Q. What are pigs and pipeline pigging?
A pig is a device inserted into a pipeline which travels
freely through it, driven by the product flow to do a specific task within
the pipeline. These tasks fall into a number of different areas: (a)
Utility pigs which perform a function such as cleaning, separating products
in-line or dewatering the line; (b) Inline inspection pigs which are used
to provide information on the condition of the pipeline and the extent and
location of any problem (such as corrosion for example) and (c) special
duty pigs such as plugs for isolating pipelines.
Q. Why is it called “pigging”?
One theory is that two pipeliners were standing next to
a line when a pig went past. As the pig travelled down the line pushing
out debris, one of them made the comment that it sounded like a pig squealing.
The pig in question consisted of leather sheets stacked together on a steel
body. Without doubting the authenticity of the story, it does indicate that
these tools have been around for some time. Another theory is that PIG stands
for Pipeline Intervention Gadget.
Q. What is the purpose of pigging?
Pipelines represent a considerable investment on behalf of
the operators and can often prove strategic to countries and governments. They
are generally accepted as being the most efficient method of transporting
fluids across distances. In order to protect these valuable investments,
maintenance must be done and pigging is one such maintenance tool.
During the construction of the line, pigs can be used to remove debris that
accumulates. Testing the pipeline involves hydro-testing and pigs are used
to fill the line with water and subsequently to dewater the line after the
successful test. During operation, pigs can be used to remove liquid hold-up
in the line, clean wax off the pipe wall or apply corrosion inhibitors for
example. They can work in conjunction with chemicals to clean pipeline from
Inspection pigs are used to assess the remaining wall thickness and extent
of corrosion in the line, thus providing timely information for the operator
regarding the safety and operability of the line. Pigs (or more specifically)
plugs can be used to isolate the pipeline during a repair.
Q. How is the correct pig selected for a given pipeline?
There are many different pigs available in the market place
and many different suppliers (see PPSA membership list). Choosing the correct
pig is an involved process but if performed in a methodical way, the right
choice can be made. It is important to set the objective and define the task
that the pig has to perform. This may be removal of a hard scale in an 8” line
for a cleaning pig or the location of corrosion pits in a 24” sour gas line for
an inspection pig for example. Operating conditional can sometimes dictate the
type of pig that must be considered. For example, an ultrasonic pig requires
a liquid couplant around the pig and this may be difficult to achieve in a gas
The pipeline layout and features will dictate the geometry of the pig largely.
The pig must be long enough to span features such as wyes and tees yet must be
short enough to negotiate bends. Changes in internal line diameter will
influence the design effort required for the pig. In summary, the correct
pig type is chosen for the task but then the pipeline design and operating
conditions will affect the actual design of the pig.
Q. What inspection Techniques are there?
The main inspection methods that are used are MFL (Magnetic
Flux Leakage) and UT (Ultrasonics). MFL is an inferred method where a strong
magnetic flux is induced into the pipeline wall. Sensors then pick up any
leakage of this flux and the extent of this leakage indicates a flaw in the
pipe wall. For instance, internal material loss in the line will cause flux
leakage that will be picked up by the sensors. Defect libraries are built
up to distinguish one defect from another.
With Ultrasonic inspection a transducer emits a pulse of ultrasonic sound that
travels at a known speed. What is measured is the difference
in time between the return of the echoes from the first surface and second surface.
The structure of the pipe wall is inferred by using this time and the speed of sound in the pipe
material. The technique needs a liquid through which the pulse can travel. The
presence of any gas will affect the output.
Q. What are the differences between offshore and onshore pipelines and
their intelligent pigging procedures?
Offshore pipelines are of thicker wall than onshore-sometimes up to 35mm thick.
Offshore pipelines can have greater operating pressures, particularly the deepwater pipelines
offshore Angola, Brazil or Gulf of Mexico. Maximum operating pressures onshore can be 100barg but
offshore can be 300barg.
Flowrates of products both onshore and offshore are the same dependant upon the type of pipeline or
its position with regard to transporting product either between offshore platforms or from platform
Offshore pipelines tend to be protected by a concrete outer coating and sacrificial anodes fitted
to the pipeline every 100 metres so the outside of offshore pipelines tend not to suffer corrosion
but may get damaged by sea bed movement or anchors from ships.
Inspection of offshore pipelines tends to look for internal problems.
The most favoured inspection methods are either ultrasonic or magnetic flux inspection.
Ultrasonic can inspect very thick wall pipe but magnetic flux is limited because of how strong
the magnets need to be to get enough magnetism in the wall of the pipe to enable good results
to be obtained. Sometimes some pipelines can only be inspected using ultrasonic techniques
because of the wall thickness.
Generally running pigs in
offshore pipelines is very similar to running in onshore lines, after the
wall thickness and higher pressures are taken in to consideration.
One very important thing to realise with offshore inspection is that the pig must not get stuck
in the pipeline as retrieving it will be much more expensive than from an onshore pipeline.
Q. What is a Plug?
A plug is a specialist pig that can be used to isolate
a section of pipeline at pressure while some remedial work is undertaken.
For example, a valve can be changed out while the pipeline remains at
pressure. This can be done by setting two plugs either side of the valve.
Work can then proceed on removing the existing valve and installing the
new one. In complex systems, this can allow production to continue while
maintenance work proceeds at a platform for example.
The plugs can withstand pressures up to 200 bars typically. The plug
works by gripping into the line pipe and then having a separate sealing
system. Lower pressure techniques include High Friction pigs, which
provide a barrier for depressurised systems.
