter. Example, a 3-D, 12-inch bend would have a 36-inch bend radius. It is also important to know the degree of bend. Is the bend a 30-degree, 45-degree, 90-degree? Pigs may traverse sharper bends if the degree of bend is less than 90-degree.
Normally, we know the pipe wall thickness but not the wall thickness of the fittings. Pigs may not traverse bends with a very heavy wall. ILI tools have a tendency to become damaged or get stuck in heavy wall bends.
Any piggable valves installed in a pipeline should be full-bore, full opening. The valve should not have any restrictions within the valve.
Extremely dirty pipelines may cause a pig to become stuck or cause sensor lift-off on ILI tools, which may decrease the tools accuracy. The cleaner a pipeline the better the ILI tool sizing accuracy. Very dirty lines should be cleaned using the progressing cleaning technique or using cleaning fluids coupled with mechanical cleaning pigs before running an ILI tool.
Unbarred tees with flow existing through the side connection tend to pull the pig into the outlet. This could cause the pig to become stuck and cause damage to the pig.
Before running an ILI tool through a pipeline that has not been inspected, a caliper type pig should be used to determine piggability. The pig will determine if the line has any major restrictions or contains significant amounts of debris that may cause an ILI tool to become stuck or damaged.
Another possible hindrance to pigging a pipeline could be low flow or low pressure. ILI tools require a higher pressure to propel them more so than standard pigs. ILI tools also have high-speed limitations, as most can only collect data at speeds less than 10-mph.
In summary, there are many reasons that could make a pipeline unpiggable. A pipeline may be piggable for certain types pigs and unpiggable for others. Pipelines can also be modified to remove unpiggable features or add piggable features. Pipeline operators should be very cautious in selecting and determining the correct pig and pig type before attempting to pig their line. P&GJ
Author: Larry Payne has over 50-years of experience in the oil and gas pipeline industry.
A pipeline could be unpiggable because:
- It’s not equipped with pig launcher/receiver or not long enough to allow the installation or removal of a long pig such has an ILI tool
- Sharp radius bends or bends welded back to back
- Very heavy wall pipe, bends or tees
- Unpiggable valves such as plug valves or possibly wedge gate valves
- Extremely high build-up of internal debris
- Unbarred tees – primarily the ones with pipeline flow exiting through the tee
- Type of pig to be used
- Low flow/low pressure conditions or possible high flow
conditions
Pipelines are normally pigged for one or
more of the following reasons:
- to clean the pipeline
- to batch different products within a common pipeline
- to displace something within a pipeline with something else (as in new construction when air must be displaced with water for hydrotesting purposes; then the water is displaced, and the line is filled with the product for which it was intended to transport.)
- to inspect the pipeline with an ILI tool for
integrity purposes
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