Agrell, Christian (DNV GL Pipelines and Materials) | østby, Erling (DNV GL Pipelines and Materials) | Levold, Erik (Statoil ASA) | Hauge, Mons (Statoil ASA) | Bjerke, Steinar Lindberg (DNV GL Pipelines and Materials)
Design against fracture in pipeline girth welds deals with hypothetical defects. A logical step would be to apply probabilistic assessment procedures including an assumed distribution of weld flaws. The major part of fracture assessments carried out today is semi-deterministic. Even if such an approach might be applicable, it is proposed that it may not always lead to optimum result in terms of combination of safety level and cost-effectiveness. The risk of girth weld fracture is influenced by several parameters like weld defect size, tearing resistance, weld metal mismatch, misalignment, yield to tensile strength ratio, etc. The likelihood of the most detrimental values of each distribution occurring at the same position is usually very low. Further, fracture assessment of pipelines often requires that the different steps in the loading history are considered in the analysis. In this paper we illustrate how probabilistic assessment can be used to obtain the overall reliability of pipelines wrt girth weld fracture. We discuss statistical representation of the different key input parameters and how the analysis can be used to determine characteristic values to be used in design assessment to meet target reliability levels. Further, we illustrate how the procedure can be used to investigate the significance of previous load steps, such as the effect of installation loads on the strain capacity during operation.
The field of strain-based design of pipelines, with special focus on tensile strain capacity due to potential defects, has received considerable attention the last 10-15 years. The ISOPE SBD symposium, having its 10th anniversary at this year’s conference, has been one of the main arenas for exchange of ideas in this respect. In this period there has been made significant contributions and developments within the field. An improved understanding of the main physical features controlling pipeline failure has been established, and several models for estimation of tensile strain capacity have been proposed, (see e.g. Østby (2007), Fairchild et al. (2014), Tang et al. (2014), Wang et al. (2012)). As a part of this development, increased use of FEA, both w.r.t. obtaining new knowledge and to development of quantitative models, has taken place (see review paper by Østby (2015)). There have also been several large-scale testing campaigns to validate models, and in general the models have been found to very well reproduce the average trend in the experimental results. It is underlined that there are still uncertain areas, e.g. embedded defects, where there is a need for a better understanding of the phenomena and how to include this in models. However, in general it is fair to say that we are in possession of quite accurate models for assessing tensile strain capacity of pipelines with defects.
This state-of-the-art paper is devoted to testing and evaluation of microstructural crack arrest. Testing and analysis of crack arrest have developed in the last decades, enhancing our understanding of the mechanisms behind crack arrest in a continuum mechanics perspective. Understanding crack arrest is important when operations are moving towards Arctic regions as low temperatures are detrimental to most steel’s fracture toughness. Large-scale testing is expensive and unpractical, and current methods fail to reflect the microstructural and micromechanical features of the fracture process. In order to increase the effectiveness of characterizing crack arrest properties, small-scale tests, as well as numerical methods, have been developed. The mechanical basis and mechanisms behind crack arrest are presented. Global and micro-arrest is considered. Key methods for understanding, evaluating and obtaining arrest parameters are presented: (i) statistical treatment of experimental results, (ii) barrier models for separating fracture and arrest sequences, and (iii) numerical tools for determining arrest behaviour. Brief presentations of the main mechanisms of crack arrest are presented with focus on the micromechanisms of arrest. The effect of grain boundaries, lattice orientation and second-phase particles upon propagation controlled cleavage are discussed, as well as their role in the arrest mechanism. Developments in arrest testing and evaluation are presented. Experimentally and numerically obtained results are linked to relevant mechanisms and theory, exhibiting the predictability and importance of crack arrest properties, and the understanding of the governing mechanisms behind crack arrest. The potential for increased understanding of the brittle fracture arrest phenomenon associated with new methods for nanomechanical testing of the material properties inside individual grains, and over grain boundaries, as well as the rapidly improving capabilities of atomistic modelling of deformation and fracture, is presented to pave the way for the future research within this field. Areas where further research could enhance our knowledge of crack arrest are listed.
Crack arrest is considering running cracks that are halted due to increasing resistance to crack propagation and/or reduced crack driving force. The former may be due to microstructural barriers or thermal gradients in the material. The latter may occur under partly displacement controlled loading, where the crack extension may increase the compliance of the structure and reduce the local crack driving force, or as a result of dynamic effects caused by impact loading or stress oscillations in the structure. This paper is mainly concerned with aspects related to the material’s resistance to crack propagation, i.e. the arrest toughness. Further, crack propagation is assumed to be dominated by cleavage fracture, i.e. ductile fracture and fatigue are not considered. The relative importance of these factors depends on the scale of which the arrest is considered. Further, the arrest can also be considered for different scenarios ranging from arrest of single grain sized microcracks up to arrest of macroscopic cracks on in the centimeter to meter range. In the first group the arrest happens locally, probably highly influenced by local microstructural features like grain boundary orientation, and would rather be categorized as avoidance of cleavage initiation on the macroscopic scale. In the latter group the problem is more of a conventional engineering fracture mechanics issue, ideally assessed through knowledge or measurements of the macroscopic arrest toughness, Kia. Ultimately, the two groups are part of the same problem, and there is a research aim to establish quantitative relations at different scales in orderto arrive at a general treatment of the problem.
Hauge, Mons (Statoil) | Maier, Mark (Shell Global Solutions International BV) | Walters, Carey L. (Structural Dynamics, TNO) | Østby, Erling (Det Norske Vertias, AS) | Kordonets, Sergei M. (Hull department, Head Office of Russian Maritime Register of Shipping) | Zanfir, Christian (Office of Public Safety, CWB) | Osvoll, Harald (FORCE Technology Norway AS)
An ISO subcommittee was set up in 2011 to improve the existing standards and norms with respect to arctic offshore operations for the petroleum, petrochemical, and natural gas industries. Within this subcommittee, a specific working group was established to address the application of materials in the environment of the arctic and cold regions. The work is focusing on a number of specific aspects related to the application of ferritic steels.
Østby, Erling (SINTEF Materials and Chemistry, Trondheim, Norway) | Nyhus, Bård (SINTEF Materials and Chemistry, Trondheim, Norway) | Hauge, Mons (StatoilHydro ASA, Trondheim, Norway) | Levold, Erik (StatoilHydro ASA, Trondheim, Norway) | Sandvik, Andreas (StatoilHydro ASA, Trondheim, Norway) | Thaulow, Christian (Norwegian University of Science and Technology, Trondheim, Norway)
Horn, Agnes Marie (Det Norske Veritas, and SINTEF, and Statoil) | Østby, Erling (Det Norske Veritas, and SINTEF, and Statoil) | Hauge, Mons (Det Norske Veritas, and SINTEF, and Statoil) | Aubert, Jean-Michel (Det Norske Veritas, and SINTEF, and Statoil)
Østby, Erling (SINTEF Materials and Chemistry) | Kolstad, Gaute T. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (SINTEF Materials and Chemistry, and NTNU (Norwegian University of Science and Technology)) | Hauge, Mons (NTNU (Norwegian University of Science and Technology), and Statoil ASA)