Assessment of Operational Degradation of Pipeline Steels

Abstract

This paper summarizes a series of the authors’ research in the field of assessing the operational degradation of oil and gas transit pipeline steels. Both mechanical and electrochemical properties of steels are deteriorated after operation, as is their resistance to environmentally-assisted cracking. The characteristics of resistance to brittle fracture and stress corrosion cracking decrease most intensively, which is associated with a development of in-bulk dissipated microdamages of the material. The most sensitive indicators of changes in the material’s state caused by degradation are impact toughness and fracture toughness by the J-integral method. The degradation degree of pipeline steels can also be evaluated nondestructively based on in-service changes in their polarization resistance and potential of the fracture surface. Attention is drawn to hydrogenation of a pipe wall from inside as a result of the electrochemical interaction of pipe metal with condensed moisture, which facilitates operational degradation of steel due to the combined action of operating stresses and hydrogen. The development of microdamages along steel texture was evidenced metallographically as a trend to the selective etching of boundaries between adjacent bands of ferrite and pearlite and fractographically by revealing brittle fracture elements on the fracture surfaces, namely delamination and cleavage, indicating the sites of cohesion weakening between ferrite and pearlite bands. The state of the X52 steel in its initial state and after use for 30 years was assessed based on the numerical simulation method.

Methodology of hydrogen embrittlement study of long-term operated natural gas distribution pipeline steels caused by hydrogen transport

“…This phenomenon can be explained only by taking into account the regularities of the operational degradation of structural steels with the intensive development of dissipated damage in the bulk of the pipe wall. Similar results on low brittle fracture resistance (KCV ≤ 20 J/cm 2 ) were reported for pipeline steels and structural steels of portal cranes [19,29].…”

“…In addition, specimens of a small thickness (1.2 mm) were tested, which could also affect the results [31]. Plasticity is only slightly decreased due to operation; however, this is accompanied by strength loss, which corresponds to the main regularities of the operational degradation [19,25,26] -a simultaneous decrease of both strength and plasticity indicates the development of operational microdamages in the material. Impact testing results averaged three specimens are (98±4) J/сm 2 and (9±4) J/сm 2 respectively for the as-received and operated steel.…”

“…The main issue is possible hydrogenation of the pipe wall from its inner surface because of humidity of the transported gas which causes electrochemical processes leading to hydrogen evolution [16,17]. Hydrogen transportation by pipelines is assumed to intensify the metal hydrogenation for two reasons: (i) due to hydrogen dissociative adsorption [18] and (ii) an increase in the amount of electrochemically formed hydrogen [19] absorbed by the pipe wall. On the other hand, gas pipeline networks usually have been operating for a long time, which leads to the essential deterioration of initial (as-received) physico-mechanical properties of steels, mostly affecting their brittle fracture resistance [20][21][22][23][24][25][26].…”

“…On the other hand, gas pipeline networks usually have been operating for a long time, which leads to the essential deterioration of initial (as-received) physico-mechanical properties of steels, mostly affecting their brittle fracture resistance [20][21][22][23][24][25][26]. In the case of hydrogen transportation, a decrease in the resistance to hydrogen embrittlement is especially important since it can cause hardly predictable pipeline failures [19,20,27]. Developing the study [6], this work presents a set of experimental techniques adapted to assessing the technical state of long-term operated steels of distribution gas pipelines for hydrogen transport in a mixture with natural gas.…”

Comparison of J Integral Assessments for Cracked Plates and Pipes

“…The technical state of gas pipelines is therefore periodically assessed. This becomes increasingly significant when the planned lifetime of gas pipelines is close to expiring [ 26 ]. It is then necessary to increase the frequency of inspections of pipelines to identify in time the most dangerous of defects in pipelines—cracks.…”