Study On Seawater Stress Corrosion Behavior Of X70 Welded Joint

Offshore pipeline mainly deliver oil and gas as the welded structure. During the processing and using, inevitably the structure will generate cracks and other flaws. These flaws determine the service life of structure once occur stress corrosion under specific environment. In order to simulate and reflect mentioned above objective situation which the structure occur stress corrosion crack in seawater, use prefabricated fatigue crack samples to research the behavior of stress corrosion for X70 pipeline.The critical stress intensity factor KISCC and crack propagation velocity for base metal, welded seam and HAZ were obtained by the Wedge-open loading constant displacement specimens. Their respective KISCC are 55.7093 MPa·m and 37.9989 MPa·m ;Respective Crack propagation velocity are 3.38×10-9mm/s and 3.9×10-9mm/s. Test indicates that the welded seam is more sensitive than the base metal for stress corrosion.Base on"Fitness for Purpose"to assess the significance of a known flaw, KISCC determination and stress corrosion crack velocity determination are considered. If the applied value of KI< KISCC, this flaw is safety; If KI exceeds KISCC, the possibility of stress corrosion crack growth should be recognized. In this case, an assessment may be made to determine whether the flaws would grow to an unacceptable size within the design life of the structure or within the appropriate inspection. This assessment should be based on available stress corrosion crack velocity data. If, as a result of this growth, the flaws do not reach the maximum tolerable size for other failure mechanisms, the flaws are acceptable. If, on the other hand, the calculated crack size at the end of the design life or inspection interval exceeds the tolerable size for other failure mechanisms, the flaws are unacceptable.