Thesis--University of Finland, 1939. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M. Sc.)--University of Hong Kong, 1960. / Mimeographed. Includes bibliographical references (p. 71-74).

No description available.

A three-dimensional numerical model was developed to simulate the stability of wellbores and perforations in poorly consolidated sandstone formations. The model integrates the post-yield plastic behavior of granular materials in order to investigate the mechanical instabilities associated with wellbores completed in such formations. Fluid flow and poroelastic stresses are computationally coupled with mechanical calculations to generate pore pressure and stress distribution in the sand. The sand erosion model developed by Kim (2010) is adopted to predict the rate of sand production based on the proposed erosion criterion. It has been widely reported in the literature that sanding can be greatly influenced by in-situ stress anisotropy, completion geometry, wellbore placement, and perforation orientation. Through advanced modeling and meshing techniques, the model developed in this thesis is capable of simulating complex completion configurations and operational conditions for the purpose of researching the impact of these factors on the wellbore and perforation stability. Accordingly, the model can be utilized to design a completion that minimizes sand production and optimizes the mechanical stability of the wellbore for a specific in-situ state of stress. Results obtained from the model show that vertical wellbores produce less sand compared to horizontal wellbores in the case where the overburden stress is the maximum in-situ stress. In addition, orienting the perforation in the direction with the least plastic strain development results in a more stable perforation tunnel with less sand production. Therefore, in a horizontal wellbore, vertically oriented perforations are more stable than horizontally oriented perforations and can withstand higher drawdown pressure before sand is produced. The model was extended to simulate the impact of mechanical and hydraulic interference from adjacent perforations on the evolution of plastic strain. Results from simulation runs show that the perforation spacing has an influence on both the magnitude and the spatial spread of the plastic strain. The model combines the effect of the wellbore diameter, shot density, and the phasing angle to determine the completion configuration with the least sanding risk. / text

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Not available / text

a b s t r a c t A periodic steady state is a familiar phenomenon in many areas of theoretical biology and provides a satisfying explanation for those animal communities in which populations are observed to oscillate in a reproducible periodic manner. In this paper we explore models of three competing species described by symmetric and asymmetric May–Leonard models, and specifically investigate criteria for the existence of periodic steady states for an adapted May–Leonard model: x˙ = r(1 − x − ˛y − ˇz)x y˙ = (1 − ˇx − y − ˛z)y z˙ = (1 − ˛x − ˇy − z)z. Using the Routh–Hurwitz conditions, six inequalities that ensure the stability of the system are identified. These inequalities are solved simultaneously, using numerical methods in order to generate three-dimensional phase portraits to illustrate the steady states. Then the “stability boundary” is defined as the almost linear boundary between stability and instability. All the mathematics discussed is suitable for advanced undergraduate mathematics or applied mathematics students, offering them the opportunity to incorporate a computer algebra system such as Mathematica, DERIVE or Matlab in their investigations. The adapted May–Leonard model provides a practical application of steady states, stability and possible limit cycles of a nonlinear system.

No description available.

The work outlined in this thesis describes how rheological techniques can be used to gain insight into the behaviour of complex hydrating systems. These techniques are currently used to elucidate interparticle interactions of concentrated colloidal dispersions. Those used here were stress relaxation, pulse shearometry and oscillation. These enabled parameters such as the dynamic moduli, relaxation moduli, relaxation spectrum and limiting moduli to be obtained. These all give a measure of the strength of interaction of the colloidal system under different conditions, without reducing these interactions by inducing flow. Calorimetric data was obtained for a hydrating cement paste. The particle and floc size of this cement was also measured. The data were found to be in accord with one another, and with current theories of the hydration processes of cement. In combination with the rheological parameters measured this enabled the extent and strength of attraction between the particles of a cement paste to be determined, as a function of the age of the paste. The adsorption characteristics of surfactant on cement were measured, as was the effect of surfactant on the particle and floc size of hydrating cement. Coupled with a measure of the rheological parameters of the cement-surfactant system, an understanding of the effect of surfactant on a hydrating cement paste was obtained. Similarly the effect of the addition of styrene-butadiene polymer latex particles, with and without excess added surfactant, was able to be determined. This information was obtained for hydrating cement pastes, both in the presence and absence of added polymer, at both ambient and non-ambient temperatures.

The interaction between floor slabs and the vertical elements in a tall building structure results in a more efficient lateral load resisting system. The study examines the structural behaviour of the floor slab in the following situations: (i) Slab coupling a pair of shear walls in a cross-wall structure under cantilever bending action. (ii) Slab in composite action with a lintel beam connecting a pair of shear walls. (iii) Slab restraining a centre core against torsional warping. (iv) Slab coupling a centre core to a peripheral framed tube in a hull-core structure under cantilever bending action. (v) Slab connecting a row of columns in a flat-plate frame structure under lateral loading. The resistance of the floor slab against the deformation of the vertical elements, and the actions induced in the slab in the various situations are determined by finite element and influence coefficient techniques. The finite element technique idealises the slab-wall structure as an assembly of plate and shell elements and uses an established stiffness analysis to determine the displacements, forces and stress resultants in the slab. The influence coefficient technique approximates the interaction forces between the slab and walls as a system of discrete nodal forces which are determined from the solution of compatibility equations written in terms of plate influence coefficients and prescribed wall displacements. The results of parameter studies are presented in a series of design curves and tables to enable, the effective slab stiffness and slab actions to be readily determined for a wide range of structural configurations. Experimental tests on small-scale shear wall-slab models are carried out to substantiate the general accuracy of the theoretical methods of analysis.