Engineering Critical Assessment Research Articles

A modification of reference strain approach for thin-walled submarine pipelines under large-scale plastic strain and internal pressure

Thin-walled pipelines are widely used in submarine oil and gas transportation. During the process of reel-lay and service, pipelines are subjected to large-scale plastic strain and internal pressure. Reference strain approach takes advantage of natural strain-controlled boundary condition directly. However, it is only reliable for small cracks by the assumption of equality between the reference strain and nominal strain. The effect of internal pressure is not considered in traditional failure analysis diagram (FAD). In this study, a theoretical solution of reference strain considering the influence of internal pressure is proposed. A corresponding failure assessment curve (FAC-New) considering the influence of internal pressure is also constructed based on the derived nominal strain. The proposed FAC-New is in contrast with FAC-R6. It is proved that FAC-New presents a more explicit classification of the influence of internal pressure in Engineering Critical Assessment (ECA).

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).

Research Progress of Engineering Critical Assessment of Submarine Pipelines with Large Deformation

Research Progress of Engineering Critical Assessment of Submarine Pipelines with Large Deformation

Engineering critical assessment for a sandwich pipe field joint

This paper seeks to apply a combination of techniques with the aim of outlining a finite element (FE) based methodology for carrying out Engineering Critical Assessment on the swaged weld for J-lay installation. The critical potential defect position during installation is identified and its severity is evaluated using the Stress Concentration Factor (SCF). Closed-form parametric equations for quantifying the geometric SCF as a function of the swaged weld dimensions are derived using large-scale parametric studies and statistical analysis for the joint under tension. The maximum allowable defect size for a swaged weld under potential installation loadings is evaluated by two proposed FE-based fracture mechanics methodologies. In the absence of tearing resistance data, the influence of the filler resin stiffness, loading type and material response on the acceptability of a defect size is studied and the conservative nature of brittle fracture design for the fracture assessment of carbon steel pipelines with significant ductility is illustrated.

Steel antenna towers – from designing to manufacturing optimization

The elaboration of a methodology for determining the acceptability of detected cracks/flaws in a structure has a major practical importance in the overall assessment and life integrity of a structure. The relation given by fracture mechanics links a parameter which describes the stress intensity at a crack tip to a material characteristic – fracture toughness. This relation provides the possibility of assessing the fracture conditions of the structural elements with defects (cracks). An Engineering Critical Assessment is an analysis based on fracture mechanics principles, of whether or not a given flaw is safe from brittle fracture, fatigue, creep or plastic collapse under specified loading conditions.The paper presents some considerations on the designing and manufacturing process, taking into account structural assessment procedures using fracture mechanics. It also presents a study case – a steel monopole tower on which the method is applied, in order to assessing the possible discovered flaws in the designing phase. Ten types of flaws which can be discovered in the service time of the structure were assessed. Different types of locations were taken into account, thus resulting groups of flaws which were then assessed and compared.

Crack driving force prediction in heterogeneous welds using Vickers hardness maps and hardness transfer functions

Flawed welds often require an Engineering Critical Assessment (ECA) to judge on the necessity for weld repair. ECA is a fracture mechanics based prediction of the integrity of structural components with defects under operating conditions. Adding to the complexity of a weld ECA is the occurrence of local constitutive property variations in the weldment (‘weld heterogeneity’). Their quantification allows for a more accurate assessment compared to common (standardized) practice, which assumes welds to be homogeneous. Hardness measurements allow to quantify weld strength heterogeneity given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of relations (‘transfer functions’) between hardness and strength, and recognize substantial scatter in hardness based predictions of strength. Within this context, this paper investigates the suitability of Vickers hardness mapping to perform an accurate weld ECA for high strength low alloy steel. To overcome the scatter associated with standardized transfer functions, this paper suggests an experimental calibration procedure based on all weld metal tensile tests. Finite Element (FE) analysis has been conducted on welds originating from steels to simulate their crack driving force response in Single-Edge notched Tension (SE(T)) specimens. Vickers hardness maps and hardness transfer functions are combined to assign element-specific constitutive properties to the model. The transfer function calibrated by all weld metal tensile tests yields a better agreement with experimental load-CTOD curves than transfer functions mentioned in standards and codes. Finally, a step-by-step procedure facilitates a practical adoption of the methodology.

