Pipeline & Gas Journal
January issue 2000:

Advances In Pigging

Effects Of Pipeline Debris On Geometry Tool Data

by Francisco Valentine, Engineer, Columbia Gas Transmission Corp., Strasburg, VA

The use of geometry and metal loss In-Line Inspection (I.L.I.) tools is growing at a furious rate. The data obtained from these tools is used to assess the integrity of natural gas, oil, and products pipelines. The geometry inspection is used to determine whether or not the metal loss inspection tool will safely pass through the pipeline.

Ideal conditions are usually assumed, but this is rarely the case for most pipelines. Pipeline debris is usually present and can affect the quality of geometry tool results. Recent pipeline cleaning activity at Columbia Gas Transmission Corporation, utilizing the online chemical cleaning method, has led to success by increasing pipeline efficiency, reducing maintenance costs, and by providing the ability to acquire quality inspection data.

When the new geometry inspection data was compared with the old, subtle differences were noted. These differences prompted a study into the reason for the differences in the before and after geometry data. Could the debris, or lack thereof, have caused the difference? The availability of before and after data has allowed the examination of the effect of pipeline debris on geometry tool survey results. For the remainder of this article, debris is defined as any foreign matter in the natural gas pipeline system.

Geometry Tool Description
The geometry tool collects inner diameter (ID) data by measuring the amount of deflection in the trailing cup. Mechanical linkages are arranged so that deflections of the cup at any clock position will cause a proportional deflection in a stylus, which writes to a graduated strip chart. The geometry tool is calibrated to the predominant wall thickness in the pipe system, so any deviations from the calibrated measure register as a change in magnitude on the strip chart. When the tool is retrieved from the pipeline, this data is reviewed to determine the minimum bend radius and ID measurements.

Bend radius data is gathered by measuring the deflection of two adjoining sections of the geometry tool. In straight pipe the angle between the two tool sections is zero. As the angle between the two sections increases, such as when traversing a bend, the angle between the tool sections increases. The displacement transcribed on the strip chart is directly proportional to the degree of the bend radius.

Every pipeline system, no matter how “clean,” will generate debris. It may come from compressor oils, glycol, sand, paraffin, iron oxide, iron sulfide, black powder, salt, or combinations thereof. This debris proceeds down the pipeline until it finds a place where it can drop out of the gas stream or is no longer able to overcome resistive forces. Due to gravity, debris in natural gas pipelines naturally tends to collect at the low elevation points in the line between the 4 o’clock to 8 o’clock positions. This type of debris deposition is called trough deposition. This can create a prime environment for concentration cells, under deposit corrosion, and microbiologically induced corrosion (MIC) as this is the place where the presence of fluid is most likely.

Debris can also collect at pipeline features such as welds. Every pipeline accumulates debris differently and this is dependent on the particular pipeline’s flow characteristics, operating mode, gas quality, filtration, and operating history. Debris can be deposited on the internal pipe surface in a concentric layer of debris or as a combination of the previously mentioned fashions (see Figure 1)  As the debris accumulates around pipeline features, it hinders the ability of the geometry tool to produce an accurate representation of the internal pipe surface.

Debris accumulations can cause velocity excursions, which are deviations from ideal geometry tool velocity. As the tool approaches a restriction (caused either by a wall thickness change, dent, ovality, or debris), the tool speed reduces or goes to zero. Pressure differential builds until the tool progresses through the restriction. Once past, the frictional forces are significantly reduced, so the tool accelerates (sometimes achieving very high velocities) until the differential across the tool stabilizes and normal tool speed resumes. Velocity excursions caused by pipeline features such as tees, ells, valves, flanges, et cetera cannot be alleviated, but the intensity of the resulting velocity excursion can be reduced with regular pipeline cleaning. This will prevent detrimental accumulations of debris along the pipeline.

ID Pipeline Debris
ID measurements are affected by pipeline debris. As the cup traverses an ID reduction, the cup deflects. Reduced ID indications can result from dents, ovalities (resulting from overburden), buckles, wall thickness changes, or pipeline debris. The tool does not have the ability to distinguish between them. The cups are designed to run over any debris it may encounter. This reduces the chance of debris accumulating in front of the tool, which can slow, cause velocity excursions, or even stop its progress.

Pipeline debris can also lead to large radius bend indications. This occurs as the tool attempts to negotiate the debris accumulation. The cups deflect in order to get around the debris and cause an increase in the angle between the two tool sections. The result is the indication of a bend. Figure 2 shows an anomaly resulting from the accumulation of debris. This anomaly was originally interpreted as a large radius 90 degree bend with a 2.3-inch total ovality over a 35-foot length.

Upon excavation and inspection, no external signs of an ovality were evident. A very slight sag was present. A section of this pipe was removed and internally inspected. A large accumulation of debris was discovered and is shown in figure 3.

Photograph provided by Rick Meeks, Columbia Gas Transmission.
The following is an example of how debris can manifest itself as short sections of heavy wall pipe. This line being inspected is 20-inch with a wall thickness of 0.312-inch. The data, in figure 4, displays an anomaly that was interpreted to be two short pups (7-foot and 9-foot sections of 0.375-inch pipe).

