COMPARATIVE RESISTANCE OF OIL INTERFIELD PIPELINE STEELS TO RILL CORROSION

Abstract

Background The main cause of accidents at interfield oil pipelines, as experience in their operation shows, is the destruction due to corrosion. Aims and Objectives - to consider the main hypotheses on the breakage mechanism of interfield oil pipelines under the influence of rill corrosion; - to identify, on the basis of the published data and in-house experimental research, the main factors contributing to the initiation and behavior of rill corrosion; - to propose a methodology for testing pipeline steels for resistance to rill corrosion. Results It was concluded that the initiation of rill corrosion requires concurrent presence of such conditions as presence of solid abrasive particles in a water-oil emulsion, enrichment of the emulsion with oxygen, including its layers that are in direct contact with the bottom surface of the pipe, and the wall metal being under stress. Methodology is proposed to test pipeline steels for resistance to rill corrosion. Testing is proposed to be carried out on plates of 150 x 50 x 3 made from the rolled products of analyzed steels soaked in a 3 % aqueous NaCl solution with continuous aeration with air. The middle part of the plates is exposed to residual stresses close to the level of maximum equivalent stresses in the oil pipe wall, which is achieved by bending the plates in clamps until the required deflection arrow is reached, which is determined based on the value of the residual stresses and the type of tensile diagram of the analyzed steel. A notch is made in the middle part of the inner side of the plates, which serves as the initiator of additional mechanical stresses. After soaking the plates in a corrosive medium, the increase of the notch radius due to the walls corrosion initiated by the aggressive environment is analyzed for each of them; based on the analyzed value, with an account for the duration of the tests, the steel corrosion rate is calculated as per the mechanism of the grooving corrosion. Corrosion rates for four grades of pipeline steels (08kp, 17GS, 10G2FBYU, 10G2FBYU-U), determined during the steels testing according to the proposed method, are presented. Based on the comparison of the values obtained, it may be concluded that the steel 17 GS has a higher resistance to the rill corrosion, which complies with the published data on the breakage rates of interfield pipelines.