Rolf A. Jakobsen, Yong Bai") and Ivar Langen
Stavanger University College, *) JP Kenny NS
Stavanger, Norway
ABSTRACT
Circumferential failure may occur in defective girth weld when it is under
high longitudinal tension. This paper presents methodology and design
criteria for assessment of pipeline girth weld defects (circumferential
failure). For limit state design, the paper covers major failure modes
such as plastic collapse and fracture. Both Corrosion defects and cracklike defects are considered.
Strength equations are reviewed and
compared.
Keywords:
Pipeline, girth weld, corrosion, plastic collapse, fracture.
a AX = 1- P(l-7]J + 2 sin oJ [0.5(1-7] )sin P]
ajlaw
Jr
Chell Method
A conservative. method for estimating
the strength of pipe
circumferential
corrosion is the Chell method (Chell 19i9).
minimum longitudinal tensile stress to cause failure is calculated as
aAX
---
INTRODUCTION
a jlow
Circumferential failure may occur in defective girth weld when it is under
high longitudinal tension (Rosenfeld and Kiefner, 1995). The defects in
girth welds may be corrosion defects or due to lack of fusion in welding
process. Internal corrosion defects are primarily confmed to the lower part
of the pipe, e.g. from 0400 to 0800 o'clock, and longitudinal failure under
internal pressure may be assessed using B31G (ASME 1993) or its
modified version (Bai et al 1997). However, B31 G are not suitable for
evaluating whether circumferential failure will occur. The purpose of this
paper is to develop methods of assessing girth weld defects.
(2)
'With
The
7]
1- (1-7])/f
f=~1+(W~/
(4)
W = circumferential width of defect
Miller (1988) compared capacity equations with experimental data for
pipe containing circumferential girth weld defects. His conclusion is that
Kastner's equation agreed well with experimental data
Kastner's Local Collapse Criterion
The plastic collapse behaviour of girth weld corrosion can be predicted
using a failure criterion proposed by Kastner et al (1981):
7][1l - p(l-7])]
7]Jr + 2 (1 -7]) sin
a jlow
where
P
=
d defect depth
t = wall-thickness
11=I-d!t
c = half defect (circumferential)
R = pipe radius
cIR (in radians)
length
~=
allow
and in which
= flow
a AX
stress
denotes the total axial stress.
Schulze's Global Collapse Criterion
Schulze et al (1980) proposed a net-section
collapse):
collapse
formula
(global
The above collapse equations consider axial tension alone. For plastic
collapse of pipes under combined loads, reference is made to Bai and
Hauch (1998).
Possible Cracks in Girth Weld
Various types of imperfection are known to occur in girth welds. The
most damaging types are cracks, inadequate penetration of the root bead,
and lack of fusion. The imperfections are particularly damaging if they
occur in a weld that undermatches the yield strength of the base material.
The most frequent type of planar/crack like defects in one-sided Shielded
Metal Arc Welding (SMAW) is lack-of-fusion defects. Such defects can
be located near the surface or be surface breaking and may have gone
undetected following NDT procedures according to API 1104.
Based on these observations it is reasonable to assume that some defects,
typically y,ith a height equal to one weld pass, i.e. 3-4 mm and length
between 50 and 150 mm, may exist in the pipeline. These cracks will be
assumed to be surface breaking or becoming surface breaking due to
preferential corrosion of the root area of the girth weld.
General
Longitudinal tensile strain in the pipelines may be induced by the
subsidence of an oil field (or soil movement). This strain can, combined
v.ith a possible initial weld defect in the weld-zone, be of a critical level
\lith respect to the Unstable Fracture and Plastic Collapse (UFPC) failure
modes.
Defects in girth weld can be addressed on one of three levels, depending
upon the quality of affected welds, the availability of certain material data,
and the difficulty of making repairs.
Level] Assessment - Worhnanship Standards
Pipeline welding codes establish minimum weld quality standards based
on inspection of a welder's workmanship.
The flaw acceptance criteria
evolved through industry experience.
Hence, most workmanship
standarrls are similar, though not identical, in terms of allowable
imp.:n'.xtion types and sizes. The advantage of workmanship standards is
thar they are time-tested, they are compatible with normal levels of l\'DE
quality, they do not require material strength and toughness property data,
and they are easy to apply. However, it has been recognized that some
rejectable flaws may not necessary pose a real threat to pipeline integrity.
A flawed girth weld that would be extremely costly to repair or replace
should not be rejected solely on the basis of workmanship standards.
A workmanship standard that is presently recognized by gas pipeline
regulations is that contained in API Standard I 104 (API 1988).
Level 2 Assessment - Alternative Acceptance Starufards
Alternative acceptance standards were developed to facilitate acceptance
of flaws that do not meet workmanship standards.
Incentives for
alternative standards are usually economic,
arising due to the
inaccessibility or quantity of welds that would otheIWise be repaired.
Alternative standards recognize that the true severity of a flaw is
dependent on material toughness and applied stress levels, and can only be
determined using fracture mechanics principles.
In pipeline industry the crack-tip opening displacement (CTOD) is most
commonly used as the toughness measurement of welds, CTOD is
established from destructive tests performed on weldments. If the pipeline
is yet to be constructed, CTOD tests can be performed as part of the
welding procedure qualification. If the pipeline is already in service and
CTOD data are pot available, the welding procedures, consumable, and
base materials used in construction may be used to duplicate welds for the
purpose of CTOD testing. If anyone of these elements is no lonuer
available, it ",ill be nccessary to obtain a representative weld for testing."
