Girth Weld Defects Research Articles

Local approach to fracture applied to the analysis of a full size test on a pipe containing a girth weld defect

Abstract A welded pipeline segment containing a girth weld defect is tested under combined internal pressure and bending. The test is then analyzed with respect to both overall elasto-plastic behavior and crack propagation up to failure. The analysis is carried out using the finite element method. Constitutive equations for the plastic behavior of both weld metal and base metal are determined using tests carried out on smooth and notched bars. The cracking behavior of the weld metal is characterized using SENT specimens. The detailed analysis of the test allows to better understand boundary conditions and to represent the pipe overall behavior. The GTN model is used to represent damage growth up to failure in the weld metal. The simulation then allows representing the pipe behavior up to failure due to through thickness crack propagation.

ExxonMobil SENT test method and application to Strain-Based Design

Strain-Based Design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Large ground movements may impose significant longitudinal strains to the pipelines. One of the key elements of the SBD approach is a Tensile Strain Capacity (TSC) prediction model for pipelines, which is usually provided in the form of an equation based on Finite Element Analysis (FEA). ExxonMobil's TSC prediction model has been validated against more than 100 full-scale tests. A key input to the prediction model is the tearing resistance of pipe and weld materials in terms of a Crack Tip Opening Displacement Resistance (CTOD-R) curve. Therefore, a test methodology is needed to appropriately characterize the ductile tearing resistance of pipes and girth welds. To this purpose, in recent years, ExxonMobil has developed a Single Edge Notched Tensile (SENT) test method, which has been shown to provide a level of crack tip constraint close to what is observed for defects in pipeline girth welds. In addition to providing input to TSC prediction, SENT testing is an integral part of Strain-Based Engineering Critical Assessment (SBECA) for pipelines.This paper describes the details of the ExxonMobil SENT test procedure and its application to carry out TSC predictions for SBD. Particularly, the paper covers the following aspects: (1) test procedure including analysis and interpretation of results; (2) validation of selected specimen geometry through comparison with data from pipe Full-Scale Tests (FST); (3) use of the CTOD-R curve as input to the ExxonMobil TSC prediction model; (4) finally, recent challenges related to crack path deviation are discussed with potential ways to address this issue.

Research on Full-Scale Hydrostatic Burst Testing of Different Pipeline Girth Weld Defects

Recently incidents related to cracking of girth welds in oil and gas pipelines occurred frequently. With highly concentrated stresses, girth weld defects may significantly reduce bearing capacity of pipelines. Many girth weld defects were detected in the in-line inspection and excavated verification. How to assess the severity of these defects to avoid the excessive and unnecessary repair was a technical challenge. Four different kinds of girth weld defects were manufactured in the 32-inch pipeline and full-scale hydrostatic burst testing were carried out to assess the pressure performance of these defects. Many fitness-for-purpose assessment methods were applied to verify the accuracy of experimental results. After the testing, pipelines with failure defects were cut out to carry out the microanalysis. The research results make us understanding the severity of these defects and avoid the excessive and unnecessary repair of these defects to ensure the pipeline safety.

Fitness for Purpose Assessment of Girth Weld Defect Based on In-Line Inspection

The girth weld defect is one of the most common types of defects on oil and gas pipelines, which can have a strong impact on the operation safety. Several girth weld failure accidents have occurred on PetroChina’s pipelines in recent years. In this paper, PetroChina’s current work on inspection and fitness for purpose assessment of girth weld defects is summarized. The in-line inspection has been proved to be the best practice for oil and gas pipeline defect inspection, but there are still some technical issues such as defect characterization and parameter selection. Fitness for purpose assessment methods for girth weld defects include strength assessment method based on plastic collapse, FAD method based on both plastic collapse and fracture, simplified factor method and numerical analysis method based on finite element. It is critical to identify the actual type of defects detected in in-line inspection to select an appropriate assessment method. The identification of various loads and the selection of appropriate material parameters are also important issues in assessment procedure. The key techniques to be developed include defect characterization, load identification, assessment of defects on high grade steel pipes, reliability-based assessment, etc.

