The Fracture Peculiarities of Trunk Pipeline Steels after their Long Term Service

Long term service of oil and gas trunk pipelines in corrosion andhydrogenated environments can cause a degradation of the mechanical properties andcorrespondingly a decrease its lifetime.Effect of service about 30 years on the impact strength and hydrogen cracking of oiland gas trunk pipeline low alloyed steels is considered.In the case of the “bottom” section of the oil pipe being in service, it has been notpossible to evaluate the impact strength, due to the deflection into cracking parallel tothe pipe wall. This is a symptom of so called hydrogen induced stratification, wellknown degradation phenomenon of industrial pipe lines transporting the crude oilcontaining the higher amount of the hydrogen sulfide and the sulfide-reductionbacteria. The different susceptibility to hydrogen cracking and impact strength for theupper and bottom of parts of exploited pipe has been explained as a result of hydrogencharging and hydrogen degradation of steel during the service.The results discussed in the paper show that inspection of the surfaces ofinstallations to find corrosion or mechanical damage alone may not be sufficient forproper evaluating risk of failure. Since material can loss its initial mechanicalproperties, a thorough investigation of the properties of the metal being in serviceunder the hydrogenation conditions for the long time is suggested.