Wide Plate Tests Research Articles

Comparison of tensile strain capacity models of oil and gas pipelines

In the field of strain-based pipeline design, the widely used pipeline tensile strain capacity models mainly include CSA Z662, PRCI-CRES, and EXXON MOBIL. In this paper, the methods for calculating the ultimate tensile strain, toughness, and other key parameters of various models are emphatically introduced, and their applicable ranges are compared. Ninety-six sets of full-scale test and curved wide plate test data are collected in this paper. The prediction accuracy of various tensile strain capacity models under different steel grades is compared and verified by the test results. The CSA Z662 model has few parameters to consider, the TSC prediction value is relatively low, and EXXON MOBIL has more nonconservative situations when the strain is less than 4%. The PRCI-CRES model has the highest prediction accuracy.

Uncertainty quantification of the failure assessment diagram for flawed steel components in BS 7910:2019

The failure assessment line (FAL) describes the interaction between plastic failure and fracture of flawed steel components subjected to tension or bending. This paper quantifies the model uncertainty of the FAL as provided in the internationally used British Standard BS 7910:2019 by comparing the assessment with the actual failure load of 82 wide plate and 4 tubular joint tests. In line with findings of others, it is demonstrated that the accuracy of the assessment is significantly improved if the crack tip constraint is considered in the assessment. Irrespective of this crack tip constraint consideration, a non-negligible number of wide plate tests has a lower failure load than the one predicted by the FAL in BS 7910:2019, if based on three fracture toughness tests. A penalty or safety margin on the FAL is advocated to compensate for this. It appears advantageous to base the assessment on the average instead of the minimum of three equivalent fracture mechanics tests together with an associated (quantified) safety margin.

J and CTOD-Based Tensile Strain Capacity Prediction for Pipelines With a Surface Crack in Girth Weld

Abstract Long-distance pipelines for the transport of oil and natural gas to onshore facilities are mainly fabricated by girth welding, which has been considered as a weak location for cracking. Pipeline rupture due to crack initiation and propagation in girth welding is one of the main issues of structural integrity for a stable supply of energy resources. The crack assessment should be performed by comparing the crack driving force with fracture toughness to determine the critical point of fracture. For this reason, accurate estimation of the crack driving force for pipelines with a crack in girth weld is highly required. This paper gives the newly developed J-integral and crack-tip opening displacement (CTOD) estimation in a strain-based scheme for pipelines with an internal surface crack in girth weld under axial displacement and internal pressure. For this purpose, parametric finite element analyses have been systematically carried out for a set of pipe thicknesses, crack sizes, strain hardening, overmatch, and internal pressure conditions. Using the proposed solutions, tensile strain capacities (TSCs) were quantified by performing crack assessment based on crack initiation and ductile instability and compared with TSCs from curved wide plate tests to confirm their validity.

Compact crack arrest testing and analysis of EH47 shipbuilding steel

It is vitally important to measure the brittle crack arrest properties of shipbuilding steels to ensure that accidental damage will not result in total structural failure. Wide-plate test methods allow for direct measurement of the crack arrest toughness but this kind of testing is incredibly expensive. Therefore, there is a need for cheaper and simpler test methods which are able to measure a material’s brittle crack arrest toughness. In this work, Compact Crack Arrest (CCA) testing, which is standardised in ASTM E1221, has been successfully used to measure the crack arrest toughness of thick sections of EH47 shipbuilding steel. The results from this study have been compared to small-scale test methods. It was found that instrumented Charpy testing gives an overprediction of the CCA results, and nil-ductility transition temperature (NDTT) from Pellini tests gives a conservative estimate. The results presented in this study are discussed in terms of the effectiveness of the CCA test method for measurement of brittle crack arrest toughness and integrity assessment of large-scale structures.

