Pipeline Integrity Assessment Research Articles

Structural integrity assessment of hydrogen-mixed natural gas pipelines based on a new multi-parameter failure criterion

Due to non-pollution and high combustion efficiency, hydrogen energy is promising as a clean and renewable energy source to gradually replace fossil fuels and reduce carbon emissions. However, the low-cost, large-scale and long-distance transportation of hydrogen has always been a problem perplexing the development of hydrogen energy. Mixing hydrogen with natural gas and transporting it in natural gas pipeline is an effective solution to this problem. In this paper, the structural integrity of hydrogen-mixed natural gas pipelines under extreme loads are studied. Firstly, a mechanical model of hydrogen-mixed natural gas pipeline was established. Secondly, a new multi-parameter failure criterion containing the ratio of yield and tensile strength (Y/T) is provided. Thirdly, based on the multi-parameter failure criterion, a novel burst pressure equation of the hydrogen-mixed natural gas pipelines is presented. Fourthly, the accuracy of the burst pressure equation is verified by experiment and finite element method. The results show that the new multi-parameter failure criterion can be used to accurately predict the burst pressure of hydrogen-mixed natural gas pipelines with high accuracy. Our research has important theoretical significance and application value for the development of classical strength theory and the integrity assessment of mixed hydrogen natural gas pipeline.

Constraint effect on fracture toughness resistance curves of an X60 pipe steel

The fracture toughness resistance curve (e.g. J-R curve and crack tip opening displacement (CTOD) or δ-R curve) is important in facilitating strain-based design and integrity assessment of oil and gas pipelines, where influence of ground movements and presence of planar weld flaws must be accounted for. In this paper, duplicate single edge bend (SE(B)) and clamped single edge tension (SE(T)) specimens with two different initial crack sizes were prepared from an X60 pipe steel. The longitudinally-oriented specimens were notched in the through-thickness direction and experimentally tested at room temperature. In general, the results showed the expected trends of higher resistance curves for both shallow-cracked versus deeply-cracked specimen types and tension versus bending loading modes. Eleven constraint parameters (i.e. QHRR, QSSY, QLM, QBM, A2, A2BM, h, Tz, Cp, Ap, Vp) were calculated based on three-dimensional (3-D) finite element analyses (FEA) for various tested SE(B) and SE(T) specimens.The results and analysis indicated that the shallow-cracked SE(B) specimens exhibited a constraint condition similar to the intrinsically low-constraint deeply-cracked SE(T) specimens. Among all of the constraint parameters, the Ap parameter based on the normalized equivalent plastic strain contour was found to have the most promising trend in terms of correlating the material’s toughness to constraint level for currently tested specimens. Further on-going investigations are underway to extend the evaluation to resistance curves for specimens made from X100, X80 and X70 pipe steels to allow for the characterization of strain hardening effect, as well as the evaluation of their associated weld metals and heat affected zones (HAZs).

Influence of additional factors on the integrity of pipelines with small corrosion defects

The ASME and DNV criteria mainly assess large area defects on pressure pipelines. For small corrosion defects, the use of these approaches leads to unduly conservative results for the assessment of pipeline integrity. In the operation of piping systems, the most common cause of accidents is the synergy of multiple effects on the piping. This paper analyse the cases of small corrosion defects when additional bending acts together on a pressure pipeline. Through experimentally validated simulations, diagrams can be produced to assess the safety of pipeline operation: e.g. for nominal stress from internal pressure, corrosion defect size and length of the deflected pipeline, an increase in stress with increasing deflection size from the original pipeline centerline can be seen

Additively manufactured tensile ring-shaped specimens for pipeline material fracture examination - influence of geometry

