Dented Pipes Research Articles

A Semi Empirical Regression Model for Critical Dent Depth of Externally Corroded X65 Gas Pipeline.

External corrosion dent is a common type of compound dent. On the one hand, this type of compound dent reduces the bearing area and bearing capacity of the pipeline. On the other hand, it leads to an increase in the stress–strain concentration in the dent and reduces the anti-fatigue load capacity of the pipeline, which is more harmful to the service safety of the pipeline than the simple dent. In this study, the reliability of the modeling method was verified by the numerical inversion of the full-size dented pipe test. A three-dimensional finite element model for a pipe with a small corrosion dent was established by analyzing the internal detection data on corrosion defects of pipes with a diameter of 813 mm. The failure criterion of the corrosion dent pipe and the calculation method of the critical dent depth were determined. The influence of corrosion depth, length, width, internal pressure load, curvature radius of indenter, and diameter–thickness pipeline ratio on critical dent depth was investigated. Finally, a critical dent-depth prediction formula was developed based on the numerical results. This study provides a reference and significant guidance for the applicability evaluation of corroded sunken pipelines.

Strain-based reliability analysis of dented pipelines using a response surface method

Dent defects can decrease the life span of oil and gas pipelines. Therefore, they need constant monitoring and maintenance to ensure the pipeline’s safety and integrity. Subsequently, this paper performs a strain-based reliability analysis on pipe dent defects using a response surface method (RSM) with a quadratic response surface (RS), including the interaction terms between the RS variables. The analyses are performed to determine the factors controlling the dent defects’ probability of failure (POF). Different pipe configurations, pipe lengths, indenter sizes, and dent depths are considered in this study. A suitable finite element (FE) model for the reliability analysis was developed for this study using the FE analysis software ABAQUS. The uncertainties in the pipe wall thickness, the dent depth, the yield strength of the pipe material, and the strain capacity are considered for the reliability analysis. The first-order reliability method (FORM) is used in the RSM as the reliability method to calculate the POF and the most probable point (MPP). The POFs of several dent defects were calculated. It has been found that the POF, which is highly related to the nominal value of the maximum equivalent plastic strains generated in the dent defect, is not only related to the indentation depth or the size of the indenter. Thus, the dent depth criterion used in the engineering practice can lead to inconsistent reliability levels in dented pipes.

Strain response analysis of API 5L X80 pipelines with a constrained dent subjected to internal pressure

Constrained dents normally cause serious plastification on the pipe under internal pressure due to the restraint of the indenter. Therefore, in this work, the strain response of the API 5 L X80 (referred to as X80 in the remainder part) steel pipeline containing a constrained dent was investigated. Firstly, the formation process of the constrained dented pipeline was numerically simulated by establishing a 3D finite element (FE) model. The model was then verified against the test results of other researchers. The validated model was employed to investigate the equivalent plastic strain distributions of the constrained dent created during the pipe construction and pipe operation stages. Finally, an extensive parametric study was conducted to examine the effects of the internal pressure, dent depth, diameter-to-thickness ratio, indenter size and shape on the strain response of the constrained dented pipe in detail. The results show that the maximum equivalent strain of shallow dents locates at the pipe inner wall at the dent center. As the dent depth increases, the maximum strain position deviates from the dent center and moves outwards along the pipe circumferential direction. Compared with the constrained dent formed during the pipe construction period, the maximum equivalent strain of the in-service constrained dented pipelines is larger under the identical dent depth. The maximum strain increases significantly as increasing the dent depth and internal pressure. It is noted that the wall thickness has a greater influence on the equivalent plastic strain distributions of the constrained dented pipe than that of the pipe diameter. The equivalent plastic strain caused by a small indenter is more serious than the large one due to the high strain concentration. It is noteworthy that circumferential dent (dent length is less than dent width) can be considered as the most severe form of dent defects on the pipe in comparison to the axial dent (dent length is greater than dent width) and dome dent (dent length is approximately equal to dent width).

Analysis of ductile damage changes of pipelines with unconstrained dents in rebound process

Dent is one of the most common defects in mechanical damage of oil and gas pipelines. Most of the dents generated during pipeline construction are unconstrained dents, and rebound will occur during service of the pipeline. The dent defect can be considered as ductile damage. Taking X52 steel pipe as the research object, the rebound process of unconstrained dents in service is numerically simulated. Based on the Oyane ductile fracture criterion, the ductile damage degree of dented pipes affected by multiple factors was studied. The change of the ductile damage integral value with the change of parameters during each loading process of the pipeline and the relationship between the pipeline damage and the depth of the dent were analyzed. The results show that the analysis results based on the ductile damage model are in accordance with the conclusions of the existing research, and the ductile damage integral value of the pipeline changes with the depth of the dent in a reasonable law, which shows that the ductile fracture criterion can be used for the safety evaluation of dented pipeline.

Stress and strain analysis of spherical sealing cups of fluid-driven pipeline robot in dented oil and gas pipeline

Pipeline inspection gauge (PIG), as a kind of pipeline robot, Pipeline inspections gauges (PIGs) have been the main equipment during different stages of operation to perform functions such as pigging and inner inspecting. However, due to the failure of the sealing cup, the PIG is often stuck when it runs in the oil and gas pipeline especially in dented pipe. The distribution of stress and strain on the outer edge of the spherical sealing cup is complex but rewarding for understanding the blockage problems. When the PIG is in a straight pipeline section, it can be abstracted as an interference problem between polyurethanes rubber and steel. In this study, the numerical solution and the analytical solution of the polyurethane rubber and steel interference problem was first compared with each other to verify the numerical solution. Then, effects of the interference (δ), thickness (t), and radian (θ) of the spherical sealing cup on the stress and strain distribution on the outer edge of the cup were discussed using MSC Marc 2016 when the PIG was in dented pipe. Results indicate that the interference of the sealing disc has the greatest impact on the stress and strain distribution when PIG runs in the straight pipeline section. However, the thickness of the sealing cup has the greatest impact on the stress and strain distribution when PIG runs through the dent. The conclusions obtained in this study are beneficial for the selection and design of the spherical sealing cups PIG.

Mechanical Behavior of Dented Steel Pipes Subjected to Bending and Pressure Loading

The presence of dents on steel pipeline wall may constitute a threat for pipeline structural safety. Experimental testing results supported by numerical simulations are reported, in an attempt to assess the structural integrity of smoothly dented (nongauged) steel pipes. Ten experiments on 6 in diameter X52 steel pipes are reported, where dented steel pipes are subjected to bending and pressure loading, in order to estimate their residual strength and remaining fatigue life. Six specimens were subjected to cyclic bending loading, whereas four dented pipe specimens, following cyclic pressure loading, have been pressurized to burst to determine their ultimate pressure capacity. Numerical simulation of the testing procedure and, in particular, the loading pattern of each specimen (denting and cyclic loading) has also been performed so that local stress and strain distributions at the dented region are calculated accurately. Based on the finite element results, a simple and efficient fatigue assessment methodology is adopted, to estimate the remaining fatigue life and the predictions were found to compare with the experimental results. Finally, following a parametric numerical study, strain concentration factors (SNCFs) for dented pipes subjected to bending are calculated, to be used in fatigue life assessment.

Improvements to the ASME B31.8 Dent Strain Equations

Pipelines used to transport oil and gas products are often subjected to external forces during its construction or operation, which can result in the formation of dents in the pipe. Various pipeline codes have stipulations on how a dent's severity can be ascertained in order to prioritize repairs. The most prominent being the depth-based criterion, which determines the severity of a dent by its depth. The depth-based criterion fails to consider the fact that the geometry of the dent can result in high strain concentration and eventually lead to integrity issues at the dented region. Alternatively, the strains associated with the dent can be an indicator of the dent's severity. Nonmandatory codified equations are available for evaluating the strains at the dented region of the pipe. The current implementation of these equations might fail to capture the strains that are not aligned with the most severe deformation profile of the dent and as such a global view of the strain distribution of the dented profile would be more informative as per the localized strain distribution. The study presented herein is the implementation of ASME B31.8 formulations together with the suggested modifications to evaluate the three-dimensional (3D) strain state of the dented pipe. The strain distributions obtained are compared against the strains predicted by finite element analysis (FEA) model. The correlation in the predicted strains indicates the possibility of the rapid and concise strain based characterization of dented pipes with the proposed technique.

Three-Dimensional Strain-Based Model for the Severity Characterization of Dented Pipelines

Oil and gas pipelines traverse long distances and are often subjected to mechanical forces that result in permanent distortion of its geometric cross section in the form of dents. In order to prioritize the repair of dents in pipelines, dents need to be ranked in order of severity. Numerical modeling via finite element analysis (FEA) to rank the dents based on the accumulated localized strain is one approach that is considered to be computationally demanding. In order to reduce the computation time with minimal effect to the completeness of the strain analysis, an approach to the analytical evaluation of strains in dented pipes based on the geometry of the deformed pipe is presented in this study. This procedure employs the use of B-spline functions, which are equipped with second-order continuity to generate displacement functions, which define the surface of the dent. The strains associated with the deformation can be determined by evaluating the derivatives of the displacement functions. The proposed technique will allow pipeline operators to rapidly determine the severity of a dent with flexibility in the choice of strain measure. The strain distribution predicted using the mathematical model proposed is benchmarked against the strains predicted by nonlinear FEA. A good correlation is observed in the strain contours predicted by the analytical and numerical models in terms of magnitude and location. A direct implication of the observed agreement is the possibility of performing concise strain analysis on dented pipes with algorithms relatively easy to implement and not as computationally demanding as FEA.

Nonlinear finite element analysis of the limit state and pressure of dented X60 steel pipeline

As a main defect in pipeline, dents have brought great challenge to the integrity of oil and gas pipelines. Numerical study was presented in this paper to investigate the stress and strain state of dented X60 pipe. Suitable failure criteria is adopted and validated by full scale experimental results. And the failure process of dented X60 pipe is elucidated in detail. Based on the established model, effects of dent depth, indenter diameter, initial internal pressure, ratio of diameter to ratio thickness on limit pressure of dented pipe was discussed. Finally, a regression based prediction equation for dented X60 pipe was proposed. Results show this equation predicts accurate results with much less effort and time with regard of 3D finite element models. DOI: http://dx.doi.org/10.5755/j01.mech.23.6.17338

Fatigue Life Analysis of Dented Pipes Subjected to Internal Pressure

Steel pipelines that transport pressurized fluids can contain a dent defect which is a permanent deformation of the outer wall due to an impact with a foreign body. This defect induces a high local stress concentration. This paper presents an optimized numerical model based on finite elements to assess the stress concentration factor around constrained and unconstrained dents in an API X52 steel pipe subjected to internal pressure. The simulation of the finite element model is conducted in two phases; the first phase concerns the realization of the dent, while the second one takes as initial geometry the dented pipe and subjects it to an internal pressure. The validation results of this finite element model show that it can precisely provide Von Mises stress around the defect needed to calculate the stress concentration factor. The fatigue life is then estimated based on S-N curves with the Gerber criterion to account for the mean stress effect for various pipe geometries, dent depths and dent types. These dents are generated using spherical and rectangular indenters in longitudinal and transverse orientations under constrained and unconstrained configurations. A parametric study conducted on 150 cases of dented pipes subjected to internal pressure between 0% and 50% MOP (maximum operating pressure) enabled us to analyze the effect of the various types of dents on the integrity of the structure.

Computational simulation of ratcheting in dented pipes due to monotonic and cyclic axial loading

This article presents a computational simulation framework developed to capture the ratcheting response of dented straight pipes. The results are compared against the results obtained from our previously conducted experiments. Ratcheting occurs when a structure is subjected to a primary axial load along with a secondary cyclic load, provided that the applied loads are sufficiently large in magnitude to cause the material to yield. The dented pipes investigated in this study were initially subjected to a monotonic axial compression load, which caused the initiation of small amplitude wrinkles, and were subsequently subjected to an axial cyclic loading regime. A nonlinear finite element analysis, using a combined hardening model, was adopted to model the response of the material during the entire loading regime. The parameters required by the material model were obtained from cyclic tests conducted on representative coupon specimens. The results of the numerical simulations were compared to experimentally obtained data. The results demonstrated that the ratcheting response of dented pipes could be numerically simulated with a reasonable accuracy. The results also revealed that the surface imperfections exerted a very pronounced effect on the ratcheting response of the dented pipes. The computational model was also used to investigate the influence of some key parameters, such as the initial strain level, stress amplitude, mean stress, loading regime, and material hardening properties, on the resulting ratcheting.

Ratcheting response of dented pipes under monotonic and cyclic axial loadings

Offshore pipelines are usually subject to hostile environments. Temperature changes and repetitive start-up/shutdown in offshore pipelines generate cycles of axial compression and relaxation. Consequently, the pipes may become subject to loading magnitudes that could cause them to locally buckle and wrinkle, and if subject to repeated loading, they could undergo ratcheting. Pipelines could also be subject to damage in the form of dents. This form of damage would have a profound effect on a pipe’s ratcheting response when subjected to cyclic loading. In this article, the ratcheting response of locally dented pipes that were subject to axially applied cyclic loading has been experimentally investigated. Scaled pipes (tubes) were first laterally indented with a cylindrical indenter and then were subjected to a monotonic axial compressive load that initiated small amplitude wrinkles on them. The pipes were subsequently subjected to repeated axial strain. The experiments were conducted on low carbon steel pipes (API 5L-grade B), with Do/t ≃ 35, which were indented to three different indentation depths. Significant variation in pipes’ responses was observed, and the number of cycles prior to collapse dramatically decreased by as much as 75% when the indentation depth was marginally increased.

Investigation and Validation of Finite Element Analysis Material Modeling for Integrity Assessment of Indented Pipe Under Static and Cyclic Loading

Mechanical damage in transportation pipelines is a threat to its structural integrity. There are many parameters that affect the severity of the mechanical damage which are related to the pipe geometry and material properties, the defect geometry and boundary conditions, the loading cycle, and the pipe state of stress. To understand those effects, the utilization of numerical finite element analysis (FEA) has been used extensively to supplement the expensive; and thus, limited full-scale tests. The actual pipe material exhibits a number of special features including nonlinear elasticity, anisotropy, and cyclic softening which need advanced material modeling techniques. However, the success of the numerical material model to actually simulate the pipe material behavior could not be studied in detail previously due to the insufficient experimental data especially in cyclic pressure loading. The objective of this paper is to investigate the effect of material modeling using FEA on the integrity assessment of dented pipe under static and cyclic loading by simulating pipe denting followed by subsequent pressure cycles. Several material models are tested and calibrated against the measurements of full-scale tests to find the effects of material modeling assumptions (e.g. isotropy, yield point, hardening rule). The results show that a combined material model simulating all special features of nonlinear elasticity, anisotropy, and cyclic softening gives a very close representation of experimental data in terms of strain values and fatigue cycles to failure. Therefore, detailed material properties are needed to conduct accurate integrity assessments of dented pipes especially under cyclic conditions.

Load bearing capacity of API X65 pipe with dent defect under internal pressure and in-plane bending

► Opening bending had a higher load bearing capacity than closing bending. ► TES moment decreased with increasing dent depths and internal pressures. ► Max moment in the closing bending mode increased with increasing internal pressures. The objective of this study was to investigate the effect of the dent magnitude on the collapse behavior of a dented pipe subjected to a combined internal pressure and in-plane bending. The plastic collapse behavior and bending moment of the dented pipe containing several dent dimensions were evaluated using elastic–plastic finite element (FE) analyses. The indenters used to manufacture the dents on the API 5L X65 pipe were hemispherical rods with diameters of 40, 80, 160 and 320 mm. Dent depths of 19, 38, 76, 114 and 152 mm were introduced to the pipe with a diameter of 762 mm and a wall thickness of 17.5 mm. A closing or opening in-plane bending load was applied to the dented pipes pressurized under an internal pressure equivalent to atmospheric pressure as well as pressures of 4, 8, and 16 MPa. The FE analyses results showed that the plastic collapse behavior of the dented pipes was significantly governed by the bending mode and the dent geometry. Moment-bending angle curves for the dented pipe were obtained from computer simulations and evaluated with a variety of factors in the FE analyses. The load bearing capacity of the dented pipes under the combined load was evaluated by TES (Twice Elastic Slope) moments. The load bearing capacity of the pipe containing up to a 5% dent depth of the outer diameter was not reduced in comparison to that of the plain pipe. The opening bending mode had a higher load bearing capacity than the closing bending mode under the combined load regardless of dent depth. The TES moment decreased with increasing dent depth and internal pressure regardless of the bending modes.

