Pipeline Integrity Assessment Research Articles

Recent development in low-constraint fracture toughness testing for structural integrity assessment of pipelines

Fracture toughness measurement is an integral part of structural integrity assessment of pipelines. Traditionally, a single-edge-notched bend (SE(B)) specimen with a deep crack is recommended in many existing pipeline structural integrity assessment procedures. Such a test provides high constraint and therefore conservative fracture toughness results. However, for girth welds in service, defects are usually subjected to primarily tensile loading where the constraint is usually much lower than in the three-point bend case. Moreover, there is increasing use of strain-based design of pipelines that allows applied strains above yield. Low-constraint toughness tests represent more realistic loading conditions for girth weld defects, and the corresponding increased toughness can minimize unnecessary conservatism in assessments. In this review, we present recent developments in low-constraint fracture toughness testing, specifically using single-edge-notched tension specimens, SENT or SE(T). We focus our review on the test procedure development and automation, round-robin test results and some common concerns such as the effect of crack tip, crack size monitoring techniques, and testing at low temperatures. Examples are also given of the integration of fracture toughness data from SE(T) tests into structural integrity assessment.

Toward a non-destructive diagnostic analysis tool of exercises pipelines: models and experiences

Strategic networks of hydrocarbon pipelines, in long time service, are adversely affected by the action of aggressive chemicals transported with the fluids and dissolved in the environment. Material degradation phenomena are amplified in the presence of hydrogen and water, elements that increase the material brittleness and reduce the safety margins. The risk of failure during operation of these infrastructures can be reduced, if not prevented, by the continuous monitoring of the integrity of the pipe surfaces and by the tracking of the relevant bulk properties. A fast and potentially non-destructive diagnostic tool of material degradation, which may be exploited in this context, is based on the instrumented indentation tests that can be performed on metals at different scales. Preliminary validation studies of the significance of this methodology for the assessment of pipeline integrity have been carried out with the aid of interpretation models of the experiments. The main results of this ongoing activity are illustrated in this contribution.

Ductile fracture of advanced pipeline steels: study of stress states and energies in dynamic impact specimens - CVN and DWTT

The development of robust protocols for assessing the structural integrity of gas pipelines is of paramount relevance, since failures can lead to financial and human losses. In this scenario, the material’s ability to slow down the propagation of a running crack (crack arrest) becomes a design requirement. Several empirical models and criteria, calibrated by real pipeline burst tests, have been developed, being the Battele Two Curve Method (BTCM) one example of technique widely employed during decades. With the evolution of steels, there was a significant increase of ductility and toughness, in a way that such semi-empirical models usually based on the energy absorbed in the Charpy impact test (ISO 148-1, ASTM E-23) began to present unsatisfactory predictions. This may be explained by the fact that in current high-ductility and high-toughness materials (e.g.: API-5L X65, X80, X100), the dominant mechanism of fracture propagation is plastic collapse. Consequently, the energies involved in deforming and fracturing a laboratory specimen are remarkably altered and transferability to pipelines by means of the aforementioned models can be lost. Therefore, for a better phenomenological comprehension of the ductile fracture process under such circumstances, this work investigates Charpy and DWTT (ASTM E-436) dynamic tests assessing stress fields and respective energies involved in deformation and fracture. It is of great interest to evaluate the energy associated to steady state ductile fracture and thus try to characterize the energy available to slow down an ongoing fracture. Pipelines are references for the developments and support assumptions and some conclusions. Based on these golas, numerical analyses including damage models (XFEM and GTN) were implemented, including parameters’ calibration and sensitivity analyses. The methodology closely reproduced available experimental results. Besides that, stress fields and energies could be quantified for the studied geometries and such analyses indicated the potential and limitations of Charpy and DWTT specimens to characterize the energies required to describe steady state ductile crack propagation and crack arrestability. Results support further developments related to pipeline integrity assessments.

Implementing Genetic Algorithms to Assist Oil and Gas Pipeline Integrity Assessment and Intelligent Risk Optimization

Oil and gas industry, worldwide, needs to monitor, control and assess the elements that are involved in the general oil transportation and production processes. However, these processes are not risk free. The project proposes an intelligent support system that provides optimized projections for effective risk management. The project focuses on the development of a set of Genetic Algorithms (GAs), a branch of AI systems that assists to optimize the usage and distribution of resources. GAs will reduce the latent risks and potential dangers as much as possible. The main purpose is to minimize the risk levels in a pipeline segment based on their condition and by detecting optimal variable configurations: their Risk of Failure (RoF), Probability of Failure (PoF), Consequence of Failure (CoF), and their sub elements (threats and impacts). The heuristic results generated by this set of GAs show a significant reduction on the risk assessment measures, by finding “optimized” configurations of these variables.

