Pipeliners Dictionary

The term unpiggable, best describes pipelines that are difficult to inspect internally with conventional in-line tools such as smart pigs

Also, unpiggable pipelines are known as nonpiggable / non-piggable pipelines in the industry. There are a number of ways in which a pipeline is characterized as unpiggable:

Small diameters: The size, or diameter, of the pipe can be restrictive to pigging tools, since most pigs are designed to travel through larger-diameter pipelines (12 inches and above).

Multi-diameters
Small diameters with tight bends
Fabricated (‘mitre’) bends
Thick- or thin-walled pipe
Repair sections in a different size
Over- or under-sized valves

Bends and connections: Bends and connections can cause a pig to get ‘stuck’. Examples of problematic bends/connections include:

Back-to-back bends
Unbarred tees
Y connections
Short-radius or mitred bends
Deadlegs, crossovers, and laterals.

Flow and access: The flow within a pipeline affects pigging, as without sufficient product flow a pig is more likely to stall or become stuck. The internal cleanliness of the pipe can also cause problems with movement inside the pipe. Companies have developed processes to help with these issues, such as pigs that physically clean the pipe, or chemicals or gels that can be pumped through the pipe for the same purpose. Below are examples of pipelines with flow or access restrictions:

Difficulties of access (such as offshore platforms, subsea)
Low or no flow
Cleanliness of the pipe
Pipeline without a pig launcher or receiver.

There must also be sufficient access facilities at each end of a pipeline section in order to launch pigs into the line and retrieve them following a cleaning or inspection run.

Older pipelines: Other restrictions on pipelines are related to how the pipeline was constructed, especially in older, longer pipes. There has been some research and development devoted to enabling pigs to travel longer distances and navigate through some of the restrictions listed below: