Remaining Life of a Pipeline

From the above consideration, it is clear that the pipeline strength “Sp strength “ is affected by the
presence of corrosion defects. Regarding this particular issue, several researchers have proposed
different equations to predict the Sp strength of a pipeline containing a finite longitudinal corrosion defect.
Among these models the most widely used was developed by Mok ( reference1)
Sp strength
Sf.
1
1
A
Ao
A
( Ao . M )
(iii)
where:
Sp strength = Predicted hoop stress level at failure
(Mpa)
Sf = flow strength of the pipe material
A = Projected area of defect on an axial plane through the wall thickness (mm 2 )
Ao = Original cross-sectional area of the pipe at the defect (mm 2 )
M = Folias or bulging factor
About equation (iii) there are other considerations.
• Consideration #1
Areas “A” and “Ao” can be obtained by the following relationships:
Ao = L . T (iv) ( see fig. 4)
A = L . d (v) ( see fig. 4)
Equation (v) is suggested in B31G Method, (see apendix #1) to calculate the metal area loss from
the overall axial length (L) and the average depth (d) of the defect.
Fortunately, corrosion defects in pipelines can be detected by the use of a high-resolution magnetic
“pic, which can be used to locate and to measure the size of a corrosion defect. Through periodic
inspections, the growth of a corrosion defect can also be monitored using ultrasonic techniques.
Substituing equations (iv) and (v) into equation (iii), it becomes:
Sp strength
Sf.
1
1
d
T
d
( TM . )
(vi)
• Consideration #2
The flow strength (Sf) is a material property and is related to material yield strength (Sf). There are
several relatipnships avalable in the literature but the most recent researches about this topic (
references 2,3,4 and 5) propose the following experimentally obtained expression:
Sf = Sy + 68.95 Mpa (vii) ; where Sy = Yield Strength of the pipeline material
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