In order to define a procedure for integrity assessment of pipelines and determining the fracture mechanics parameters, a new type of specimen with a sharp notch, Pipe Ring Notched Tension (PRNT) specimen, is tested. The aim of the study is to determine the influence of the specimen geometry (cylindrical shape, as well as number and length of stress concentrators) on parameters such as force, Crack Mouth Opening Displacement CMOD, Crack Tip Opening Displacement CTOD and J integral. The specimens were fabricated by an additive production method - selective laser sintering (SLS). EOS Formiga P100 machine is used, and the material is polyamide PA12. In addition to Pipe Ring Notched Tension specimens, Single Edge Notched Tension (SENT) specimens with identical cross section were also considered. All specimens were tested on a universal tensile testing machine. A tool specially designed to apply contact pressure to the inner walls in the tension direction was used, simulating the internal pressure. For determining the field of displacement and strain on the surface of the tested samples that occur during the loading, Aramis GOM 2M optical measurement system was used. Aramis is applied for determination of geometry fracture mechanics parameters: CMOD and CTOD (based on δ5 concept). In addition to the examination of fracture properties of additively manufactured PA12, the main topic of this work is the development of the non-standard testing procedure, which will be subsequently applied to the specimens cut from metallic or non-metallic pipes. As an important part of this procedure, the calculation of fracture parameters will be conducted based on the results presented here.

Offshore pipeline integrity assessment considering material and parametric uncertainty

This paper presents a methodology that integrates the semi-empirical corrosion models with probabilistic analysis to study steel structural failure behavior considering material and parametric uncertainties. The semi-empirical models are used to assess the asset’s susceptibility, system degradation rate, and defect growth over time under harsh corrosive environment. The developed model is translated into a limit state function in a probabilistic framework to define the asset’s safe operating envelope. The probabilistic framework is simulated considering the variations in the material properties of steel grades, corrosion response parameters, and types of susceptibility models. The variabilities in the ultimate tensile strength, operating pressure, and wall thickness exhibit the highest contributions to pipeline failure behavior in a harsh offshore environment. It is also observed that the failure probability of the pipeline increases with an increase in the coefficient of variation at the lower bound of failure, while it decreases at the upper bound of failure. The coefficient of variation for the tensile strength shows a 32.2% (the highest) impact on the limit state function performance as the year of exposure progresses. The proposed approach offers a systematic framework for an appropriate material selection and risk-based integrity management strategy for offshore structures, including pipelines.

Buckling analysis of subsea pipeline with idealized corrosion defects using homotopy analysis method

Comprehension on the bucking failure of corroded subsea pipelines is important. Those corrosion defects usually exhibit complex geometries and will complicate the buckling failure behavior of subsea pipeline under external pressure. In this paper, a semi-analytical approach named homotopy analysis method (HAM) is introduced to investigate the buckling failure of subsea pipeline with three idealized corrosion defects. The proposed solutions are proven to be capable of giving accurate and efficient predictions in comparison with finite element result as well as analytical results. Numerical examples indicate that corrosion shapes have pronounced effect on the buckling pressure as well as buckling mode of pipeline with severe corrosion defects and those cases should be carefully concerned in the structural integrity assessment of subsea pipelines.

Reliability analysis of intact and defected pipes for internal pressure related limit states specified in CSA Z622:19

The importance of reliability-based design and assessment has been widely recognized by the pipeline industry. Thus, this paper aims at providing comprehensive reliability assessment of intact and defected pipes subjected to internal pressure based on the CSA Z662:19. Various limit states related the pipe design, pre-commission hydrostatic testing, and operation are studied. Specifically, both corrosion and crack defects are considered for pipeline integrity assessment based on different defect scenarios. Reliability results, or probabilities of failure, are reported with respect to design factors, hydrostatic test pressure factors, and safety factors, which can be used in designing new pipes, determining the applied pressure in hydrostatic tests, and operation pressure control of defected pipes, respectively. The effects of pipe grade, pipe dimensions (i.e., diameter and wall thickness), corrosion or crack defect sizes (e.g., length and depth), and internal pressure on POFs for different limit states are also investigated.

Creep rupture assessment of cyclically heated 3D pressure pipelines with volumetric defects using a direct numerical approach

