PSI - Issue 42

4

Mimoun Elboujdaini et al. / Procedia Structural Integrity 42 (2022) 1033–1039 Mimoun Elboujdaini / Structural Integrity Procedia 00 (2019) 000 – 000

1036

Fig. 2. Crack development in an X- 65 specimen at σ max = 70%YS in NS-4 solution, showing the corrosion pit and the crack at the bottom of the pit .

Fig. 3. ( a ) Failure in hydrostatic testing, SCC as root cause; ( b ) Magnetic Particle Inspection (MPI) picture of SCC colony found next to a pressure failure. The outside pipe surface shows additional, longitudinal cracks adjacent to the fracture origin.

3.4. Stages in Crack Development The start of cracking involves the development of discontinuities on the surface. A lot of the early work in initiation in NNPH solutions involved studies on polished surfaces to see how these discontinuities developed . The work showed that in fact it was not that easy to produce cracks on smooth surfaces and mechanical conditions far in excess of those experienced by gas pipelines were required to give appreciable discontinuities in the surface. These initial discontinuities tended to involve either dissolution of inclusions leading to pits (Fig. 1). These pits or discontinuities can arise also from other conditions. The next stage is the development of cracks associated with the pits or discontinuities. This stage is clearly shown in Fig. 2. The subsequent development of this small crack into a crack at least 1mm deep growing by the ‘SCC’ mechanism is still not understood. There are several implications of the above factors on the operation of a pipeline. The source of stresses that are exerted on the pipe during its operation must be considered, e.g., the operating pressures in the pipeline and more importantly the pressure variation during operation . Other implications are concerned with the type of soil the pipe and ground movements, including frost heave and thaw settlement (in the north cold area), which could apply external stresses to the pipe. The type of soil and how it could affect the acidity, or pH, of the groundwater around the pipe is also important. Metallurgical factors relate to nonmetallic inclusions in the steel from which the pipe is made, and the steelmaking practices used in its production. Other metallurgical factors relate to the mill welds used to make the linepipe and the field welds that are used to join the pipes.