Pipe uplift in saturated sand: rate and density effects

Williams, Elizabeth; Byrne, Byron W.; Blakeborough, A

doi:10.1680/geot.12.p.067

Cited by 25 publications

(14 citation statements)

References 12 publications

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“…The transition to a flow-around failure as reported in the companion paper (see Fig. 5(b) of the companion paper (Williams et al, 2013)) could well be related to this vertical stress limit. (Interestingly if one checks the above vertical stress limit based on the horizontal stress at the centre line of the pipe, rather than at the crown -that is, replacing the factor (H À D/2) in the above by H -and considering the friction angle of the soil tested, one obtains a transition from vertical slip model to the vertical stress limit being the governing condition as H/D rises above 2 .…”

Section: Vertical Stress Limitsupporting

confidence: 60%

“…The paper by Byrne et al (2013) together with the companion paper by Williams et al (2013) present perhaps the most comprehensive and systematic set of upheaval resistance tests in saturated clean sand. The results also appear very consistent with the theories and explanations provided, which further attests to the quality of the experimental work, and the power of the relatively simple model for development and dissipation of excess pore pressure, and the vertical slip model for uplift resistance.…”

Section: Contribution By R Peekmentioning

confidence: 99%

“…The authors thank the contributor for his observations and further thoughts relating to the two Oxford papers Williams et al, 2013) on uplift resistance of shallowly buried pipes in saturated fine sand. The following points are made in response to the four sections of the discussion.…”

Section: Authors' Replymentioning

confidence: 99%

“…

Contribution by R. PeekThe paper by Byrne et al (2013) together with the companion paper by Williams et al (2013) present perhaps the most comprehensive and systematic set of upheaval resistance tests in saturated clean sand. The results also appear very consistent with the theories and explanations provided, which further attests to the quality of the experimental work, and the power of the relatively simple model for development and dissipation of excess pore pressure, and the vertical slip model for uplift resistance.

STABILITY UNDER FORCE-CONTROLLED LOADING?

The paper shows that for loose backfill, to the point where it contracts when sheared, significant and predictable uplift resistance is developed for slow displacement-controlled loading of the pipe, ensuring essentially drained conditions, but that rapid displacement-controlled loading essentially liquefies the soil, resulting in near zero or even negative uplift resistance.

…”

mentioning

confidence: 99%

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Uplift of shallowly buried pipe sections in saturated very loose sand

Byrne¹,

Schupp²,

Martin³

et al. 2014

Géotechnique

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Contribution by R. PeekThe paper by Byrne et al. (2013) together with the companion paper by Williams et al. (2013) present perhaps the most comprehensive and systematic set of upheaval resistance tests in saturated clean sand. The results also appear very consistent with the theories and explanations provided, which further attests to the quality of the experimental work, and the power of the relatively simple model for development and dissipation of excess pore pressure, and the vertical slip model for uplift resistance. STABILITY UNDER FORCE-CONTROLLED LOADING?The paper shows that for loose backfill, to the point where it contracts when sheared, significant and predictable uplift resistance is developed for slow displacement-controlled loading of the pipe, ensuring essentially drained conditions, but that rapid displacement-controlled loading essentially liquefies the soil, resulting in near zero or even negative uplift resistance. But what would happen under force-controlled loading, which is more representative of the loading the pipe exerts on the soil?The authors conclude that 'It would therefore [i.e. in view of the test results] be prudent to avoid this behaviour [i.e. contractive behaviour of the soil leading to a rise in pore water pressure] by ensuring that soil backfill is sufficiently densified after placement, or sufficiently free draining, that loss of strength does not occur.' This discussion endorses that conclusion even when the rate of heating of the pipe is slow compared to the time needed to dissipate excess pore water pressures. The reason for this is that the upheaval loads in a heated pipeline are force-controlled rather than displacementcontrolled as in the tests. Indeed, as the yielding of the soil due to the uplift force increases the out-of-straightness, this also increases the uplift force, so the type of loading on the soil is even more severe than force-controlled. (At least this applies for small amounts of uplift, typical of those that develop up until upheaval buckling instability.) The implications of such an increasing-force loading condition are not clear from the results of the displacement-controlled tests reported.Based on the results in the paper it takes about 50 s for the excess pore pressure to dissipate for a burial depth of H ¼ 0 . 25 m to the centre line of the pipe. For typical flowline the burial depth might be 1 m, so that, for the same soil, the pore pressure dissipation time is 50 s 3 (1 . 0/0 . 25) 2 ¼ 13 . 3 min. The heating of a flowline, from ambient to near the maximum operating temperature, could happen that fast, for example when dual flowlines are used and fully developed production is switched from one flowline to the other, but typically heating takes longer. The question is whether under such slower loading conditions one can rely on the significant uplift resistance for very loose soils under drained conditions established in this paper.In theory if a test under displacement-controlled loading produces an uplift resistance F ¼ F(t ), then applying ...

