Page 267 - Pipelines and Risers

240 Chapter 14

Corroded Welds
Corrosion in submerged-arc seams (longitudinal welds) should be handled in the same
manner as corrosion in the body of the pipe. Corrosion in girth welds (circumferential) should
be assessed using the Kastner's local collapse criterion. The level 2 (or level 3 analysis
implemented in PD 6493 (1991) should be applied for assessing corroded welds. The
corroded groove could be considered as a crack of the same depth and length. The effect of
the material's fracture toughness (in ductile and low toughness pipes) could be taken into
account in the assessment procedure of the material fracture toughness.

Safety Factor
Traditional safety factors are given based on engineering experience and judgement. Within
Kiefner and Vieth (1989, 1993) studies, several modified B31G criteria were developed. In all
cases, the safety factors are assumed to be 1/0.72=1.39, as the original B31G criterion.
However, the safety factor for the new criteria is calibrated based on reliability methods. It is
around 1.8 and dependent on the accuracy of inspection tools and corrosion depth.

14.4 Evaluation of New Criteria

The evaluation of the new criteria is conducted in this section to compare with the test data
from AGA database, NOVA tests, British Gas tests, and Waterloo tests. In the comparison, a
model uncertainty parameter XI, is introduced as:
(14.28)

where Xm, is the true strength in the tests and XPd is the capacity predicted by a given
criteria (existing or new). Table 14.1 is the statistical parameters for XM (mean and COV
v). It is demonstrated in the table that the uncertainty of the new criteria is much smaller than
that of the existing criteria.

Table 14.1 Statistics for Different Criteria and Test Data.
I 1 B31G I NG18 I New Criteria
I I I I n
(Mean I 1.74 I 1.30 I 1.07
COV 10.51 10.19 10.18 I

14.5 Reliability-based Design

The reliability-based design is to develop a Load Resistance Factor Design (LRFD) equation
where bursting is taken as the failure criterion. This includes the following items:

Specification of a target safety level
Specification of characteristic value for the design variables