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3.2.1. Single defect vs. interacting defects per ABS Guide (ABS, 2000)

For longitudinal defects to pipe axis: if the distances between two longitudinal defects of length l₁ and l₂ is greater than either l₁ or l₂, then the length of corrosion defect l is l₁ or l₂, whichever is greater. It can be assumed that there is no interaction between the two defects and it can be defined as a single (isolated) defect as shown in the illustration (Fig. 1). If the distance s between two longitudinal defects is less than either l₁ or l₂, it is to be assumed that the two defects are fully interacted and
the length of the corrosion defect l is to be taken as l = l₁ + l₂ + s.

Fig. 1. Interacting defects dimension (API, 2000).

3.2.2. Single defect vs. interacting defects per DNV RP-F101 (DNV, 1999)
A defect can be treated as a single defect if either of the following conditions is satisfied:
1. The circumferential angular spacing between adjacent defects, Ø:
Ø> 360(t /D)^0.5 degrees                                                                                                                                                            (1)
where t is the uncorroded nominal pipe wall thickness and D is the nominal outside diameter of the pipe.
2. The axial spacing between adjacent defects, s is given as
s> 2.0(t*D)^0.5                                                                                                                                                                              (2)
3.2.3. Corrosion rate estimation                                                                                                                                                        Corrosion rate in pipeline made of carbon steel may be evaluated using industry accepted equations that preferably combine contributions from flow independent kinetics of the corrosion reaction at the metal surface, with contribution from flow
dependent mass transfer of dissolved CO2. The details of the corrosion rate estimation can be found in Appendix D of ABS (2000).
3.2.4. MAOP for corroded pipes
ABS (2000) and ASME (1991) state that all defects deeper than 0.8 times the wall thickness are to be repaired. For less severe defect depths, ASME B31G (1991) may be applied to estimate the maximum allowable operating pressure for the pipeline. For the 24-in. pipe, the possible operating pressures (p) were calculated to match the 72% of SMYS for various wall thicknesses:
     (3)
where s is the pipe hoop stress and h is the usage factor of 0.72.