1642 research outputs found
Sort by
Investigation of scour onset under seabed pipelines with geometric irregularities
Get PDFBased on previous studies, it is well understood that the hydrodynamic pressure difference across the pipeline is responsible for initiation of piping and the onset of scour. However, the influence of geometric irregularities such as field joints on the onset of scour has not previously been investigated. This new study has been stimulated by field observations of subsea pipelines that have indicated that scour holes tend to occur at field joint locations. This paper summarizes a series of experimental results using The University of Western Australia’s (UWA)’s unique O-tube test facilities investigating onset of scour underneath scaled model pipelines with a field joint in steady currents. The results of this work are benchmarked against experimental tests and published results for prismatic / uniform pipes. It is demonstrated that periodic geometric irregularities such as field joints have a significant influence on the onset of scour underneath a pipeline.
KEY WORDS: Scour, piping, subsea pipeline, field joint, anode, STABLEpipe JIP
Supporting decision-making for channel conveyance maintenance
No full textChannel maintenance activities, understood as vegetation and sediment management, have a potential to change the conveyance capacity of a watercourse and thus, to provide a benefit from a flood risk perspective. Vegetation and its overgrowth is one of the main factors influencing flow capacity and hence river conveyance. Despite the undesirable consequences associated with flooding, it is rare that sufficient funds are available to undertake all necessary maintenance activities. In the face of limited budgets, flood risk managers must take difficult decisions in order to decide how and where limited resources should be invested. This paper presents a national scale approach to identify the strategically important watercourses where conveyance related works produce the greatest benefit. This is done by identifying watercourses where maintenance works may have a potential to positively maintain or increase conveyance capacity and watercourses where the attribution of area or receptors benefiting is important. Assembling together this information produces a picture of the strategic lengths of watercourse to be maintained
Relationship between soil erodibility and engineering properties
Get PDFDifferent soils exhibit different erodibility (sand, clay) therefore erodibility is tied to soil properties. On the other hand, many researchers have attempted to develop such equations without much success. One problem is that erodibility is not a single number but a relationship between the erosion rate and the water velocity or the hydraulic shear stress. This erosion function is a curve and it is difficult to correlate a curve to soil properties. The main purpose of this study is to develop correlations between the elements of the erosion function (critical velocity, critical shear stress, slope of the erosion function) with elementary soil properties (plasticity index, mean grain size, unit weight, shear strength, and others). There are many tests to evaluate the erodibility of the soil in laboratory and in situ and to identify the erosion function (i.e. Jet Erosion Test, Erosion Function Apparatus, Hole Erosion Tests, etc.). This study deals with developing correlating equations between erodibility parameters obtained from many years of testing experience with Erosion Function Apparatus (EFA) and geotechnical properties of the soil
Numerical modeling of particle migration in granular soils
Get PDFSuffusion is the process of internal erosion where fine particles migrate under water seepage through a coarser soil matrix. Relevant models of suffusive phenomena must reproduce the poromechanical effects that result from the two-way coupling between the deformation of the solid matrix, the fluid pressure and the flow. In this work, an advanced computational method is used to study the particle migration in granular soils. The so called coupled Discrete Element Method - Pore scale Finite Volume (DEM-PFV) is based on a microscopic hydromechanical approach. It couples the discrete element method that solves the equations of motion for the solid fraction, with a PFV method that solves the fluid flow equations. We use this method to study particle transport through coarser granular assemblies that do not evolve with time. These simulations allow us to obtain, for different cases, the parameters to include in a general advection-dispersion equation (ADE). We paid particular attention to the role played by the intermittent formation of blockages of transported particles in the constrictions of the granular assembly. These temporary and collective trapping events change local fluid flows and affect the particle transport on short time or length scales. As the transport time between consecutive blockages and the duration of blockages have exponential decays, sink and source terms can be added to the ADE
Characteristics of scour and flow field beneath a forced vibrating circular cylinder
