The functional safety of ageing magistral gas pipeline is associated with the strength of pipelines. The mechanical characteristics deteriorate during the operation time, what causes the development of local defects and as a result, the safety indicators of system deterioate. The goal of promotion thesis is the development of model and methodology for calculation of strength of ageing gas pipeline at certain time of service. To achieve the set objective, the laboratory experiments with steel samples are performed, which were taken from ageing gas pipelines. To evaluate the strength of magistral pipelines during the operation time, the factors of impact of ageing and loads were implemented. Based on data, gained through laboratory experiments, the mathematical model for evaluation of strength of ageing pipelines (with service period of 43 years) was created. Creating the model, the changes of elastic features depending on age and load were taken into consideration. Based on developed model, the new calculation algorithm was created, as well as the methodology of evaluation of strength of ageing gas pipelines. The developed mathematical model is adapted also to practical application and was applicated in the magistral gas pipeline Rīga – PaĦevezža in year 2010, evaluating and predicting the strength of nonstationary loaded gas pipelines. The results of research are currently in use for analysis of defect danger of magistral gas pipeline for enterprise Stock Company «Latvian Gas» in the operation department „Gas transport” where author have been working and engaged in safety problem resolutions of gas transportation system since the year 2005. The application of methodology is the determination and predicting of strength of gas pipeline, for determination of destructive pressure in the certain period, for exact calculation of defects and it is especially important, if the investments in the reconstruction of gas pipelines, which have served the operation time, are small. The results of scientific research are applicable also in the engineering systems of oil and water pipelines.