Antonio Carlucci

Crack Initiation and Growth in Bimetallic Girth Welds

Bimetallic girth welds are characteristics of clad pipe technology. When dealing with propagation issues, fracture mechanics concepts usually are no longer applicable as a result of the extensive and non-homogeneous plastic deformation along bi-material interface that occur at the crack tip even below design allowables. In this study, ductile crack initiation and propagation in bi-material girth welds was investigated using a Continuum Damage Mechanics (CDM) model proposed by Bonora [1]. For the base, weld and clad metal, ductile damage model parameters have been determined by means of inverse calibration technique using fracture data obtained on smooth and round notched tensile bar specimens. Firstly, the damage model was validated predicting ductile crack growth occurring in single end notch (SEN(T)) geometry sample comparing the applied load vs crack mouth opening displacement with experimental measurements. Successively, the model was used to investigate ductile crack initiation and propagation for under clad circumferential weld crack under remote tension.

Simplified Approach for Fracture Integrity Assessment of Bimetallic Girth Weld Joint

Recent extensive use of Corrosion Resistance Alloy (CRA) as internal protection layer of standard carbon-steel pipes (clad and lined pipe) in the oil and gas industry requires an intensive use of bimetallic welds.Since some degree of defects in welds is inevitable, and in codes and standards (such as BS7910) the case of bi-metallic joint is usually not considered, some R&D’s activities are ongoing to define specific design guidance for an Engineering Critical Assessment (ECA) aimed at determining flaw acceptance criteria for fabrication of bimetallic joints.Based on the limited guidance in the literature, proposed procedures for ECA on CRA welds seem not cover the root/hot pass weld region, for which the requirement of “zero defect” became mandatory. As direct consequence, it penalizes the weld fabrication rate, particularly if “J-lay” or “S-Lay” methods are adopted. Furthermore, they are investigating on cases where weld material is overmatching the base metal or for a limited partial overmatching, despite for CRA welds, such conditions, seem quite difficult to be fully met, if current consumable materials present in the marked are selected.Aim of present paper is to describe how any standard ECA procedure (ordinarily used to assess carbon-steel welds) may be alternatively adopted to assess CRA welds for clad & lined pipe material, if specific conditions are respected. For this purpose a few number of elastic-plastic Finite Element Analysis (FEA) is required to identify and/or extends the validity limits which have to be met in order to be conservative in the use of selected standard procedure. Outer, inner and under clad flaws, located along the weld fusion line, were investigated. Such approach, certainly leads to a quite conservatism, but gives the advantage to provide a safe flaws acceptance criterion in root/hot pass weld, and it may be also applied for any level of weld partial overmatching condition.Despite proposed simplified approach is suitable until moderate plastic straining, it may be appropriated for any ECA on CRA pipe when “J-lay” or “S-lay” installation method is adopted, and/or for many riser’s configuration, and/or for several flowline routing also if exposed to post-buckling condition. It is demonstrated that the proposed simplified approach, when applied under moderate plastic strain conditions, provides accurate J-integral solutions compared to the complex method as proposed by current R&D.Copyright

Integrity Assessment of Clad Pipe Girth Welds

Current design standards and codes do not provide specific guidance how to perform engineering criticality assessment with bi-metallic girth weld in lined or clad pipe. Recently, Bonora et al. (Proc. ASME 2013 32nd OMAE conf.) proposed the equivalent material method (EMM) which allows one still to use current design assessment routes. The method consists in considering instead of three materials in the weld joint, a single “equivalent” material with a flow curve defined as the interpolated lower bound of the three weld joint material flow curves. In this work, the applicability of the EMM was verified considering the effect associated with inner pressure loading and weld residual stresses. To this purpose, two flaw geometry configurations have been investigated. Particular relevance was given to the multi-pass weld process simulation. Numerical results indicate that the EMM always provides conservative results in terms of applied J with respect to those estimated considering the effective multi-materials configuration in the weld joint.© 2014 ASME

The role of weld processing on the integrity assessment of bimetallic girth welds

Current design standards and codes do not provide specific guidance how to perform engineering criticality assessment with bi-metallic girth weld in lined or clad pipe. Recently, Bonora et al. (Proc. ASME 2013 32 nd OMAE conf.) proposed the equivalent material method (EMM) which allows one to still use current design assessment routes. The method consists in considering instead of three materials in the weld joint, a single “equivalent” material with a flow curve defined as the interpolated lower bound of the three weld joint material flow curves. In this work, the applicability of the EMM was verified considering the effect associated to weld residual stresses. To this purpose, two flaw geometry configurations have been investigated. Particular relevance was given to the multi-pass weld process simulation. Numerical results indicate that the EMM always provides reasonable results in terms of applied J with respect to those obtained considering the effective multimaterials configuration in the weld joint.

