Different stressed states of steel pipes in the practice of stressed state inspection on residual magnetic field strength of metal

Zagidulin, Timur; Spector Sci-Tech LLC; Russian Federation

Zagidulin, T.R.; Spector Sci-Tech LLC; Russia
Zagidulin, R.V.; Spector Sci-Tech LLC; Russia

ID: ECNDT-0430-2018

Each steel pipe exposed to number of different types of loads, depending on its application: main pipelines works generally under high pressure load, production pipelines also affected by bending actions, drilling pipes undergoes a torsion deformations, etc. Gradual accumulation of stress results to occurrence of crack and sudden disruption of steel pipe can follow it.

General origin of micro-cracks in steel pipes is significant stress concentration indicated by locally increased level of stressed state of metal. These potentially dangerous local areas are detecting by metal strain indicators IN-01m and scanners IN-02 on residual magnetic field strength of metal.

Readouts of metal strain indicators are directly proportional to effective mechanical stress of metal σeff which determined in strength theory by tangential τxy, τyz, τxz and normal components σx, σy, σz of stress tensor for each complex case stress taken place in the practice. According to theory, the reason of crack occurrence is when effective mechanical stress in specified local area reaches close to permissible stress value. Therefore the readouts of metal strain indicators are comparing directly with permissible stress established.

Inspecting stressed state of metal near the welds along and around of pipe the defects are detecting inside the welds. Defects detected were verified by ultrasonic flaw detector, some of them considered allowable according to inspection standards.

Most effective is magnetic inspection of stressed state of drilling pipes, working in conditions of extremely high stresses: in strong and at the same time fragile metal of drilling pipe the crack occur and progress in relatively short timespan. In the practice most stressed areas of metal can not be detected by flaw detectors because there are no defects yet. However, they are reliably detecting in early stages by magnetic inspection of stressed state of metal techniques, preventing sudden breakdowns of equipment.