Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Robert B. Gilbert and Roy E. Olson. Vita. Includes bibliographical references.

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This thesis is concerned with the analysis of large offshore gravity structures used for oil exploration and recovery in the Beaufort Sea. Because of the high ice loads and the water depths involved, these structures comprise a large steel box infilled with a sand core for stability. One such structure was subjected to severe ice loading in April 1986 causing portions of the sand core to liquefy and bring the structure to a near failure condition. This structure was heavily monitored and thus serves as a case study against which the proposed analysis procedure can be checked. The behaviour of these soil-structure systems is highly complex depending upon the characteristics of the soil, the structural elements and the soil-structure interface. In this thesis a three-dimensional Finite Element computer program with soil, interface and structural elements is developed. Emphasis is placed on the three-dimensional stress-strain constitutive law both in terms of its ability to model observed laboratory response as well as the determination of the constitutive law parameters from in situ testing. The results obtained in terms of displacement, acceleration and zones of liquefaction by the analysis were then compared with the field measurements obtained during the April 1986 ice load event. The good agreement obtained between predicted and observed response is a validation of the proposed procedure. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

The problem of the dynamics of a flexible offshore structure in either a regular or random sea is considered in this thesis. A simple single degree of freedom model of the structure is assumed and the relative velocity formulation of the Morison equation is used to describe the fluid force. The resulting equation of motion is a nonlinear ordinary differential equation with either harmonic or stochastic forcing depending on the wave description. Solutions are obtained for regular deterministic waves by numerical integration, various linearization methods and a new perturbation method developed in this thesis. The numerical solution is used to assess the accuracy of each of the approximate solution methods. Of these, the perturbation method is found to give the best approximation to the numerical solution over the complete frequency range of interest. For random seas the response spectrum and the mean square response are obtained by various linearization methods, the method of equivalent linearization, and by the new perturbation method. The perturbation method and the method of equivalent linearization are very similar in that they both yield the same values of effective damping. Comparison of the results obtained by a numerical simulation method with the results of the perturbation method and the widely used method of equivalent linearization shows that the perturbation method gives a better estimate of the response mean square value than does the method of equivalent linearization. For all of the approximate solution methods that are discussed it was found that the use of Hermite polynomials to represent the solution is very effective in obtaining various expected values required in the computational procedure. In addition to the average response statistics, such as the response mean square value, the probability density of the response is also considered. It is well known that the response of a linear system to Gaussian forcing is itself Gaussian. The wave force given by the Morison equation is non-Gaussian and therefore the response is also non-Gaussian but of unknown form. The hypothesis that for a linear equation, the probability density of the response is of the same form as the probability density of forcing, even for the case of non-Gaussian forcing, is investigated and verified using the results of numerical simulations. Design considerations of interest which follow from the response probability density are also discussed. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

The objective of the work described in this thesis is to evaluate the significance of the ambient fluid on the motion of an ice mass in the vicinity of an offshore structure and during the subsequent impact mechanism. Models for iceberg drift are first reviewed. The changes in flow field around an ice mass drifting in a current near an offshore structure are investigated by potential flow theory. The proximity effects and current interactions are generalized by introducing the added mass and convective force coefficients for the ice mass. A two-dimensional numerical model based on the boundary element method is developed to calculate these coefficients over a range of separation distances up to the point of contact. A numerical model based on ice properties and geometry is developed to simulate the impact force acting on the structure. Both the 'contact-point' added masses estimated in this thesis and the traditionally assumed far-field added masses are used in the impact model separately. The results from the two cases are compared and the crucial roles played by the ambient fluid during impact are discussed. Finally, a number of related topics is proposed for further studies. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

A series of laboratory experiments was conducted to examine the oscillatory forces" and the wave runup on a vertical, circular, surface piercing, rigid cylinder in the presence of both waves and a colinear current. It was found that a current which ran opposite to the direction of wave propagation reduced the oscillatory force and the runup on the cylinder. The results for a current running in the same direction as the waves were more scattered, with some cases indicating an increase in force while others a decrease. The runup, however, increased in all cases. An innovative numerical technique which is currently under development was applied to this problem. The loads on the cylinder were obtained by a time stepping procedure in which the flow at each time step was calculated by an integral equation method based on Green's theorem. The general results of the numerical method agreed quite well with the experimental observations, within the constraints of some simplifying assumptions. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

A new technique for controlling the pitching motion of a floating platform is proposed in this study. The floating platform is assumed to be a simplified model of the columnar type rectangular platform supported by buoyant force from four cylindrical legs. The control arrangement consists of water jet streams immerging horizontally from two points located some distance apart on each leg to form a restoring couple. The water jet streams can be shifted t0 opposite horizontal positions or to the vertical downward ,positions according to control requirements. They are governed by angle control criterion as well as velocity control criterion. The goal is to attain the platform stability within a desirable range of angles about the equilibrium position. The mathematical model governing the motion of the floating platform consists of all pertinent forces along with a control variable. It is a second order nonlinear differential equation having no known exact solution. The state variable technique is employed to solve this equation numerically. The state transition equation is established and reduced to a sampled-data system. Two Fortran computer programs were written for the numerical process involved in the solution of this nonlinear equation. This theoretical study shows that the platform motion under investigation is controllable by the proposed technique. The study also shows that major concern of this technique is the high energy consumption that would be required to maintain the stability of the structure.

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Thesis (Ph.D.), Memorial University of Newfoundland, 1998. / Bibliography: leaves 230-240.