PTC-2011- Hannover 4-5 April 2011

Advances in Mechanical Damage Management

Author: Martin Fingerhut

Abstract

The threat to pipeline integrity arising from mechanical damage can result in leak or rupture when critical conditions are present such as plain dents, dents with coincident stress concentrators, wrinkles or buckles. Pipeline Operators, world-wide, have routinely employed in-line inspection technologies such as mechanical caliper, magnetic flux leakage and ultrasonic in-line inspection tools to assess mechanical damage. When the population of in-line predictions for mechanical damage is nil then condition discovery and remediation can be straightforward and safety insured with minimal impact to pipeline integrity resources. When significant numbers of mechanical damage conditions are reported by in-line inspection an understanding of the performance of those in-line tools for detection and discrimination of mechanical damage can be used to insure immediate and future safety while optimizing resources employed responding to the in-line tool predictions. Current in-line tools technologies are not able to provide claimed performance specifications for all mechanical damage conditions hence comparisons of assessment predictions with field results is the only way to determine true in-line tool performance. Significant applied research has been conducted to understand the capabilities of inspection tools to detect and discriminate mechanical damage. This paper reviews recent research programs and resulting developments of new direct examination technologies, specifically portable laser profilometry for mechanical damage and the application of direct measurement data for making repair determination and determination of true in-line tool performance. This paper also addresses the application of true in-line performance data to in-line tool mechanical damage features left in a pipeline in order to manage the risk of false acceptance of in-line tool predictions and justify re-inspection intervals.

Author Biography

Martin Fingerhut is Manager of the Asset Integrity Engineering Services USA for Applus RTD in Houston, Texas USA where he is responsible for the transfer of technical knowledge and capability, between and within different geographical regions, as well as providing Asset Integrity Management services for clients worldwide. Martin has a BSc Metallurgical Engineering from the University of Alberta and is a Professional Engineer and active member of ASME, API, NACE and PRCI.

6th Pipeline Technology Conference 2011

Introduction

Decisions to recoat or repair fully discovered integrity conditions within pipeline excavations require reliable direct examination and appropriate Non-Destructive Examination (NDE) technologies and techniques. In addition, effective pipeline integrity management often depends on understanding the capabilities of in-line inspection (ILI) tools in order to apply appropriate technologies in response to potential pipeline threats in the future and respond to ILI predictions appropriately. There is significant potential to leverage data available within ILI response excavations for the purposes of understanding ILI performance when advanced NDE tools and techniques are employed.

Pipeline Operators should understand the performance of the ILI tools in order to guard against false acceptance of ILI predictions as safe and to manage the false rejection of ILI predictions which can adversely affect response resource allocation. The value in validating performance of ILI tools for metal loss threat has been widely recognized by both regulators and pipeline operators. ILI tool capability for detecting and discriminating mechanical damage threat has recently been demonstrated through research sponsored by the USDOT PHMSA. [1] The governing integrity regulations and Codes describe mechanical damage and prescribe acceptable limits for conditions detected by in-line tools. Models for predicting the severity of mechanical damage that could affect oil and gas pipelines have also been under development within the pipeline industry and those models consider measurement data beyond that normally reported with conventional NDE and ILI. The prescriptive requirements and the assessment models, along with supporting analytical tools, enable pipeline operators to decide whether damage indicated by ILI could be a threat to a pipeline on the basis of information from an in-line inspection and other data sources.

The conditions arising from mechanical damage are normally divided into two categories: deformations (dents, kinks, buckles) and gouges. The definitions for dents and deformations vary among many industry standards. Some of the more accepted definitions for mechanical damage include:

ASME B31.8S defines mechanical damage as a type of metal damage caused by the application of an external force, possibly resulting in denting, coating removal, metal removal, moved metal, cold working of the underlying metal, and/or introduction of residual stresses.

API 1160, 1163, 1156 and PDAM define dents as local changes of surface contour with differences quantifying the extent of the deformations. These criteria vary from “perceptible” to “gross” deformation caused by contact with a foreign body.

The PHSMA integrity management regulations in North America define mechanical damage conditions requiring repair or evaluation as:

Dents with any indication of metal loss, cracking or stress riser (response time is orientation dependent)

Dents with depths greater than 2% depending on orientation and location

Dents with depths greater than 6% depending on orientation and location

Gouges greater than 12.5% of nominal wall

There are no ILI tools with the singular capability to detect and discriminate all conditions associated with mechanical damage. Often, data integrated from multiple sensors (inertial, deformation and metal loss) are required to make predictions, with full discovery of conditions possible only after direct examination within excavations.[1] An understanding of the true performance of ILI systems, for detecting and discriminating mechanical damage, is valuable for justifying the response schedules (excavation and re-inspection intervals) to ILI predictions and to insure the correct ILI technology is employed in response to the applicable integrity threats.

