Scientific and Methodological Provision of Safety of Offshore Subsea Pipelines with Defects according to Risk Criteria

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.