alternate anomaly response criteria for dent … dinovitzer... · 2019. 4. 9. · 2. dent...

22nd JTM, 29 April – 3 May 2019, Brisbane, Australia of 13 Aaron Dinovitzer et al

ALTERNATE ANOMALY RESPONSE CRITERIA FOR DENT MANAGEMENT

Aaron Dinovitzer* and Sanjay Tiku BMT Canada Limited, Ottawa, Canada Mures Zarea Engie, Paris, France Mark Piazza Colonial Pipeline Company, Atlanta, USA * presenting author

ABSTRACT Pipeline dents can occur from the pipe resting on rock, rock strikes during backfilling, and during pipeline transportation and construction amongst other causes. The long-term integrity of a dented pipeline segment is a complex function of pipe geometry, indenter shape, dent depth, indenter contact, the presence of coincident features and pipeline operating pressure at and following indentation. US DOT regulations (49 CFR 192 / 195) include dent repair and remediation criteria broadly based on dent depth, dent location (top or bottom side), and interaction with coincident features. These criteria are prescriptive and simple to apply to pipeline integrity and repair programs. However, they do not fully capture the range of factors that need to be considered in assessing dent fatigue/failure. For this reason, PRCI MD-4-13 is being implemented to define Alternate Criteria for Dent Assessment. Using the PRCI Mechanical Damage Research Roadmap as the foundation, this paper presents the results of work carried out by PRCI, US DOT, BMT and others showing the importance of dent shape, operational pressure spectra, and presence and extent of coincident features in determining the fatigue life of a dent. Full-scale testing and FEA modelling provided data needed to establish the alternate criteria. This paper defines techniques to geometrically characterize mechanical damage, including dent shape and interacting features. This information is used in identifying what is and is not a dent and characterizing the dent feature shape based upon in-line inspection pipe deformation data and evaluating its restraint condition. Screening tools are used to separate potentially injurious and non-injurious mechanical damage features based on dent shape, pipe geometry and operational pressure conditions. The dent shape and associated strains in the pipe steel are also employed to consider the potential for crack initiation during indentation. The life assessment techniques estimate when and by how much coincident features affect dent fatigue life. Based upon these results, it is possible to manage dents with coincident features, rather than treating them as an immediate repair condition, as currently required by USDOT PHMSA. Further information on mechanical damage response and remediation approaches, based upon past research and experience are also described. The results of this project will provide pipeline operators with guidance to manage dents, considering identification, characterization, screening, assessment, and remediation. The paper will also provide insights into the development of an industry Recommended Practice through API (API RP 1183) for assessment and management of dents which is being developed based upon this research.


1. INTRODUCTION Federal regulatory standards [1][2] require the repair of dents that exceed certain prescriptive thresholds that are based on the dent depth and the presence of interacting secondary features, including corrosion and cracks. However, leaks have been known to occur at dents with depths less than the regulatory thresholds, and dents interacting with secondary features have been known to survive in service for extended periods of time. Similar results have been seen in the full-scale dent fatigue test project, MD 4-2 [3] and DoT 339 [4], as shown in Figure 1.1 where no correlation is evident between dent depth and fatigue life to failure (i.e. to the formation of a leak). The fatigue life observations in Figure 1.1 include a range of dent depths, pipe materials and restraint conditions demonstrating that while dent depth is an important characteristic, it cannot be used in isolation to determine dent severity or fatigue life.

Figure 1.1: Dent Depth Versus Fatigue Life Under 10% to 80% Specified Minimum Yield Strength