These indications presented no restriction for the metal loss tool. However, after the online chemical cleaning was performed and geometry tool rerun, these indications were reviewed. These evidently were accumulation of debris deposited in such a way that they could be interpreted as heavy-wall pipe sections.

Data Analysis
Examination of weld indications has provided much insight. Welds typically produce a signature on the strip chart resembling a “spike” at intervals of approximately 40 feet. Review of the geometry tool data is subjective and based on experience in reviewing this type of data. Interpretations are based on visual inspection of the data. The more abrupt the change, the more easily it is detected. Debris attenuates or reduces these changes and spreads it over a larger distance. When significant amounts of debris are present, features can either be misrepresented or totally missed.
As debris collects around the girth weld, it produces a ramping effect caused by the collection of debris on the fore and aft sides of the weld. This ramping effect increases the amplitude and span of the weld indications.

When this debris has been removed from the weld the signal is much more discernable and features more easily identified, figure 5, Wall thickness changes were found after the cleaning program that went unnoticed by the geometry inspection before the cleaning program. This happened in one of two ways. Either wall thickness changes were identified which were not found on the original inspection or additional wall thickness changes were identified upon reinspection indicating the use of a transition fitting. If we were to draw a diagram of the wall thickness change with a layer of debris superimposed it might resemble, Figure 6. It is obvious that the debris prevents the cups of the geometry tool from following the actual contour of the pipe surface. Therefore, wall thickness changes that take place at road crossings, tees, and transition fittings can be missed or misrepresented.

Loose debris in pipelines such as welding rods, skids, weld slag, sand, rocks, and salts introduce extraneous signals into the data. Usually these signals do not represent a substantial ID reduction and are often ignored if they are under the minimum ID specifications of the metal-loss tool vendor. However, these extraneous signals could be interpreted as dents, buckles, or some other anomaly that may prompt an unnecessary search for an anomaly that does not exist. This results in wasted effort, time, and dollars. To reduce these extraneous readings it is important to properly prepare the pipeline for inspection.

Analysis Results
Availability of before and after geometry tool data provided a prime opportunity to examine what effect removal of pipeline debris has on geometry tool data. The following list contains a summary of the noted observations:
1. Fewer anomalies were recorded
2. Reduction in the size and/or length of previously identified
anomalies
3. Data is clearer and features more
distinguishable (less noise in data)
4. Welds are more identifiable (less
exaggeration)
5. More wall thickness changes
were detected
6. Fewer velocity excursions were
experienced
7. Debris can manifest itself as indications of short pups of heavy-wall pipe
These observations overwhelmingly point to the fact that pipeline debris has negative effects on geometry tool data and that there is value in pipeline cleaning activities.
The increased number of wall thickness changes detected by the geometry tool after the cleaning is attributed to the fact that the debris masking or hiding the transition is now gone. In the grand scheme of things, detecting the wall thickness changes is not especially important. However, it provides the investigator with one more piece of information to use as a reference or benchmark when locating more serious anomalies. To that end, this data is valuable.
After review of the available geometry tool data, indicators were identified which may provide a means to assess a pipeline’s cleanliness. The following have been identified as indicators that significant amounts of debris may be present in a pipeline.
1. Known pipeline features (espe-
cially wall thickness changes) are
not identified.
2. Unknown features such as large
radius bends being identified.
3. The presence of extraneous sig-
nals in the data.
4. Identified features lack clarity
(excessive noise).
5. Velocity excursions at places
where there is little or no reason.
6. Exaggerated or unidentifiable
weld indications.
The more of these indications present, the greater the chance that significant amounts of debris are in the pipeline.

Summary & Conclusion
The cleanliness of the pipeline has been found to directly affect the accuracy and clarity of the geometry tool data. Pipeline debris can show up on the data log as anomalies (sharp, flat, or ovality), noise, a general reduction in ID, or missed pipeline features. The anomalies vary in percent ID reduction and span depending on how the debris has been deposited in the line and the pipeline geometry.

A lone geometry inspection can provide an indication of a pipeline’s cleanliness. Before and after geometry tool data can also be used to give an indication of the improvement in a pipeline’s cleanliness. It is recommended that before and after data comparisons be made, by the same vendor and, if possible, using the same tool. The overall effect of the debris is to hide wall thickness changes and introduce extraneous signals into the geometry tool results. This has no detrimental effect to the pipeline’s integrity, but it can affect the operator’s ability to detect an anomaly and accurately characterize the data.

While it is true that a “clean” pipeline provides the highest quality geometry inspection data, the degree of cleanliness should be balanced against the information to be obtained. There are a number of pipeline cleaning methods available and the choice of which method to use should coincide with the desired results. Pipeline cleaning reduces the number of velocity excursions, reduces the chance of extraneous indications, and increases the accuracy of the geometry tool data. Inspection costs can be substantial, so it is all the more important that the operator maximizes the return on its investment- quality inspection data. P&GJ