Plain Dents Research Articles

Standards and methods for dent assessment and failure prediction of pipelines: A critical review

Dent, a common mechanical damage on pipelines, is associated with a significant local plastic deformation. Dents can cause pipeline failures, especially when they are combined with other types of defects such as gouges, fatigue, corrosion, and cracks. In this work, a systematic review of various assessment methods and standards for pipeline dents, including the combination of a dent with other defects, is conducted. Generally, the methods available today are not sufficiently accurate and reliable to assess pipeline dents, especially the dent-defect combinations. For plain dents on pipelines, both the depth-based criterion and the strain-based criterion are commonly used in engineering. Their main problems include inaccuracy and conservatism. For a dent combined with other defects, the existing assessment techniques are not mature enough to give reliable results. Both experimental testing and numerical modeling through finite element (FE) analysis are capable of investigating the influence of dents and dent-defect combinations on burst failure pressure of the pipelines, although an approximation to the reality is still the main difficulty existing in the experimental testing and FE analysis. Nowadays, relevant studies on assessment techniques for plain dents, a dent with fatigue and a dent with a single gouge have been common in literature. The combinations of a dent with corrosion or cracks have been rarely assessed due to complicated mechanisms involving a multi-physics coupling effect. Development of novel assessment methods by integrating mechanical stress and strain, electrochemical reactions and steel metallurgy will be a key topic to accurately assess the dent-defect combinations for improved pipeline integrity.

Assessment of Stress Concentration Factors of Dented Rigid Risers Under Fatigue Loadings

Abstract In the development of oil and gas fields, subsea pipes are used in various applications, like pipelines and risers. During operation, risers can be subjected to accidents, such as collisions with other risers, anchors, rocks, or any heavy equipment or objects, which may lead to mechanical damages. These mechanical damages are commonly characterized as dents. The objective of this work is to study the effect of the introduction of plain dents on the fatigue life of rigid risers under fully reversed bending with the conduction of resonant bending tests. A three-dimensional finite element model was developed to estimate the stress concentration on dented risers under bending. Numerical simulations and experimental tests were carried out to evaluate the resulting stress concentration factors (SCFs). These SCFs can be used in the prediction of the remaining fatigue life of dented rigid risers.

Multi-dimensional mechanical response of multiple longitudinally aligned dents on pipelines and its effect on pipe integrity

Dents have been identified as among the most common deformation defects in oil and gas pipelines. In this study, the multidimensional mechanical responses, springback, and rerounding behaviors of dents as well as the interaction of multiple dents on pipelines and their effect on pipe integrity were investigated through the three-dimensional finite element method and full-scale burst tests. The burst test demonstrates that a plain dent has a minimal effect on the pipeline burst pressure, and the burst failure position is not located in the dented area. Generally, when a dented pipeline that is subjected to monotonically increasing internal pressure bursts, the dent does not fully reround, and a convex protuberance forms in the axial shoulders on both sides of the dent. During pressurization, the maximum equivalent plastic strain in the dented pipe always occurs on the inner surface near the dent center in which no necking is found in the radial direction of the wall thickness. However, a larger equivalent stress is observed around the edge of the dent center, whereas the stress at the center is relatively small, resulting in the formation of a plastic hinge in the dented area. If internal pressure fluctuations occur, this can easily lead to fatigue failure. Finally, the interaction among multiple longitudinally aligned dents was investigated, and the critical spacing affecting the integrity of the pipeline was determined. The investigation reveals that when the dent spacing exceeds the pipe diameter, the interaction between two dents may be ignored, and the dent effect on the pipe integrity may be separately evaluated. However, if the dent spacing is less than 0.5 times the pipe diameter, then the dents must be treated as a combined dent. Further, if the spacing is 0.5–1.0 times the pipe diameter, then dent interaction must be considered.

The role of a plain dent on the failure mode of a crude oil pipeline

A failure of a subsea crude oil API 5L X52 welded steel pipeline with an 18-inch outside diameter has caused oil leakage and this event was reported after 20 years in service. Based on the post-failure investigation, it was found that there was a circumferential through-wall crack in the center of the plain dent at the leak point of the pipeline. This study investigates the role performed by the dent in the failure mode of the pipeline. Macroscopic and microscopic observation revealed that the internal surface of the pipeline contains a local erosion around the fracture zone while the outer surface contains a lot of surface cracks. The results of the investigation showed that crack was started on the outer surface at the bottom of the severely deformed (concave) zone. The concavity of the deformed pipe outer surface could result in sufficient stress cycles and residual stresses, the condition necessary for the initiation of a corrosion fatigue crack. Through thickness propagation of fatigue crack continued toward the pipe inner surface until operating stresses exceeded the yield strength of the material. As the cracks progressed, the stresses exceeded the strength of the remaining intact pipe, and a failure occurred.