Q. Is it possible to pig multi-diameter pipelines?
For economic reasons, a number of dual diameter pipelines
have been designed and built in recent years. An existing riser or J-tube at
a platform may require that there is a difference between the pipeline and
the riser diameters. Tying a line into an existing pipeline may result in
a change in diameter from one to the next. Dual and Multi-diameter pigs
have had to be designed and tested to allow such systems to be pigged.
These include pre-commissioning pigs for dewatering the lines; operational
pigs to allow liquid hold-up to be removed from gas lines and inspection
pigs to provide information on the line. Typical examples of dual diameter
lines include a 10” x 8” line, a 20” x 16” and a multi-diameter
line 11” x 12” x 14”. The biggest line is the Åsgard gas export line, which
is 28” x 42” in the Norwegian sector of the North Sea. This can be both
pigged and inspected.
Q. How often should a pipeline be pigged?
Pigging frequency depends largely on the contents of the
pipeline. Some sales gas pipelines for example are normally never pigged.
This is since there is little by way of liquid to remove or debris / corrosion
products in the line. On the other hand, production oil lines can suffer from
wax deposition, which must be managed in order to allow production to continue.
It is difficult to give general guidance on this, as the pigging frequency must
be set for each specific pipeline. The general advice would be that a pig is
a valuable flow assurance tool and a decision should be reached with the operator
on the frequency of pigging based on the flow assurance analysis of the line and
in conjunction with the pigging specialists. Likewise, inspection intervals
should be based on discussions between integrity management and the pig vendors.
Q. What is an ‘unpiggable’ pipeline?
Traditionally an ‘unpiggable’ pipeline is one that may not have been
designed with pigging in mind and may lack dedicated launch/receive facilities, or have
complex geometry, such as significant variations in bore, tight radius bends (particularly
for smaller lines) or have bends in close proximity (separation less than the pig length).
However, due to recent advancements in technology and a greater understanding of pig behaviour,
the industry is adapting in such a way that there are fewer ‘unpiggable’ pipelines, just
challenging ones which require the correct level of engineering and planning in order to be
pigged. A thorough feasibility study and tool selection process will ensure that all options
have been considered when assessing the piggability of a pipeline. (Answer by Jee Ltd)
Q. Why has there been an increase in market interest in cleaning complex and challenging pipelines?
The market interest in cleaning ‘unpiggable’ pipelines has been growing in
recent years for a number of reasons. Firstly, operators increasingly need to inspect
unpiggable and challenging pipeline systems and this often comes with the requirement
to clean and prepare the pipe wall for the inspection technology. This has led pigging
vendors to develop new innovative designs and techniques to clean or sweep debris away
from the inspection region to accompany bi-directional or tethered inspections. The
alternative to these techniques is subsea launch which in the majority of cases has
significantly greater associated costs and modifications required.
Additionally the number of decommissioning projects, particularly in the
North Sea, has increased and some of these involve complex pipeline systems that are challenging
to clean and flush via conventional pigging techniques. This again has driven vendors to explore
new technologies utilising gel and even ice pigs. (Answer by Jee Ltd)
Q. What course of action should you take if you suspect that you have a stuck pig?
Pigging is a high risk operation and a stuck or stalled pig can have significant operational,
safety, environmental and commercial consequences. If a pig were to get lost in the pipeline, then, without
detailed preparation, the options for retrieval may be limited and complex. Prior to any pigging campaign,
it is good practise to review the geometry and the operating conditions of the pipeline for any areas that
have potential to impair the movement of the pig, such as damaged barred tees, tight bends, or
intermittent/unreliable flow rates and to assess any mitigations or preventative measures that can be
implemented prior to the run. Typically the first steps after indication of a stuck or stalled pig would
be to identify the likely location and thus probable cause. This can be achieved by collating and analysing
pig run operating data (pressure, flowrates etc.) to identify any unusual signals or trends as well as the
use of trackers to locate the pig. Before resorting to physical retrieval options such as chaser pigs, it
is prudent to investigate retrieval of the pig by altering process parameters only as in certain circumstances
sending a chaser pig could exacerbate the situation. (Answer by Jee Ltd)
Q. Is it possible to inspect pipelines without dedicated receiving sites e.g. a production pipeline from well to platform?
For conventional pigging techniques this would pose a considerable problem. However, bi-directional
pigs and crawler tools enable such pipelines to be inspected as long as there is an existing launcher which can be
utilised as a receiver or the potential to tie-in a temporary PLR (pig launcher/receiver). Bi-directional tools
will require the ability to reverse the flow, but crawler tools (inspection vehicles that are driven by mechanical
mechanisms, such as wheeled or inchworm systems) can be used in the absence of flow in both directions.
The feasibility of these and other potential options should be assessed as part of the tool selection
process to ensure that the chosen tool is suitable for the specific conditions and geometry present in the pipeline
as well as the preferred pigging method. (Answer by Jee Ltd)
Q. What should be considered in the design of a pipeline to enable pigging?
The geometry of a pipeline is one of the key factors in determine the piggability of a system.
As such, to ensure that future pigging is less complex and less reliant of specialist tools and techniques the
following, as a minimum, should be considered:
- Bends should be typically no tighter than 1.5D
- Variations in bore should be kept to a minimum
- Back to back bends should be avoided where possible
- Tees greater than 50% of the pipeline diameter should be barred
- Dedicated tie-in points for launchers and receivers should be provided if not installed at either end of the pipeline
Pigging has often been overlooked in the design of many pipelines which has resulted in some
of the complex challenges we face today. (Answer by Jee Ltd)