Rudi Denys

Statistical detection of defects in radiographic images in nondestructive testing

In this paper, we investigate applicability of statistical techniques for defect detection in radiographic images of welds. The defect detection procedure consists in a statistical hypothesis testing using several nonparametric tests. A comparison of rules derived for image thresholding for a given level of false alarm is presented. In this work we consider circular defects such as cavities and voids. Numerical experiments with real data are performed.

A Preliminary Strain-Based Design Criterion for Pipeline Girth Welds

The strain capacity of girth welds containing surface-breaking welding defects is examined through numerical analysis and experimental verification under a PRCI (Pipeline Research Council International) funded project. Some important insights on the various factors affecting the girth weld strain capacity are generated. The defect size is identified as one of the most important factors in determining strain capacity of a girth weld. Other factors, such as the strain hardening rate of the pipe and weld metals, weld strength mismatch, fracture toughness, and weld cap height, can play a significant role if the defect size is within certain limits. It is discovered that the girth weld response to the remotely applied strain may be characterized by a three-region diagram. For a given set of defect size and weld strength mismatch conditions, the crack driving force may be bounded, unbounded, or gradually changing, with respect to the remotely applied strain. A set of parametric equations is developed that allow the computation of allowable strains with the input of defect depth, defect length, CTOD toughness, and weld strength mismatch. The comparison of the developed strain criteria with full-scale bend tests and tensile-loaded CWPs (curved wide plates) shows the criteria are almost always conservative if lower bound CTOD toughness for a given set of welds is used. However, the criteria can significantly underpredict strain capacity of girth welds with short defects. Although defect length correction factors were added to the strain criteria based on the comparison of axisymmetric finite element (FE) results and full-scale bend test results, a more thorough investigation of the effects of defect length on strain capacity is needed. Future investigation that incorporates the finite length defects is expected to greatly reduce the underprediction. The influence of other factors, such as strain hardening rate, should be further quantified.Copyright

Investigation of strain measurements in (curved) wide plate specimens using digital image correlation and finite element analysis

Some pipelines face global plastic straining due to the nature of their installation process or harsh environmental conditions during operation. The ability of the girth welds to withstand these plastic strains is often evaluated on the basis of wide plate tests. Key for the validity of these tests is a representative measurement of remote strain, mostly obtained by linear variable differential transformers and/or strain gauges. The outcome of the remote strain measurement depends on the specimen geometry and the position of these sensors. In an attempt to investigate a specific geometric design of wide plate specimens and to find appropriate remote strain sensor positions, the authors have performed a series of tension tests on medium-sized wide plate specimens, supported by digital image correlation strain measurements. In addition, finite element simulations have been performed to evaluate whether the experimental observations can be extrapolated to a wider range of conditions. The results indicate that the strain distribution is mostly influenced by the weld strength mismatch, which governs the lateral restraint. For all experiments and simulations, nevertheless, the strain field was highly uniform in an identified zone, resulting in simple guidelines regarding specimen geometry and sensor positioning.

Parametric finite element model for large scale tension tests on flawed pipeline girth welds

The structural response of a pipe with a flawed girth weld, subjected to global plastic deformation is influenced by a large number of geometrical and material properties. Finite element models that aim to simulate this need to include all relevant influence factors, which causes high challenges in the creation of such models. In search for a high degree of flexibility, automation and ease of use, the authors have developed a parametric script that creates geometries for two common pipeline girth weld tension tests: the curved wide plate test and the full scale pressurized pipe tension test. The developed model allows to modify pipe geometry, test specimen geometry, flaw position (weld metal centre or heat-affected zone), flaw size, weld misalignment, pipe thickness variations, weld fusion line profile, and weld cap profile. The desired geometry is obtained by a coordinate transformation scheme that starts from a flat plate with a simplified weld geometry. A deliberate partitioning strategy is applied to obtain flexibility in the flaw location and full independence between a fine flaw mesh and a coarse body mesh. This article describes the approach, structure and governing equations of the model. An example geometry is discussed to illustrate the various possibilities. The proposed model provides inspiration for all who seek to develop parametric finite element models with a similar flexibility and ease of use.

