There are many stories about how the term ‘pipeline pigging’ came about. Some claim ‘pig’ is an abbreviation for ‘pipeline intervention gadget’ or ‘pipeline inspection gauge.’ Other theories state that, in the past, leather sealing discs were sent through pipelines and that these squeaked against the pipe wall, making it sound like a pig squealing.
Whatever the truth of the matter, one thing is for certain: build-up in a pipeline can slow flow or plug it, and even generate cracks or flaws, which, under extreme circumstances, can prove catastrophic. To preempt such a situation arising, ‘pipeline pigging’ brings a form of flow assurance, particularly for oil and gas pipelines and flow lines, that ensures the process runs smoothly.
So, how do they work? First, the pipeline pig is introduced into the line via a pig trap, which includes a launcher and receiver. Without interrupting flow, the pig is then forced through the pipe by product flow; or it can be towed by another device or cable. Usually cylindrical or spherical, pigs sweep the line by scraping the sides of the pipeline and pushing debris ahead.
As they travel, there are a number functions the pigs can perform, from clearing the line to inspecting the interior. Inspection pigs, also referred to as ‘in-line inspection pigs’ or ‘smart pigs’, gather information about the pipeline from within, such as pipeline diameter, curvature, bends, temperature and pressure, as well as corrosion or metal loss.
Specialty pigs, such as plugs, are used to isolate a section of the pipeline for maintenance work to be performed, while gel pigs can be employed for product separation, debris removal, hydrotesting, dewatering and condensate removal – as well as removing a stuck pig!
The engineers at FTL know all too well how increasingly complex pipeline systems are becoming and the many demands for pigging solutions. “We’ve worked in the oil & gas sector for many years, and understand the importance of providing pipeline pigs with increased flexibility, adaptability, operational repeatability and improved efficiency,” says the company’s strategic marketing manager Morgan Riding. “It is for this very reason that we developed a multi-diameter pigging system that is capable of meeting these increased challenges in difficult-to-pig pipelines. While build-up in a pipeline can cause product transmission to slow, a major concern is where there are plugs in a line, cracks or flaws, all of which can be dangerous.”
Sweeping the line’s walls helps remove built-up solids like wax and/or precipitated water to help prevent a corrosive environment from occurring. Common pigging problems he highlights include:
- Pigging multi-diameter pipelines
- Large percentage diameter pipeline changes
- Keeping drive pressure low
- Transitions past complex features – wyes, full-bore offtakes and tees
- Tight bends
- Long-distance pipelines.
To solve these challenges, and as a direct result of the functional requirements for the Statoil Åsgard Pipeline RFO (Ready For Operation) commissioning project in Norway (see also www.is.gd/itiluh), FTL developed the multi-diameter VariPig (pictured, above and below).
“Among the challenges to successfully pass through the pipeline system is the initial requirement for VariPig to be launched within the small-diameter pipeline, then increase into the larger-diameter pipeline, before subsequently re-entering into a smaller diameter section of pipeline,” explains Riding. “The inclusion of 3D bends within the smaller diameter pipeline provided additional design challenges for the VariPig system.”
Using the information gathered, a pigging strategy was established to include additional inspection and assessment runs in both the onshore and offshore pipeline sections. It has since installed a series of enhanced sensors on the VariPig, he adds. “Following completion of this new series of runs, VariPig will have successfully completed in excess of 1,250km of pipeline, with just basic routine field maintenance required to maintain operational reliability.”
PIGS ON ICE
Beyond pigging itself, there are other associated methodologies that have proved to be equally effective. Take what might be called ‘Pigs on ice’, for example. Ice pigging, an innovative technique for cleaning pipelines, forces preformed ice into and along a pipe, collecting within the ice harmless natural sediment as it travels through.
The approach was identified and embraced by Northumbrian Water to help drive forward the company’s Trunk Mains Cleaning programme, in order to improve tap water quality for its customers (pictured, left). According to the company’s project manager Richard Johnston, the process is faster and uses less water than traditional methods of cleaning, such as jetting.
It all started more than a decade ago for Northumbrian Water. “As part of a continuous five-year business cycle, we undertook the cleaning of the major pipes from our water treatment works,” he says. “At that time, discolouration was a big problem, as we were dealing with pipes between 50 to 100 years old, and that had led to the accumulation of naturally-occurring organic material, such as manganese, iron and aluminium, on the pipe walls. This material isn’t normally a problem, as it remains in situ, unless a pipe bursts or there is a reverse flow, in which case the material is disturbed and sent into the water supply, and can occasionally end up in customers’ taps.”
TARGETS UNDER THREAT
While the material in the pipes is perfectly safe, adds Johnston, it is unsightly and at that time affected around 1 million people. “The problem we had was that, by just using the standard jetting method of cleaning, we were in danger of not meeting our targets and deadline. We were already aware of ice pigging, provided by a company called Suez Aqualogy, so we invited it for trial on a small section of pipework that was proving quite tricky to cleanse conventionally.”
Unlike traditional pigs, which can be inflexible when navigating the more complicated internal workings of pipes, the ice pig forms a soft plug from its consistency, which pushes through the pipe, scouring it effectively, adapting its shape to fill even complex pipework.
Johnston says the outcome of the trial was very successful, and that ‘planted the seed’. Within 18 months, Suez Aqualogy was back, working with Northumbrian Water on a full-scale ice pigging strategy. “Typically, we isolate a section of pipe up to 1.5km long and depressurise it sufficiently for a train of ice to be injected into the upstream section of pipeline. The downstream section is closed and the upstream valve is opened for the ice to flow through at velocity, then collected at the downstream collection point.”
How effective has ice pigging proved in action? By way of example, after four tonnes of slushy ice was inserted into a pipe during Northumbrian Water’s cleansing operations, 22kg of debris was removed, pumped into a tanker and taken away. Ice pigging enabled the company to tackle the more challenging stretches of pipework at speed and put itself back on track to hit its deadlines. “Ice pigging made a significant difference to the programme delivery,” says Johnston. “Without it, we would almost certainly have been running late.”
Meanwhile, ice pigging itself has taken off, with Suez Aqualogy having grown from a small company, based in Bristol, to an international concern.