True ILI Performance

The determination of true ILI performance is customarily accomplished by comparing ILI predictions (location and physical dimensions) with actual conditions discovered by direct examination (i.e. field comparisons). In order to discern the true systemic ILI error from field comparisons the error associated with the field measurements must be known or the field measurement error must be minimized. [2]

For validation of ILI predictions by direct examination there are a number of available technologies and systems that can be applied, all are comprised of characteristic fundamental errors:

Locating and Matching Error

Validation Measurement Error

The methods of analysis in API 1163 and Ref. [2] assume measurement errors are normally distributed with means zero and unknown standard deviations. The following describes a simple situation where an assumed distribution of the field measurement error is known:

Error Observed = (d/t)ILI – (d/t)Field = [(d/t)ILI – (d/t)Actual] – [(d/t)Field – (d/t)Actual ] = Error ILI – Error Field (1)

where d is the measured depth and t is the nominal wall thickness.

Assuming that the ILI and Field measurement errors are uncorrelated, so that;

2Field

2ILI

2Observed

(2)

where 2 denotes the variance of the error associated with the subscripted quantity,

or, equivalently;

2Field

2Observed

2ILI

(3)

It is important to note that equation (3) is based solely on the assumption that the ILI and Field measurement errors are uncorrelated. There are no other modeling

assumptions. In particular, it is assumed measurement errors are normally distributed with a mean of zero.

The following scenarios should be considered regarding the effect of the in-ditch error and the resulting ability to understand the true ILI error:

1. If the verification errors are larger than the expected error for the ILI tool then little can be learned regarding the true ILI error.

2. If the verification error is equal to or less than the expected ILI tool error then the true ILI error is indeterminate and the apparent validation error requires correction for in-ditch error.

3. If verification error is minimized then the validation result will provide the most accurate understanding of the the true ILI error.

It follows directly from equation (3) that, for practical purposes, ILI=Observed whenever

Field0.1*ILI. Therefore, there is a limit in the accuracy of field measurements

beyond which there is no useful advantage in evaluating ILI. It follows, that if

Field0.1*ILI then ILI=Observed can be assumed even if the precise value of Field is

unknown. Alternatively, if Field>0.1*ILI then Field must be known in order to evaluate

ILI.

In-Ditch Mechanical Damage Measurement

Prior research into the capabilities of current ILI technologies to detect and discriminate mechanical damage identified a lack of a reliable and consistent industry in-ditch NDE protocol capable of measuring three dimensional dent shape and dimensional details for co-incident damage, such as metal loss within dents. [3] This observation, in turn, limited the ability to reliably evaluate the performance of ILI with respect to dents with coincident metal loss or stress risers. Dents and coincident metal loss as shown in Figure 1 are difficult to measure due to difficulties in identification of an un-deformed frame of reference. Recent research identified new developments in three-dimensional measurement technology applied to the assessment of mechanical damage. Figure 2 shows an overview of the Handy Scan(sm) laser profilometry system. The scanner is portable, lightweight (0.9 kg) and commercially available to a wide variety of industrial applications. The specifications of the portable laser profilometry system are shown in Table 1, the single point accuracy is 50μm and 40μm, and resolution is 0.1mm (0.004in) or better, such as 0.05mm (0.002in) depending on the model of the scanner. Specialty software was developed for mechanical damage applications by Applus RTD to build a standard frame of reference from the center of the pipe relative to random positioning targets placed on the pipe surface prior to scanning. [4]

The laser scanning system has auto positioning stereo vision and is able to determine its relative position to the targets by triangulation. When the surface is scanned, the curves and laser lines are recorded by the scanner and an optimization loop is initiated between the scanner and the collected data. The output of the scan is not a point cloud, but rather an optimized surface. This is an improvement over several non-portable laser scanner devices. The accuracy, reliability and repeatability of the laser scanner device were tested by repeatedly measuring a step

wedge where measurements were within +/- 0.003 inch (+/-0.08 mm) 80% of the time.

Table 1. Specifications of the laser scanning system

A sample of pipe containing multiple mechanical damage features is shown in Figure 3. The pipe has an OD of 16 inch and nominal wall thickness of 0.25 inch (6.35mm). The pipe contained five dents, one dent contained a gouge.

