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.

No description available.

Thesis (M. S.)--Texas A&M University, 2004. / Vita. Abstract. "Major Subject: Petroleum Engineering" Includes bibliographical references.

Safety leadership is emerging as a key factor in determining organisational safety performance at all levels of management (Zohar, 2002; 2004). This PhD addresses the relevance and challenges of conducting safety interventions in the workplace. It started out as an evaluation of a safety leadership course in the oil and gas industry, and moved on to conceptualise the underlying difficulties inherent in those conversations and the success factors that help supervisors and managers overcome these challenges. Study One and Study Two focused on attitudes and attitude changes in course participants with increasing focus on attitudes towards safety interventions. Utilising questionnaires designed according to the theory of planned behaviour (Ajzen, 1985) and Bandura’s concept of selfefficacy and analysing qualitative data, the studies showed that participants’ general safety attitudes, attitudes towards rules and procedures, control beliefs, intentions to perform safety interventions, general self-efficacy and self-efficacy to perform safety interventions would increase from before to after the course. Study Three and Study Four aimed to evaluate participants’ behavioural changes with regard to performing safety interventions. A behavioural rating tool and statistical analysis were utilised in the third study. Results obtained showed a skill gap in managers’ and supervisors’ ability to perform safety interventions 6-12 months after they had attended the course. This skill gap indicated that – despite acknowledgement of the importance of safety interventions and participants’ intentions to frequently perform safety interventions – people did not perform these conversations at the worksite as often as they had intended. Results also indicated that two particular communication strategies, the use of open ended questions and the creation of ‘what-if’ scenarios, were crucial for a positive safety conversation outcome. In the fourth study, discourse analysis techniques and the application of a derived framework on social interaction allowed for a further understanding of the success factors and challenges of safety interventions. Results obtained emphasised particular face keeping strategies that were associated with the successful performance of safety interventions. However, strategies which, once applied, would lead to the failure of a conversation could also be extracted. It could also be shown that the conversation ‘scheme’ that had been taught during the training course was not fit for purpose as it did not enable participants to successfully conduct safety interventions without upsetting their conversation partner.

Ce travail de thèse s’articule autour de la problématique de l’évolution de la première fréquence propre d’une éolienne en mer au cours de sa durée d’exploitation. Ce type de structure élancée est soumis à des chargements cycliques et dynamiques tels que le vent, les vagues, les courants marins, la rotation des pales. Ces chargements ont des fréquences de sollicitation proches de la première fréquence propre de l’éolienne, celle-ci s’inscrivant dans un intervalle restreint entre la fréquence du rotor et celle des pales. Afin d’éviter tout phénomène de résonance, il convient d’évaluer précisément à la fois la première fréquence propre de l’éolienne juste après son installation ainsi que son évolution au cours de son exploitation. Ces deux problématiques ont été considérées dans cette étude.Dans ce contexte, un travail expérimental a été réalisé autour de deux modèles physiques 1g d’éolienne en mer fondée sur monopieu et installée dans un massif de sable de Fontainebleau. En se basant sur une modélisation de la fondation à l’aide d’un ensemble de ressorts, latéral et de torsion, une série d’essais a été réalisée afin d’évaluer à la fois la raideur de ces ressorts, mais aussi la fréquence propre du modèle physique considéré. Cette méthode a été comparée à des méthodes existantes et a aussi permis le développement de méthodes originales. Réaliser les essais sur deux modèles à échelle différente (1/60 et 1/120) a permis d’adapter les résultats obtenus aux cas d’une éolienne à échelle 1. Enfin, dans le cadre de l’étude de l’évolution de la structure au cours de son exploitation, des essais de sollicitations cycliques ont été réalisés. Une étude paramétrique se concentrant sur l’influence de la fréquence de la sollicitation et de la force globale appliquée a été proposée. Les résultats obtenus permettent d’analyser le comportement des modèles réduits, soumis à un maximum d’un million de cycles, en se concentrant sur l’évolution du déplacement, de la rotation et de la fréquence propre de ces structures. Ces résultats sont comparés aux limites imposées pour l’état limite de service d’une éolienne grandeur réelle / This PhD thesis deals with the main issue which is the evolution of the first natural frequency of an offshore wind turbine. These slender structures are submitted to cyclic and dynamic loads such as wind, waves, currents, and the blade rotations. The frequencies related to these loads are close to the first natural frequency of the turbine, which lay in a narrow interval between the frequencies of the rotor and the blades. In order to avoid any resonance phenomenon, one needs a precise evaluation of not only the natural frequency of the wind turbine after its installation but also its evolution during the operation of the turbine. These two issues are considered in this work.In this context, an experimental work was developed considering two 1g physical models of an offshore wind turbine with a monopile foundation installed in Fontainebleau sand. Based on the modelling of the foundation as a set of lateral and rotational springs, an experimental program was developed in order to evaluate the stiffness of these springs and the natural frequency of the scaled models. This method was first compared to the existing methods and has allowed to develop some original methods to evaluate the natural frequency of the considered scaled model. The tests, conducted on two physical models with different scales (1/60 and 1/120), also allowed us to adapt the obtained results to a real offshore wind turbine. As a part of the study of the turbine’s evolution during its operation, cyclic load tests were conducted. A parametric study is proposed with a focus on the influence of the load’s frequency and its amplitude. The obtained results allowed us to analyze the behavior of the scaled models submitted up to one million cycles considering, mainly, the evolution of the displacement, the rotation, and the natural frequency of the structures. These results were then compared to the limits imposed by the serviceability limit state of a real offshore wind turbine