(SMYS) Pressure Loading In the full-scale dent fatigue tests carried out in the MD-4-2 project [3] and in numerical simulations completed in MD-4-9 project [5] it was observed that the location, orientation and initiation surface of fatigue cracks depended upon the dent restraint condition, dent depth1 and the indenter size. The observation was consistent with earlier full-scale test results [6]. The above results illustrate that knowledge of the depth of a dent alone is not adequate to determine its severity or predict its fatigue life and that dent behaviour/response is different based on restraint condition. These full-scale test results and numerical simulations also demonstrated techniques to define the restraint condition of a dent using ILI dent shape data and the importance of techniques to consider restraint condition in fatigue life estimation. Furthermore, these results demonstrated techniques to assess and manage the fatigue life of dent features coincident with other features (e.g. corrosion, welds). Using these techniques, the integrity of dents with interacting features may be managed rather than considering them as features requiring immediate remediation. Through research sponsored by PRCI and other research organizations, these results have led to the development of screening and assessment tools and demonstrated that opportunities to inform and enhance codes and standards exist. For example, these results illustrate that US DOT regulations (49 CFR 192 / 195) can be enhanced considering knowledge gained within the course of the research completed in the past 10 years. Table 1.1 provides a comparison of existing US DOT regulations and enhancements that can be considered. To support the enhancement of current mechanical damage assessment and management in operating company integrity management programs, provide input for the development of an industry standard recommended practice (e.g. API RP 1183), and develop a basis for modifying existing regulations, PRCI has sponsored a research project assembling the current state of knowledge. The goal is to develop alternate response criteria and treatment for mechanical damage (MD-4-13). This paper provides an overview of this program and parallel work developing an API (American Petroleum Institute) recommended practice 1183.

1 While there is no correlation between dent depth and remaining fatigue life, the dent depth does affect the

location and orientation of fatigue cracks that form within the dent due to pressure cycle fatigue.


General Comments 49 CFR 192 49 CFR 195 Research

Dent Depth • Response based on

dent depth with deeper dents considered more significant

Dent Location • Top side dent likely

unrestrained, 3rd party damage with a gouge, and more critical

• Bottom side dent assumed restrained rock dent and less critical

• Depth Greater than 6% of nominal pipe diameter requires immediate remediation

• Depth Greater than 6% of nominal pipe diameter

• Upper 2/3 of the pipe requires Immediate remediation

• Lower 1/3 of the pipe requires remediation in 180 days

• A dent with a depth greater than 3% of the nominal pipe diameter on the upper 2/3 of the pipe requires remediation in 60 days

• A dent with a depth greater than 2% of the nominal pipe diameter on the upper 2/3 of the pipe requires remediation in 180 days

Dent Shape • Evaluate fatigue life using 3 Level dent assessment approach to

determine priority • In general unrestrained dents of the same shape and depth as

restrained dents will have lower fatigue lives • Note that shallow restrained dents can have shorter fatigue lives

than deeper restrained dents • Response based on dent shape calculated from ILI data and

operational line pressure history from SCADA Restraint Parameter • Determine restraint condition based upon dent shape from ILI data

using the restraint parameter

Operational Severity (Pressure Cycling)

• Liquid line pressure cycling is more severe than gas lines, resulting in different responses

Operational Severity (Pressure Cycling) • Consider pressure history evaluating operating severity of the line

regardless of product based upon rainflow counting • Consider operational pressure severity attenuation with distance

from station

Dents Interacting With Welds

• Dent depth >2% affecting pipe curvature at girth weld or long. seam requires remediation in one year

• Dent depth >2% that affects pipe curvature at a girth weld or a longitudinal seam weld requires remediation in 180 days

Dent (half length < 6ft (2m)) Interacting with Welds • Girth Weld will not affect dent fatigue life if located at a distance

defined by OD from dent peak • Long seam weld will not affect dent fatigue life if located outside of

90 degree sector (3 clock positions) centered on dent peak • If welds interact with dent, conservatively reduce fatigue life of

plain dent with the same shape by 10x

Dents Interacting with Corrosion

• Dent with any indications of metal loss, cracking or stress raiser requires immediate remediation

• Dent with indications of metal loss, cracking or stress raiser

• Upper 2/3 of the pipe requires Immediate remediation

• Lower 1/3 of the pipe requires remediation in 60-days

Dents Interacting with Corrosion • Developed fatigue life reduction factor related to wall thinning and

surface finish effect. Result supported by testing observations: • Unrestrained dents with < 30% reduction in wall thickness,

reduce fatigue life of plain dent with same shape by 4x • Restrained dents with < 30% reduction in wall thickness, over a

small portion of the pipe surface, consider fatigue life of plain dent with the same shape

Table 1.1: US DOT regulations (49 CFR 192 / 195) Enhancement Opportunities from Mechanical Damage Research


2. DENT ASSESSMENT AND MANAGEMENT The current state of practice for mechanical damage characterization, assessment, management and remediation includes a variety of treatments that are not consistent in their terminology, treatment and recommendations. This paper provides an overview of the work being completed by PRCI (MD-4-13) to develop a comprehensive mechanical damage (Dent) assessment and management process. This process commences with the establishment of terminology to support the process outlined in Figure 2.1. The PRCI project is being completed in parallel and supporting the development of an industry recommended practice for assessment and management of dents in pipelines (API RP 1183). The plan is to have this recommended practice completed in 2019 and be adopted by reference in regulations and standards at some later date.

