While the previous approaches have been effective in integrity assessments of corroded pipelines and form the basis for industrial codes and guidelines for fitness-for-service analyses of local thinned areas of piping components, the semiempirical nature of Eq. (8) has raised some points of criticism. In particular, the industry experience suggests that defect assessment procedures based upon Eq. (8) may be overly conservative depending upon pipeline steel grade and corrosion defect geometry. The analyses described subsequently employ a failure stress criterion based upon plastic instability of the defect ligament to produce less conservative burst strength predictions。
3. Finite element procedures
3.1. Parametric studies of flawed pipes
Nonlinear finite element analyses are described for plane-strain models of axially flawed pipes with D508 mm(20 in.), t15 mm (D/t34) and external surface defects having a/t0.2 (shallow defect) and a/t0.5 (deep defect). Here, a is the crack depth, t is the pipe wall thickness and D is the pipe outside diameter. These configurations typify common geometries and flaw sizes in high pressure, high strength pipelines. To verify the effect of flaw shape and flaw width on corrosion defect assessments incorporating stress-based criteria, the matrix analysis considers groove-shaped defects with groove width, dg0.2, 0.5, 1, 3 and 6 mm, and rectangular- shaped defects with defect width, wc25, 50, 75, 100 and 200 mm. Fig. 3 illustrates the flaw geometry adopted in the analyses.
Fig. 4(a-c) shows the finite element models constructed for the pipe configurations having a/t0.2 with dg1 mm(groove shape) and wc25 mm (rectangular shape). Symmetry conditions permit modeling of only one-half of the analyzed pipes. Square elements of uniform size are defined in the defect region and along the remaining ligament to provide similar levels of mesh refinement for all numerical models. Because no strong stress gradients arise in the defect region (which contrasts to conventional crack-tip problems – see, for example, Ruggieri and co-workers [25,26] for illustrative analyses), the evolving stresses ahead of ligament with increased pressure are resolved adequately for element sizes of z0.1w0.5 mm. The half-symmetric model has one thickness layer of 1770 8-node, 3-D elements (3970 nodes) with plane-strain constraints (w 0) imposed on each node. Very similar finite element models and mesh details are employed for other pipe configurations.
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