Scientific and methodological provision of safety of subsea pipelines with defects using risk criteria

Scientific and methodological aspects of ensuring the safety of offshore subsea pipelines proceeding from the accident risk criteria are considered. A conceptual approach to assessing the defect hazard by risk criteria is formulated. This approach is based on the provisions of modern norms and requirements for the calculation of pipelines. A probabilistic model for assessing the risk of accidents at offshore subsea pipelines is developed with allowance for a random nature of defects and damage from accidents. The criterion conditions for the admissibility of defects in pipelines are formulated. Two conceptual directions for developing a methodological base for calculation of the permissible defects by risk criteria are thus proposed. The first direction is the development of semi-probabilistic calculation procedures using differentiated safety factors, with allowance for the level of the accident risk. The second direction consists in solving the problem of the probability of the pipeline destruction taking into account the restrictions in the form of a given risk value. The probability and scale of accidents are linked by a risk matrix. The calculation methodology was developed using a semi-probabilistic concept for the most typical defects in subsea pipelines. The suitability of the pipeline for operation after in-line diagnostics is determined using a three-level assessment of the permissible size of defects. The first, basic level, determines the permissible size of defects according to the strength criteria for the pipelines exposed to the action of the main loads, i.e., internal overpressure and hydrostatic external pressure. The second, extended level, determines the permissible dimensions of defects based on the strength criteria, taking into account the impact of additional longitudinal and bending loads on pipelines. The third, special level, determines the permissible dimensions of cracks and crack-like defects according to the characteristics of the crack resistance of the pipeline metal. The novelty of the methodology consists in the justification of the safety factors through the levels of failure probabilities corresponding to a given class of damage and losses. A scheme for making decisions on the admissibility of defects by risk criteria has been developed. An example of hazard assessment of defects in subsea pipelines is presented.

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