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3.5. Dented pipe with cracks (ABS, 2000; ASME, 1991)
A dent is defined as a depression that is more than 2% of the nominal
diameter of the pipe ASME (1991). Mechanical damages occur frequently to
pipes. These dents are caused by operation activities, fabrication errors
etc. ABS guideline (2000) is used to calculate the reduced MAOP for a
dented pipe with cracks. The critical stress (sc) at failure is given by:
 (5)
The plastic failure stress (sp) is given by:
 (6)
where a is the maximum depth of pipe wall thickness defect, sf the flow
stress as defined the mean value of SMYS and tensile value, Y the geometry
function defined in ABS (2000) and Kmat is the fracture toughness defined
in API (2000).
The reduced MAOP for pipe with a dent and a crack based on ABS guideline
are presented in Fig. 4. The reduction of MAOP of the pipe is very
sensitive to the crack depth and dent depth. According to Fig. 4, a crack
depth of 0.2 in. with a dent depth greater than 4 in. are not acceptable
for an operating pressure of 10.3 barg (150 psig).
3.6. Out-of-roundness and collapse buckle (API, 1998)
Collapse water depths for various wall thickness and ovalities are checked
per API RP 1111 (1998). Combined bending strain and external pressure load
should satisfy the following:
 (7)
where g(d) = (1 + 20d) 1 is the collapse reduction factor, d = (Dmax Dmin)
/ (Dmax+ Dmin) the ovality, the maximun installation or in-place bending
strain, eb = t / 2D the buckling strain under pure bending, po
the external hydrostatic pressure, pi the internal pressure in
the pipe, Dmax the maximum diameter at any given cross section of the pipe
and Dmin is the minimum diameter at any given cross section of the pipe. |