Figure 2.1: Mechanical Damage (Dent) Assessment and Management Process Flow

The objective of the process is to collect pipe wall observations and determine conditions where geometric variances exist, identify which of the variances meets the definition of a dent, screen the dent feature population to identify those which are potentially injurious, assess the fatigue life and failure pressure of the potentially injurious dent features, make decisions on the need and timing for remedial actions and finally define and implement required remedial actions. The sections that follow provide addition information on each step in the process and provide examples of the techniques implemented in the process.

2.1 Feature Observations In the process outlined in Figure 2.1, ILI or field observations of pipe wall geometric variances are collected to characterize the observed features. At this point in the process, all manner of pipe wall geometric variances may be collected including dents, buckles, wrinkles, ovality, pipe long seam misalignments, amongst others. In this first step, the features are sorted based upon their geometric shape to identify those features that should be considered as dents. The criteria for identifying a feature as a dent can consider:

▪ The direction of the pipe wall deformation (i.e. radially inward or outward) ▪ The aspect ratio of the pipe wall deformation (i.e. features with large circumferential extent

relative to their axial dimension may be wrinkles) ▪ The interaction of features with metal loss and the nature of the metal loss (i.e. does the metal

loss pattern look like gouges) ▪ The interaction of pipe wall deformation features with magnetic anomalies (i.e. localized

plasticity) ▪ The depth of the feature (i.e. are they so shallow that they can not be reliably measured) ▪ The length of the pipe wall deformation feature (i.e. are they localised, or do they extend over the

entire pipe joint)


2.2 Feature and Operational Characterization Prior to the application of dent screening tools it is recommended that the dent features be characterized and the operating conditions of the pipeline in which they are resident in be characterized. A dent is a three-dimensional feature that must consider its entire shape. Traditional integrity assessment treatments for dents characterized them exclusively in terms of depth, often normalized by the nominal pipe outside diameter. While dent depth is a parameter that influences the behaviour of a dent, the response of a pipeline dent depends on its shape. In some instances, screening tools may consider upper bound behaviours of a wide range of dent shapes of a given depth and thus develop a conservative assessment result. The shape of a dent feature will change with the removal of the indenter and changes in pipeline internal pressure. As such, dent shape information should be gathered along with information describing the dent restraint and internal pressure condition at the time of measurement. The maximum pressure historically experienced by the dent should also be reported because this will define the maximum pressure for the stabilized dent shape. The geometry of a dent may be measured using ILI systems or field measurement techniques. In both cases, a three-dimensional description of the dent shape and the pipe surrounding it is developed. The recommended practice for characterizing a dent feature involves the development of two-dimensional longitudinal and transverse profiles of the dent shape through the deepest point of the dent. These dent profiles have proven useful in defining the dent restraint condition and shape parameter for fatigue life assessment of single peak dents. The axial and transverse dent profiles are also useful in demonstrating the performance of FE model-based dent simulations in matching measured dent shapes. For single peak dent assessment, the axial and transverse profiles are characterized in terms of dent characteristic lengths and areas calculated from the axial and transverse profiles as illustrated in Figure 2.2 and described more completely in the PRCI MD-4-9 report [5].

Figure 2.2: Sample Dent Characteristic Length Measurement

2.2.1 Dent Restraint Condition Dents are formed by external indenter contacting the pipe wall and applying a force. If the indenter remains in contact with the pipe at the indentation point while the pipeline is in service, the dent feature is considered to be restrained. If the indenter is removed from contact with the pipe after the dent is formed and the pipeline is in service, the dent feature is considered to be unrestrained.

Thickness

Dia.