The positioning targets for scanning were applied to the pipe surface with a random spacing of 3 to 4 inches as shown in Figure 4. The laser scanner shown in Figure 1 was was employed to scan the pipe surface.

Upon completion of scanning the areas of interest the supporting software is used to import the 3D geometry from the scanner into a compatible format and is used to generate the profile of the dent and any associated feature (in this case, gouge using the 3D model geometry). Figure 5 shows the ability of software to generate the profile for a dent and gouge, and determine the length, depth, and width of the gouge. Similarly, any information regarding dent characterization, length, depth and width, can also be generated from the software.

The supporting software is subsequently utilized to generate the axial and circumferential sections as shown in Figure 6. The coordinate positions along the cut can be generated in polar coordinate array as shown in Figure 7. Once the profile is generated, the axial and circumferential data can be used to characterize the dent length, depth and width. Strain assessment software has been developed that directly utilizes the detailed axial and circumferential displacement output from the laser scanner. The bending strain assessment tool can generate the raw and filtered dent profile data. Figure 8 shows the axial profile of the dent at various axial cuts along the z axis. It also shows the uneven axial profile of the gouge in dent at an axial distance of 126mm, and the profile at the dent deepest point at an axial distance of 146 mm. The profile at dent’s deepest point can be used to determine the depth and length of the dent.

The circumferential profile is generated by the strain assessment tool using a circumferential cut at a distance of 126mm and 146 mm. The profile at 126mm shows circumferential profile of the gouge, Figure 9a. The profile at 146mm shows the circumferential profile at the deepest location in the dent. This profile is used to determine the circumferential angular span and width of the dent, Figure 9b.

The axial and circumferential profile generated can be directly used to calculate the equivalent strain along the dent profile. ASME B31.8 2007(Gas) mentions the depth-based assessment and strain-based assessment method for dents. According to

Technical

SpecificationsModel 1 Model 2

Weight 1.25Kg (2.75lb) 980 g (2.1 lb)

Dimensions

172 X 260 X 216 mm

(6.75 X 10.2 X 8.5 in)

160 X 260 X 210 mm

(6.25 X 10.2 X 8.2 in)

Measurements 25,000 measures/s 18,000 measures/s

Laser Class II (Eye-Safe) II (Eye-Safe)

Resolution in Z axis 0.05mm (0.002in) 0.1mm (0.004in)

Accuracy Upto 40μm (0.0016in) Upto 50μm (0.002 in)

ISO 21 μm + 100 μm/m 20 μm + 200 μm/m

Depth of Field 30 cm (12 in) 30 cm (12 in)

Length

2.487”

Width =0.5008”

Depth = 0.1219”

Length

2.487”

Width =0.5008”

Depth = 0.1219”

ASME B31.8 2007, plain dents are defined as injurious if they exceed a depth of 6% of the nominal pipe diameter. It is also mentioned that plain dents of any depth are acceptable, provided strain levels associated with the deformation do not exceed 6% strain. The technology helps in evaluating the depth- based criteria and strain-based criteria for dent assessment and mitigation. A Dent Strain Assessment Tool has been developed that utilizes the high resolution deformation measurements from the Laser scanning tool to assess for plain dents. [4] It should be noted that when an anomaly is associated with dent, the strain assessment is performed assuming the feature is a plain dent. Any anomaly related strain concentration is not considered in the bending strain assessment.

The strain within a dented area of thin wall pipe consists of two main components; longitudinal and circumferential strain. Each of them can be further separated into bending and membrane strains. The membrane strain is constant through the wall while the bending component changes linearly from the inner wall to the outer surface. Figure 10 is a sketch of pipe wall showing the four principal strain components.[5]

The equations used for computing circumferential and axial bending strain at each point of the pipe surface followin ASME B31.8-207;

Circumferential (4)

Longitudinal (5)

A “t/2” correction to all radii of curvature (R) is used to convert the surface radius to the mid –section of the wall (neural axis). The equations for axial and circumferential membrane strains are derived based on the general formula:

(6)

Where Lo and Ls are the lengths before and after deformation at and around each point of the pipe surface. The membrane and bending strains are then combined together into the equivalent strain for thin-wall pipe under conditions of plain stress.

(7)

Equation (7) is the modified (Czyz et.al) equation for equivalent strain that is different from that found in ASME B31.8-2007 as described by Arumugam [7]. The ASME B31.8-2007 equation for equivalent strain is:

(8)

Arumugam noted that equation (8) is inconsistent with plasticity theory, as a result, the use of equation (8) could either under-estimate or under-estimate equivalent

strain. Equivalent strains calculated by both Equations (7) and (8) are employed by the strain assessment tool used by Applus RTD.