The Effect of Internal Pressure on the Tensile Strain Capacity of X52 Pipelines With Circumferential Flaws

Wide plate testing has been traditionally applied to evaluate the tensile strain capacity (TSC) of pipelines with girth weld flaws. These wide plate tests cannot incorporate the effect of internal pressure, however, numerical analysis in recent studies showed that the TSC is affected by the level of internal pressure inside the pipeline (Wang et al. 2007, "Strain Based Design of High Strength Pipelines," 17th International Offshore and Polar Engineering Conference (ISOPE), Lisbon, Portugal, Vol. 4, pp. 3186–3193). Moreover, most of the past studies focused on the effect of circumferential flaws on the TSC for pipelines of steel grade X65 or higher. The current Oil and Gas Pipeline System Code CSA Z662-11 provides equations to predict the TSC as a function of geometry and material properties of the pipelines. These equations were based on extensive studies on pipes having grades X65 or higher without considering the effect of internal pressure. This paper investigates the TSC for pipelines obtained using an experimental technique considering the effect of internal pressure and flaw size. Eight full-scale tests of X52 NPS 12 in. pipes with 6.91 mm wall thickness were conducted in order to investigate the effect of circumferential flaws close to a girth weld on the TSC for vintage pipelines subjected to eccentric tensile forces and internal pressure. The tensile strains along the pipe length and on the outer circumference of the pipe were measured using biaxial strain gauges and a digital image correlation (DIC) system. Postfailure macrofractography analysis was used to confirm the original size of the machined flaw and to identify areas of plastic deformation and brittle/ductile fracture surfaces. From the experimental and numerical results, the effect of internal pressure and flaw size on the TSC and the crack mouth opening displacement (CMOD) at failure were investigated and presented.

Framework for Key Influences on Tensile Strain Capacity of Flawed Girth Welds

A key influence factor in the strain-based assessment of pipeline girth weld flaws is weld strength mismatch. Recent research has led to a framework for tensile strain capacity as a function of weld flow stress (FS) overmatch. This framework is built around three parameters: the strain capacity of an evenmatching weldment, the sensitivity of strain capacity to weld FS overmatch, and the strain capacity at gross section collapse (GSC). A parametric finite element study of curved wide plate (CWP) tests has been performed to identify the influence of various characteristics on each of these three parameters. This paper focuses on flaw depth, tearing resistance of the weld, stress–strain behavior of the base metal, and weld geometry. Influences of these characteristics are mostly found to be limited to one or two of the three framework parameters. A preliminary structure is proposed for equations that further develop the strain capacity framework.

Parameters affecting the weld defect acceptance criteria of clad submarine pipelines installed by S-lay or reel-lay

This case study presents a methodology to determine the criticality of defects in submarine pipeline girth welds made of clad carbon steel and to establish weld defect acceptance criteria. The maximum target acceptance criteria are usually given by the applied design code often with additional requirements set by the client. ECA calculations are carried out in order to demonstrate that the target criteria can be met using the relevant material data, geometry and load input. The ECA methodology must follow the defect development from welding fabrication through pipeline installation from the lay barge, as well as consider loads in the as-laid condition during pre-operational and operational phases until the end of design life and verify that leakage or failure will not occur.

Comparison of girth weld tearing resistance obtained from Curved Wide Plate and Single Edge Notch Tensile testing

Curved Wide Plate and Single Edge Notch Tensile tests are commonly considered for girth weld defect assessment. It is however not clear which test yields the least conservative estimate of the tearing resistance. A combined numerical–experimental procedure has been established to measure the crack extension, based on the direct current potential drop technique. This procedure appears accurate with respect to testing standards. In addition, the tearing resistance of the SENT specimens appears noticeably lower compared to wide plate specimens. This implies that SENT testing introduces conservatism in a girth weld defect assessment.

