Browsing by Subject "sand"
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Item Examination of Granular Material Behavior in a Laminar-type Direct Simple Shear Device using Laboratory Validated Discrete Element Method Simulations(2013-08-29) Bernhardt, Michelle LeeSimple shear testing is used to study a number of practical geotechnical prob?lems including: soil conditions directly below a loaded surface, adjacent to a driven pile shaft, soils deposited on a slightly inclined slope, and most notably the response of soils subjected to earthquake-type loading. While each of these problems still have important questions to be answered, earthquakes and earthquake triggered geohaz?ards are the most complex and also pose the highest risk. An important aspect of assessing the risk associated with earthquakes is the need to accurately predict soil behavior. True field loading conditions involve multi?directional shearing and the rotation of principal planes and are much more complex than the triaxial laboratory testing methods and models often used to describe them. Simple shear testing allows for the in situ conditions to be replicated; however, several limitations of the device make data interpretation difficult. The inability to apply complementary shear stresses and the inability to measure the horizontal normal stresses results in non-uniform stresses across the boundaries, as well as an undefined stress state during shearing. This, in turn, requires assumptions to be made about the failure conditions before any state parameters can be determined. Even when only monotonic testing is conducted, there are still many important questions to be answered about the actual severity of the non-uniform stresses on the boundaries, as well as the internal stresses and the microscopic response of granular soils. Discrete element method (DEM) modeling has the advantage of being able to examine particle-to-particle interactions. Once validated with the measured labora?tory data, these models provide a vast quantity of information about the fundamental mechanisms underlying the observed complexity of the response of the soil mass as a whole. The goal of this research is to gain insight into the particle-to-particle interac?tions driving the overall response of granular samples subjected to multi-directional cyclic simple shear conditions. The main objectives of this proposed project are to (1) characterize the macroscopic response of metal ballotini representing idealized sand under simple shear loading conditions and (2) model the physical element tests using DEM simulations to gain insight into the microscopic response of the gran?ular material. Findings from this study showed that the DEM simulations could be successfully validated by laboratory data and that the overall trends observed agreed reasonably well with the experimental data from this study, as well as previ?ous studies by other researchers. Analyses showed that density not only influences shear strength of a sample, it also affects the angle of shearing resistance, the mag?nitude of principal stress rotation, the angle of non-co axiality, and the orientation of the principal fabrics for strains below those needed to reach critical state. Vertical effective stress was instead shown to have very little influence on these parameters. The initial fabric appears to play the largest role in the behavior of samples tested at different vertical stresses. The simulations also showed the non-coaxial behavior of the granular samples in terms of principal stress and strain rate orientations, as well as particle displacements. A number of other sensitivity studies were conducted to examine the influence of the model simplifications on the observed response. Several of these simplifications were shown to affect the shear strength obtained and should be included in future analyses.Item Flow assurance and multiphase pumping(2009-05-15) Nikhar, Hemant G.A robust understanding and planning of production enhancement and flow assurance is required as petroleum E&P activities are targeting deepwaters and long distances. Different flow assurance issues and their solutions are put together in this work. The use of multiphase pumps as a flow assurance solution is emphasized. Multiphase pumping aids flow assurance in different ways. However, the problem causing most concern is sand erosion. This work involved a detection-based sand monitoring method. Our objectives are to investigate the reliability of an acoustic sand detector and analyze the feasibility of gel injection as a method to mitigate sand erosion. Use of a sand detector coupled with twin-screw pumps is studied under varying flow conditions. The feasibility of gel injection to reduce slip and transport produced solids through twin-screw pump is investigated. A unique full-scale laboratory with multiphase pumps was utilized to carry out the experimental tests. The test results indicate that acoustic sand detection works in a narrow window around the calibration signature. An empirical correlation for predicting the twin-screw pump performance with viscous fluids was developed. It shows good agreement in the practical operational limits ? 50% to 100% speed. The results indicate that viscous gel injection should be an effective erosion mitigation approach as it reduces slip, the principle cause of erosive wear. To correlate the performance of viscous fluid injection to hydroabrasive wear, further experimental investigation is needed.