JPE - Sept09 - cover2-4.pmd - Pipes & Pipelines International ...

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Conclusion
Our presented non-linear elastic-plastic fracture mechanics
analysis of the fifth spill incident indicates that the high
toughness of both the pipe material and the weld material
necessitated the combination of extremely high axial loads
(which exceeded the uniaxial yield strength of the material)
and a biaxial stress state to propagate defects at the
circumferential girth welds. The biaxial stress state, caused
by the internal pressure, essentially increases the load
needed to cause yielding in the pipe and can allow ductile
tearing to occur in the girth welds before the load reaches
this higher yield point for biaxial stress. A substantial axial
load can cause ductile tearing of defects that are considered
allowable by API 1104, since girth welds may be code
compliant, but may still contain allowable deviations from
the ideal condition allowed in the standard. These minor
defects will ultimately provide the stress risers to initiate
cracks as high loads arise. Although the exact propagation
rate was not determined, it seems likely that this tearing
could occur over a short time: this limits the ability of (ILI)
tools in identifying these defects in a timely fashion.
Currently only an ultrasound ILI with a modified sensor
arrangement would reliably detect circumferential cracks
within an actionable timeframe to safeguard the pipeline
against potential geotechnical hazards. However, at present,
insufficient evaluation has been conducted to establish the
relationship between the onset of tearing and the shape,
size, and orientation of welding defects. The timescales of
the ductile tearing have also not been established sufficiently
to rely upon periodic inspection. Hence, detailed
geotechnical studies prior to construction, careful
monitoring of the pipeline’s RoW, and geotechnical risk
assessments appear currently to be the most comprehensive
way to safeguard the integrity of the pipeline. Constructing
the necessary geotechnical stabilization measures can
typically mitigate geotechnical hazards.
When designing pipeline systems, particularly for use in
susceptible geological environments, it is important to
recognize that axial loads generated from soil movement
The Journal of Pipeline Engineering
can be high enough to propagate relatively-small
circumferential flaws and cause failure. Currently-available
commercial pipeline inspection methods do not appear to
provide a suitable means of detecting such flaws and
guarding against failure due to propagation of these
relatively-small flaws. Therefore, in susceptible geologic
environments, extra attention must be given during the
design phase to specifically include suitable and conservative
assumptions of the external loading.
References
1. EGIG, 2005. Report 1970-2004. Gas pipeline incidents.
6th Report of the European Gas Pipeline Incident Data
Group, Doc. Number EGIG 05.R.0002, December.
2. PRCI, 2004. Guidelines for the seismic design and assessment
of natural gas and liquid hydrocarbon pipelines. Catalog No.
L51927, October.
3. Exponent Failure Analysis, 2007. Report: Integrity analysis
of the Camisea transportation system, Peru. Submitted to
the Inter-American Development Bank, June.
4. A.P.S. Selvadurai, J. J. Lee, R.A.A. Todeschini, and H.F.
Somes, 1983. Lateral soil resistance in soil-pipe interaction.
Proc. Conf. Pipelines in adverse environments 11, San
Diego, California, November.
5. M. Sweeney, A. H. Gasca, M. G. Lopez, and A. C. Palmer.
Pipelines and landslides in rugged terrain: a database, historic
risks and pipeline vulnerability.
6. Germanischer Lloyd, 2007. Report: Auditoria integral de
los sistemas de transporte de gas natural y liquidos de gas
natural del proyecto Camisea. Final audit report of the
Ministerio de Energia y Minas del Peru, No. GLP/GLM/
MEMP/726-07, October.
7. Mauricio Carvalho Silva, Eduardo Hippert Jr., and Claudio
Ruggieri, 2005. Experimental investigation of ductile tearing
properties for API X70 and X80 pipeline steels. Proc. PVP2005
2005 ASME Pressure vessels and piping division conference,
July 17-21, Denver, CO, USA.
8. C. Ruggieri and E. Hippert Jr., 2002. Cell model predictions
of ductile fracture in damaged pipelines. Fatigue and Fracture
Mechanics: 33rd Volume, ASTM STP 1417, Walter G. Reuter
and Robert S. Piascik, Eds, ASTM International.
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