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The cleaner the pipe, the better the data: progressive pigging helps ensure inspection success

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World Pipelines,

When it comes to pipelines, “clean enough” isn’t good enough. And while most operators understand the value of clean pipelines when it comes to optimising throughput, it’s easy to overlook just how clean the line needs to be prior to in-line inspection, (ILI) a crucial part of any integrity programme.

To record high-quality, accurate information about dents, corrosion, metal loss and other anomalies, the delicate sensors on ILI tools have to adequately contact the pipe wall. When debris, scale or contaminants get in the way, they interfere with the sensors’ ability to do their job. That can result in a failed ILI tool run with costly consequences: Additional tool runs, increased downtime, lost production and tool recovery or repair efforts can all add considerably to the ILI budget and schedule.

To prevent this sensor “lift off” and improve the odds of a successful inspection, operators perform a pre-cleaning protocol, typically progressive pigging—running increasingly aggressive cleaning pigs in sequence until the operator’s cleanliness specifications are met.

There’s no one-size-fits all approach to pipeline cleaning

Progressive pigging is widely considered the recommended practice for pre-ILI cleaning. The process typically begins with a pig made of urethane foam that’s used to prove the pipeline is piggable. After that, however, things become decidedly more variable. There’s no one-size-fits-all approach to pipeline cleaning: it takes experience, expertise and a thorough understanding of pipeline conditions to determine the right pigs, the right order and the right frequency to achieve the right level of cleanliness.

Even then, the process isn’t fool-proof.

Although each pig in the series must achieve cleanliness standards based upon the amount of pipeline liquids, scale, debris or solids received before the next pig is launched, it is impossible to predict with 100 percent certainty exactly how many total cleaning runs it will take to ensure a successful inspection. For one thing, If a pig doesn’t reach its cleaning acceptance benchmark, the run is repeated. Sometimes, the sequence actually goes back a step or two and a less-aggressive pig is launched, essentially re-starting the series mid-stream.

There have even been instances when the pipeline seemed clean enough for ILI, but it wasn’t—and, unfortunately, it took a failed inspection to reveal that further cleaning is necessary. That was the case for a sour gas operator in the United States, whose pipeline required nearly 50 percent more cleaning passes than their initial plan called for.

“Clean enough” isn’t the same as clean

Prior to inspecting a 1.6-mile segment of a 10-inch pipeline, the operator implemented a progressive pigging program using a variety of cleaning tools and services from global pipeline solutions provider T.D. Williamson (TDW).

The initial scope of work called for running five pigs multiple times, including:

  • A high-density foam pig to determine pipeline piggability
  • Increasingly aggressive cleaning pigs, including ones with steel bristles to scrape away hard deposits
  • A one-piece molded urethane pig with cups and discs to push residue through the pipe and into the trap
Pig RunSIZE (in)DescriptionType
410X-Pig® (1C5D)X-Pig®

The original scope of work called for sending five pigs on multiple runs.

After 14 cleaning runs with little debris accumulating ahead of the pigs, the operator believed the pipeline was clean enough for ILI.

However, TDW was still finding debris sticking to the bottom of brush-equipped pigs and cautioned the operator to consider additional pigging. Facing schedule pressure, the operator proceeded with ILI, instead. Unfortunately, the inspection tool run returned degraded data, meaning the inspection failed. At this point, without time to spare, the operator agreed to more pigging. To race against the ticking clock, TDW suggested using chemical batches to enhance the additional mechanical cleaning.

Chemical batches help ensure first-time ILI run success

Just like soaking a fry pan in soapy water helps loosen stuck-on foods, chemical batching works by reducing the cohesion of debris and contaminants attached to the pipe wall. The chemical slug—generally a customised solution of detergents—travels through the pipe between two urethane pigs. Not only do those pigs help transport the chemical batch, their discs help disrupt the debris before and after the chemical slug, boosting the overall cleaning power.

In the end, it took an additional six cleaning runs—three mechanical, three chemical—to dissolve and remove the remaining debris. However, moving past “clean enough” to truly clean added only one day to the project timetable—time well spent considering it enabled a successful ILI run.

Getting to the right level of cleanliness for your pipeline

Having the cleanest pipe possible is a first step to ILI success. In this case, getting to the the appropriate level of cleanliness took chemical batching, but that’s hardly a universal requirement. Often, progressive pigging is sufficient by itself. Determining the best pre-ILI cleaning approach isn’t a matter of guesswork or starting over with a clean slate: experience can guide operators to ideal pipeline conditions for capturing the most useful ILI data.

Author: P.J. Robinson

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