Full-scale Burst Test and Finite Element Simulation of 32 Inch Oil Pipe with Girth Weld Defects

Recently several leak accidents occurred due to girth weld defects in long distance transmission pipelines of PetroChina. To avoid possible failure accidents, in-line inspections were performed on some certain pipelines. In some new built pipelines, a large quantity of girth weld defects were detected in the inspections. Many fitness-for-service (FFS) assessment methods have been proposed for various kinds of metal loss in pipeline, however, it is still difficult to obtain a confident assessment of girth weld defects. Several types of girth weld defects were manufactured on 32 inch oil pipe in this study and full-scale burst tests were performed combined with 3D finite element simulation to study the influence of girth-weld defect geometry on the failure pressure. These defects were manufactured with different opening width and orientation. After full-scale burst tests, the real morphology of the girth weld defects were examined by fracture analysis. The defect data and experimental results were obtained and used to FFS assessment, where FFS methods included API 579, BS 7910, Miller, Kastner etc. Error analysis results were illustrated by comparing the experimental data and the assessment results, and suggested that fracture failure should be considered for girth-weld defects assessment in a conservative view at last.

Multiaxial fatigue analysis on reeled deepwater steel catenary risers with girth weld defects

In the present study, we simulated the reel-lay installation process of deepwater steel catenary risers (SCRs) using the finite element method and proposed multiaxial fatigue analysis for reeled SCRs. The reel-lay method is one of the most efficient and economical pipeline installation methods. However, material properties of reeled risers may change, especially in the weld zone, which can affect the fatigue performance. Applying finite element analysis (FEA), we simulated an installation load history through the reel, aligner, and straightener and analyzed the property variations. The impact of weld defects during the installation process, lack of penetration and lack of fusion, was also discussed. Based on the FEA results, we used the Brown-Miller criterion combined with the critical plane approach to predict the fatigue life of reeled and non-reeled models. The results indicated that a weld defect has a significant influence on the material properties of a riser, and the reel-lay method can significantly reduce the fatigue life of SCRs. The analysis conclusion can help designers understand the mechanical performance of welds during reel-lay installation.

J-integral analysis of heterogeneous mismatched girth welds in clamped single-edge notched tension specimens

Flaw assessment procedures require a quantification of crack driving force, and such procedures are generally based on the assumption of weld homogeneity. However, welds generally have a heterogeneous microstructure, which will influence the crack driving force. This paper describes a stress-based methodology to assess complex heterogeneous welds using a J-based approach. Clamped single-edge notched tension specimens, representative of girth weld flaws, are analyzed and the influence of weld heterogeneity on crack driving force has been determined. The use of a modified limit load for heterogeneous welds is proposed, suitable for implementation in a ‘homogenized’ J-integral estimation scheme. It follows from an explicit modification of an existing solution for centre cracked tension specimens. The proposed solution provides a good estimate of crack driving force and any errors in the approximation may be accounted for by means of a small safety factor on load bearing capacity.

Combined numerical-experimental framework for strain based design and flaw assessment of girth welds

An increasing number of transmission pipelines have to be installed and operated in harsh conditions, due to the diminishing reserves of exploited fossil fuel sources. Under certain environment related circumstances, longitudinal plastic strains are imposed. When tensile, these may induce failure in girth welds in conjunction with the inevitable presence of weld defects. A large number of factors related to environment, material, geometry and operating conditions influence the tensile strain capacity and, hence, the acceptability of girth weld flaws. Therefore, a project specific development of guidelines in this strain based context is recommended. This paper provides systematic frameworks on (a) how to design pipelines under strain based conditions, and (b) how to assess girth weld flaws that were detected using non destructive testing. Attention is given to several technical and economical aspects related to the selection and qualification of pipe and weld metals, and to the evaluation of flaw acceptability. Both frameworks comprise a combined experimental-numerical approach, collecting project development reports from literature with research carried out at Soete Laboratory. Concretely, four in-house realizations are adopted: the UGent equation for strain capacity, the curved wide plate tension test, the UGent stress-strain equation and a finite element model of full scale pressurized pipe tension testing. The proposed frameworks aim to facilitate in performing thorough and economically justifiable strain based design and assessment processes.