Influence of forge pressure on properties of friction welded pipelines using intermediate ring

Abstract An innovative welding method for fully automatic joining of pipelines has been developed. The proposed welding procedure is a variant of the conventional friction welding process. A rotating intermediate ring is used to generate heat necessary to realise the weld. The working principles of the welding process are described. The influence of the forge pressure on the mechanical properties of the welds and the heat affected zone microstructure was experimentally investigated. It was found that the forge pressure had no influence either on the mechanical properties or on the weld microstructure, which is in contradiction with the published data in literature concerning conventional friction welding.

Pressure Correction Factor for Strain Capacity Predictions Based on Curved Wide Plate Testing

Strain-based girth weld defect assessment procedures are essentially based on large scale testing. Ever since the 1980s curved wide plate testing has been widely applied to determine the tensile strain capacity of flawed girth welds. However, the effect of internal pressure is not captured in curved wide plate testing. Accordingly, unconservative predictions of strain capacity occur when straightforwardly transferred to pressurized pipes. To address this anomaly, this paper presents results of finite element simulations incorporating ductile crack growth. Simulations on homogeneous and girth welded specimens indicate that a correction factor of 0.5 allows to conservatively predict the strain capacity of a pressurized pipe through wide plate testing under the considered conditions.

EPRG Tier 2 Guidelines for the Assessment of Defects in Transmission Pipeline Girth Welds

This paper presents the proposed revisions of the EPRG guidelines for the assessment of defects in transmission pipeline girth welds. The revisions cover Tier 2 of the guidelines, in particular (a) the extension of the guidelines to include Grade L555 (X80) material, (b) the assessment of surface-breaking defects with heights up to 5mm and (c) the assessment of multiple co-planar defects. Since the welds should be, at least, matching the pipe material in yield strength, the paper also defines the required levels of weld metal yield strength for the safe application of the guidelines.

BS5500 appendix D : An assessment based on wide plate brittle fracture test data

Abstract This paper summarises an internationally funded assessment of the low temperature fracture toughness requirements of the British Standards Institution Document BS5500:1976, ‘Specification for unfired fusion welded pressure vessels’. The assessment was based on an analysis of wide plate brittle fracture tests, supporting chemical compositions and small-scale mechanical test data from worldwide sources, all these data having been collated in a programme of work involving The Welding Institute (UK), Gent University Research Centre of Welding (Belgium) and Delft University of Technology Metallurgical Department (Netherlands). The assessment showed that the BS5500 requirements are generally safe. However, attention is drawn to the relatively few wide plate test data that are available for section thicknesses less than 25 mm.

Effect of Pipe and Weld Metal Post-Yield Characteristics on Plastic Straining Capacity of Axially Loaded Pipelines

Girth welds in pipelines subject to longitudinal plastic tensile strains are critical regions of the pipeline. As girth welds might contain flaws of some form or other, it is of paramount interest to have a thorough understanding of the deformation characteristics of girth welds in the post-yield loading range. The response of a defective weld to plastic strains depends on many variables. While toughness is an important variable, large-scale tests demonstrate that the plastic straining capacity is directly affected by the mechanical properties of the materials surrounding the defect. The purpose of this paper is to describe the effect of the interrelation between the pipe and weld metal post-yield characteristics on the straining capacity of girth welds containing a defect.Copyright

Curved wide plate testing with advanced instrumentation and interpretation

Curved wide plate testing is a valuable experimental tool to determine the strain capacity of flawed pipeline girth welds under tension. However, its design, test procedure and analysis are not standardized to date. In an effort to contribute to these three aspects, the authors have executed medium scale (curved) wide plate tests with full field strain measurements and unloading compliance crack extension measurements. This paper discusses specifications, possibilities and limitations of both features and provides representative results. Full field strain measurements and unloading compliance analysis support the validation of a finite element model for curved wide plate testing, and confirm the ability to obtain uniform strain fields in the pipe sections. It is expected that these results may contribute to a future standardization of the curved wide plate test.Copyright