Residual strength of pipelines with dents

Abstract

We show the urgency of developing methods for prediction of the ultimate load of ductile fracture for defects of the dent type in pipelines. We propose an idealized model of the limiting plastic state of a pipe with infinite dent along it. We obtain formulas for engineering calculations and analyze them.Present-day methodologies of evaluation of the strength of very plastic materials [ 1,2] demand that one carry out calculations by the methods of limit equilibrium. However, the evaluation of defects in pipelines is still based on empirical approaches. The well-known formulas for axial surface and through cracks in pipes [3] were proposed at the end of the 60s for atomic power engineering. These formulas were taken as a basis of criteria of acceptability for oil and gas pipelines with corrosion defects [4]. Theoretical analysis with the help of the methods of limit equilibrium [5,6] showed that these formulas are rather efficient although, in our opinion, they give a conservative estimate in the case of very deep surface defects. The efficiency of formulas [3] can be attributable to the use of a vast amount of experimental data for their construction and a small number of parameters (input data) affecting the limiting state. Apparently, it is impossible to modify these formulas experimentally in order to estimate strength with a greater number of input parameters (different types of cracks, the presence of additional axial stresses in a pipe, the consideration of the third dimension (width) in a defect under study, close location of defects, etc.). In our opinion, just a theoretical analysis can prompt the required solutions [6][7][8][9].The worst situation has arisen with defects of the dent type. Normative documents used for their analysis are based sooner on the subjective perceptions of the designers than on a detailed consideration. It is sufficient to note that the formulas for calculation of dents accepted by the British Gas and Shell corporations and by the American Petroleum Institute give discrepant results [10,11 ]. Methods of calculation of dents accepted in the CIS [12] are based on the concept of theoretical stress-concentration factor. However, such an approach is incorrect, because the well-known expert Peterson [13] as early as at the beginning of the 70s emphasized that such approaches are unfit for the static analysis of plastic steels. In all basic documents of the oil and gas industry of the former USSR, the expediency of analysis according to limiting states (instead of ultimate stresses) is declared [14]. Obviously, numerical calculations of dents by the methods of plasticity theory cannot be widely used in engineering practice. This is connected, first, with the inapplicability of the used theories of plasticity for large deformations, second, with significant consumptions of computer operating time and memory resources, and, third, with the fact that numerical calculations do not give a qualitative comprehension of the danger of defects.The aim of our investigation is to develop a theoreti...

Residual ultimate strength of offshore metallic pipelines with structural damage – a literature review

“…For the dented metallic pipe, Orynyak et al (1999) developed a simple analytical equation to predict the bursting strength with respect to dimensionless length of dent (λ = l d / √ Rt). However, only a single dimension of dent was taken into account.…”

Numerical investigation of residual ultimate strength of dented metallic pipes subjected to pure bending

“…The research found that the collapse capacity of pipes was relatively insensitive to the detailed geometry of a dent such as the shape and size, but was critically dependent on the maximum ovalisation of its most deformed cross-section. Orynyak et al (1999) developed an ana-lytical equation to predict the bursting strength with respect to dimensionless length of dent (λ l = l d / √ Rt). Bjørnøy et al (2000) investigated the bursting capacity of dented pipes subjected to internal pressure through experiments.…”