Sensitivity of Plastic Response of Defective Pipeline Girth Welds to the Stress-Strain Behavior of Base and Weld Metal

One of the key parameters influencing the acceptability of a pipeline girth weld defect subjected to remote plastic deformation is the strength mismatch between weld and base metal. However, no single definition exists for weld strength mismatch, as it can be defined either on the basis of yield stress, ultimate tensile stress or any intermediate flow stress. To investigate the relevance of such definitions, the authors have performed a series of analyses of curved wide plate tests, using a validated parametric finite element model. The results indicate that, whereas yield stress overmatch determines crack driving force for small plastic strains, ultimate tensile stress overmatch is the more important parameter for advanced plastic strains and determines the eventual failure mode. Further, the strain capacity and exact crack driving force curve are additionally determined by uniform elongation and crack growth resistance.

Validation of a wide plate finite element model using digital image correlation

To investigate the influence of global plastic deformations on girth weld defect tolerance inpipelines, a parametric finite element model has been developed. This paper provides an experimentalvalidation of the model. It describes the test setup and instrumentation used for the evaluation of plasticstrain fields around a notch in a tension loaded non-welded X65 mini wide plate. LVDT measurements anddigital image correlation (DIC) results are compared to each other and to the results of finite elementsimulations. Whereas some deviation is observed owing to unavoidable experimental uncertainties andlimitations of finite element modelling, the overall correspondence is more than satisfying.

Influence of Fatigue Loading on the Engineering Critical Assessment of Steel Catenary Risers in Sour Deepwater Oil and Gas Developments

Steel catenary risers (SCR) are used in deepwater oil and gas developments to transfer produced fluids from the seabed to surface facilities. SCRs can be subject to fatigue loading from a variety of sources including wave and tidal motion, vortex induced vibration (VIV) and operating loads. When the produced fluids are sour (ie contain water and H2S) higher fatigue crack growth rates (FCGR) are expected, and this can have a significant effect on defect tolerance. The aim of this paper is to provide guidance on the current best practice methods for performing engineering critical assessments (ECA) on internal surface breaking defects in SCRs operating in a sour environment and subject to VIV fatigue loads. Example ECA calculations are presented for circumferential girth weld flaws, based on the failure assessment diagram (FAD) approach within the framework of BS 7910 [1]. The influence of certain key input variables is demonstrated, including the FCGR, determined from recent sour test data generated as part of this research.

Automated Ultrasonic testing of Pipeline Girth Welds, its Present Status and Future Developments

Mechanized or Automated Ultrasonic Testing of pipeline girth welds is now in common use in the on - and offshore industry. Automated ultrasonic testing (AUT) is globally seen as more than just an alternative to the standard radiographic inspection technique not only because it does not poses safety hazards but also because it is faster, more reliable and has better detection capabilities of critical Lack of Fusion defects in pipeline girth welds. One of the reasons to use the AUT technique is due to its possibility to use acceptance criteria which are based upon ECA (Engineering Critical Assessment) instead of the so-called "Good Workmanship". Usually the AUT systems are mounted on a band strapped around the pipe. From the weld, ultrasonic data is collected from which the defect sizes and positions can be determined by experienced operators using dedicated software algorithms. This paper discusses the RTD Rotoscan system, which is the first worldwide AUT inspection system for new construction pipeline girth welds. The principles of AUT, the conventional RTD Rotoscan as well as the RTD phased array Rotoscan and its advantages in comparison with conventional (multi probe) AUT are discussed. Furthermore challenges regarding the use of AUT on Austenitic welding having a corrosion-resistant alloy layer are presented. This paper describes also latest improvements made on AUT during the last years in order to optimize inspection philosophy and minimize the system's "Uncertainties". In addition a brand new method, RTD IWEX, is briefly described that allows the detection and sizing of weld imperfections in 3D.