The strain assessment tool is capable of performing both depth and strain based analysis, however, only the strain based analysis is discussed in detail in this paper. The following structured process is incorporated within the strain assessment tool:

Step 1: The axial and circumferential profile data generated by LaserScan supporting software is utilized as an input by the Dent Strain Assessment Tool.

Step 2: The profile is subjected to filtering and smoothing to remove the noise from the data.

Step 3: The processed data is used to calculate radius of curvature along the dent profile.

Step 4: The component and equivalent strains are calculated along the dent profile using the ASME B31.8 and a modified Equation (7).

Step 5: The strain values are plotted along the profile of the dent to indicate the location of equivalent maximum strain in the dent.

Comparison of In-Ditch with In-Line Inspection

The follow is an example of strain assessment of a bottom-side dent with metal loss. The ILI tool reported it as a plain dent, where as in-ditch was characterized as a dent with metal loss. Figure 11 shows the axial profile with the deepest point for both ILI and in-ditch using Model 2 LaserScan from Table 1. It is seen that the ILI measured dent depth is greater than that of excavated dent. This may be due to the removal of constraint and slight pressure reduction during the excavation of dent.

Figure 12 shows spikes along the axial profile of the dent. The spikes are the metal loss features, which are captured in the profile using LaserScan. These spikes are subjected to filtering and smoothing before equivalent strain calculations are performed in order to avoid apparent high strain due to irregular profile. It is to be noted that equivalent strain is calculated assuming the dent with associated anomaly is a smooth dent and the irregularity of corrosion profile does not contribute to the dent strain. This assumption is valid if the corrosion profile follows the dent profile on a micro scale.

Figure 13 shows the maximum equivalent strain using the external LaserScan and ILI data. The equivalent strain based on LaserScan and ILI caliper profile is 8.0% and 3.6%, respectively. In-ditch profile laser based maximum strain value is 2.2 times the calculated value using LaserScan. This is due to the fact that ILI calipers are low resolution tools compared to in-ditch laser tools and may not provide the exact dent profile of the dent which in turn could cause ILI to underestimate the value of the strain and increase the risk of false acceptance of ILI indications.

Currently, there are no quantitive criterion for assessing the severity of gouges coincident with dents. The European Pipeline Rearch Group (EPRG) and published in the Pipeline Defect Asssessment Manual has developed emprical dent gouge models based on the results of burst tests.[6] This model predicts acceptable gouge depths coincident with dent depths. The applciation of such a model necessarily requires the ability to accuractely measure dent depth and relatively small metal loss associated with gouges (<10%wt metal loss). The new three dimensional laser

scanning technology has demonstrated its capability to provide relevant measurements to the dent-gouge assessment model.

Other researchers have employed high resolution, three dimensional laser dig validation data and the capability to evaluate strain and compare with ILI predictions to identify bias in ILI data and adjust ILI data or prioritize the response to ILI predicted conditions. Their approach suggests using high reolution laser mapping in conjunction with bending strain computing models to establish a correction factor based on a correlation study (unity plot) of the computed dent maximum strain between ILI and in-ditch measurement dent profile data. A correction factor was developed to address ILI error for caliper type ILI measurements to provide an engineering basis or guidance for remediation and integrity management of small and shallow plain dents in accordance with current ASEM B31.8-2007 strain based repair code.

Summary

Portable Laser profilometer technology is a promising for precise in-ditch characterization of dents, double dents and dents associated with other anomalies. The detailed profiling can be used to accurately determine the equivalent strain, whose value may guide the mitigation decision. This technology provides accurate and more reliable dent profile data than the traditional technologies, where profile gage was used to measure the dent profile. The application of the technology would: (1) facilitate to evaluate the performance of current ILI technologies for mechanical damage characterization and (2) enhance understanding of the capabilities of the current ILI technologies to detect and discriminate mechanical damage, and (3) make accurate and real time decisions in the ditch.

Figure 1: Example of dent with gouge damage identified by a current ILI technology and assessed by direct examination within a pipeline excavation.

Figure 2: An overview of the Handy Scan laser profilometry hardware.

Figure 3: Pipe containing five dents.

Figure 4: Positioning targets for scan image and laser scanning

Dent 1 (Dent with Gouge)

Figure 5: (a) Scanned image of the dented pipe section and (b) Gouge profile with depth, length and width.

Figure 6: Axial and circumferential dent profile using laser scan system and supporting specialty software

.