Three alternative criteria that are recognized by their respective national
regulating agencies and that are often cited elsewhere are the appendix to
API Standard 1104, Appendix K to CSA-ZI84, and BSI PD-6493 (pD6493 Level I is comparable to what is referred to herein as Level 2.). All
three standards are based on the CTOD Design Curve approach developed
by The Welding Institute, which extends Linear Elastic Fracture
Mechanics (LEFM) concepts into the elastic-plastic regime. In spite of
their common origins, they differ in their treatment of residual stresses,
summation of stress components, minimum toughness level, and factors
of safety.
Level 3 Assessment - Detailed Analysis
Flaws that are not pennitted by Level 2 assessment may be evaluated by
detailed fracture mechanics analysis. PD 6493 provides an appropriate
Level 3 procedure based on R-6 FAD methodology. PD 6493 Level 2 and
Level 3 are both comparable to what is referred to herein as Level 3. In
case accurate information about the whole stress-strain relationship for the
weld material is lacking, the default Failure Assessment Diagram (FAD)
specified in PD6493, based on yield stress and tensile stress of weld
material, is applied to model the acceptance criteria against UFPc. The
default FAD for level 3 is generally conservative.
The internal pressure
v.ill have negligible influence on the UFPC capacity of transverse cracks,
and is not accounted for in the analysis.
In the assessment of the UFPC capacity of the weld due to longitudinal
strain of the pipe, it is assumed that there exist a semi-elliptical surface
weld defect of depth a and total length 2c. In the determination of the
stress intensity at the crack tip, established empirical expressions are
applied to describe the stress distribution over the weld defect (geometry
functions) for both the membrane and bending stress distribution
(Newman and Raju 1981).
The critical stress levels with respect to UFPC failure can be obtained
from FAr? analysis, for the different pipelines as a function of the degree
of corrosion wall-thickness reduction. The corresponding critical strain
level is estimated using Ramberg-Osgood
curve for the stress-strain
relationship.
Fracture of Corroded and Cracked Girth Welds
Corrosion of the weld material will lead to an increase in the stress level
over the weld zone due to reduced local thickness and stress concentration
when the pipeline is exposed to longitudinal tensile strain.
In a UFPC capacity modeling of the corroded weld due to longitudinal
tension, it is assumed that the weld defect is present together with the
reduced wall-thickness of the weld due to corrosion.
Due to the high
number of welds (weld lengths) being exposed to a similar high strain
level, the combined effect of having a surface weld defect and a reduced
weld thickness due to corrosion is considered realistic.
The thickness reduction due to corrosion is assumed only to affect the
weld material, and not the base material, causing a high stress
concentration. The corrosion reduction of the weld material will probably
affect the weld only within the lower circumference of the pipe, but is for
simplicity in the modeling assumed to be evenly distributed around the
pipe circumference.
It is further assumed that the weld defect is a semielliptical surface crack (defect depth a, total defect length 2c) located at
the worst possible location with respect to the stress concentration,
independently of the degree of corrosion.
The corrosion results in a material reduction in the weld zone and thereby
a stress concentration in the weld zone under longitudinal tension. To
account for the stress concentration in the UFPC analysis, a fInite element
analysis is carried out in order to determine the stress distribution over the
weld thickness.
The stress distribution over the weld obtained from the finite element
analysis is represented through a combined membrane and bending stress
distribution over the weld thickness.
Schelze H.D., Togler G. and Bodmann E. (1980): "Fracture Mechanics
Analysis on the Initia~on and Propagation of Circumferential and
Longitudinal Defects in Straight Pipes and Pipe Bends," Xuclear
Engineering and Design, Yo1.58, pp.l9-31.
The critical stress levels with respect to UFPC failure can be obtained
from FAD analysis, for the different pipelines as a function of the degree
of corrosion defect depth. The corresponding critical strain level is
estimated using Ramberg-Osgood curve for the stress-strain relationship.
The UFPC analysis can be carried out considering
parameters:
stress-strain relationship of the pipeline material
yield and tensile strength of the weld material
crOD value of the weld material
depth of weld defect
length of the weld defect
residual stress
the following
This method could, however, be over-conservative, because it assumes
worst combination of corrosion defect, crack and loads.
The paper presented criteria for assessment of circumefrential failure of
girth weld, considering failure modes of plastic collapse and fracture.
For plastic collapse, three alternative equations are presented and
Kantner's equation is recommended.
For girth weld cracks, both
cracks and combined cracks and corrosion defects are considered.
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MeCTHLP.>IH
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paccMoTpeliLl B "Bal and Hauch" (1998).
ASME B&PY Code, Section VIII, Division I, Article UW-51,
Radiographic Examination of Welded Joint; Appendix 4, Acceptance
Standards for Radiographically Deternuned Rounded Indications in
Welds; and Appendix 12, Ultrasonic Examination of Welds (UT).
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(Schulze's Global Collapse Criterion);
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Chell Method.
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MII,l.lep (1988) peKOMell,lryCTIICnOIlL30BaHlle BLlp3lKeHHlIKaCTllepa,
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Requalification
Criteria
for Longitudinally
Corroded
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