Fatigue performance of pre-strained pipes with girth weld defects: Local deformation mechanisms under bending

One of the most effective installation methods of metallic risers for offshore oil and gas applications is the reel-lay process. Pipe segments are first welded onshore and subsequently bent over a cylindrical rigid surface (reel) on a laying vessel. During installation, the line is unreeled, straightened, and then laid into the sea under tension. In this process, material properties change and eventual weld defects may increase, thus reducing the fatigue life of those joints under operational loads. Therefore, welded joints must be manufactured based on strict weld acceptance criteria. These criteria should guarantee reliable fatigue life standards of the joints without impairing the feasibility of weld manufacture (high cost). This paper describes the apparatus designed and built at COPPE/UFRJ to simulate the cyclic bending process as it occurs during some installation processes such as the reel method. Experimental results from bending tests on full-scale pipes are presented, including the effect on pipe initial geometry, weld defect dimensions, and local strains experienced in the weld region. The reeling process is then simulated through a nonlinear finite element model that incorporates weld defects. The results are then used as guidelines to experimentally obtain fracture mechanics parameters of typical weld under pre-strained conditions.

Fatigue performance of pre-strained pipes with girth weld defects: Full-scale experiments and analyses

The local deformation mechanism of pre-strained welded joints with defects was studied in the first part of this work. [Netto TA, Botto A, Lourenço MI. Fatigue performance of pre-strained pipes with girth weld defects: Local deformation mechanisms under bending. Int J Fatigue (2007), doi:10.1016/j.ijfatigue.2007.08.001]. It was shown that highly localized strains may develop at the tip of these defects, depending on their type (lack of fusion and lack of penetration) and size. Different levels of pre-straining do affect the fracture mechanics material parameters and subsequently can deteriorate the fatigue performance of these joints under operational cyclic loads. Here, the fatigue lives of pre-strained joints with different defects were obtained experimentally by means of full-scale specimens. Subsequently, both a finite element model and an algorithm based on linear fracture mechanics that accounts for the changes in the material properties due to pre-straining were used to estimate the fatigue life of typical joints. The results were compared and used to suggest guidelines to the development of weld acceptance criteria of reeled steel catenary risers.

Improved focusing for thick-wall pipeline girth weld inspections using phased arrays

Since Engineering Critical Assessment (ECA) became a standard procedure for assessing pipeline girth weld defects, sizing has become a major issue. Despite claims and requests for better sizing, most pipeline trials have shown that sizing accuracies are only within about I mm at best in the vertical (through-wall) direction, and typically worse in the horizontal (circumferential) plane due to beam spread. Defect sizing requirements are becoming more stringent as new high-strength pipes and new applications are being introduced. This applies especially to thick-walled offshore pipelines, particularly tendons and risers, where fatigue life is critical. This project has developed a curved 1.5D matrix array, primarily for improving focusing in the horizontal direction on thick-walled risers and tendons. Initially, the focusing was modelled for a variety of arrays, including the standard flat array. Modelling showed the optimum array to be 120 elements long, with three rows, with a slight curvature for initial focusing. Detailed focusing can be performed using the electronic capability of phased arrays. Normal unfocused Pipe WIZARD phased arrays use 60-element arrays on either side of the weld; in contrast, the new array uses 360 elements for one-side only inspection. The results from this array were compared with the standard unfocused 1D array on a typical calibration block; significant improvements were obtained.

Prediction of the Tolerable Defect Size for Strain Based Design

By using data of Curved Wide Plate (CWP) tests, a simple procedure to derive tolerable girth weld defect sizes is presented. The procedure involves prediction of the maximum tolerable defect size as a function of the applied remote plastic strain and the actual pipe and weld metal properties. Application of the methodology requires a reliable non-destructive testing technique that can detect the critical defect dimensions.