Figure 7: Polar coordinate array extracted from the axial and circumferential dent profiles obtained by the laser scanner and specialty software in the

cylindrical coordinate system =degree-circumferential coordinate, z = axial distance, the numbers in the table are the radius of the pipe

Figure 8: Dent axial profile Point to Point.

0 2 4 6 8 10

0 204.0 204.2 204.4 204.7 204.9 205.2

2 204.0 204.2 204.4 204.7 204.9 205.2

4 204.1 204.2 204.4 204.7 204.9 205.2

6 204.1 204.2 204.4 204.7 204.9 205.2

8 204.1 204.3 204.4 204.7 204.9 205.2

10 204.1 204.3 204.4 204.7 204.9 205.2

12 204.1 204.2 204.4 204.7 205.0 205.2

14 204.0 204.2 204.4 204.7 205.0 205.2

16 204.0 204.2 204.4 204.7 205.0 205.2

18 204.0 204.2 204.4 204.7 205.0 205.2

20 204.0 204.2 204.4 204.7 205.0 205.2

22 203.9 204.2 204.5 204.7 205.0 205.2

θ

Z

0 2 4 6 8 10

0 204.0 204.2 204.4 204.7 204.9 205.2

2 204.0 204.2 204.4 204.7 204.9 205.2

4 204.1 204.2 204.4 204.7 204.9 205.2

6 204.1 204.2 204.4 204.7 204.9 205.2

8 204.1 204.3 204.4 204.7 204.9 205.2

10 204.1 204.3 204.4 204.7 204.9 205.2

12 204.1 204.2 204.4 204.7 205.0 205.2

14 204.0 204.2 204.4 204.7 205.0 205.2

16 204.0 204.2 204.4 204.7 205.0 205.2

18 204.0 204.2 204.4 204.7 205.0 205.2

20 204.0 204.2 204.4 204.7 205.0 205.2

22 203.9 204.2 204.5 204.7 205.0 205.2

θ

Z

(a) (b)

Figure 9: Dent circumferential section ‘Point to Point’ for Dent 1 (a) at an axial distance of 126mm, where gouge is located (b) at a distance of 146mm, where the deepest point of the dent is located

Figure 10: Strain components in pipe wall.[5]

-250

-200

-150

-100

-50

0

50

100

150

200

250

-250 -200 -150 -100 -50 0 50 100 150 200 250

Deformed Undeformed

38 degrees

Figure 11: Comparison between axial profiles along the maximum dent depth measured by the laser scanner and an ILI caliper tool

Figure 12: Dent with metal loss profile

Coarse steps with a

small flat peak

Coarse steps with a

small flat peak

Figure 13: Maximum equivalent strains for (a) Laserscan external dent profile and (b) ILI dent profile

References

1. Investigate Fundamentals and Performance Improvements of Current In-Line Inspection Technologies for Mechanical Damage Detection, USDOT/PHMSA DTPH56-06-T-00016, May 2008

2. McNealy, R. McCann,R. et al, In-Line Inspection Performance, Effect of In-itch Errors in Determining ILI Performance, 8th International Pipeline Conference IPC2010-31269, ASME, Sept 2010, Calgary AB, Canada

3. Gao, M., Krishnamurthy, R., Investigate Performance of Current In-Line Inspection Technologies for Dents and Dent Associated with Metal Loss Damage Detection, 8th International Pipeline Conference IPC2010-31409, ASME, Sept 2010, Calgary AB, Canada

4. Arumugam,U.,Tandon, S., Gao,M.,Krishnamurthy,R.,Hanson, B.,Rehman, H.,Fingerhut,M., Portable Laserscan for In-Ditch Profiling and Strain Analysis: Methodology and Application Development, 8th International Pipeline Conference IPC2010-31336, ASME, Sept 2010, Calgary AB, Canada

5. Lukasiewicz, S.A., Cyz, J.,Sun C., Adeeb, S., Calculation of Strains in Dents Based on High Resolution In-Line Caliper Survey, , 6th International Pipeline Conference IPC 2006-1010, Sept 2006, Calgary, AB, Canada

6. Cosham, A,,Hopkins, P., The Pipeline Defect Assessment Manual (PDAM), Joint Industry Project, Andrew Palmer and Associates, November 2001

7. Arumugam, U., Kendrick, D., Tapia., S., Gao, M., An Approach for Evaluating and Prioritizing Dents for Remediation as Reported by ILI Tools, 8th International Pipeline Conference IPC2010-31401, ASME, Sept 2010, Calgary AB, Canada