Creep rupture assessment is a significant issue in the field of high-temperature structural integrity. In this paper, the creep rupture assessment of 3D pressure pipelines with volumetric defects subjected to cyclic thermo-mechanical loadings is done using a direct numerical approach within the framework of stress compensation method (SCM). For calculation of creep rupture limit, an extended shakedown analysis scheme with yield strength correction is essentially performed, where the revised yield strength is determined as the minimum of the plastic yield strength and the creep rupture strength of material corresponding to a specified elevated temperature and rupture time. This direct numerical approach performs the creep rupture assessment by using the given creep rupture data of material rather than simulating the degenerative process of material based on creep damage constitutive equations under a specified loading history. The numerical approach is incorporated into commercial finite element software of Abaqus. Parametric studies on geometric dimensions of part-through slot affecting the creep rupture limit of 3D pressure pipelines with volumetric defects are conducted. Numerical results shown in this study indicate that the direct numerical approach accurately identifies the creep rupture limit boundary of the pressure pipeline, verified by the elastic-plastic cycle-by-cycle analysis. Failure mechanisms, such as local creep rupture, global creep rupture and global plastic rupture are detected under different loading combinations. These results give the helpful information for in-service integrity assessment of defective pressure pipelines at elevated temperature, and demonstrate the applicability and application prospect of the direct numerical approach in solving questions relevant to design or life assessment of other engineering structures.

A new evaluation method for burst pressure of pipeline with colonies of circumferentially aligned defects

Burst pressure prediction of pipelines with interacting defects is essential in the integrity assessment of steel pipelines. A series of evaluation criteria have been proposed to predict the burst pressure. However, the burst pressure is not accurately estimated by the existing evaluation methods for some defect distribution cases, especially for circumferentially aligned defects. In this study, a new evaluation method of burst pressure is established by developing a reliable calculation expression of the effective defect depth. A burst pressure study is performed by considering the effects of the defect distribution, defect size and circumferential spacing between two adjacent defects based on finite element analysis. Then, the detailed expression of effective depth of circumferentially aligned defects with different sizes is obtained. Full-scale burst tests for corroded pipelines are conducted to validate the accuracy of the new evaluation method. The predictions of burst capacity by the new method used for the pipeline with circumferentially aligned defects are in good agreement with experimental results for varying defect sizes and pipeline steel grades. Finally, the proposed evaluation method is verified equally applicable to various defect distribution modes.

Development of a Tensile Strain Capacity Predictive Model for American Petroleum Institute 5L X42 Welded Vintage Pipelines

Abstract Pipelines can be exposed to a wide variety of loads, depending on the environments and the area of application. These loads may impose large longitudinal plastic strain on pipelines, which could constitute a significant threat to the structural capacity of the pipeline. Reliable calibration of the strain capacity of pipelines plays an important role in the strain-based design (SBD) method. In this paper, a tensile strain capacity (TSC) predictive model (an equation) for welded X42 vintage pipes has been developed by conducting nonlinear parametric analysis followed by nonlinear regression analysis. First, our previously validated extended finite element method (XFEM) model was used to demonstrate the applicability of the XFEM in simulating full-scale ductile fracture response of pipelines subjected to biaxial loading, using pressurized American Petroleum Institute (API) 5L X42 vintage pipes subjected to four-point bending. Second, a parametric study investigating the effects of pipe and defect geometries as well as loading on the pipe TSC is presented. The nonlinear parameterization using XFEM was conducted in abaqus/standard. The TSC trends obtained for the various parameters considered were examined to derive appropriate individual variable functions for each parameter while taking any significant interactions between the parameters into consideration. Also, a nonlinear regression analysis is employed to develop a nonlinear semi-empirical model for predicting the TSC. The results obtained from the developed TSC predictive model (TSCvin.) was compared with those evaluated using the validated XFEM models. The results showed good agreement. Finally, statistical analysis was conducted to ensure the model is unbiased and predicts conservative TSCs by modifying the model using probabilistic error analysis. The modified model is capable of increasing the confidence level in the predicted TSC hence becoming a practical tool for reliable prediction of TSC of X42 vintage pipes needed for conducting pipeline integrity assessment. This modified predictive model is useful in practical applications because it provides a quantifiable degree of conservatism and reliability to the predicted TSCs.