Validation of BS 7910 fracture assessment procedures; wide plates and cylinders

BS 7910 provides guidance on the fitness for service of a flawed component. It covers a wide variety of metals in broad applications, thus the integrity of many components in various safety critical industries depends on it. Its detailed and rigorous validation, therefore, is of utmost importance. In this study, validation of the fracture assessment procedures of BS 7910 is carried out through the use of historical large test data, where the failure conditions are known in detail. Hundreds of test results from large-scale wide plate and cylinder tests are collected from the literature and assessed with BS 7910. The outline of the analysis and the tests analysed are given in this study in a concise manner. The exercise validates the fracture clauses and associated annexes that concern plate, pipe and curved shell geometries, together with other equations included in the procedure and used during the analyses.

Strain-Based CTOD and J-Integral Estimations for Pipelines With a Surface Crack Under Large Plastic Strain and Internal Pressure

Abstract Engineering solutions for crack-tip opening displacement (CTOD) and J-integral estimations for pipelines with a surface crack are proposed based on parametric finite element (FE) analyses for various geometries, material properties, and internal pressure conditions. Two kinds of CTOD definitions are considered in relation to strain-based estimation solutions for dealing with confusion regarding the definition of CTOD and to extend the applicability of tensile strain capacity (TSC) assessment. Moreover, influence functions of internal pressure are also suggested to take account of the effect of internal pressure on TSC. Using the proposed solutions, TSCs for cracked X65 and X70 pipes were assessed based on initiation and ductile instability. Curved wide plate tests were performed to obtain experimental TSCs, which were compared with those from the proposed solutions. Moreover, TSCs from the proposed solutions were also compared with those from other TSC-predicted models in order to assess their validity.

A simplified method for evaluation of brittle crack arrest toughness of steels in scaled-down bending tests

The wide-plate crack arrest test under tensile load for characterizing the brittle crack arrest toughness of high strength shipbuilding steel requires high economical cost and long lead-time, so there is still a substantial industry need for the simplified evaluation using the scaled-down specimen. The alternative method using a single edge-notched bending specimen, which assumes the complete load redistribution during the crack propagation, was recently proposed. However, it was confirmed that the results obtained using this method exhibited a low correlation with the arrest toughness obtained using the wide-plate tests. A new scaled-down version of the crack arrest test under bending load and a series of simplified evaluation based on the dynamic elasto-plastic FEA were introduced to characterize the crack arrest performance of the two types of steel plates in this investigation. The specimen is a single edge-notched tapered plate subjected to a three-point bending load in an isothermal environment. The geometry was selected so that the dynamic crack driving force calculated using FEA decreases monotonically with the crack propagation. For adopting a simplistic analytical approach considering the dynamic effect, the dynamic FEA simulation assumed the constant crack velocity and flat crack front and output the opening stress distributed ahead of the growing crack. The experimentally obtained values of the arrested crack length and the analytically obtained transition curves of the dynamic SIF were employed to characterize the crack arrest performance of the two steels. The results obtained using the simplified method developed in this investigation exhibited a high correlation with the arrest toughness obtained using the wide-plate crack arrest test.

Effect of Biaxiality on Engineering Critical Assessments

Common practice for the integrity assessment of structures, follows the use of Fitness for Service procedures, such as BS 7910/R6. There are many common structures (e.g. pipes) that are under biaxial loading. Even though the load biaxiality has been proven experimentally to have an effect on both the fracture toughness and the limit load of a component, the influence of biaxial stresses is not always directly addressed in the procedures. In order to address the effect of biaxiality on engineering critical assessments, BS 7910 and R6 fitness for service procedures are applied to four biaxially loaded wide plate tests previously conducted on A533B steel. The plates had been loaded in biaxial tension, with different biaxiality ratios, while the temperatures of the plates tested correspond to lower transition fracture toughness. The assessments include failure assessment lines created using Option 1, which requires the knowledge of the yield stress and tensile strength, Option 3 which is a material, structure and load dependent option, as well as a constraint modified Option 3 failure assessment line. To obtain Option 3 failure assessment lines, finite element analyses are conducted for the specimens. The tests are first assessed with the proximity to plastic collapse (Lr) and fracture (Kr) being calculated with the use of the main fracture clauses of BS 7910/R6, which do not take into account the effects of biaxiality. The conventional assessment is then followed by a more detailed assessment using the values calculated from Finite Element Analyses (FEA).