A principal motivação da seguinte pesquisa foi conseguir estabelecer um ganho referente ao custo total da programacão de atendimento das plataformas de petróleo, partindo da premissa de cooperação dos navios de diferentes portos. Por tanto nesta pesquisa será estudado o problema de roteirização periódica na entrega de suprimentos a plataformas de petróleo, com a consideração de compartilhamento de recursos entre os diferentes portos. Isto significa que uma ou mais embarcações poderão ser deslocadas entre os portos, sendo aproveitadas nos dois portos. Esta forma de operação se contrapõe a ter uma frota dedicada em cada porto, sem a possibilidade de compartilhamento. A pesquisa se propõe a construir modelos matemáticos de programação linear inteira, e resolvê-los por meio de pacotes computacionais. Este problema foi inicialmente estudado como sendo uma extensão do problema de localização e roteirização periódica, que também é pouco explorado na literatura. No caso da distribuição física urbana, o compartilhamento de recursos consiste em possibilitar que a rota de um veículo inicie em um depósito, e termine em um depósito diferente do qual iniciou. Para o caso da distribuição urbana, também serão apresentados 3 modelos matemáticos. Instâncias extraídas e adaptadas da literatura serão testadas para mostrar a aplicabilidade dos modelos. / The main motivation of the following research was to establish a gain related to the total cost of scheduling service of oil platforms, based on the premise of cooperation of ships from different ports. Therefore In this research the periodic routing problem will be studied in the context of delivering supplies to oil platforms at the sea. A novel aspect will be considered which is the sharing of resources (supply vessels) among the ports. This means that one or more vessels can be transferred between ports, being used by all ports. This modus operandi contrasts with a dedicated fleet operating at each port, without the possibility of being shared. The research is focused on proposing integer linear mathematical models, and have them solved with commercial optimization codes. The problem was initially considered as an extension of the periodic location routing problem, which also has been seldom studied in the existing literature. In the case of the urban physical distribution, the resource sharing aspect consists in allowing a distribution route to end at a different depot, other than the one where the vehicle started. For the urban physical distribution, it will be proposed 3 mathematical models. Instances extracted and adapted from the literature will be tested to demonstrate the models\' applicability.

No description available.