Distances from the Dent Peak

Dmax

95% Dmax

50% Dmax

25% Dmax

15% Dmax

𝐿15%𝐴𝑋

𝐿25%𝐴𝑋

𝐿50%𝐴𝑋

𝐿95%𝐴𝑋


The behaviour of restrained and unrestrained pipeline dent features are different. The deformation of the pipe wall within the dent is affected by the support provided by the indenter contact. The stress and strain distribution and fluctuation within the dent feature at any internal pressure condition or due to a change in pressure, will be different in restrained and unrestrained dents. These differences will result in changes in failure pressure and fatigue life. The critical location for fatigue crack initiation will be affected by the dent restraint condition, as demonstrated through full scale testing and numerical modelling. These observations are summarized in Appendix A. Restrained dents are dents where the indenter is constantly in contact with the pipe wall, restraining the dent from re-rounding significantly under internal pressure changes. Unrestrained dents are dents where the indenter is removed after the initial indentation process such that the dent is free to re-round when subjected to internal pressure. Traditionally, the clock position of the dent feature has been used to consider the restraint condition of a dent. Dent features located on the top-side of the pipe (between 10 and 2 o’clock positions) have been considered to be more likely related to third-party damage and thus unrestrained dents. Similarly dent features having signs of gouging, are considered to be third party damage and thus unrestrained dents. Dent features located on the bottom-side of the pipe (between 4 and 8 o’clock positions) have been considered to be more likely to be rock dents and thus restrained dents. It has been demonstrated that the shape of the dent may be used to understand the feature restraint condition since the shape of the dent feature will change in response to the removal of the indenter. Based upon an understanding of this change in dent shape, a restraint parameter (RP) using the previously identified dent characteristic lengths and areas has been developed [5] to predict the dent restraint condition, as defined below:

𝑅𝑃 = max {18 ∗ |𝐴15%

𝐴𝑋 − 𝐴15%𝑇𝑅 |1/2

𝐿70%𝑇𝑅 , 8 ∗ (

𝐿15%𝐴𝑋

𝐿30%𝐴𝑋 )

1/4

∗ (𝐿30%𝐴𝑋 − 𝐿50%

𝐴𝑋

𝐿80%𝑇𝑅 )

1/2

}

The restraint parameter defined above is a dimensionless parameter, where values greater than 20 indicate restrained dents, whereas, values below 20 indicate unrestrained dents. 2.2.2 Coincident and Interacting Features The failure pressure or fatigue life of dent features may be affected by the interaction with other features such as welds, corrosion, gouges and cracks. The size and position of these features within or nearby the dent will define their significance. These features may be geometrically coincident with the dent feature, however, if they do not affect the dent failure pressure or fatigue life, they are not considered interacting. Pipeline longitudinal seam and circumferential girth welds coincident with a dent feature may reduce the dent failure pressure and fatigue life. Assuming that the weld is of good quality, dent-weld interaction criteria have been developed to define the axial distance (dc) from the deepest point in the dent within which the girth weld will reduce the fatigue life of the dent, as follows:

𝑑𝑐 = 𝑎 ∗ 𝑂𝐷 + 𝑏

Where The coefficients, a, and, b, are listed in Table 2.1 for restrained and unrestrained dents.

Restraint Condition

Girth Weld Interaction Constants For 𝒅𝒄and OD in inches

Girth Weld Interaction Constants For 𝒅𝒄and OD in mm

a b A b

Restrained Dents 0.418 3.723 0.418 94.6

Unrestrained Dents 0.129 4.314 0.129 109.6

Table 2.1: Coefficients for Dent Girth Weld Interaction The interaction of a longitudinal weld seam with a dent is defined by the weld seam falling within an angular sector centred on the deepest point of the weld. Table 2.2 illustrates and defines the weld interaction criteria for dent features. The weld will not affect the dent fatigue life if it is located outside


the area included within a defined sector for interaction defined for restrained and unrestrained dent features in Table 2.2.

