Copyright 2001 Scientific Surveys Ltd. All rights reserved. 1

Cleaning and geometry inspectionprior to corrosion inspectionby Larry D Payne

TD Williamson, USA

Contents of this Paper:

• Introduction

• Internal corrosion inspection pigs

• Cleaning for corrosion inspection

• What makes a good cleaning pig?

• Geometry inspection

Pipeline Pigging and Integrity Monitoring Conference: Kuala Lumpur, 1998

2 Copyright 2001 Scientific Surveys Ltd. All rights reserved.

Introduction

Today, all pipeline companies are concerned with the internal condition of their pipelines. U.S. federal regulationshave mandated routine internal inspections to monitor internal conditions. Aging pipelines are starting to fail.Leaks, ruptures, and pipeline blowouts are failures that pipeline companies cannot afford. With these concerns,most companies have established programs to routinely inspect their pipelines. These programs, such as riskmanagement, pipeline rehabilitation, pipeline integrity, and corrosion management are common. Job titles such ascorrosion engineer, corrosion technician, and manager of pipeline integrity are becoming critical positions. Weare also seeing conferences, trade shows, and seminars directed toward corrosion control technology. Thetechnology for corrosion detection with internal inspection pigs has improved tremendously and has proven to bean effective method for finding internal corrosion problems.

This paper will concentrate on preparing a pipeline for an internal pig corrosion inspection.

Internal corrosion inspection pigsBefore we can properly understand why a line must be prepared for a corrosion inspection, we must understandwhat a corrosion inspection tool is and how it works. Corrosion tools are massive in size and weight. It is notunusual for a 30-in tool to weigh 5,000 pounds and be 12 to 15ft long. These tools carry on-board computers tocollect, analyze, and collate the data. Most tools work on the magnetic flux principle. This requires that amagnetic field be induced into the pipe. Any disruption of this magnet field is analyzed and reported as a flaw.Today's tools are highly accurate in reporting the percent of metal loss and location. The accuracy in sizing thecorrosion deterioration is dependent upon the cleanliness of the pit and pipeline. Therefore, dirty lines not onlyrisk not being able to properly size metal loss corrosion but the debris gathered with the corrosion tool may causethe tool to malfunction and/or possibly become stuck.

Also, corrosion tools, because of their size and mass, have a hard diameter requirement and a minimum bendrequirement. Hard diameter is defined as the minimum opening the tool will traverse. Most tools will onlytraverse diameter restrictions from 7 to 10%. For example, a 30-in tool could only pass a diameter restriction of2.1in to 3in before damaging or hanging the tool. The other requirement that can affect the type of tool to be usedis the minimum bend radius. Some inspection tools are limited to 1.5 D bends while others can handle 3 D bends.A 1.5 D bend is a bend that is described by having a center radius equal to 1.5 times the diameter of the pipe. Forexample: a 30-in pipeline with a 1.5 D bend would have a center bend radius of 45 ins; a 3D bend in the same sizewould have a center radius of 90 ins.

Therefore, if a line is not clean or contains reductions or bends not compatible with the corrosion tool, the linecannot be properly inspected.

Cleaning for corrosion inspection

Most companies think running one or two pigs prior to a corrosion inspection is adequate. In some cases, this maybe true. However, in most cases it is not. The cleaner the line, the more accurate the corrosion inspection results.The corrosion pig has been recognized as one of the best cleaning pigs on the market. The reasons are:

• it is equipped with 4 to 8 cups;

• the magnetic flux tools have heavily loaded brushes to maintain contact with the pipe wall for magneticpipe contact;

• the pigs are very heavy, which creates a heavy load on the pipe wall; and

• the magnetic field attracts ferrous material. Therefore, it is not uncommon to have a corrosion toolremove more internal debris than a standard cleaning pig. If you are going to clean a line for a corrosionsurvey, you need an aggressive cleaning pig that cleans as well as or better than a corrosion pig.

Cleaning and geometry inspection prior to corrosion inspection

Copyright 2001 Scientific Surveys Ltd. All rights reserved. 3

What makes a good cleaning pig?

A good cleaning pig is equipped with several or all of the possible cleaning elements available. Cleaning elementsinclude scraper discs, scraper blades, brushes, magnets, and scraper cups. The arrangement and design of thesecleaning elements are primary factors that make them effective.

Scraper disc

This is normally a flat disc that attacks the pipe wall with a scraper edge. The harder the disc, the more effective.The larger the diameter and the thicker the disc, the more effective. However, the hardness, thickness, anddiameter must be balanced to give the disc the flexibility and wear characteristics to make it effective.