An efficient procedure for reducing in-line-inspection datasets for structural integrity assessments

In-line inspection (ILI) has become a routine procedure in the Oil and Gas industry for performing cost-effective pipeline integrity assessments, allowing continuing monitoring and providing a basis for informed decisions in terms of repair, maintenance or a change to the operating conditions. The amount of ILI data is however immense and dealing with these data from a fitness-for-service point of view poses a significant challenge to the industry. Thus, smart methods for using ILI data in the assessment of the integrity of oil and gas transmission pipelines are essential. The aim of this paper is to propose a screening approach for reducing the amount of ILI inspection data requiring detailed structural integrity assessment. The screening approach has two main stages: (1) a geometry based filter assessing the shape of the flaw and (2) an elastic stress based filter that uses the point method, as in the Theory of Critical Distances (TCD), to identify the most severe flaws. The methodology uses the outputs from ILI (dimensions of flaws, orientation and distance from starting point) to generate a visualisation of the pits within the pipeline, a ranking of pits in terms of sphericity (roundness) and depth, to evaluate pit density and generate the models for finite element analysis. The method was tested on actual ILI data, where the number of pits in a 12.75in. riser of 11km length was reduced significantly (i.e. two/three orders of magnitude), such reduction depending on the level of conservatism introduced by the analyst. The tool will allow Oil and Gas owners and operators to reduce the immense amount of data obtained during pigging to a much less time-consuming set for flaw assessment.

Effect of the Y/T on the burst pressure for corroded pipelines with high strength

Pipelines are the fifth type of transportation, which has the highest efficiency and the least environmental damage for gas or oil transmission. Recently, the pipeline failures due to corrosion defects have attracted people's attention in maintaining pipeline integrity. Based on the double circular arc (DCA) model and elastic-plastic theory, an analytical solution of the corroded pipelines under internal and external pressures was obtained. And then, a new burst pressure equation of the corroded pipelines with high strength was further provided according to the von Mises criterion and previous equations. Compared with previous equations, this equation greatly extends the application scope and increases the calculation accuracy. The accuracy of our equation is validated by comparing with the full-scale experimental data. The effects of the thickness to diameter ratio, corrosion ratio and yield strength on the burst pressure are carefully investigated. The results reveal that when the thickness to diameter ratio is less than 0.1, the calculated results show a good agreement with the experimental data for the corroded and high strength pipes. Our research can provide a foundation for replacing the corroded pipelines at the bottom of the sea. In addition, our research is beneficial for the engineering design and integrity assessment of oil and gas pipelines.

Revised burst model for pipeline integrity assessment

Abstract Failure of pipeline is often caused by corrosion. It affects the health, safety, environment and economy considerably. To prevent pipeline failure, it is necessary to focus on the corrosion and pressure of the structures. Burst pressure is a key factor to assess the integrity of a pipeline. There are three means to determine burst pressure. These are lab testing, evaluation criteria and the Finite Element Method (FEM) modeling. However, the results of the burst pressure assessment using evaluation criteria are too conservative compared to the results of an actual pipe burst experiment and FEM modeling. The objectives of this paper are to analyze the changing trend of burst pressure of pipelines with different defect dimensions; to compare the FEM results with those of the evaluated criteria (DNV model) to indicate the conservative property; to revise the DNV model and verify its validity with actual burst experiments. The revised DNV model predicts burst pressure more reliably that could result in better engineering design of pipelines.

Assessing Gaseous Hydrogen Assisted Fatigue Crack Growth Susceptibility of Pipeline Steel Weld Fusion Zones and Heat Affected Zones