Restraint Condition Long Seam Weld Interaction Sector Half Angle (θ) From the Dent Deepest Point

Degrees Clock Positions

Restrained Dents 40 1.5

Unrestrained Dents 30 1

Table 2.2: Coefficients for Dent long Seam Weld Interaction When dent weld interaction is defined using the above criteria to exist, the fatigue life is reduced by a multiple of 10 [5]. Corrosion features may be identified by ILI systems or with in-ditch NDE. The corrosion feature will reduce the pipe wall thickness making it more flexible, reduce the remaining ligament for fatigue crack growth and act as a stress riser on the pipe surface. These effects are considered in terms of a surface finish effect and the local wall thickness reduction effect. The combined fatigue life reduction factor (𝑅𝐹𝐿𝑇𝐴) due to both the surface finish effect (RFsf) and the local wall thickness reduction (RFWT) effect is given by:

𝑅𝐹𝐿𝑇𝐴 = 𝑅𝐹𝑊𝑇 ∗ 𝑅𝐹𝑠𝑓 = (𝐾𝑠𝑓 ∗ 𝑡𝑛𝑜𝑚/𝑡𝐿𝑇𝐴)3

where 𝐾𝑠𝑓 is the fatigue strength reduction factor (dependent on UTS) 1.24, 𝑡𝑛𝑜𝑚 is the uncorroded pipe

wall thickness and 𝑡𝐿𝑇𝐴 is the wall thickness at the locally thinned area on the pipe. For calculating fatigue life for dents interacting with metal loss, first plain dent fatigue life will be calculated then the

fatigue life reduction factors will be applied by dividing the plain dent fatigue life by the reduction factor. The fatigue life reduction assessment process may be applied to both restrained and unrestrained dents. If the expected crack initiation surface is on the opposite pipe wall surface as the corrosion

feature (e.g. OD corrosion at a restrained dent) the surface finish fatigue strength reduction factor (𝐾𝑠𝑓)

may be omitted. 2.2.3 Operational Severity The operational severity of a pipeline system is characterized based upon maximum operating pressure and operating pressure time history data. The maximum operating pressure is required to compare against the dent burst pressure. The maximum pressure experienced by a dent historically is also of importance in evaluating the fatigue response of a dented pipe segment. Dent features deform as the pipeline internal pressure increases. As long as the dent restraint condition remains the same and the internal pressure remains below previously experienced maximum pressure magnitudes, the dent deformation process repeats itself and no permanent deformation in the dent occurs. The repeatable dent deformation process defined in conjunction with the maximum historic operating pressure is used to define the fatigue damage accumulation process. In evaluating the fatigue life of a dent feature both the past and future operational pressure time history must be defined. The historic operational pressure time data is used to estimate the fatigue damage accumulation to date and the future operational pressure time data is used to estimate the remaining life of the dent feature.

Dent fatigue crack growth is a result of the application of cyclic internal pressure to the dent feature. A rainflow counting procedure (ASTM E1049-85) [7] is often applied to convert SCADA operating pressure time histories to cyclic pressure histograms. In developing the cyclic pressure spectrum for dent fatigue life evaluation in liquid pipelines, the amplitude of operational pressure cycles has been demonstrated to attenuate between stations and techniques have been developed to define location specific pressure spectra [8]. Due to the complex, variable amplitude nature of an operating pressure time history, it is difficult to quantify the cyclic fatigue severity associated with any given time history, even after developing the pressure range histogram through rainflow counting. The Spectrum Severity Indicator (SSI) is a parameter that quantifies the cyclic fatigue severity associated with a given pressure time history.

θ θ Dent-weld interaction zone


As illustrated in Figure 2.3, the SSI is the number of cycles of a characteristic stress (or pressure range) that results in the same fatigue damage (i.e. crack growth) as the actual pressure time history. Although any characteristic stress range can be used as the basis for the SSI, the SSI is based on the hoop stress range of 90 MPa (13 ksi) [5]. The SSI is presented on an annual (i.e. yearly) basis, regardless of the duration of the actual pressure time history. For time histories that are for a duration that is shorter or longer than one year, the damage accumulated over the entire time history is scaled to represent one year of operation.

Figure 2.3: Spectrum Severity Indicator Description

2.3 Feature Screening The third step in the mechanical damage assessment process involves the application of conservative screening tools to sort features into two groups identifying those that are potentially injurious and those that are benign, considering both fatigue life and failure pressure. The potential for a feature to be injurious can consider:

▪ pipeline operational severity (e.g. cyclic and maximum pressure), ▪ pipe wall stiffness (e.g. flexibility characterized by D/t), ▪ dent restraint condition, and ▪ dent shape.

