Page 257 - Pipelines and Risers

230 Chapter 14

14.2 Review of Existing Criteria
The design criteria for corroded pipelines is generally expressed as equations to determine the
operating parameters:

1. the maximum allowable length of defects;
2. the maximum allowable design pressure for uncorroded pipelines;
3. the safe maximum pressure.

A number of criteria exist to determine these operating parameters.

14.2.1 NG-18 Criterion
The NG-18 criterion developed in the late 1960s and early 1970s is to evaluate the remaining
strength of corroded pipe (Maxey et al. (1971)). It was developed for a pipe with a
longitudinal surface flaw:
I-AREA/ AREA0
s, =%ow (14.1)
1- M-'(AREA I AREAO)
where:
S, = predicted hoop stress level at failure
Sflow = flow stress of the material
AREA = area of through thickness profile of flaw
AREA0 =
L= maximum axial extent of the defect
t = nominal wall thickness of the pipe
M= Folias factor which is determined by:
2.51(L /2)* O.O54(L/ 2)4
(14.2)

where:
D= nominal outside diameter of the pipe.

Equation (14.2) can be further simplified as (Kiefner (1974)):

(14.3)

The calculation of AREA is simplified by assuming the shape of corroded area is parabolic
for short corrosion and rectangular for long corrosion (Kiefner (1974)). The maximum
allowable length Lllow, and the failure pressure P is solved from a formula which equates
predicted bursting hoop stress Sp to 1.1 SMYS (Specified Minimum Yield Stress) assuming
that the flow stress is 1.1 SMYS (Bai et al. (1994)).