Milenin O. Reliability of welded pipelines with revealed corrosion-erosion damages under operation and in-service repair.

The Thesis presents a set of methods for predicting the subcritical and critical damage of welded pipeline elements with three-dimensional metal discontinuity defects and determining the way of influence of assembly and repair welding on their reliability, strength and operability. Thus, complex methods for predicting the kinetics of physical and mechanical processes in welding and operation of typical welded structures that take into account the fracture of metal by a ductile mechanism have been further developed. This allowed development and implementation of the methods for assessing the permissibility of isolated and multiple defects such as gas inclusions and wall thinning of pipeline elements in the region of welding by taking into account the features of the assembly welding process. Methods of numerical assessment of the probability of fracture of welded pipeline elements (pressure vessels) with revealed surface defects of local corrosionerosion metal loss have been developed. The integration of the fields of principal stresses within Weibull statistics was assumed as its basis, which allows taking into account the residual postweld stress-strain state. In addition, it has been proposed an alternative approach based on the integral analysis of the plastic strain intensity fields. It allows fulfilling more adequately analysis of the tendency to fracture in case of the significant development of plastic strains before the limiting state, in particular, at high-temperature creep. Weibull distribution coefficients were obtained for typical pipe steels and aluminum alloys to determine the probability of failure of welded pipeline elements with detected three-dimensional metal discontinuities. Based on the developed methods, ways of wider application of typical methods for repairing pipeline elements with detected corrosion-erosion defects using up-todate technologies of in-service repair (controlled grinding, multi-pass surfacing, installation of metal and composite reinforcing structures) are demonstrated. For analysis of the results of inline diagnostics of long sections of the pipelines, a multilevel technique of defect ranking for determination of the sequence of their elimination by in-service welding repair was developed. Three-level approach for the distribution of defects by danger level was proposed. It is based on the division into groups according to the requirements of regulatory documents, by assessment the residual safety factor as well as by estimating the probability by the Monte-Carlo method. In addition, within the third level, it is proposed to rank not only the revealed defects, but also the areas with multiple defects to increase the efficiency of repair of the sections of a certain length. The numerical procedure has been developed for quantitative assessment of the effectiveness of bearing capacity renewing of pipeline with corrosion-erosion metal losses by means of reinforcing structures. As a quantitative parameter of repair effectiveness the function of the ratio of the probabilities of defective structure failure before and after repair was offered. It allows taking into account the features of interaction in a contact pair using up-to-date techniques for numerical prediction of the stress-strain state of critical structures. To carry out the corresponding calculations, a new generation of efficient software for finite element modeling of welding processes and predicting the operability of welded structures was implemented. They are based on the proposed complex models of welding processes and algorithms of prediction of the welded structures workability, the corresponding finite element description and solvers, which involve the use of multiprocessor systems of different levels, namely supercomputers, clusters, GRID systems, computers of hybrid architecture. It is shown that the use of these approaches to solve the typical problems of determining the residual life of pipeline elements and pressure vessels with revealed defects allows accelerating the numerical calculations up to 80 times.