Engineering Critical Assessment for Offshore Pipeline with Semi Elliptical Surface Cracks in Girth Weld – Comparison of FEM and BS7910 Guideline

Engineering Critical Assessment for Offshore Pipeline with Semi Elliptical Surface Cracks in Girth Weld – Comparison of FEM and BS7910 Guideline

Experimental investigation of mechanical and fracture properties of offshore wind monopile weldments: SLIC interlaboratory test results

AbstractS355 structural steel is commonly used in fabrication of offshore structures including offshore wind turbine monopiles. Knowledge of mechanical and fracture properties in S355 weldments and the level of scatter in these properties is extremely important for ensuring the integrity of such structures through engineering critical assessment. An interlaboratory test programme was created to characterise the mechanical and fracture properties of S355 weldments, including the base metal, heat‐affected zone, and the weld metal, extensively. Charpy impact tests, chemical composition analysis, hardness tests, tensile tests, and fracture toughness tests have been performed on specimens extracted from each of the 3 material microstructures. The experimental test results from this project are presented in this paper, and their importance in structural integrity assessment of offshore wind turbine monopiles has been discussed. The results have shown a decreasing trend in the Charpy impact energy and Jmax values with an increase in yield stress from base metal to heat‐affected zone to weld metal. Moreover, the JIC fracture toughness value in the heat‐affected zone and weld metal is on average around 60% above and 40% below the base metal value, respectively. In addition, the average Charpy impact energy value in the heat‐affected zone and weld metal is around 5% and 30% below the base metal value, respectively. The effects of mechanical and fracture properties on the critical crack size estimates have been investigated, and the results are discussed concerning the impact of material properties on structural design and integrity assessment of monopiles.

Determining the crack acceptability in the welded joints of a wind loaded cylindrical steel shell structure

Abstract Engineering critical assessment can be considered as a mandatory request in the civil engineering designing and manufacturing process. The paper is presenting the procedure for determination of crack acceptability based on fracture toughness with failure assessment methods (FAD-1 and FAD-2) which is applied to a cylindrical steel shell structure with welded joints which is having the wind as the main load [7]. The stresses in the structure were determined based on a structural analysis. According to [3] an in depth finite element analysis was applied to the segment joint of the steel shell element (pillar), thus revealing the stresses in the joint (as well in the welded joints). There were assessed common types of flaws met at steel shell cylindrical structure elements using failure assessment diagrams – level 1 – FAD-1. The results are presenting the acceptability level for each type of flaw with comparative graphs, determining also the critical dimension of the flaw. For each flaw was calculated the failure assessment diagram level 2 (FAD-2). Comparisons between groups of flaws were done, revealing the critical crack like flaw. Also the critical values of flaw dimensions were calculated for each flaw type [7]. The methodology establishes clear rules for assessment of structural elements with cracks, determining the initial flaws, assessed flaws and critical values of the cracks. The conclusion of the research reveals the fracture resistance failure susceptibility of different parts of the segment joint given an existing discovered flaw. Based on the detailed procedures described in the paper, on conclusions to the assessment done on each type of flaw, the method can be applied from the design phase on these types of structure elements.