Impact Analysis of Inline Inspection Accuracy on Pipeline Integrity Planning

Abstract Integrity planning methods and inline inspection (ILI) tool performance have a great impact on a pipeline integrity management program. In pipeline integrity planning, risk and integrity assessments are performed to schedule integrity activities like ILI for the purpose of reducing risks and ensuring reliable and safe operations. In this paper, a method is developed for analyzing the impact of ILI tool accuracy on pipeline integrity planning, which is of great importance but has not been systematically studied before. Crack inspection and threat of fatigue cracking are used as the working case for the analysis, although the approach could potentially be used for any pipeline threat type. The Paris' law degradation model is used for the crack growth and subsequent severity and risk assessment. We investigated the impact of ILI tool accuracy on the cost rate, as well as the associated inspection intervals. The impact on long-term cost rate was also investigated considering new defect generation and continuous growth. Sensitivity analyses were performed. The optimal inspection intervals and the corresponding total cost rates with respect to different ILI tool accuracy and different input parameters were obtained and compared. The proposed method can support integrity management program planning by linking risks with integrity plan costs associated with ILI accuracy and optimal re-assessment intervals. The contributions of this paper mainly include the investigation of the problem of how ILI tool accuracy impacts integrity planning, the development of the method for analyzing pipelines with cracks, and the verification and validation with the examples.

Determination of Folias Factor for Failure Pressure of Corroded Pipeline

AbstractCorrosion assessment and burst pressure prediction of line pipes with corrosion defects are essential for the integrity assessment of steel transmission pipelines. The failure assessment methods proposed in codes or handbooks may be overly conservative or exhibit significant scatter in their predictions. In this paper, the effects of two key parameters—the flow stress and Folias bulging factor, on predicting the failure pressure of pipelines with defects are studied. The Folias bulging factor is suggested by fitting the results from finite element (FE) analysis. Then, a new prediction method for the failure pressure of pipelines with defects is proposed. The failure pressures predicted by the proposed method are in better agreement with the experimental results than the results by the other methods such as B31G, MB31G, Det Norske Veritas (DNV), and rectangular parabolic area (RPA).

Nature and magnitude of operating forces in a horizontal bend conveying gas-liquid slug flows

Operating forces and magnitude of loads from gas-liquid slug flows exerted on a horizontally orientated 90o bend are investigated. The distributed forces are either Newtonian, associated with the fluids motion or Configurational, inherent to the internal distributions of the phases. The forces are derived through the conventional balances of mass and linear momentum arising from the volume of fluid (VOF) description of gas-liquid flows. The study uses the integral form of the momentum balance to estimate the operating forces budget. Invoking dynamical time scales separation discloses the connection of the Lamb vector (vortex-force) to the local time rate of momentum. An interesting outcome being an explicit expression for Favre-Reynolds stress that reveals the contribution of void fraction fluctuations in the redistribution of the stress across the interface. Numerical simulations are performed to determine the magnitude of Newtonian loads on bend using a segmented domain technique to represent the fully established slug flow regime. The time-dependent traces of the relevant flow variables such as liquid hold-up, flow rates and resultant forces on the bend are recorded and analysed. Compared to the isotropic component, the deviatoric stresses are shown to have a marginal contribution to the total forces. It is also shown that loading cycles on bends are much higher than slugging cycles; this is an important feature for the structural integrity assessment of pipelines with bends.

Calculation of burst pressure of pipeline with local defect

In the operation of transport piping systems, there are situations when the wall of the pipeline is locally weakened by corrosion, weld defects or other defects. There is practically no analytical solution to such cases in terms of pipeline integrity. FEM simulation is a good tool for solving such cases. In particular two questions will be discussed: Which material model should be used and what will be the pipeline destruction criterion. The propose methodology of numerical simulations of burst pressure is experimentally confirmed on the steel X52. Good conformity with the experimentally achieved bursting pressures gives a presumption of its use also for the assessment of pipeline integrity in connection of local defects with other operational influences.

Investigating the Effects of Cyclic Thermo-Mechanical Loading on Cyclic Plastic Behavior of a Ninety-Degree Back-to-Back Pipe Bend System

Abstract Pipe bends are generally employed for routing piping systems by connecting to straight pipes but back-to-back pipe bends are often necessary for confined space applications. In order to achieve safe operation under complex loading, it requires a thorough pipeline integrity assessment to be commenced. This paper investigates the effects of cyclic thermo-mechanical loading on cyclic plastic behavior of a 90-deg back-to-back pipe bend system, including temperature-dependent yield stress effects. Structural response interaction boundaries are determined for various different combinations of cyclic and steady loading. Constructed structural responses are verified by full cyclic incremental, step-by-step, finite element analysis. The numerical studies provide a comprehensive description of the cyclic plastic behavior of the pipe bends, and semi-empirical equations for predicting the elastic shakedown limit boundary are developed to aid pipeline designers in the effective assessment of the integrity of the pipe bends without a requirement for complex finite element analysis.