Crack Initiation and Propagation Clad Pipe Girth Weld Flaws

At present, design standards and prescriptions do not provide specific design routes to perform engineering criticality assessment (ECA) of bimetallic girth welds. Although the authors has shown the possibility to implement ECA in accordance with available prescriptions of such flawed weld joint following the equivalent material method (EMM), when dealing with ductile crack initiation and propagation — as a result of the large scale yielding occurring at the crack tip for high fracture toughness material operating in the brittle-ductile transition region — fracture mechanics concepts such as JIc or critical CTOD may breakdown. In this work, the possibility to accurately determine the condition for ductile crack growth initiation and propagation in bi-metallic girth weld flaws using continuum damage mechanics is shown. Here, the base metal as well as the clad and the weld metal have been characterized to determine damage model parameters. Successively, the geometry transferability of model parameters has been validated. Finally, the model has been used to predict crack initiation for two bi-material interface circumferential crack configurations.Copyright

Fracture Integrity Assessment of Flawed Bi-Metallic Girth Weld Joint

The objective of the work is to establish, for a bi-metallic girth weld joint, up to which level of remote stress/strain is still conservative the use of the standard approach to perform an Engineering Criticality Assessment (ECA) considering the joint made of a single equivalent material. Several flaw types located at the interfaces between the joint materials were considered. Extensive finite element analysis was performed to derive the crack driving force using domain integral method. The possibility to use, with appropriate meshing, the CTOD as parameters to directly derive the J-integral in the numerical simulation is also demonstrated. Computational results indicate that the use of a material curve, obtained as the lower bound of all joint materials curves, lead to conservative results.

Strain capacity assessment of API X65 steel using damage mechanics

Strain-based design for offshore pipeline requires a considerable experimental work aimed to determine the material fracture toughness and the effective strain capacity of pipe and welds. Continuum damage mechanics can be used to limit the experimental effort and to perform most of the assessment analysis and evaluation in a simulation environment. In this work, the possibility to predict accurately fracture resistance of X65 steel using a CDM model proposed by the authors, is shown. The procedure for material and damage model parameters identification is presented. Damage model predictive capability was demonstrated predicting ductile crack growth in SENB and SENT fracture specimens.

Use of Circumferentially Cracked Bar sample for CTOD fracture toughness determination in the upper shelf regime

In this work, the use of circumferentially cracked bar (CCB) sample to determine material fracture toughness in the upper shelf regime for carbon steels has been investigated. Since high fracture toughness materials are known to exhibit extensive crack tip blunting before ductile crack initiation, accurate specimen design is required to provide realistic fracture toughness measurement. Here, a CCB was designed to have similar loss of constraint as for SENT sample. Continuum damage mechanics was used to predict the occurrence of ductile crack initiation and propagation. Finite element analysis was performed to predict specimen response and to compare computed J-integral crack driving force with measured CTOD. Finally, experimental tests were performed on X65 carbon steel and the measured critical CTOD was compared with available fracture data obtained with SENT.

Simplified Approach for Fracture Integrity Assessment Procecedure of Flawed Weld Joint Under Non-Conventional Condition

Welded pipes are widely employed in many oil and gas applications. Engineering Critical Assessment (ECA) shall be performed in order to establish acceptance levels for revealed or postulated flaws in new or existing constructions. Although methods for assessing the acceptability of flaws in all types of structures are presented in codes and standards, such as BS7910 [1], API579-1 [2], R6 [3], DNV OS-F101 [4], the approaches are typically derived by simplified geometries as plate solutions, simplified material assumption, and simplified load condition as uniaxial load condition. Dedicate numerical solution are more accurate and would improve the assessed results. But the use of appropriate conditions in the full ECA requires several specific Finite Element Analysis (FEA) which are able to identify the Crack Driving Force (CDF) for each postulated defect geometry, material assumption and load conditions. The purpose of this paper is to propose a simplified method, into a standard procedure (similar to BS7910 [1]), minimizing numerical analyses, to guaranty the safety against fracture of many kind of weld joint under non-conventional condition (such as generic weld joint geometry and/or weld joint subjected to combined axial force, bending moment and internal over pressure which are not contemplate in current code and for which dedicated FEA are recommended).© 2014 ASME

Simplified Toughness Resistance Curve

This paper presents a method in order to plot a simplified toughness resistance curve (R-Curve) from three single conventional toughness tests.The simplified toughness resistance curve will be used to carry out a tearing assessment as part of Engineering Criticality Assessment (ECA) (ref. to Level 3B, [2]). This level of assessment takes into account of stable tearing in order to reduce over-conservatism, but it usually requires additional toughness tests to identify the toughness resistance curve. This present method allows to reach this assessment level without impacting the welding procedure qualification (WPQ).Copyright