Study of the influence of dent depth on the critical pressure of pipeline

The origin of more than 90% in service pipeline failures is attributed to the presence of stress concentration areas such as defects in forms of gouges, dents, or when these defects are present together. These defects are provoked by external factors interferences, for instance by excavation machines. The influence of the dent depth on the burst pressure of the pipeline was evaluated. An approach based on a simple local strain criterion has been proposed to predict the damage and the failure of evaluated dented pipe. To validate a purely empirical rule, which specifies that a critical depth dent is equal to 10% of outside diameter of the pipeline, a series of mechanical tests (denting tests and burst tests) were conducted. Finite element analysis used in order to compare the numerical and experimental results. Experimental results showed that a single dent did not affect the burst pressure of the investigated specimens; therefore the rule of a critical depth of dent equal to 10% was found very conservative.

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.

Pipe Wall Damage Detection in Buried Pipes Using Guided Waves

It is well known that cylindrical guided waves are very efficient for detecting pipe wall defects when pipes are open in the air. In this paper it is investigated how efficient the guided waves are for detecting pipe wall damage when the pipes are embedded in the soil. For this purpose guided waves were propagated through pipes that were buried in the soil by placing transmitters on one end of the embedded pipe and receivers on the other end. Received signals for both defect-free and defective pipes were recorded. Then the received signals were subjected to wavelet transforms. To investigate whether embedding the pipe in the soil makes it more difficult to detect the pipe wall defects, the same set of defective and defect-free pipes were studied before and after burying them in the soil. In both cases the defective pipes could be easily identified. Interestingly, contrary to the intuition, it was observed that under certain conditions defective pipes could be identified more easily in buried conditions. For example, the difference between the strengths of the initial parts of the received signal from defect-free and dented pipes was found to be greater for the buried pipes. Some qualitative justification for easier detection of buried dented pipes is provided.

Fracture assessment of dented pipes with cracks and reliability-based calibration of safety factor

Mechanical damage, including dents and cracks to pipes, occurs frequently, mainly caused by operational activities and fabrication errors. Structural assessment of dented pipes with cracks is of importance, but reliability-based assessment is necessary to properly account for the uncertainties invloved. This paper proposes a new methodology for calculating the fracture strength of dented pipes with cracks. Comparisons between the predicted fracture strength based on the presented approach and test results are made and good agreement is observed. A fracture reliability model of dented pipes with cracks is predicted. Fracture reliability analysis of a damaged pipe is performed based on a design example and detailed parameter studies are also conducted. Calibration of the safety factor based on the reliability methods is carried out.

Strain Concentrations in Pressurized Dented Pipes

Knowledge of the strain distribution in a dent is crucial in assessing the severity of damage to pipes. Accordingly this paper presents the results of a series of tests to measure strains and displacements in previously dented aluminium pipes subject to increasing internal pressure. Indentation of unpressurized and pressurized pipes was caused by a rigid sphere and resulted in short smooth dents of depths up to 16 per cent of pipe diameter. Strain changes on the external pipe surface were monitored by strain gauges and a photoelastic coating. The results identify two stationary crescent-shaped regions of high external strain at the axial extremities of the initial dent, where strain concentration factors of 10 were recorded in the early (elastic) stages of pressurization, increasing to a maximum of 28 in one particular dent. The results have immediate application to the assessment of a gouge within a dent, which has long been recognized as the most severe form of mechanical damage to pipelines and can result in failures at very low internal pressures. A parallel series of burst tests by the authors revealed that gouges located near the axial extremities of the initial dent may cause premature failure. The strain concentration results presented here help to explain this behaviour, and indicate the size, shape and location of the regions of high external strain.