Strain ratcheting failure of dented steel submarine pipes under combined internal pressure and asymmetric inelastic cycling

This paper presents the results of an experimental study in which for the first time the progressive inelastic deformation of high strength steel pressurized pipes with plain and gouged dents was analyzed. The effect of some key loading and geometrical parameters such as initial pre-strain (εmon), dent depth ratio (δ/D) and the presence of the gouge was studied. Local dents with a hemispherical shape and gouges with different dimensions were introduced on the surface of the specimens. Specimens with plain or gouged dents were first internally pressurized, axially compressed to an initial pre-strain then subjected to cyclic nonlinear stressing.The mechanical defects were found to drastically degrade the inelastic cyclic response of the pressurized steel pipes. The presence of a dent, even with small amounts of initial pre-strains, made the pressurized pipes vulnerable to ratcheting failure. Furthermore, the ratcheting responses of the gouged dented specimens significantly differed from those for the corresponding specimens with plain dents. In fact, the combination of a dent and gouge was the severest form of the defect and demonstrated the lowest number of cycles before failure. The degradation effects of gouge were strongly dependent upon the gouge geometry, dent depth and initial pre-strain level.

Local buckling failure analysis of high strength pipelines containing a plain dent under bending moment

This work investigated the local buckling failure of high-strength and large-diameter pipelines containing a dent under bending moment. An improved three-dimensional numerical model was developed by considering the multiple nonlinearities of pipe geometry, material and the boundary conditions. The effect of yield strength and strain hardening parameters of pipeline steel on the critical buckling load at the dent was analyzed in detail. The results show that, the critical buckling moment of the pipeline decreases with the increase of n. However, when the internal pressure is low, the influence of internal pressure on the critical buckling moment is limited, and the hardening exponent affects the critical buckling moment only when n < 24. The greater the internal pressure, the faster the critical bending moment decreases with the strain hardening exponent, especially when the circumferential stress of the pipeline exceeds the yield strength of the steel. The sensitivity analysis of the size parameters of dents indicates that the large and deep dent can significantly reduce the bending moment resistance of a pipeline.

Strain evolution characteristics of X80 line pipes with plain dents

In the process of construction and service, high-grade line pipes will get defective, e.g. dents, which will change its stress and strain distribution characteristics and impact its service reliability. In this paper, a X80 line pipe was taken as the research object. The distribution characteristics of the strain field in the X80 line pipe with plain dents with the change of dent depth under external load were analyzed using the finite element analysis software ABAQUS. Then, the strain distribution and microstructure characteristics in the dent zone were explored by conducting prefabrication test on physical dent. Finally, combined with the finite element simulation results, the strain distribution laws of the X80 line pipe with plain dent were discussed. And the following research results were obtained. First, under the same internal pressure, the strain distribution characteristics in the dent zone at different dent depths are similar, i.e., the strain increases with the increase of the distance from the center of the dent, and decreases rapidly with the increase of the distance after the peak strain. Second, the strain increases with the increase of dent depth, and under the same internal pressure and dent depth, the axial strain is larger than the radial strain at the same location. Third, the greater the dent depth, the stronger the superposition effect of internal pressure and depth on the strain. Fourth, strain hardening occurs on the materials in the initial stage of the dent deformation. With the aggravation of deformation and the extension of dent radius, the strain response ability of materials increases, the grains at the bottom and side walls of the dent zone are elongated along the direction of maximum deformation, the lattice is distorted and strain hardening occurs. As a result, the dislocation density in this zone increases and the interaction occurs between dislocations, as a result, the strength of line steel is enhanced. In conclusion, the research results do well in predicting the stress–strain evolution laws in the process of dent, and provide a theoretical foundation and an experimental basis for studying the influence of mechanical damage on the service safety of pipelines.

