Small diameter pipelines are routinely used to transport oil and gas between offshore production plants and the mainland, or between remote subsea well-heads and a centralised production facility. The pipelines may be placed on the soil surface but it is more usual that they are placed into trenches, which are subsequently backfilled. For the buried pipelines a well established problem has been that of upheaval buckling. This occurs because the fluid is usually pumped through the pipes at elevated temperatures causing the pipeline to experience thermal expansion which, if restrained, leads to an increase in the axial stress in the pipeline possibly resulting in a buckling failure. A secondary phenomenon that has also been identified, particularly in loose silty sands and silts, involves floatation of pipelines through the backfill material, usually shortly after burial. At the University of Oxford a project sponsored by EPSRC and Technip Offshore UK Ltd has commenced to investigate in detail the buckling and floatation problems. The main aim of the research programme is to investigate three-dimensional effects on the buckling behaviour. The initial experiments involve the more typical plane strain pipeline unburial tests to explore the relationship between depth of cover, uplift rate, pipeline diameter and pullout resistance under drained and undrained conditions. The second and main phase of experiments involves inducing a buckle in a model pipeline under laboratory conditions and making observations of the pipe/soil response. This paper will describe the initial findings from the research including a) plane strain pipe unburial tests in loose dry sand, and, b) initial small scale three-dimensional buckling tests. The paper will then describe the proposed large scale three-dimensional testing programme that will be taking place during 2006 and 2007.
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.Analysis of the spray field development on a vertical surface during water spray-quenching using a flat spray nozzle Abstract -The aims of this study were (i) to conduct experimental spatially and time-resolved measurements of flow development on large heated surfaces during transient spray cooling operations, and (ii) to investigate and discuss the influence of spray cooling mechanisms such as bubble formation and the flow field development of the cooling fluid and how this affects heat transfer. Quartz plates were heated to above 500°Cand then sprayed with pressurised water subcooled to 80K. High speed images of the quench process were collected at a rate of 3000 Hz making it possible to track the movement of the quench front as the plate cools below the Leidenfrost temperature of the fluid at that location. Observations showed that the relative importance of droplet-surface interactions decreases once the Leidenfrost temperature is reached on the plate:It was found that once the water contacts the surface, a water pool develops rapidly which grows larger as the pool floods the heated surface. Comparisons between the spatial flow development and heat transfer on the plate are made in order to describe these interactions more accurately. This information not only provides crucial input into process simulations, but is also useful to develop theoretical models of fluid-solid interaction describing the wetting of a heated component due to water spraying.
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...
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 ...
This paper is concerned with the interaction of a buried pipeline and the surrounding soil during various unburial processes. This follows a three year research program at Oxford University investigating mechanisms of pipeline unburial including upheaval buckling and pipeline floatation. The results include 2D plane strain uplift tests where a section of pipe is pulled upwards through soil, with the resulting forces and displacements being measured. A loose fine uniform sand is the focus of the study as this is often found in the North Sea and also reflects the soil conditions after pipe installation such as by jet trenching. The soil was tested in a dry state to benchmark drained capacities and in a saturated state to explore rate dependent responses. Particle image velocimetry was used to identify failure mechanisms at different depths as the pipe is pulled from the soil. A second theme of larger scale experiments, using a slender length of pipe, was also carried out to identify 3D effects that might not be captured in the conventional 2D tests. One set of experiments was carried out to determine the axial buckling load of a pinned and buried strut. The tests explored responses in dry sand and saturated sand showing that buckling loads are a function of soil cover depth. Further work was carried out in a specially designed pipeline testing tank (8m long) to explore buckling and floatation responses of a long slender pipe. The experiments were instrumented so that pipe displacements along the length were acquired as well as loads applied to one end of the pipeline. Summary conclusions from the work so far are presented.
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