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.

Structural integrity of corroded girth welds in vintage steel pipelines

Girth welds of old steel pipelines and their surrounding heat affected zones are susceptible to corrosion attack. The resulting reduction in wall thickness may reduce the axial load or internal pressure bearing capacity to an unsafe level. Since standards provide limited guidance on weld corrosion assessment, the authors have executed an extensive experimental program to evaluate the axial load bearing capacity of corroded girth welds. To this end, curved wide plate tests have been executed and were analyzed by means of 3D digital image correlation. This paper discusses key influence factors related to weld geometry and material (strength and toughness). Then, the results are used to develop an assessment approach, based on Annex G of BS 7910:2013 and modified to account for the elastic–plastic stress–strain concentration resulting from weld misalignment.

Influence of pipe steel heterogeneity of the upper bound tensile strain capacity of pipeline girth welds: A validation study

The strain capacity of flawed girth welds is influenced by the constitutive properties of the connected pipes. Although most defect assessment procedures assume equal properties for both pipes, line pipe steel standards recognize significant variability. A recent theoretical framework provides an upper bound equation of tensile strain capacity based on pipe steel heterogeneity (and regardless of weld properties). This paper validates the equation using 64 curved wide plate test results. A strain capacity prediction method adjusted for heterogeneity is developed, allowing to adopt existing strain capacity equations for welds connecting homogeneous pipes. Its applicability to pressurized pipelines is discussed.

Toughness of X80 Grade Pipe Steel Girth Welds by Curved Wide Plate Testing

To confirm pipeline designs and validate the weld toughness, extensively works of Curved Wide Plate (CWP) test methods was carried out. The method of Two Clip Gauge-CTOD was firstly introduced in CWP test in this paper. The test results of CWP analyzed by Two Clip Gauge-CTOD method and Compliance—J method was compared. The CTOD-R curves achieved by these methods showed the good consistence with each other. Furthermore, the difference of the test results among SENB(single edge-notched bend), SENT(single edge-notched tensile) and CWP was discussed. It shows that the toughness obtained by SENB is relatively conservative, and CWP and SENT are recommend to be chosen for pipeline steel welds toughness test.

Strain capacity of girth weld joint cracked at “near-seam zone”

Cracking occurs often at “near-seam zone” in the girth weld joint of an X70 pipeline in tensile test, which is considered unacceptable for strain-based design pipelines according to some current standards. The tensile strain capacity of girth weld joints for X70 pipelines with “near-seam zone” cracks has thus been studied via the approach of crack driving force and fracture resistance curve. The high strain capacity has been demonstrated by resistance curve tangency approach and curved wide plate test. The results prove that the girth weld joint has considerable fracture resistance and is thus of high strain capacity.

Application of Weibull Stress Criterion to Brittle Fracture Assessment of Heat-Affected Zone-Notched Welds With Residual Stress

A constraint loss correction procedure using the Weibull stress criterion is specified in ISO 27306. However, this standard is applicable only to structural steel components with defects, not to welded joints. Therefore, we propose a method for estimating the brittle fracture limit of a weld with a notch in the heat-affected zone (HAZ) and residual stress based on the Weibull stress criterion. Three-point bending (3PB) tests and wide-plate (WP) tension tests of HAZ-notched welds made of 780-MPa class high-strength steel were conducted at −40 °C. The minimum critical crack tip opening displacement (CTOD) of the WP specimen fracturing at the coarse-grained region of the HAZ (CGHAZ) was approximately four times that of the 3PB specimen. Then, the effects of specimen geometry, residual stress, crack-front shape, and HAZ microstructure classification on the Weibull stress were investigated by using a finite element analysis (FEA). The results of these analyses showed that the specimen geometry, the welding residual stress, and HAZ microstructure affect the Weibull stress of HAZ-notched welds as crack driving force. Based on above results, the CTOD–Weibull stress curves for 3PB and WP specimens fracturing at CGHAZ were calculated by using an FEA. It was confirmed that the brittle fracture limit of an HAZ-notched weld with residual stress could be predicted from the Weibull stress criterion because predicted critical CTOD of WP specimens obtained by Weibull stress included experimental critical CTOD of WP specimens.

