Low temperature tensile properties of line pipe steels

Given the expected increase in Arctic oil and gas exploitation, there is a demand for high-strength line pipe steels able to cope with the Arctic climate. The state-of-the-art of the tensile properties of API 5L steels at low temperatures is reviewed and discussed. Well-known characteristics such as an increase in strength and Young’s modulus with decreasing temperatures are confirmed. The Y/T ratio is fairly unaffected by changes in temperature. Lüders elongation manifests itself at low temperatures where the Lüders plateau tends to increase. Conflicting statements about the relation between ductility and temperature were found. Altogether, quantifiable test results are scarce, especially for the high strength grades from API 5L X90 grade onwards. The urgent need for more tensile strength and ductility data of these steels at low temperatures is stated and defended

The influence of material anisotropy and spiral welding on tensile strain capacity of spiral welded pipes

The longitudinal strain capacity of spiral welded pipelines displays to some extents unexplained behaviour. Therefore, they are not (yet) used extensively in offshore applications and harsh conditions, demanding a strain based design. An important factor that influences the tensile strain capacity is the quantity of anisotropy in terms of strength and toughness. Starting from an anisotropic hot rolled highstrength steel skelp, the process of helical forming and post-treating of the pipe adds heterogeneity and changes the level of anisotropy of the product. A parameter that should be examined with respect to anisotropy is the crack driving force, a measure for the toughness of the pipeline steel. Additional to the mode I loading (opening of the crack), the mode III component drives the in-plane shear motion of a crack in the spiral weld when the pipe is subjected to longitudinal deformation. This action, not present in longitudinal welded pipes, shows a decreasing contribution with increasing plasticity. FE simulations have demonstrated a rise of crack driving force in anisotropic cases with respect to an isotropic reference. However, exact data and variation of various parameters, along with experimental testing need to be conducted. The outcome analysis of such simulations and tests can validate existing models, or help create a better understanding of anisotropic and heterogenic influences on the tensile strain capacity of spiral welded pipes

Strain based design considerations for spiral welded pipelines

Pipelines are constructed in hostile environments where the occurrence of imposed plastic deformations can necessitate a strain based design approach. Under such conditions not only the strength and toughness properties have to be considered; also the strain capacity of pipe and weld metal become crucial. Considering the use of spirally welded linepipe sections, the helical seam weld and anisotropic material properties pose real challenges to pipeline designers. In our work, the tensile strain capacity and defect tolerance of high strength, high toughness spiral pipes will be investigated. This paper briefly discusses the different steps in the spiral pipe manufacturing process and their influence on the mechanical properties of the pipe. The forming angle is a key parameter as it determines (a) the anisotropy in strength and toughness of the pipe steel, and (b) the orientation of possible seam weld defects. Each mechanical operation (forming, expansion) and each thermal operation (welding, coating) will affect local or global strength, toughness and ductility properties of the pipe metal. A thorough material characterization at each process step is needed for a qualitative and quantitative understanding of these effects

Sensitivity study of crack driving force predictions in heterogeneous welds using Vickers hardness maps

Weld flaws 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 welds under operating conditions. Adding to the complexity of an ECA is the occurrence of local constitutive property variations in the weldment (‘weld heterogeneity’). Their quantification is important to allow for an accurate assessment. Hereto, hardness measurements are widely adopted given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of different hardness transfer functions and additionally recognize substantial scatter in predictions of strength. Within this context, this paper investigates the suitability of hardness mapping to perform an accurate weld ECA. A finite element analysis has been conducted on welds originating from steel pipelines to simulate their crack driving force response using 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 resulting crack driving force curves are probed against experimental results. The variable agreement between simulations and experiments highlights the need for further research into the characterization of local constitutive properties of heterogeneous welds. A hardness transfer procedure based on all weld metal tensile testing appears to be particularly promising

Study of SENT specimens with a tilted notch to evaluate ductile tearing in spiral welded pipeline applications

There is an increasing interest for the use of spiral welded pipelines in strain based design applications. Environmentally imposed loads are able to plastically deform the pipelines, meaning that their structural response is of the utmost importance. However, since the influence of the spiral weld is not fully grasped, further investigation is necessary. The mechanical response of the pipeline is not only influenced by its material properties, but also by the angular position of the welds. Subsequently, the effect of mixed mode loading is a crucial aspect when assessing the helical welds. To evaluate the ductile tearing of the pipeline material, multiple single edge notched tensile (SENT) tests - each with a tilted notch of 25° with respect to the transverse direction - were executed. The extension of the crack is assessed by means of potential drop measurements and finite element simulations. Resistance curves were realized by combining the crack opening displacement with the associated crack extension. This is an ongoing investigation and in this paper a first set of five tests are evaluated

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

Fracture mechanics analysis of heterogeneous welds : validation of a weld homogenisation approach

Flaw assessments assume homogeneous material properties, but welds are heterogeneous. The authors have developed a method to address heterogeneity in the estimation of crack driving force. This paper presents a numerical validation of the proposed approach. Weld heterogeneity is characterized in three pipeline girth welds by means of hardness mapping. The resulting distributions are input into a finite element model of a clamped SE(T) specimen. Following, crack tip opening displacement responses of heterogeneous welds are compared with predictions. The approach is found to strongly simplify analyses involving heterogeneous welds while maintaining an acceptable level of accuracy with respect to load

R-curve evaluation of pipeline girth welds using advanced measurement techniques

A strain-based flaw assessment procedure is recommended for girth welded pipelines subjected to large deformations. To evaluate the allowable defect dimensions, the tearing resistance needs to be characterized. This paper investigates the effect of weld metal strength mismatch on the resistance curve using Single Edge Notched Tension (SENT) specimens. Several advanced measurement techniques are applied during the tests in order to obtain a continuous measurement of crack extension and to visualize the deformation fields near the crack. The resistance curves are determined using a single specimen technique. The unloading compliance method and the potential drop method result in similar predictions of ductile crack extension, yielding similar resistance curves. Next to these measurements, the full field deformations are determined using digital image correlation. The experiments indicate that the position of the applied notch in the weld has the potential to influence the strain fields