Stress-strain assessment of plain dents in gas pipelines

Paper presents the analytical solution of the stress-strain state for a dented pipeline, based on the method of equivalent loads. First off all solution for a harmonic imperfection is found, then using Fourier series expansion a semi-analytical procedure is proposed to assess single dent. Comparison between analytical and numerical results for the axial force and pressure load is given. Influence of the dent dimensions, shell radius to thickness ratio and initial loading to stress concentration factor are discussed.

Mechanical behavior investigation on the formation of the plain dent of an API 5L L245 pipeline subjected to concentrated lateral load

As the main geometric defect in pipeline maintenance, dent has been paid more attention by pipeline managers because of its strong stress concentration. To investigate the formation mechanism of a pipeline dent, an indentation test of a pipe subjected to concentrated lateral load was carried out in the laboratory to simulate the formation process of the dent, and a 3D numerical model corresponding to the test was established to verify the results between the experiment and simulation. Subsequently, the stress and strain response of the dented pipeline was analyzed in detail in various stages of dent formation process. Deformation analysis revealed that the residual stresses state formed in the dented area after unloading is related to the depth of the dent, which may be in tension or in compression. During the formation of the dent, the pipeline presents “∽”-type wave-like bending state in the axial direction. These dent formation mechanisms may provide a guidance for dent assessment studies.

Generalized expressions for stress concentration factors of pipeline plain dents under cyclic internal pressure

The present work follows a previous study devoted to developing a new methodology for fatigue life assessment of steel pipelines with plain dents, under cyclic internal pressure, based on the estimation of SCFs to modify appropriate S–N curves. SCFs were estimated from a non-linear finite element model and the methodology was validated according to experimental results. A comprehensive parametric study was then carried out for spherical, longitudinal and transverse plain dents, with varying pipe and dent dimensions, and analytical expressions to provide SCFs were developed for each dent type. The aim of this work is to simplify these expressions, which have the disadvantage of being related to particular dent types (geometries). Here, a new analytical formulation is developed and simpler expressions are obtained to estimate SCFs. The proposed methodology becomes then more practical and easy to be used in the fatigue life assessment of steel pipelines with plain dents of different types, based only on measurements of dent dimensions without the need of classifying them according to their types.

Effects of Loading Sequences on Remaining Life of Plain Dents in Buried Liquid Pipelines

AbstractThe integrity assessment of dents in liquid pipelines, as regulated by codes and standards, is based mostly on depth and threat integration, with strain analysis incorporated as a nonmandat...

Simulation Analysis of the Pressure Carrying Capacity of X80 Pipe with Plain Dents

In order to study the pressure carrying capacity of X80 pipe with plain dents, the formation process and the hydraulic test were analyzed by finite element simulation. Based on this, the sensitivity analysis of the factors affecting the pressure carrying capacity of the pipeline, such as the internal pressure, the confinement state and the material performance, is carried out. Research results show that springback amount of the pipeline decreases due to the initial internal pressure, and constraint state has little effect on the pressure carrying capacity while increases with the increasing of the material tensile properties. When the depth of the dent is less than 6% pipe diameter or the strain of the dent is less than 6%, the dent has little impact to the pressure carrying capacity of the pipe.

The analysis of damage degree of oil and gas pipeline with type II plain dent

Abstract Dent is a common type of defect in oil and gas pipelines and the assessment on dented pipelines is carried out with major international standards and specifications based on dent depth as the evaluation criteria. However, such evaluation criteria based on depth does not account for the impact of various parameters (e.g. parameters of dent, parameters of pipeline and the internal pressure of pipeline) on the evaluation result, as a result, many dented pipelines lose their efficacy, even though they meet the depth-based standards. The influence of parameters changes on the damage degree of II-type dented pipelines is investigated on the basis of Oyane's ductile fracture criterion and by the method adopting finite element numerical calculations. Finally, the non-linear regression analysis is conducted and based on the outputs, a specific expression of dent depth and pipeline damage degree is also acquired within a certain scope of application.

The Analysis on Damage Degree of API X52 Sensor Pipe Caused by the Defect of Plain Dent of Type I

The Analysis on Damage Degree of API X52 Sensor Pipe Caused by the Defect of Plain Dent of Type I

Fatigue life assessment of damaged pipelines under cyclic internal pressure: Pipelines with longitudinal and transverse plain dents

This ongoing work aims to refine a methodology for fatigue life assessment of dented steel pipelines under cyclic internal pressure. A numerical model is developed to obtain stress concentration factors on dented pipes under internal pressure. A comprehensive parametric study is accomplished and analytical expressions to estimate stress concentration factors for spherical, longitudinal and transverse plain dents as a function of selected geometric parameters are developed. The proposed fatigue assessment methodology is validated from fatigue test results. Here, the analytical expressions are refined to include a more complete dent geometry definition for transverse, longitudinal and short longitudinal dents.