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.

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.

Weld Strength Mismatch in Strain Based Flaw Assessment: Which Definition to Use?

Weld strength mismatch is a key factor in the strain based assessment of flawed girth welds under tension. A strength overmatching weld shields potential flaws within the weld itself from remotely applied deformations and consequently reduces crack driving force. Although this effect has been recognized for decades, different weld strength overmatch definitions exist, and it is not yet fully established which of those is most relevant to a strain based flaw assessment. In an effort to clarify this unsolved question, the authors have performed a large series of parametric finite element analyses of curved wide plate tests. This paper provides an experimental validation of the model and subsequently discusses representative results. It is found that crack driving force is influenced by the shape of the pipe metals' stress–strain curves, which influences the representativeness of two common mismatch definitions (based on yield strength and on ultimate tensile strength). Effects of strength mismatch on strain capacity of a flawed girth weld are best described on the basis of a flow stress, defined as the average of yield and ultimate tensile strength. Based on the observations, a framework for a new strain capacity equation is proposed.

Strengthening of Misaligned Welded Pipes with Outer Circumferentially Crack Using FRP Bandage Finite Element Analysis

Finite element analysis is used to investigate the effectiveness of Fiber Reinforced Polymers (FRP) bandage in rehabilitating misaligned welded pipes with outer circumferential surface crack. It is assumed that the pipes are subjected to an axial tensile stress. Linear Elastic Fracture Mechanics (LEFM) regime is adopted to estimate the stress intensity factor (SIF). ANSYS software is used in the study through 3D-axisymmetric finite element analysis. The results demonstrated that stiffening with FRP bandage has a great influence on the reduction of the stress intensity factor at the crack tip improving the fracture integrity of the misaligned welded pipes. The most common solution of the rehabilitation process for damaged pipes fabricated from metals is very important mater. The rehabilitation technique for pipes is needed because of external erosion and corrosion for pipes or due to external damages .Usually such process requires using heavy machines and skilled workers in addition to the time needed. By using Fiber Reinforced Polymers (FRP) techniques to increase the strength for existing pipes, it is very important for the corroded and damaged pipes to increase the resistance to the surrounding environment. The conventional techniques for repairing damaged pipes need operation of cutting, welding and inspecting in addition. This may also require stopping the system during the repair period. Furthermore, for many cases the repair process is undesirable because of the residual stress induced in the pipe during the welding process which tends to decrease the fatigue performance of the pipe material in addition to the weakness of the pipe wall due to any of corrosion types such as pitting. Therefore, the final rehabilitation cost using FRP is low compared with the traditionally usual trends using welding techniques. Pipeline welding contractors are very often used in mechanized and semi-automatic welding processes. It is generally accepted that the most welded structures enter service containing tiny cracks or flaws (1). Moreover, these flaws may extend to a size where the safety of the structure becomes dangerous. It is therefore essential that engineers and designers have methods of assessing the significance of flaws detected during manufacture or after the structure has entered its service life, especially the structures that are connected with the human safety, such as pipes. Generally, cracks in piping system often occur at welding joint and around it (2). Most of these cracks are located in circumferential direction. For this reason circumferential cracks play an important role in the safety analysis of pipes (3). Strengthening techniques have drawn attention by many researchers to investigate the effectiveness of the stiffeners in the rehabilitation processes through using experimental as well as finite element methods. Ahmed et al. (4-6) investigated the effect of using FRP strengthening techniques to rehabilitate structural members under numerous loading conditions through FEA. Mourad et al. (7-18) have studied the effect of stiffening on crack growth through three-point bending TPB, compact tension CT and compact shear CS specimens. Chang-Young Oh et al. (19) investigated existence methods to estimate stress intensity factors due to welding residual stresses and compared with finite element (FE) solution for welding residual stress profiles, generated by simulating repair welds and compared with analytical solution. On the other hand, finite element analysis (FEA) was carried out to obtain the elastic stress distribution at cylinder to cylinder junction in pressurized shell structures that have applications in space vehicle design (20). The peak stress value is found to reduce due to filleted butt joint as expected and also confirmed through test results. Pipes with a flawed girth weld for two common girth weld tension tests: the curved wide plate test and the full scale pressurized pipe tension test, and subjected to global plastic deformation were studied by finite element models (21). Xiaohui Chen et. al. (22) presented an overview of recent progresses in experimental investigation and finite element analysis (FEA) of ratcheting behaviour of pressurized piping. The stress-intensity factor for the circumferential semi-elliptical surface cracks in a hollow cylinder's cross