Subsea Pipeline Lateral Buckling Design—Strain Concentration or Strain Capacity Reduction Factors

A strain concentration factor is typically incorporated in the higher-pressure and high-temperature (HPHT) pipeline lateral buckling assessment to account for nonuniform stiffness or plastic bending moment. Increased strain concentration can compromise pipeline low cycle fatigue and lateral buckling capacity, leading to an early onset of local buckling failure. In this paper, the philosophy of local buckling mitigation using the strain concentration factor is examined. The local buckling behavior is evaluated. Global strain reduction and evolution against buckling are analyzed with respect to varying joint mismatch level. The concept of a strain reduction factor (SNRF) due to joint mismatch is developed based on the global strain capacity reduction with reference to the uniform configuration. It is demonstrated that the SNRF in terms of strain capacity reduction is a unique characteristic parameter. As opposed to strain concentration, it is an invariant insensitive to evaluation methods and design strain demand level, hence more representative as a limiting design metric to maintain the safety margin. The rationale for its introduction as an alternative to the strain concentration factor is outlined and its benefits are established. The method for obtaining the SNRF and its application is developed. The discernible difference and scenarios for application of either factor are discussed, including low and high cycle fatigue, linearity and stress concentration, engineering criticality assessment (ECA), and lateral buckling. Additional causal factors giving rise to mismatch such as pipe schedule transition and buckler arrestor are also discussed. Iterations of finite element (FE) analyses are performed for a pipe-in-pipe (PIP) configuration in a case study.

An extended engineering critical assessment for corrosion fatigue of subsea pipeline steels

Abstract Engineering critical assessment (ECA) is widely used to assess the structural integrity in offshore industry. But industry standards provide limited guidance on ECAs of structures subjected to corrosion fatigue (CF). In this paper, a critical stress intensity factor (SIF) derived from a corrosion-crack correlation model is proposed to improve the traditional ECA for steel structures in seawater. The proposed critical SIF extends the traditional ECA for CF in that it accounts for the influence of load frequency and initial crack size on the model selection within current ECA guidelines for the ECA of marine structures under CF. The extended ECA is applied for × 65 carbon pipeline steels subjected to CF. The crack growth curves are built using a three-stage CF crack growth model and the experimental data. Fatigue lives are calculated based on those curves as well as traditional ECA models. Results show that the critical SIF can effectively improve the ECA for × 65 carbon pipeline steels under CF. The extended ECA provides a reasonable assessment with reduced conservatism in contrast to the traditional ECA for CF.

Welding residual stresses in a strip of a pipe

Residual stress was measured through the thickness of a strip sample of a girth welded pipe. The motivation behind this measurement was to investigate whether the segment test, commonly used in validation of Engineering Critical Assessment results, is representative of full girth welds in terms of the welding residual stress. A major rationale was to try to understand how much residual stress will be relieved due to the removal of the strip sample from the pipe. The reported results in this paper are based on aJoint Industry Project (JIP) led by DNV GL on the treatment of residual stress on pipes undergoing high plastic deformation. The paper presents measured residual stresses data in the hoop and axial direction of the pipe. In the current study, the measured residual stresses in the segment sample are found to be similar to those at similar full girth welds but showed more compressive stresses in the weld cap, more tensile stresses in the weld root, and a similar profile through the thickness. This re-distribution of residual stress can be attributed to the release of the “far field” bending stresses when the pipe was sectioned into the strip in both the hoop and axial directions.

A modified engineering critical assessment method for deeply-embedded cracks in metallic pipelines subjected to large plastic strain

The results of three-dimensional finite element analyses indicate that the normalized J-integral values of surface cracks are almost 3 times higher than those of deeply-embedded cracks, provided the sizes of the surface and deeply-embedded cracks are the same. In this paper, we introduce a factor F, which serves as an adjustment factor to reduce the degree of conservatism. The results show that F is the function of the non-dimensional deeply-embedded crack height (2ae/t), since F is markedly affected by 2ae/t. Furthermore, after introducing the factor F, the crack tip constraint of the deeply-embedded cracks is found to be lower than that of the surface cracks, which gives a further conservatism in using the factor F for assessment of embedded cracks.

