Pipe dreams

By Diana Milne, Editor

Oil and Gas companies battling pipeline corrosion are finally winning the war thanks to the next generation of pigging technology. Diana Milne meets Tom Sowerby of the PPSA to find out how.

Pigging technology has revolutionised the way companies inspect and maintain pipelines. Where once the conditions inside pipes were a source of mystery and guesswork for oil companies, pigging provides them with crucial data which could save them millions of dollars in lost revenue through leakage or pipeline corrosion.

Tom Sowerby is a technical advisor of the PPSA, an international trade association which serves the pipeline industry. The organisation, which was formed in 1990, has over 90 members representing the pigging industry across the world and its main aim is to promote knowledge of pigging worldwide and to provide a channel for communication between the different PPSA members.

Describing how the oil and gas industry has benefited from In-Line Inspection Tools or Smart Pigs,  Sowerby says: “The use of In-Line Inspection Tools has helped to take away the guesswork about the condition of pipelines. The use of new generation inspection tools has allowed operators to understand exactly what is going on in their lines and therefore helped them to either uprate the lines with Flowrate or pressure with confidence or extend the safe working life of their pipeline investment.“

The availability of data on the conditions of the pipelines has greatly increased companies’ ability to detect and tackle corrosion within their pipes – a condition with the potential to cause serious damage to pipelines and result gas leakages as seen at Prudhoe Bay oil field in Alaska, where severe corrosion and leakages were discovered through data from a smart pig in 2006. As a result oil production on the site was reduced by around 400,000 barrels a day. Expanding on how such data is processed following a pipeline inspection Sowerby says: “The data recorded by In-Line Inspection Tools is carefully analysed using many complex software programmes by experienced analysts and a report is produced for the operator showing the exact details of corrosion, dents etc in the pipeline.” He goes on to say that this data can be combined with information on products being transported in the line to help provide companies with the comprehensive information on the state of their pipelines: “Many other tools can be used to back up the in line tool data such as fluid composition, corrosion coup on readings from the pipeline as well as programmes that can look at the corrosivity of the products being transported in the line and help predict areas of concern in the pipeline.”

Sowerby says he believes that operators can be confident in the integrity of information produced about the conditions of pipelines by the pigging products: “Because the major operators of pigs have many years experience with their pigs they can give, with confidence, reports on the exact state of the pipeline.  On land based pipelines the results can easily be verified by “digging” a feature of corrosion to verify the data.”

However he admits that there are not currently any standardised tools that have been developed to assess the performance of pigs  - therefore there is no accurate way for operators to tell whether the pigs are performing as required. “No specific standards exist for pig performance but operators can look at test results from the pig when pulled through pipelines with known defects machined in them at the pig operator’s workshops,” says Sowerby.

Another traditional shortcoming of pigs is that they are not suitable for all pipes, for instance if the pipe contains butterfly valves. However Sowerby says technology is being developed to create suitable in-line inspection tools for those types of pipes: “Many pig vendors are developing pigs for “unpiggable” pipelines,” he says. “These pigs can be inserted into lines and then reversed out of the lines.  There are a number of crawler type devices being developed which can crawl down a line pulling an inspection head and then reverse back out of the line.  There are also tethered pigs for such pipelines.”

Since pigs were first launched they have undergone several upgrades and become increasingly sophisticated as a technology. Modern pigging systems feature global positioning systems that provide information on the exact location of the pig within the pipe. They can also come equipped with internal cameras that provide live video pictures of the inside of the pipe and the condition it is in.

However Sowerby says the innovation which has made the greatest impact on the effectiveness of pigging is Magnetic Flux Leakage – a magnetic method of non-destructive testing which is used to detect corrosion and pitting in oil pipelines. A powerful magnet is used to magnetise the steel inside the pipe then in areas where there is corrosion or missing metal the magnetic field leaks from the steel.

“Without doubt the technology that has had most affect on the pipeline inspection market is that of Magnetic Flux Technology,” says Sowerby. “To this date it is still the most reliable, most robust and most used technology in the pipeline inspection market.”

What are pigs?

Pipeline pigs are devices which are inserted into and travel through pipelines and are driven by the product flow.

Originally developed to remove deposits in the pipe which could obstruct the oil flow, they are now used for multiple purposes including: Internal inspections; to batch or separate dissimilar products; for displacement purposes.

There are three different types of pigs: Utility pigs for cleaning, separating or dewatering; In-Line Inspection Tools, to provide information on the condition of the line and the location of any problems; Gel Pigs which are used in conjunction with conventional pigs to optimise pipeline dewatering, cleaning and drying tasks.

About the company

The Pigging Products and Services Association (PPSA) was formed in 1990 and now has over 90 members, representing the pigging industry throughout the world. The Association is entirely funded by its members through their annual subscription fees. Its aims are, “To promote the knowledge of pigging and its related products and services by providing a channel of communication between the members themselves, and with users and other interested parties”. PPSA plays a major role in providing information and sourcing equipment and services, for pipeline operators and the industry generally and responds to hundreds of enquiries each year. The PPSA web site enables visitors to source the products and services they need quickly and easily and provides a link to PPSA's technical information service. PPSA runs seminars and training courses on pigging, sponsors lectures and meetings about the subject and supports relevant conferences. Full members are companies manufacturing or marketing pigging products or services. Associate members are companies connected with the industry, e.g. operators, supplier's and allied industries. Individual members are anyone with an interest in pigging.

Frequently Asked Questions

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 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.