Abstract The objective of this work was twofold: (1) measure reliable fatigue crack growth relationships for X65 steel and its girth weld in high-pressure hydrogen gas to enable structural integrity assessments of hydrogen pipelines, and (2) evaluate the hydrogen accelerated fatigue crack growth susceptibility of the weld fusion zone and heat-affected zone relative to the base metal. Fatigue crack growth relationships (da/dN versus ΔK) were measured for girth welded X65 pipeline steel in high pressure hydrogen gas (21 MPa) and in air. Hydrogen assisted fatigue crack growth was observed for the base metal (BM), fusion zone (FZ), and heat-affected zone (HAZ), and was manifested through crack growth rates reaching nearly an order of magnitude acceleration over rates in air. At higher ΔK values, crack growth rates of BM, FZ, and HAZ were coincident; however, at lower ΔK, the fatigue crack growth relationships exhibited some divergence with the fusion zone having the highest crack growth rates. These relative fatigue crack growth rates in the BM, FZ, and HAZ were provisional, however, since both crack closure and residual stress contributed to the crack-tip driving force in specimens extracted from the HAZ. Despite the relatively high applied R-ratio (R = 0.5), crack closure was detected in the heat affected zone tests, in contrast to the absence of crack closure in the base metal tests. Crack closure corrections were performed using the adjusted compliance ratio method and the effect of residual stress on Kmax was determined by the crack-compliance method. Crack-tip driving forces that account for closure and residual stress effects were quantified as a weighted function of ΔK and Kmax (i.e., Knorm), and the resulting da/dN versus Knorm relationships showed that the HAZ exhibited higher hydrogen accelerated fatigue crack growth rates than the BM at lower Knorm values.

Prediction of crack propagation and arrest in X100 natural gas transmission pipelines with a strain rate dependent damage model (SRDD). Part 2: Large scale pipe models with gas depressurisation

Part 1 of this paper described a specimen for the measurement of high strain rate flow and fracture properties of pipe material and for tuning a strain rate dependent damage model (SRDD). In part 2 the tuned SRDD model is used for the simulation of axial crack propagation and arrest in X100 natural gas pipelines. Linear pressure drop model was adopted behind the crack tip, and an exponential gas depressurisation model was used ahead of the crack tip. The model correctly predicted the crack initiation (burst) pressure, the crack speed and the crack arrest length. Strain rates between 1000 s−1 and 3000 s−1 immediately ahead of the crack tip are predicted, giving a strong indication that a strain rate material model is required for the structural integrity assessment of the natural gas pipelines. The models predict the stress triaxiality of about 0.65 for at least 1 m ahead of the crack tip, gradually dropping to 0.5 at distances of about 5–7 m ahead of the crack tip. Finally, the models predicted a linear drop in crack tip opening angle (CTOA) from about 11−12° at the onset of crack propagation down to 7−8° at crack arrest. Only the lower of these values agree with those reported in the literature for quasi-static measurements. This discrepancy might indicate substantial strain rate dependence in CTOA.

Localised assessment of pipeline integrity during ultra-deep S-lay installation

As offshore oil and gas developments move into ultra-deep water depths, the greater the impact onerous installation procedures will have on pipeline integrity. Current approaches for predicting this impact are mostly based on a global pipeline perspective, using installation analysis tools that model the entire length of suspended pipeline using a string of ‘pipe’ finite elements. While these analysis methods are proven to provide reasonable predictions for moderate installation loadings, they lack sufficient detail for considering the most extreme conditions and do not generally account for material plasticity, residual curvature and pipe twist. This lack of detail could represent a significant shortcoming in future developments where pipe joints will be subjected to more substantial loading during ultra-deep water installation. A detailed local finite element model of the pipe cross-section during installation has been developed that allows for greater insight into the actual as-laid condition of a given pipeline. Numerous installation scenarios are considered and comparisons are made with output from traditional global analysis methods. These comparisons demonstrate potential shortcomings of existing engineering design approaches for ultra-deep water. This ultimately leads to recommendations that will shape on-going developments in design methods.

Assessment on failure pressure of high strength pipeline with corrosion defects

An accurate prediction on the failure pressure of line pipe is very important in the engineering design and integrity assessment of oil and gas transmission pipelines. This paper analytically investigates the failure pressure of line pipes with or without corrosion defects, and focus on the high strength steels. Based on von Mises strength failure criterion, a classic strength failure criterion, the failure pressure of end-capped and defect-free pipe pM is theoretically deduced with the strain hardening material. In order to derive a general solution for corrosion defect assessment of high strength pipelines, an extensive series of finite element analyses on various elliptical corrosion defects was performed. Finally, a new formula for predicting the failure pressure of corroded pipe in the material of high strength steels is formulated, based on the FE models and pM, and is validated using 79 groups of full-scale burst test data, which contain the low, middle and high strength pipeline. The results indicated that the proposed formula for predicting the failure pressure is closely matches the experimental data for the high strength steels.