Displacement Analysis of Plain Dent on Pipeline Based on FE Calculation

According to the report of the United States transportation department, mechanical damage is one of the most important reasons for the pipeline accident .The most typical form of mechanical damage is indentation. Dent defect is one of the important factors affecting pipeline fatigue life, and it will greatly reduce the fatigue life of the pipeline in service. Meanwhile the dent displacement will be changed with the operation pressure fluctuations of the in-service pipeline, resulting in a circular bending stress, which directly affect the pipeline fatigue life. For typical plain dent on pipeline, the finite element models are established under different circumstances. A large number of the calculation results are sorted and inducted. On this basis, the results are analyzed by univariate. Non-linear regression analysis was utilized to fit the results, some specific expressions of the relationship between the dented pipeline displacements and the wall thickness and diameter of pipeline, the dent depth and longitudinal dent length are obtained after much calculation and analog.

FE Calculation and Analysis of Plain Dent Size Influence on Pipeline Displacement

According to the report of the United States transportation department, mechanical damage is one of the most important reasons for the pipeline accident .The most typical form of mechanical damage is indentation. Dent defect is one of the important factors affecting pipeline fatigue life, and it will greatly reduce the fatigue life of the pipeline in service. Meanwhile the dent displacement will be changed with the operation pressure fluctuations of the in-service pipeline, resulting in a circular bending stress, which directly affect the pipeline fatigue life. For typical plain dent on pipeline, the finite element models are established under different circumstances. A large number of the calculation results are sorted, inducted, drawed. On this basis, the results are analyzed by univariate. Non-linear regression analysis was utilized to fit the results by matlab, some specific expressions of the relationship between the dented pipeline displacements and the dent depth, the dent longitudinal length are obtained after much calculation and analog.

Stress‐life fatigue assessment of pipelines with plain dents

ABSTRACTThis paper presents a new algorithm for assessing the fatigue life of dented pipelines. The proposed methodology was conceived according to the current stress‐life fatigue theory and design practice: it employs S–N curves inferred from tensile test material properties and uses well established methodologies to deal with the stress concentration, the mean stress and the multi‐axial stress state that characterizes a dented pipe. Finite element analyses are carried out to model the denting process and to determine the stress concentration factors of several pipe‐dent geometries. Using dimensional analysis over the numerical results, a non‐dimensional number to characterize the pipe‐dent geometry is determined and linear interpolation expressions for the stress concentration factors of dented pipelines are developed. Fatigue tests are conducted with the application of cyclic internal pressure on small‐scale dented steel pipe models. In view of the fatigue test results, the more appropriate S–N curve and mean stress criteria are selected.

Fatigue analysis of damaged steel pipelines under cyclic internal pressure

A methodology for fatigue analysis of damaged steel pipelines under cyclic internal pressure is proposed. This methodology employs stress concentration factors, which are commonly used to modify standard S–N curves of metallic structures under high cycle fatigue loadings. Experiments are accomplished to evaluate the strain behavior of small-scale steel pipes during denting and cyclic internal pressure. A nonlinear finite element model is developed to obtain stress concentration factors induced by plain dents on steel pipes under internal pressure. Afterwards, analytical expressions are developed to estimate stress concentration factors as function of the damaged pipe geometric parameters. Finally, fatigue tests are conducted to evaluate the finite life behavior of small-scale damaged pipes under cyclic internal pressure and to validate the proposed methodology of fatigue analysis.

The effect of dents in pipelines—guidance in the pipeline defect assessment manual

The pipeline defect assessment manual (PDAM) project is a joint industry project, sponsored by sixteen international oil and gas companies, to produce a document specifying the ‘best’ methods for assessing defects in pipelines. A dent reduces the static and cyclic strength of a pipe. Plain dents, dents on welds and dents containing defects are considered here; small scale and full scale tests, theoretical analyses and assessment methods are discussed, and the ‘best’ methods included in PDAM are described.