The Dutch main gas transmission system is now some 50 years old and in the transition to renewables another 50 years of service are foreseen. Due to the transition to renewables, the operating conditions will change with respect to historical and current operating conditions. Furthermore, gases with different compositions are to be injected into the Dutch main gas transmission system. Without mitigation, these changing operating conditions and different gas composition in the ageing network can possibly result in different dominant pipeline degradation mechanisms than currently experienced. Therefore, the question arises what are, in the future, the relevant time dependent degradation mechanisms that could occur and how can they be assessed and ranked. ASME B31.8S provides a framework to help a pipeline operator to manage system integrity of gas pipelines, also for time-dependent degradation mechanisms like external and internal corrosion and stress corrosion cracking. This paper will indicate risks for time-dependent degradation mechanisms in gas transmission pipelines and facilities under changing operating conditions and gas compositions. It starts with a systematic approach for assessing the failure frequency and failure mode of degradation mechanisms in the existing gas transmission system and an outlook of the effect of changing gas compositions will be given. This approach can be used for determining mitigation measures to prevent future failures. Furthermore, it can be used to extend the life time of gas transmission pipelines at the lowest total cost of ownership. The effect of changing operating conditions in future will be investigated in the next step of the project.