Scraper blades

These are usually mounted on spring arms to meet a desired load on the pipe wall. These are non-metallic,polyurethane blades set at an angle. The harder the blade and the heavier the load, the more effective the cleaning.However, the load and hardness must be balanced to make the blades effective without wear-out or distortion. Asingle row of scraper blades is the equivalent to three discs in perimeter scraping. The spring-loaded blade makesit wear-compensating. This means as the blade wears, the spring arm keeps the blade against the pipe wall. Thisdoes not occur with a scraper disc. Scraper blades are most effective at removing large amounts of soft depositssuch as mud, sludge, and paraffin.

Brushes

Brushes are the most effective cleaning elements for removing hard internal deposits such as mill scale, blackpowder, sand, calcium, and crusting. This makes them especially effective in corrosion control applications.

Brush designs vary tremendously throughout the industry and one should pay close attention to the detail of thebrush. Brush wrap assemblies only rely upon the spring load of the individual bristles. Once these bristles areworn, the load is lessened which decreases the effectiveness. A spring-loaded wear compensating brush has amuch greater load, making it more effective. Fine wire bristles and individually loaded bristles make brush pigsvery effective for cleaning corrosion pits.

Magnets

Magnetic cleaning elements have only become available over the last few years. The magnetic shoes on thecorrosion pigs gather the ferrous material that creates problems in their information gathering process. Theseproblems have created the need for magnetic cleaning elements. The factors making a good magnet cleaningelement are magnet strength, magnet location, and mounting method. Magnets are more effective when mountedclose to the pipe periphery, and most ferrous materials are located in the lower quadrant of the pipe.

Scraper cups

The primary purpose of a scraper cup is to carry the pig through the line. However, cups do provide anothersource for cleaning, and a good cup design will assist the cleaning operation. The shape, wear surface, andmaterial are the most important factors in cup design. Conical cups with a small mounting diameter provide anexcellent seal and allow the pig to traverse major restrictions.

Pipeline Pigging and Integrity Monitoring Conference: Kuala Lumpur, 1998

4 Copyright 2001 Scientific Surveys Ltd. All rights reserved.

Pig design

The optimum pig design is a pig or multiple pigs giving you all the best cleaning element options. Since it isrecommended that some cleaning occur prior to running a geometry pig, it is important that the cleaning pig havethe capability to traverse major reductions. Lines that have not been cleaned routinely may require a moreconservative approach with a progressive pigging program. A progressive pigging program is one that starts witha less aggressive pig and progresses to the most aggressive pig. Pig trains or coupled pigs have proven to be themost aggressive cleaning pigs. A pig train may be two or three pigs connected together with a flex coupler, whichallow the train to traverse sharp bends. An excellent train pig would be a multiple cupped/disc pig with springloaded cleaning blades, followed by a multiple disc pig with spring loaded cleaning brushes, followed by amultiple disc pig with brush wraps and magnetic cleaning elements.

Once this train is received without any significant amount of debris, the line is ready for a corrosion inspectionpig.

Geometry inspection

The importance of performing a geometry inspection prior to a corrosion inspection assures:

• there are no major reductions that will prohibit safe passage of the corrosion pig.

• the line contains no sharp bends that exceed the requirements of the corrosion pig.

• the degree of cleanliness of the pipeline, and

• the information recorded by the geometry pig can help identify benchmarks and other critical locationsproven to be useful in the corrosion log analysis.

Geometry pigs (often referred to as Kaliper or caliper pigs) were the first internal inspection pigs to enter theinternal inspection market. This occurred in the early 1970s.

Their purpose was to size and locate major diameter reductions such as dents, buckles, or gouges. Most geometrypigs will traverse reductions 30 to 40%. For example, a 30-in geometry pig will pass through a 30-in pipesqueezed down to a 17-in opening. Since the first geometry pigs were introduced, refinements have improvedtheir locating and sizing accuracy, and the bend radius measurement capability has been added.

Today's geometry pigs can size reductions as small as 1/8 in and locate these reductions within a few feet. Thebend radii can be measured in bends with a radius equal to ten pipe diameters.

Geometry pigs can also find and locate internal benchmarks such as: mainline valves, check valves, pipe wallchanges, debris build-up, ovality, and side connections.

Performing a corrosion inspection to ensure a pipeline's integrity is an important decision. Your company's nextdecision should be "What does it take to do it right?"