Get PDFThe objective of the study is to enhance the understanding of scour under a steel catenary riser (SCR), in particular the influence of the SCR dynamic motions on scour. The motion of an SCR could be attributed to the motions of the attached floating platform induced by surface waves and/or currents. In this study, a circular cylinder mounted elastically on a forced vibrating rod was installed and three vibration frequencies were tested to simulate this phenomenon. Flow visualization and high time-resolved PIV measurement techniques were applied to observe and measure the flow field around the cylinder. The results show that there are two distinct types of scour processes, depending on the imposed vibration frequency even though they have the same amplitude. Based on the qualitative observation and quantitative measurement of the flow field and turbulence characteristics, the formation and transmission of the vortices are discussed in this paper
Using DEM to assess the influence of stress and fabric inhomogeneity and anisotropy on susceptibility to suffusion
Get PDFUnderfilled and gap-graded soils are known to be susceptible to suffusion; a form of internal instability in which the finer fraction of a soil is washed out from the coarser matrix under the action of seepage. This phenomenon poses a risk to embankment dams and flood embankments. The processes and mechanisms operate at the particle scale, and insight can be gained via the particulate discrete element method (DEM). Vir-tual samples can be created using DEM and simulation results can provide information on particle stresses, as well as quantitative information on the fabric of the particulate material. This is important as the amount of stress carried by the finer particles is thought to govern the susceptibility of a given material to suffusion. DEM modelling can also provide information on variation in properties within samples as well as the detailed data needed to quantify the material fabric. DEM models are, however, an idealization of reality and con-strained in particular by the number of particles used and sample preparation method. This study examines key issues relating to the development of virtual samples for use in DEM analysis and also examines the proportion of the applied stress that is carried by the finer particles
Time scales for scour below pipelines and around vertical piles due to nonlinear random waves
Get PDFThis paper provides a practical stochastic method by which the time scale for the equilibrium scour depth below pipelines and around slender vertical piles exposed to long-crested (2D) and short-crested (3D) nonlinear random waves can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall (2000) wave crest height distribution representing both 2D and 3D nonlinear random waves. Moreover, the time scale for regular waves given by Sumer and Fredsøe (2002) are used. Examples of results are also presented
Bed morphology changes at river contractions
Get PDFContraction scour is a significant problem of engineering concern which might occur when the stream encounters a reduction in flow area because of either natural constraints or hydraulic works. This paper is a part of ongoing investigations by the authors on the spatial and temporal variations in bed morphology at both short and long river contractions based on experimental investigations at laboratory scale. Experiments were carried out at University of Basilicata, Italy, in a 1 m wide and 20 m long rectangular channel. The working section was up to 16 m, depending on the length of the contraction model. Two nearly-uniform sediments were used as mobile bed, sand with median grain size d501.7 mm and gravel with d509.0 mm. The contraction length was either 0.5,1.0, 2.0 or 3.0 m. Runs were carried out under steady flow and clear-water approach flow conditions. Typically they were of long duration (up to 15 days) to achieve conditions of dynamic equilibrium. New results on the effects of contraction ratio, densimetric Froude number, and relative contraction length are given. Moreover, insights on the spatial and temporal changes in bed morphology are provided with emphasis on the thalweg profile at contracted sections and bed degradation processes downstream of the contracted area
Physical modelling of debris flow deposits in contiguous confluences
Get PDFDam formation in river channel generated by single and multiple debris flow injections from affluent basins are investigated in through flume experiments. The aim of the research is to understand the morphodynamics that governs the dam formation. A set of 14 experiments considering both single and double debris flow injections have been carried out, investigating different configurations in terms of debris flow discharge, confluence angle and water discharge along the main channel. In all cases, the debris flow consisted of a mixture of gravel and water, that reproduces stony-debris flow conditions. Results from single confluence experiments are taken as reference for better understanding the interactions between the different deposits when considering adjacent debris flow injections. The collected data are used to test the critical indexes for dam formation probability proposed in literature