Measurement techniques and procedures for standardized SE(T) testing of linepipe steel

The single edge-notched tension (SENT or SE(T)) test uses a low-constraint specimen geometry to determine the elastic-plastic fracture toughness of relevant linepipe steels. Application of the results include, but are not limited to, design methods, material selection, structural integrity assessment, fitness for service (FFS) and engineering critical assessment (ECA). Until recently, industry and government researchers have developed and published recommended practices without consensus from standard development organizations. A test standard was recently issued in 2014 by the British Standards Institution as BS 8571, and there is an ongoing effort to publish a robust standard test method within the American Society of Testing and Materials (ASTM International). Standardization of any test method should consider the influence of physical measurements on the results of the test. Generically, all measurements have uncertainty, and a standardized test method endeavors to produce results with a minimum uncertainty, as well as known precision and bias so that the results can be intelligently used for their intended purpose. This paper reviews the measurement techniques and procedures from each of the published recommended practices and BS 8571 and provides further guidance on specific techniques and procedures with respect to uncertainty.

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.

Investigation on the strain capacity of girth welds of X80 seamless pipes with defects

An experimental and numerical study has been carried out on the strain capacity of girth-welded X80 pipes subject to displacement-controlled conditions. The focus of this research is fracture limit state. This paper contains details of small-scale and full-scale experiments on X80 line pipe specimen containing girth welds and different initial defect dimensions in order to understand their fracture behaviour. The effect of internal pressure on strain capacity is also investigated through full-scale bending tests. Extensive accompanying Finite Element Analysis has been carried out with a focus on weld overmatch combined with Heat Affected Zone. The analysis results are compared with experimental results and the current Engineering Critical Assessment procedures.

Treatment of fracture toughness data for engineering critical assessment (ECA)

This paper is concerned with the choice of the fracture toughness value employed in deterministic ECAs and how this is dealt with in various codes. The use of the minimum value from three tests, the minimum of three equivalent (MOTE) concept and the use of statistical data fitting procedures are examined. In addition, the implications of the choice of fracture toughness value on the quality of the ECA are discussed. It is argued that as ECA procedures become more sophisticated and more precise, there is a need to critically examine the way fracture toughness data are treated. Simple approaches, based on testing a few specimens, may no longer be appropriate and using a characteristic value from a distribution fitted to the data will have a more reliable outcome.

Application of Engineering Critical Assessment Method in the Development Stage of Welding Consumables

Application of Engineering Critical Assessment Method in the Development Stage of Welding Consumables

A modified fracture assessment method for pipelines under combined inner pressure and large-scale axial plastic strain

Owing to ground movement, in-service pipelines may be subjected to large scale axial plastic strain. Therefore, determining the fracture capacity of pipelines with crack-like defects is a key issue that needs to be addressed. In this study, to conduct an engineering critical assessment (ECA) for in-service pipelines, the limit load solution of Kastner for pipelines with circumferential surface cracks under tension was modified based on the von Mises yield criterion. Then, the limit load solution of Kastner was adapted to the situation under combined inner pressure and axial plastic strain. Furthermore, the optimised reference stress method was applied to improve the accuracy of the ECA for in-service pipelines. Finally, the formula for the non-dimensional parameter, γ, was developed.

Measurement and prediction of CTOD in austenitic stainless steel

AbstractVariation of Crack Tip Opening Displacement (CTOD) test values can have a significant effect on the Engineering Critical Assessment of a structure. This paper examines the development of CTOD with increasing load in an austenitic stainless steel. The silicone replication method giving variation of CTOD across the specimen thickness, and Digital Image Correlation (DIC) are compared to each other, and in turn to clip gauge measurements from tests. Results from Finite Element models are also presented. Estimations of CTOD from BS 7448‐1, ISO 12135 and ASTM E1820, and a proposed modification from JWES are compared to the experimental data from the crack cast in silicone compound – assumed to be the actual CTOD. The DIC measurement showed consistency with crack replicas, and a formula is given to estimate CTOD using DIC. For high strain hardening austenitic stainless steel, both the JWES and ASTM E1820 estimations provide adequate accuracy for CTOD.