Research on probabilistic assessment method based on the corroded pipeline assessment criteria

Pipeline integrity assessments are performed using conventional deterministic approaches, even though there are many uncertainties about the parameters in the pipeline integrity assessment. In this paper, a probabilistic assessment method is provided for the gas pipeline with corrosion defects based on the current corroded pipe evaluation criteria, and the failure probability of corroded pipelines due to the uncertainties of loadings, material property and measurement accuracy is estimated using Monte-Carlo technique. Furthermore, the sensitivity analysis approach is introduced to rank the influence of various random variables to the safety of pipeline. And the method to determine the critical defect size based on acceptable failure probability is proposed.

Failure Assessment Research of Flawed Ductile Metals Used in Nuclear Plant

A new method based on Gurson model to estimate plastic collapse failure for flawed high energy pipelines is put forward. First, standard fracture toughness measurement is explored on contact tension specimen made of ductile metal of S355. With increasing load, crack tip collapse occurs instead of crack extension, and local yielding appears around crack tip not the whole ligament, which coincides with recent research that NSC (neck-section-collapse) overestimates structure loading capability. From the test results, a method based on J-P curves is used to estimate collapse loads for contact tension specimen with different crack length. Second, tension process of contact tension specimen is simulated by finite element method combining with Gurson model, which is proved to simulate structure plastic failure well. Then, Gurson model is adapted to simulate failure process of circumferential cracked pipeline made of S355 with internal pressure. Finally, relationship between crack length and collapse CMOD (crack mouth opening displacement) is built, which is important for integrity assessment of nuclear pipelines.

Evaluation of burst pressure prediction models for line pipes

Accurate prediction of burst pressure plays a central role in engineering design and integrity assessment of oil and gas pipelines. Theoretical and empirical solutions for such prediction are evaluated in this paper relative to a burst pressure database comprising more than 100 tests covering a variety of pipeline steel grades and pipe sizes. Solutions considered include three based on plasticity theory for the end-capped, thin-walled, defect-free line pipe subjected to internal pressure in terms of the Tresca, von Mises, and ZL (or Zhu-Leis) criteria, one based on a cylindrical instability stress (CIS) concept, and a large group of analytical and empirical models previously evaluated by Law and Bowie (International Journal of Pressure Vessels and Piping, 84, 2007: 487–492). It is found that these models can be categorized into either a Tresca-family or a von Mises-family of solutions, except for those due to Margetson and Zhu-Leis models. The viability of predictions is measured via statistical analyses in terms of a mean error and its standard deviation. Consistent with an independent parallel evaluation using another large database, the Zhu-Leis solution is found best for predicting burst pressure, including consideration of strain hardening effects, while the Tresca strength solutions including Barlow, Maximum shear stress, Turner, and the ASME boiler code provide reasonably good predictions for the class of line-pipe steels with intermediate strain hardening response.

Numerical modeling of ductile crack extension in high pressure pipelines with longitudinal flaws

This study examines the applicability of a micromechanics approach based upon the computational cell methodology incorporating the Gurson–Tvergaard (GT) model and the CTOA criterion to describe ductile crack extension of longitudinal crack-like defects in high pressure pipeline steels. A central focus is to gain additional insight into the effectiveness and limitations of both approaches to describe crack growth response and to predict the burst pressure for the tested cracked pipes. A verification study conducted on burst testing of large-diameter, precracked pipe specimens with varying crack depth to thickness ratio ( a / t ) shows the potential predictive capability of the cell approach even though both the GT model and the CTOA criterion appear to depend on defect geometry. Overall, the results presented here lend additional support for further developments in the cell methodology as a valid engineering tool for integrity assessments of pipelines with axial defects.

Failure assessments of corroded pipelines with axial defects using stress-based criteria: Numerical studies and verification analyses

Conventional procedures used to assess the integrity of corroded piping systems with axial defects generally employ simplified failure criteria based upon a plastic collapse failure mechanism incorporating the tensile properties of the pipe material. These methods establish acceptance criteria for defects based on limited experimental data for low strength structural steels which do not necessarily address specific requirements for the high grade steels currently used. For these cases, failure assessments may be overly conservative or provide significant scatter in their predictions, which lead to unnecessary repair or replacement of in-service pipelines. Motivated by these observations, this study examines the applicability of a stress-based criterion based upon plastic instability analysis to predict the failure pressure of corroded pipelines with axial defects. A central focus is to gain additional insight into effects of defect geometry and material properties on the attainment of a local limit load to support the development of stress-based burst strength criteria. The work provides an extensive body of results which lend further support to adopt failure criteria for corroded pipelines based upon ligament instability analyses. A verification study conducted on burst testing of large-diameter pipe specimens with different defect length shows the effectiveness of a stress-based criterion using local ligament instability in burst pressure predictions, even though the adopted burst criterion exhibits a potential dependence on defect geometry and possibly on material's strain hardening capacity. Overall, the results presented here suggests that use of stress-based criteria based upon plastic instability analysis of the defect ligament is a valid engineering tool for integrity assessments of pipelines with axial corroded defects.

