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Pipeline Integrity Management Under Geohazard Conditions (PIMG)

By
Mamdouh M. Salama
Mamdouh M. Salama
ConocoPhillips
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Yong-Yi Wang
Yong-Yi Wang
CRES
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Don West
Don West
Golder
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Alexander McKenzie-Johnson
Alexander McKenzie-Johnson
Geosyntec
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Azam B A-Rahman
Azam B A-Rahman
Petronas
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Guiyi Wu
Guiyi Wu
TWI
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Jens Petter Tronskar
Jens Petter Tronskar
DNVGL
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Jim Hart
Jim Hart
SSD Inc
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Bernt J. Leira
Bernt J. Leira
NTNU
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ISBN:
9780791861998
No. of Pages:
412
Publisher:
ASME Press
Publication date:
2020

ABSTRACT

Pipelines around the world are built through rough terrain that includes natural slopes that can be marginally stable. The standard of practice is that the stability of the slopes along the pipeline route is rarely formally evaluated to calculate the factor of safety or likelihood of landslide triggering. An evaluation that would render reliable values of factors of safety (FOS) would require a degree of characterization that cannot be achieved for these long linear structures given that subsurface conditions along the slope prior to pipeline construction are spatially and seasonably variable and can be affected by weathering over the design life of the project.

Best practice is to conduct geomorphological evaluations to identify areas that are prone to landslides, such as steep slopes or gentler slopes with relatively weak soil or rock formations. Where it is practical, areas that are subject to landslides before pipeline installation are avoided. When steep areas cannot be avoided, the pipeline is typically installed with best management practices that are intended to mitigate the landslide hazard. These include minimizing ground disturbance during pipeline construction and application of surface water diversions and subsurface drainage, along with other stabilization measures, such as re-establishing vegetation and soil compaction. When enough of these mitigation measures are applied to steep slopes disturbed by pipeline construction the result can generally offset soil strength losses attributed to ground disturbance.

In steep slope environments, such as in Appalachia, some natural slopes become unstable following periods of intense precipitation and ground saturation. When slope instability does occur, shallow landslides and soil creep are among the most common mechanisms of permanent ground displacement (PGD). Mitigating these natural processes from occurring on the pipeline right of way (ROW) is not always practical, particularly where they may have occurred naturally regardless of pipeline construction. Emphasis should be placed on the overall risk to the integrity of the pipeline and addressing those with appropriate mitigation measures, such a burying the pipe deeper than the minimum requirement, if needed, to reach stable soil or rock formations and maintaining the stability of the trench backfill.

Since the computation of a FOS is highly uncertain and may not reflect the likelihood of initiation of instability, a performance-based criterion is therefore considered to be a more appropriate approach for the design and construction of pipelines along natural steep slopes. Using this approach, the risk of landslides occurring and their possible effects on pipeline integrity, although unknown, is tolerable to the pipeline operators because the slope is expected to perform in a way that is similar to its performance prior to the installation of the pipeline.

This paper discusses the technical challenges facing construction of new pipelines on steep soil-covered slopes and provides recommendations for implementation of a performance-based design criterion to mitigate instability of steep slopes on pipeline ROWs.

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