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This article discusses the possibilities of estimation of safe sizes of integrity defects on the basis of risk criteria. Such defects occur at all stages of the lifetime of structures. In most cases the estimation of their hazard and determination of allowable sizes attract attention when the defects can lead to brittle or quasi-brittle fractures. In this case, the models of linear and nonlinear destruction mechanics are applied, when the defects are considered as internal elliptical or surface semielliptical cracks. The stochastic variety of shapes, sizes, locations, and orientations of defects has a significant influence on the failure mechanisms. Therefore, the probabilistic problem of estimating allowable sizes of defects according to the criteria of risk of failure is relevant. This paper examines a general approach to estimation of the hazards of defects according to risk criteria. Two formulations of the probabilistic problem of risk estimation are presented: on the basis of single-parameter and two-parameter failure criteria. The risk function is used as the calculated characteristic, represented as the probability of failure according to a given criterion. An equation of the risk function based on single-parameter failure criteria is presented. The main focus is on the probabilistic model based on the two-parameter Morozov failure criterion. This criterion provides a wide range of opportunities for analyzing various failure mechanisms with variations in the size of defects. An expression for the risk function based on the family of two-dimensional Lu–Bhattacharya probability distributions of Weibull type is derived. It is shown that correlations between failure mechanisms can significantly influence the probabilities of failure and, consequently, the allowable size of defects. [ABSTRACT FROM AUTHOR]

Scientific and methodological aspects of ensuring the safety of offshore subsea pipelines according to accident risk criteria are considered. A conceptual approach to assessing the hazard of defects according to risk criteria is formulated by analyzing modern norms and requirements on pipeline design. A probabilistic model for assessing the risk of accidents at offshore subsea pipelines considering the randomness of defects and damage from accidents is developed. The criterial conditions for the permissibility of defects in pipelines are formulated. Two conceptual directions for developing a procedural framework for calculating the permissible defects versus risk criteria are proposed on this basis. The first direction consists in elaborating semi-probabilistic calculation procedures using differentiated margin factors, considering the accident risk level. The second direction consists in solving the problem of the probability of pipeline failure, taking into account the restrictions in the form of a preset risk value. The probability and scale of accidents are linked through a risk matrix. The calculation methodology is developed using a semi-probabilistic concept for the most typical defects in offshore subsea pipelines. The suitability of the pipeline for operation after in-line diagnostics is determined using a three-level assessment of permissible defect sizes. The first level is basic and determines permissible defect sizes according to the strength criteria for pipelines exposed to the main loads, i.e., internal overpressure and external hydrostatic external head. The second level is extended and determines permissible defect sizes according to the strength criteria, taking into account the impact of additional longitudinal and bending loads on pipelines. The third level is special and determines permissible dimensions of cracks and cracklike defects according to the crack resistance characteristics of the pipeline metal. The novelty of the method consists in the justification of the margin factors through the failure probability levels corresponding to a preset class of damage and losses. A scheme for making decisions on the permissibility of defects according to risk criteria has been developed. An example of the assessment of the hazard of defects in offshore subsea pipelines is presented. [ABSTRACT FROM AUTHOR]

The probabilistic aspects of multiscale modeling of the fracture of heterogeneous structures are considered. An approach combining homogenization methods with phenomenological and numerical models of fracture mechanics is proposed to solve the problems of assessing the probabilities of destruction of structurally heterogeneous materials. A model of a generalized heterogeneous structure consisting of heterogeneous materials and regions of different scales containing cracks and cracklike defects is formulated. Linking of scales is carried out using the kinematic conditions and the multiscale principle of virtual forces. The probability of destruction is formulated as the conditional probability of successive nested fracture events of different scales. Cracks and cracklike defects are regarded as the main sources of fracture. The distribution of defects is represented in the form of Poisson ensembles. Critical stresses at the crack tips are described by the Weibull model. Analytical expressions for the fracture probabilities of multiscale heterogeneous structures with multilevel limit states are derived. An approach based on a modified Monte Carlo method of statistical modeling is proposed to assess the fracture probabilities with allowance for the real morphology of heterogeneous structures. The characteristic feature of the proposed method is the use of a three-level fracture scheme with numerical solution to the problems at the micro-, meso-, and macroscale. The main variables are the generalized crack driving force and crack growth resistance. Crack sizes are regarded as the generalized coordinates. To reduce the dimension, the problem of fracture mechanics is reformulated to the problem of stability of a heterogeneous structure under loading with variations of the generalized coordinates with the analysis of the virtual work of generalized forces. Expressions for estimating the fracture probabilities for multiscale heterogeneous structures are obtained using the modified Monte Carlo method. The prospects of using the developed approaches to assess the fracture probabilities and address the problems of risk analysis of heterogeneous structures are pointed out. [ABSTRACT FROM AUTHOR]

The results of unique experimental studies of the strength and service life of a metal-composite high-pressure tank are presented. The study is aimed at analyzing the fracture mechanisms and evaluating the strength characteristics of the structure. The technique included tests of full-scale samples of the tank for durability under short-term static, long-term static, and cyclic loading with internal pneumatic pressure. The generalized test results and data of visual measurements, instrumental and acoustic-emission control of deformation processes, accumulation of damage, and destruction of full-scale tank samples are presented. The strength and the stiffness of the structure exposed to internal pneumatic pressure are analyzed. The types of limiting states of the tanks are established experimentally. Variation in the stress-strain state of the tank under cyclic and prolonged static loading is considered. The specific features of the destruction mechanism of the metal-composite tank are determined with allowance for the role of the metal liner strain. The calculated and experimental estimates of the energy potential of destruction and the size of the area affected by destruction of the tank are presented. Analysis of the test results show that the tank has high strength and resource characteristics that meet the requirements of the design documentation. The experimental results are in good agreement with the results of the numerical calculations and analysis of the stress-strain state and destruction mechanisms of the metal-composite tank. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental studies of damage accumulation in a metal-composite pressure vessel by pneumatic strength tests. The deformation and fracture of the composite structure accompanied by matrix cracking and fiber rupture are analyzed. It is shown that the cracks and fractures generate acoustic-emission signals of various types. The results of acoustic-emission monitoring were used to develop a criterion for ranking vessels according to the strength characteristics of the pressure composite shell. [ABSTRACT FROM AUTHOR]

It is shown that the following tasks have to be handled in devising models, computational techniques, and a database for risk analysis for complicated technical systems (CTS): the description of CTS from the viewpoint of integrity and hierarchic structure; formulation of the data support to risk analysis; choice of risk criterion and the setting up of risk analysis techniques with given adjustable parameters. Developments in CTS risk analysis require advances in the mechanics of deformation and failure and also in computational technologies and engineering. [ABSTRACT FROM AUTHOR]

This article discusses the concept of assessing the risk of defects in offshore subsea pipelines according to risk criteria, taking into account the characteristics of methods and means of nondestructive testing used in practice. The expansion of the practice of assessing the danger of defects in pipelines according to risk criteria will clarify the assigned safety factors and increase the reliability of calculations of the permissible sizes of defects. The work describes the most promising methods of pipeline repair, and in their range, special attention is paid to the repair method using crimped composite sleeves, which performs repairs at an existing facility in real operating conditions. Based on the results of testing the proposed repair technology on a real object, the method repairs reliably most defects in the base metal and welded joints of pipelines for the entire service life and restores its full functionality. [ABSTRACT FROM AUTHOR]