Experimental Modeling of the Unburial Behaviour of Pipelines

Byrne, Byron W.; Schupp, J.; Martin, C. M.; Oliphant, J. Orin; Maconochie, Alasdair; Cathie, David

doi:10.4043/19573-ms

Cited by 3 publications

(2 citation statements)

References 13 publications

(14 reference statements)

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“…The likely reason for the development of the positive excess water pressures is the fast upward compression and shearing of the very loose soil above the pipe (e.g. see results from Byrne et al, 2008). Owing to the high uplift rate, the excess pore water pressures are unable to dissipate fully, resulting in a loss of effective stress and therefore uplift resistance.…”

Section: Experimental Equipmentmentioning

confidence: 96%

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Uplift of shallowly buried pipe sections in saturated very loose sand

Byrne¹,

Schupp²,

Martin³

et al. 2013

Géotechnique

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Offshore pipelines are often buried to protect them from damage, and to provide additional thermal insulation. In sandy soils the pipes are trenched using jet-trenching or ploughing. In both cases the nature of the trenching operation means that the backfill material can be placed over the pipe in an extremely loose state. If the pipe then undergoes a small displacement or vibration, liquefaction of the backfill material may occur, and the resistance to upward movement of the pipe can be reduced. To explore this experimentally, an instrumented model pipe section was pulled vertically upwards at different rates in very loose, saturated, fine, uniform sand (representative of a North Sea sand). The instrumentation allowed for the measurement of the force on the pipe section as well as the excess pore water pressure regime around the pipe. The results show that, for sand at a relative density of zero, there is a reduction of capacity at the faster uplift rates. A simple analytical model, using the vertical slip model (for uplift resistance) modified to account for the development and dissipation of excess pore water pressures around the pipe, is used to predict the results from the experiments. Implications for the design of buried offshore pipelines in sand are discussed. INTRODUCTIONThis paper considers the problem of a short section of pipe that moves upwards in saturated loose fine sand. This problem is very important for understanding the likely performance of offshore pipelines buried in sandy or silty seabed soils. The pipe must be buried to a sufficient depth that any upward forces are resisted by the reaction of the soil, but not so deeply that the trenching operation becomes uneconomic. The pipe may experience a net upward force due to a very low combined pipe/fluid weight, the development of excess water pressures around the pipe, or a phenomenon known as upheaval buckling. Although it is recognised that buckling or flotation of an offshore pipeline is a three-dimensional problem, the present sudy is confined, as in many other studies, to the plane-strain problem of a short pipe section moving upwards through the soil. This allows the soil resistance to be measured accurately in laboratory tests, normalised against relevant parameters and used in pipeline design. The experiments reported here have been carried out in saturated, very loose fine sand, with a particular focus on the effect of uplift rate on soil resistance. A number of studies of pipes moving through dry (and hence fully drained) sand have been published, but little information is available in the public domain relating to pipes moving through saturated sand at different rates. The recent work by Bransby & Ireland (2009) highlighted that pipes in loose to medium dense saturated sand under faster rate loading experienced increased capacities, most likely due to a dilative sand response. The work presented below focuses on very loose sand, and shows substantial reductions in capacities for faster rate loading. As well as presenting novel experi...

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Section: Experimental Equipmentmentioning

confidence: 96%

“…At greater depths, where the confining stresses are higher, there will be a transition to a flow-around mechanism (Schupp et al, 2006;Byrne et al, 2008;Cheuk et al, 2008). The depth of transition depends on the density of the sand and the peak friction angle.…”

Section: Simple Model For Pipe Uplift Resistancementioning

confidence: 97%

Uplift of shallowly buried pipe sections in saturated very loose sand

Byrne¹,

Schupp²,

Martin³

et al. 2013

Géotechnique

View full text Add to dashboard Cite

show abstract

Uplift Resistance of Offshore Pipelines Subject to Upheaval Buckling

Ismail¹,

Najjar²,

Sadek³

et al. 2017

Sustainable Civil Infrastructures

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Pipe uplift in saturated sand: rate and density effects

Williams¹,

Byrne²,

Blakeborough³

2013

Géotechnique

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A key parameter for assessing the unburial behaviour of offshore pipelines, such as during upheaval buckling, is the soil uplift resistance. Despite much research on uplift response, two variables remain relatively unexplored: the effect of low relative density backfill and partially drained soil response. To address this shortcoming, an investigation into the plane-strain uplift resistance of pipe segments in saturated sand was carried out. The model tests, targeting drained and partially drained soil behaviour, demonstrate that soil state strongly influences the pipe–soil failure mechanism, and hence the mobilised uplift resistance. In very loose sand the normalised peak uplift resistance remained constant beyond a critical embedment depth, indicating a possible transition between different failure mechanisms. Faster rate tests result in positive excess pore pressures, which serve to further reduce the uplift resistance. Tests at varying uplift rates and densities identified a transition relative density between net contraction and dilation for a given embedment. These findings suggest that a minimum backfill density should be ensured prior to pipeline operation, to avoid the negative effects associated with contractive behaviour of loose soil.

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Experimental Modeling of the Unburial Behaviour of Pipelines

Cited by 3 publications

References 13 publications

Uplift of shallowly buried pipe sections in saturated very loose sand

Uplift of shallowly buried pipe sections in saturated very loose sand

Uplift Resistance of Offshore Pipelines Subject to Upheaval Buckling

Pipe uplift in saturated sand: rate and density effects

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