Analysis of Laminations in X52 Steel Pipes by Nonlinear by Finite Element

Hydrogen induced cracking is of great interest in the mechanical integrity assessment of sour gas pipelines. Multiple stepwise cracks with internal pressure called laminations are often observed in pipelines and their interaction and coalescence may significantly affect the residual strength of the pipes. In this work, the interacting fields of noncoplanar pressurized laminations in the wall of a pipe under pressure are analyzed by nonlinear finite element, considering an isotropic hardening law and the real tensile properties of the X52 steel. The results are presented as the evolution of the stress fields in the interlaminar region as a function of the pressure inside the laminations. It is found that for two approaching stepwise laminations, the critical pressure follows a hyperbolic type law. It is observed that for two cracks with lengths of less than 6.35 mm, the interlaminar region resists a critical pressure between 110 Mpa and 124 Mpa, respectively, for thicknesses 15.8 mm and 25.4 mm. The critical pressure is defined as the pressure inside the lamination that causes plastification of the interlaminar region.

Reliability based integrity assessment of steel pipelines under corrosion

Currently, the assessment of mechanical integrity of pipelines damaged by corrosion is performed through deterministic procedures and, in special cases, by using random variables. Rigorously, the calculations require the use of probabilistic methods because they provide the proper framework to include explicitly the uncertainties involved. Some recent treatments consider the corrosion events to be independent, which may lead to errors in estimating the failure probability of the pipeline. In this paper, pipeline integrity is assessed in terms of failure probability and recent models are utilised that estimate the resistance to failure using improved representations of the corrosion defect. Also, a simplified way to consider the space correlation between different segments of a corroding steel pipeline is introduced and the procedure is applied to the specific case where the defect geometry is modelled to be of parabolic shape. The geometrical and mechanical pipeline properties are considered to be random variables; their mean values are taken from experimental results and the interdependence between them is characterised using the model's correlation matrix.

05/00599 Structural integrity assessments of high pressure pipelines with axial flaws using a micromechanics model

The mechanical behavior of metal hydride reactor and the mechanism of the advent of strain caused by Ti0.71Zr0.29Mn0.8CrCu0.2 alloy expansion have been studied using pressure vessel controlled to be no pressure difference between the inner and outer wall of the reactor tube occurred. Strain gauges were set on the various locations in the reactor. The change of strain was continuously monitored using the strain analyzer. The metal hydride in the reactor after hydriding–dehydriding cycles was subjected to optical and scanning electron micrograph study, particle size analysis and the measurement of the angle of repose to investigate the flowability of metal hydride. The results indicate that once the elastic deformation of the reactor occurred during hydriding–dehydriding cycles, its deformation did not become larger anymore because the grain size of the alloy in the reactor changed to a lesser extent after subsequent cycles. The formation of metal hydride aggregates during cycling exerts a great influence on the advent of strain imposed to the reactor because the strain caused by the expansion of the aggregate becomes larger than that of the fine grain itself. The aggregate formation tends to occur easily due to the increase of cohesion and friction within the metal hydride powder during hydriding–dehydriding cycles.

Structural integrity assessments of high pressure pipelines with axial flaws using a micromechanics model

This study extends a micromechanics approach based upon the computational cell methodology to model ductile crack extension of longitudinal crack-like defects in a high strength pipeline steel. Laboratory testing of an API 5L X60 steel at room temperature using standard, deep crack C( T) specimens provides the data needed to measure the crack growth resistance curve for the material. A simple scheme to calibrate material-specific parameters for the cells employs this measured R-curve. A central focus of the paper is the application of the cell methodology to predict experimentally measured burst pressures for longitudinally pre-cracked pipe specimens. The experimental program includes large-diameter pipe specimens with different crack depth ( a) and crack size (2 c). Plane-strain computations are conducted on detailed finite element models for the pipe specimens to describe crack extension with increased pressure. The cell model predictions of crack growth response and burst pressure predictions are in good agreement with experimental measurements for the tested pipes. The present methodology holds significant promise as an engineering tool to simulate ductile crack growth and to predict the burst pressure of high pressure pipelines containing crack-like defects.