Typically dent fatigue life is of greater concern for liquid pipelines, whereas failure pressure is of greater concern for gas pipelines owing to differences in typical operating pressure characteristics. Gas pipelines typically operate at higher internal pressures with fewer and lower magnitude pressure fluctuations than liquid pipelines. 2.3.1 Failure Pressure Screening Plain dent features with depth up to 10% of the pipe diameter, without coincident metal loss, weld or crack features have been shown to have the same failure pressure as plain linepipe. A single peak dent in the absence of a coincident feature (e.g. weld, corrosion, gouge, crack) can be considered non-injurious. To apply this screening approach, inspection data identifying the presence of these interacting features must be considered. It is also suggested that the potential for the dent formation process to form a crack be considered using dent strain assessment criteria such as those provided in ASME B31.8 [9] or the ductile failure damage indicator (DFDI) approach [10]. These approaches employ the dent shape to define the pipe wall strain


and limit its magnitude to a critical value associated with the formation of a crack. The applicability of these techniques to unrestrained dent features has not been fully demonstrated, because the unrestrained dent shape will have rerounded and thus not provide good information on the dent formation peak strain. 2.3.2 Fatigue Life - Operational Severity Screening By considering a wide range of dent shapes and pipe geometries a screening approach has been developed to consider the dent fatigue life cyclic loading severity for restrained and unrestrained dents [11]. This approach considers the dent depth, internal operating pressure spectrum severity indicator (SSI) and dent restraint condition of dent to define a lower bound fatigue life as outlined for restrained dents in Table 2.3 (similar table for unrestrained dents has also been developed for INGAA [11]. A fatigue life reduction factor (as described in Section 2.2.2) may be applied to the screening tool estimated fatigue life to consider the impact of corrosion or weld interaction with the dent feature.

Dent Depth, d [%OD] d < 1.0 d < 1.5 d < 2.0 d < 3.0 d < 4.0 d < 5.0 d < 7.0

SSI (Annual number of 13ksi hoop stress cycles)

Fatigue Life (Years)

10 5,692 5,276 4,899 3,964 3,705 3,252 3,053

30 1,897 1,759 1,633 1,321 1,235 1,084 1,018

50 1,138 1,055 980 793 741 650 611

70 813 754 700 566 529 465 436

90 632 586 544 440 412 361 339

110 517 480 445 360 337 296 278

130 438 406 377 305 285 250 235

150 379 352 327 264 247 217 204

200 285 264 245 198 185 163 153

300 190 176 163 132 124 108 102

400 142 132 122 99 93 81 76

500 114 106 98 79 74 65 61

750 76 70 65 53 49 43 41

1000 57 53 49 40 37 33 31

1250 46 42 39 32 30 26 24

1500 38 35 33 26 25 22 20

1750 33 30 28 23 21 19 17

2000 28 26 24 20 19 16 15

Table 2.3: Dent Feature Fatigue Life Spectrum Severity Criteria – Restrained Dents 2.3.3 Other Screening Tools The injurious nature of a dent can also be considered based upon the dent depth, restraint condition and pipe wall stiffness. These approaches demonstrate that low diameter pipelines can be less susceptible to dent fatigue [12]. In some instances, finite element modelling of a dent may be used as a screening tool. In some instances, a rapid assessment may be completed by creating a finite element model of the dented pipeline shape. These models do not capture the residual stresses and non-linear behaviour of the dent developed by the dent formation process, however, they can prove useful in rapidly approximating the dent response and fatigue life.

2.4 Dent Fatigue Life Assessment A three-level approach has been developed for assessing the fatigue life or cyclic pressure loading dependent failure of pipeline dents [5]. All three assessment levels draw upon information regarding pipeline operational, material and mechanical damage (dent) data and recognize the nonlinear response of the dent feature to changes in internal pressure. The three levels provide a range of alternatives for integrity management, where the appropriate method to use is dependent on the desired outcome and available information.


