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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.
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 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.
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:
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)