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Section: Vertical Stress Limitsupporting

confidence: 60%

Section: Contribution By R Peekmentioning

confidence: 99%

Section: Authors' Replymentioning

confidence: 99%

“…

STABILITY UNDER FORCE-CONTROLLED LOADING?

…”

mentioning

confidence: 99%

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Uplift of shallowly buried pipe sections in saturated very loose sand

Byrne¹,

Schupp²,

Martin³

et al. 2014

Géotechnique

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show abstract

“…However, when the rate of testing exceeds this limiting value, partially drained conditions will apply and this will affect the peak mobilisation displacement. Williams et al (2013) showed that the mobilisation displacement will increase for speeds in the range 0?002-5 mm/s. A recent full-scale experimental study was undertaken at the University of Cambridge in sandy backfill (Thusyanthan et al, 2010;Wang et al, 2012) to investigate the uplift stiffness response of pipelines buried in both saturated and dry sand.…”

Section: Introductionmentioning

confidence: 99%

Uplift mobilisation resistance of subsea pipelines in loose sand

Ivanović¹,

Oliphant

2014

Géotechnique Letters

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Upheaval buckling (UHB) is a well-known phenomenon associated with buried high-pressure hightemperature pipelines. A large number of research studies have been undertaken in the last two decades on developing reliable uplift resistance mobilisation models from which the current standards and recommendations are based. This paper examines Technip's extensive database consisting of experimental and numerical studies concerning sandy backfills only. The peak uplift mobilisation distances obtained are compared with the current DNV recommendations and a recent experimental study undertaken at the University of Cambridge. The trends obtained indicate differences associated specifically with the lower backfill cover depths and suggest that the current recommendations need to be updated. A new improved peak mobilisation distance model, which is able to capture the relevant data with a distinction made between very loose and loose sand backfill, is offered.

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Pipe–Seabed Interaction

White

Bransby

2018

Encyclopedia of Maritime and Offshore Engineering

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Subsea pipelines rest on or in the seabed, and therefore their behavior and condition depends on pipe–seabed interactions. Pipelines expand and contract during operation and can also be loaded by hydrodynamic action, turbidity flows, or debris flows. These actions are resisted by seabed reaction forces in the vertical, axial, and lateral directions. In some situations, the pipeline may be deliberately buried, for example, to allow overtrawling, provide thermal insulation, or prevent expansion‐induced buckling. However, in many cases, the pipeline may be laid directly onto the seabed. It may self‐bury through the action of sediment transport, remain only partially embedded into the soil, or some intermittent combination of these effects along its route. Assessments of pipe–seabed interaction forces for all the abovementioned burial conditions begin with estimation of the as‐laid (or as‐constructed) embedment. Classical methods such as limit plasticity underpin solutions for embedment and the limiting pipe–soil resistance in each direction. The critical uncertainties are usually the operative soil strength—which is affected by the seabed disturbance during the lay (or backfilling) process and throughout the operating life—and the pipeline embedment (or cover depth). Specific geotechnical testing technologies have evolved to reduce uncertainties in pipe–seabed assessments, including tools for determining the near‐seabed (and disturbed) soil strength and the pipe–soil interface strength at very low stresses. The associated analysis methods also aim to quantify the variability of the seabed reaction forces along the pipeline. The abovementioned predictions of pipe–seabed interaction behavior are performed to feed into structural analyses of the pipeline, which often use a reliability‐based framework. Pipe–soil interaction assessments require close collaboration between the pipeline and geotechnical experts due to the inherent influence of soil–structure interaction, as well as the aim of tailoring the geotechnical assessment to address the particular design criticalities, which vary depending on the seabed and pipeline conditions.

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Pipe uplift in saturated sand: rate and density effects

Cited by 25 publications

References 12 publications

Uplift of shallowly buried pipe sections in saturated very loose sand

Uplift of shallowly buried pipe sections in saturated very loose sand

Uplift mobilisation resistance of subsea pipelines in loose sand

Pipe–Seabed Interaction

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