▪ Level 1 Assessment - Dent Geometry Severity Ranking - The Level 1 assessment uses a geometry-based shape factors and shape parameter criterion to assess the relative severity of plain single peak dent features. The relative severity helps in the prioritization of dent features allowing operators to allocate repair or remediation resources effectively to mitigate the effect of cyclic internal pressure on the fatigue life of the dented pipeline segment. The data required for a Level 1 assessment includes detailed in-line inspection (ILI) geometric data and some knowledge of the pipeline operational pressure spectrum (dominant mean pressure and pressure range combination).

▪ Level 2 Assessment – Dent Geometry and Load Severity Ranking - The Level 2 assessment

extends the dent severity shape factors and shape parameter ranking criterion from Level 1 to further consider the effects of the detailed pipeline operating pressure spectrum. The Level 2 assessment provides fatigue life estimates of individual dent features. The data required for this assessment includes a detailed operational pressure spectrum (such as that generated through a rainflow counting algorithm applied to a detailed pressure time history) and the detailed ILI geometric data of the dent feature.

▪ Level 3 Assessment - Dent Fatigue Life Assessment - The Level 3 assessment employs detailed

nonlinear finite element analysis (FEA) model that has been validated against full-scale dented pipeline fatigue trial data. This model provides a life assessment for mechanical damage features and forms the basis for the development of the Level 1 and 2 approaches. The detailed finite element model is intended to be used in fitness for purpose assessments of high consequence dent features and/or for undertaking the assessment of dent features that at present cannot be assessed using the Level 1 or Level 2 assessment approaches.

2.4.1 Analytic Dent Life Assessment (Levels 1 and 2) The details of the calculation procedures for these methods have been previously described [13] and are presented in detail in the MD-4-9 project [5]. These analysis techniques were developed based on 170,00 finite element analysis results considering a range of dent shapes, restraint conditions, materials and operating conditions. The approach taken is unique because it recognises that the indentation process first flattens the pipe at the indenter contact point, then the pipe wall curvature reverses as the dent formation continues. The response of dent to internal pressure fluctuations will be different at depths above and below this change in pipe wall curvature. The change in response is recognized by defining the relative depth of the dent feature for restrained dents as “shallow” or “deep”. Restrained dents are defined as shallow under the following conditions:

▪ Dent depth < 4% of pipe OD [for OD ≤ 12.75 in (324 mm)] ▪ Dent depth < 2.5% of pipe OD [for OD > 12.75 in (324 mm)]

In all other cases dents are identified as having a relative “deep” depth. The significance of the dent shape (i.e. dent shape parameter) is defined differently for shallow restrained dents versus any other dent shape. This approach considers the non-linear dent response to internal pressure to evaluate dent fatigue and is applicable to single peak dents. All dents are considered as dents without interacting features and then the effect of interacting features (See Section 2.2.2) may be applied as a modifier to the estimated life (or relative risk estimate).

2.5 Dent Failure Assessment The failure pressure for dents have been evaluated using stress-based, strain-based and empirical criteria. The stress-based criteria consider fracture and plastic collapse such as those based upon failure assessment diagram (FAD) approaches. The failure assessment approaches follow techniques outlined in in BS 7910 and API 579 and can consider the presence of coincident features. Coincident features are treated as general wall thinning or cracks or stress concentration effects. There are no methods for reliably predicting the burst strength of a smooth dent on a weld, however, research work suggests that sound ERW welds do not affect the burst strength of dents. Kinked dents being dents containing high levels of curvature, are a concern for stress-based failure pressure evaluation. While the behaviour of these features has been a concern, there are no published methods for predicting the behaviour of kinked dents.


The approaches considered in the PDAM assessment manual are outlined the table reproduced from [14]. The lack of models to consider all failure pressure assessment conditions suggest that there is room for improvement in industry practice in this area.

Feature Failure Pressure Assessment Technique

Gouge NG-18 equations [15]; PAFFC [16]; BS 7910 [17] or API 579 [18]

Plain Dent Dent depth less than 7 or 10% of pipe diameter (empirical limit)

Kinked Dent No method

Smooth dents on Welds No method

Smooth Dents and gouges No method

Smooth dents and other types of defect

Dent-gouge fracture model

Table 2.4: PDAM Recommended Mechanical Damage Failure Pressure Assessment Technique

2.6 Post Assessment Remedial Action With the completion of the assessment the fatigue life and failure pressure estimated may eb used to define inspection intervals and or remedial action timing. The objective of the project is to provide guidance to consider different responses to mechanical damage including:

▪ Long Term (Monitor) Response – The conditions under which the response to a feature can be “Leave in place & Monitor” for development of coincident features.