Parametric finite element model for spiral welded pipes sections loaded in tension

Spiral welded pipes gain interest for oil and gas transportation in strain based design projects. Here, the structural response of the pipe with an environmentally imposed global plastic strain is of critical importance. However, a current lack of knowledge about the structural response of a spiral welded pipeline hinders the application in a strain based design context. This response is not only influenced by material properties, but also by geometrical characteristics. To understand these complex phenomena, finite element simulations can be considered. The finite element models need to incorporate all important factors to obtain a representative analysis. This paper describes the approach and structure used to develop such model. To obtain a high degree of automation and flexibility, the authors have developed a parametric script that allows creating a three dimensional curved wide plate test geometry taken from a spiral welded pipe. The model allows to modify pipe geometry, weld and heat affected zone geometry and material properties including Hill’s 1948 yield criterion to account for anisotropic material response. Pipe geometry includes helical forming angle, diameter, pipe profile and wall thickness with possible thickness variations. Weld and heat affected zone geometry include geometrical weld reinforcement of root and/or cap, fusion line profile and misalignment. A flaw, with variable size, is introduced in the weld metal or heat affected zone. To obtain an accurate description of the actual geometry, coordinate transformation schemes are incorporated that start from a flat plate with a simplified girth weld geometry. It is concluded that the proposed model is ready for use with a good view on mesh quality and output accuracy.

Evaluation of the effect of strength mismatch in undermatched joints on the static tensile strength of welded joints by considering microstructure

High-tensile strength 950 MPa steel, HT950, which is steel used in penstocks, was developed to provide two vital properties: fracture arrest to stop brittle fracture and high weldability. This steel has been already used for penstocks in some hydropower plants in Japan. To widely apply high-tensile strength steels in penstocks in the future, fewer restrictions against their welding conditions such as pre-heat and post-heat temperature controls are required. One proposed solution is to undermatch the strength of the filler metal to that of the base material. This allows less pre-heating, or no pre-heating at all, and the use of conventional rod and process management is easier. Previous studies have shown that there are softening conditions under which the strength of the joint can be considered as that of the base material. However, the shape and distribution of the soft region are assumed to be ideal. In this study, the method for calculating the change of the strength in heat-affected zone (HAZ) during the welding process is discussed. Then, the influence of the strength distribution of HAZ and welded zone to the strength of the joint is investigated by a wide plate test in both experiment and elastic–plastic analysis. Applicability of undermatched joints to penstock fabrication is considered by these discussions. As a result, it is concluded that the Vickers hardness distribution in the HAZ can be estimated by the method which is proposed in this study and the strength of the under-matched joints is high enough in both experiment and analysis in which the Vickers hardness distribution is considered. From these conclusions, the applicability of undermatched joints, of which the weld metal is the HT570 class, to penstock fabrication is conformed.