Summary
Provision of mitigative and control measures are necessary for a pipeline to
survive in a debris flow event, but the forces in the soil/structure
interaction must be estimated for the design. Based on physical experiments in
a flume and numerical analyses, this paper presents a method for estimating the
impact drag force on laid-on-seafloor and suspended (free-span) pipelines. The
method may be applied in practice to a wide range of debris flow impact
situations. Two conceptual mitigative and control measures for design against
submarine debris flow impact are discussed: the berm-protected laid-on-seafloor
pipeline and the cable-controlled pipeline system. The latter may be applied to
both the pipeline-on-seafloor and suspended pipeline situations. The
observations from a laboratory flume experiment with a model pipe protected by
an upstream berm, as well as complementary computational fluid dynamics (CFD)
numerical analyses results are presented. The results from the flume experiment
show that there is a possibility to protect a pipeline provided the protective
structure can withstand the basal shear and lift forces induced by the water
and debris flows on its surfaces. The results may be used for conceptual and
preliminary design purposes, and the analysis methodology may be tailored to
other situations or the detailed design. The feasibility of the two conceptual
mitigative and control measures is briefly discussed.
Introduction
Fast-moving, flow-like submarine landslides are among the most destructive
and frequent occurring geohazards, with potential to seriously damage seafloor
installations. Estimation of the forces from a submarine debris flow impact is
required for pipeline design and routing. A common practice is to protect the
pipelines by burying it in areas where it could be affected by debris flow
impact. Besides the construction difficulties associated with the burial, a
buried pipeline may become exposed during its course of service, resulting from
seafloor scour and wave-induced or operational uplift forces. Further, it is
likely that a buried pipeline in a debris flow pathway becomes exposed just
prior to the impact because of seafloor erosion from the water displaced by the
debris front or, later, by the debris flow itself.
Upon impact with a pipeline, the maximum drag force is exerted shortly after
contact, provided that the debris flow is not supplied by consecutive failures
upstream trailing at higher velocities. The magnitude of the drag force drops
quickly as the upstream debris flow velocity decreases. If a pipeline can
withstand the drag forces exerted upon and shortly after the impact, it will
most likely survive the entire event. Without provision of protective measures,
a pipeline alone cannot withstand the debris flow impact forces. The mitigative
and control measures may only be designed once the debris flow impact forces
are estimated. For pipelines, the methods available in the literature mainly
address the problem of forces on a buried line in an unstable zone, as opposed
to the debris flow impact (Zakeri 2009). However, as is discussed next, recent
developments in the field have shed new light on the problem of submarine
debris flow impact forces on pipelines (Zakeri et al. 2008, 2009). The impact
forces on pipelines may be mitigated or controlled by techniques such as
constructing protective berms (either on the upstream or both sides of the
pipe) or mooring the line to the seafloor using cables and suction caissons.
The feasibility and constructability of each technique depends on many factors
which have to be evaluated on project basis to a defined set of criteria.
This paper briefly outlines a method to estimate the impact forces induced
on suspended (free-span) and laid-on-seafloor pipelines. It then discusses two
conceptual mitigative and control measures: the berm-protected laid-on-seafloor
pipeline and cable-controlled pipeline system. The latter may be adopted for
both laid-on-seafloor and suspended pipelines. To that end, the results from a
laboratory experiment and series of CFD simulations for an upstream
berm-protected laid-on-seafloor pipeline model are discussed. The analysis
results may be used for conceptual and preliminary design purposes. The
feasibility of the two conceptual measures is also briefly discussed.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
8 December 2008
- Meeting paper published:
5 May 2008
- Revised manuscript received:
16 June 2008
- Manuscript approved:
21 June 2008
- Published online:
5 March 2009
- Version of record:
5 March 2009
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