Numerical investigation into the effects of different initial imperfections on the lateral buckling of submarine pipelines

Initial imperfections (out of straightness) constitute one of the key parameters impacting lateral buckling of pipelines as they influence both pre-buckling assessment and pipeline integrity assessment during the post-buckling stage. When adjacent imperfections exist in a submarine pipeline, the influence regularity of initial imperfections on the global buckling of pipelines will become more complicated. An elastic-plastic 3D-Explicit numerical method is proposed to investigate the global lateral buckling of pipelines with single and double imperfections. Then, an investigation is conducted on the influences of the spacing, relative direction and relative size of double imperfections on the global buckling characteristics, such as the critical temperature rise, the buckling deformation amplitude and the buckling mode. Based on the limit state criteria of global buckling pipelines in DNVGL-RP-F110, the structural integrity of pipeline with different buckling modes is assessment. The results show that pipeline failure has a certain correlation with its buckling mode type. For pipelines with the same buckling mode type, the pipeline failure parameter is approximately linearly associated with the double-imperfection spacing.

Analytical prediction of failure pressure for pipeline with long corrosion defect

Corrosion assessment analysis and failure pressure prediction of pipelines with corrosion defects are essential in the integrity assessment of transmission pipelines. A series of semi-empirical corrosion criteria have been proposed to predict the failure pressure. However, the influence of defect width on the failure pressure is not considered in theory. In this paper, an improved model for the end-capped pipe with infinite length defect is developed based on plastic instability theory and finite strain theory, by considering the corrosion depth and width. The constitutive behavior of the pipe steel material is characterized by a power-law hardening stress-strain curve. Thus, two theoretical solutions are developed based on von Mises and Tresca criteria, by considering the strain hardening effect. Finite element analysis and sixty-one full-scale burst tests for each pipe with a single longitudinal defect are performed to validate and define the application scope of these new solutions. The results show that the proposed model based on von Mises criterion makes the best prediction and is suitable for the cases of defect length longer than 20Dt.

Quality assessment and deviation analysis of three-dimensional geometrical characterization of a metal pipeline by pulse-echo ultrasonic and laser scanning techniques

Although ultrasonic-based techniques are the most widely used ones for the assessment of pipeline integrity they have some limitations, and alternative methods show potential to overcome them, such as laser scanning techniques. This paper presents the results of an experimental evaluation of pulse-echo ultrasonic and laser scanning techniques for pipeline inspection using a metal specimen which represents a damaged pipeline. Both techniques were able to detect all the defects on the inner surface of the specimen, but the defect geometry was important to define the accuracy of each technique. Foralmostall the evaluated defects, the differences between the reference and the three-dimensional representations created from the experimental data showed that the ultrasonic technique presented errors with magnitude around 0.2 mm, which is in general half the error observed for the laser scanning technique. However, ultrasonic technique demanded 8 h for specimen inspection, while laser technique required only 10 min.

A Framework for Risk-Based Integrity Assessment of Unpiggable Pipelines Subject to Internal Corrosion

Many pipelines are unpiggable, which means they cannot be examined by in-line inspections (ILI). Existing industry practice for integrity assessment of unpiggable pipelines is not risk based, which precludes optimal allocation of inspection and maintenance resources. A framework for risk-based integrity assessment of unpiggable pipelines subject to internal corrosion is presented. The framework considers localized corrosion and microbiologically influenced corrosion (MIC) by combining flow and corrosion analysis for probability and consequence analysis as part of a quantitative risk analysis. Primary results of the application of the proposed framework show that it is applicable and beneficial for inspection and maintenance cost optimization.

Residual stress in pipeline girth welds- A review of recent data and modelling

Girth weld residual stresses cannot be avoided during welding and could affect the integrity of the pipeline during installation and service. Procedure BS 7910 is widely accepted for structural integrity assessment of pipelines and provides guidelines regarding appropriate residual stress assumptions for girth welds. However, there are limitations based on the assumptions provided in BS 7910. This paper revisits recent studies of girth weld residual stress in the context of pipeline integrity to provide suggestions for future improvements of the codes and standards. The review carried out in this paper is mainly limited to published measurement data and advances in the numerical simulation of girth weld residual stress in ferritic steels during the last ten years.