▪ Medium Term Response – The conditions under which the response to a feature can be a “Medium Term” based upon the life estimates developed.

▪ Immediate Response – The conditions under which the response to a feature should be “Immediate” due a low fatigue or failure pressure estimate.

Beyond the assessment the PRCI MD-4-13 guide being developed will provide recommendation on field practice for safe dent excavation [19], an in-ditch feature documentation protocol and remedial actions to answer questions such as:

▪ How to deal with restrained dents. Remove or replace restraint? ▪ Desirable sleeve repair strategies considering steel, composite sleeves and filler materials. ▪ How to grind out surface cracks if found. ▪ How to identify and document dent stress raisers and confirm the absence of cracks after repair. ▪ How to recoat before returning to service

3. CONCLUDING REMARKS This paper provided some insights on Mechanical Damage Research by PRCI and other research organizations (e.g. CEPA, INGAA, API) that has been executing an industry roadmap to develop tools to assess and remediate mechanical damage features. The progress to data provides means of assessing the remaining life or safety of features that were previously identified for immediate repair. The information developed in this program through full scale testing, and numerical simulation is providing a basis to develop new assessment tools and ultimately standards to support integrity management for the industry. This is a work in progress, however, significant progress has been made and the next steps in the process include:

▪ Completion of the API recommended practice for assessment and management of dents in pipelines,

▪ Enhancement of the failure pressure estimation procedure dent features, ▪ Development and demonstration of in-line inspection tool performance identifying and

characterising dents and coincident features The results of this project will provide pipeline operators with guidance to manage dents, considering identification, characterization, screening, assessment, and remediation.

4. REFERENCES [1] Federal Regulations, Title 49 CFR §192 – Transportation of Natural and Other Gas by Pipeline:

Minimum Federal Safety Standards.

[2] Federal Regulations, Title 49 CFR §195 – Transportation of Hazardous Liquids by Pipeline.

[3] Full-Scale Demonstration of the Interaction of Dents with Welds and Localized Corrosion Defects, PRCI Project MD-4-2 (PR-214-073510).


[4] Full-scale testing of Interactive Features for Improved Models” DOT Final Report DTPH56-14-H-0002, 2017.

[5] Fatigue Life Assessment of Dents with and without Interacting Features, MD 4-9 PRCI Final Report, Catalog No. PR-214-114500-R01, November 2018.

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[7] American Society for Testing and Materials, Standard Practices for Cycle Counting in Fatigue Analysis, ASTM E1049-85 (Re-approved 1997).

[8] Semiga,V, Dinovitzer,A, Tiku,S, Vignal,G, “Liquid Pipeline Location Specific Cyclic Pressure Determination”, International Pipeline Conference, Paper IPC2018-78717, 2018.

[9] ASME B31.8-2018 “Gas Transmission and Distribution Piping Systems”.

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APPENDIX A EXPECTED FATIGUE CRACK FEATURES Field observations, full-scale testing and numerical simulation have demonstrated trends in the location, initiation surface, orientation and form of fatigue cracking developed in dent features. The information provided in Figure A.1 may prove useful in support of developing in-line-inspection systems and in-ditch dent inspection procedures. In this figure, the nature of cracking that could be developed in-service are described. To make use of this information, the restraint condition of the dent feature needs to be defined and the depth of the dent feature needs to be considered. Int is context the dent depth is defend based upon the observed behaviour of dent features where shallow dents are defined as:

▪ Depth < 4% of the pipe diameter [for OD ≤ 12.75 in (324 mm)] ▪ Depth < 2.5% of the pipe diameter [for OD > 12.75 in (324 mm)]

Figure A.1: Dent Fatigue Crack Location, Orientation and Surface

alternate anomaly response criteria for dent … dinovitzer... · 2019. 4. 9. · 2. dent...

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