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Frequently Asked Questions
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 various build-ups.
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 pipeline.
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.
Ultrasonic inspection is a direct measurement of the thickness of the pipe wall. A transducer emits a pulse of ultrasonic sound that travels at a known speed. The time taken for the echo to return to the sensor is a measurement of the thickness of the pipe wall. 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 to shore.
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.