Thesis. 1977. Ocean E.--Massachusetts Institute of Technology. Dept. of Ocean Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / Ocean E.

Complex nonlinear and chaotic responses have been recently observed in various compliant ocean systems. These systems are characterized by a nonlinear mooring restoring force and a coupled fluid-structure interaction exciting force. A general class of ocean mooring system models is formulated by incorporating a variable mooring configuration and the exact form of the hydrodynamic excitation. The multi-degree of freedom system, subjected to combined parametric and external excitation, is shown to be complex, coupled and strongly nonlinear. Stability analysis by a Liapunov function approach reveals global system attraction which ensures that solutions remain bounded for small excitation. Construction of the system's Poincare map and stability analysis of the map's fixed points correspond to system stability of near resonance periodic orbits. Investigation of nonresonant solutions is done by a local variational approach. Tangent and period doubling bifurcations are identified by both local stability analysis techniques and are further investigated to reveal global bifurcations. Application of Melnikov's method to the perturbed averaged system provides an approximate criterion for the existence of transverse homoclinic orbits resulting in chaotic system dynamics. Further stability analysis of the subharmonic and ultraharmonic solutions reveals a cascade of period doubling which is shown to evolve to a strange attractor. Investigation of the bifurcation criteria obtained reveals a steady state superstructure in the bifurcation set. This superstructure identifies a similar bifurcation pattern of coexisting solutions in the sub, ultra and ultrasubharmonic domains. Within this structure strange attractors appear when a period doubling sequence is infinite and when abrupt changes in the size of an attractor occur near tangent bifurcations. Parametric analysis of system instabilities reveals the influence of the convective inertial force which can not be neglected for large response and the bias induced by the quadratic viscous drag is found to be a controlling mechanism even for moderate sea states. Thus, stability analyses of a nonlinear ocean mooring system by semi-analytical methods reveal the existence of bifurcations identifying complex periodic and aperiodic nonlinear phenomena. The results obtained apply to a variety of nonlinear ocean mooring and towing system configurations. Extensions and applications of this research are discussed. / Graduation date: 1992

In this study, the semi-empirical approach is introduced to accurately estimate the probability distribution of complex non-linear random variables in the field of wavestructure interaction. The structural form of the semi-empirical distribution is developed based on a mathematical representation of the process and the model parameters are estimated directly from utilization of the sample data. Here, three probability distributions are developed based on the quadratic transformation of the linear random variable. Assuming that the linear process follows a standard Gaussian distribution, the three-parameter Gaussian-Stokes model is derived for the second-order variables. Similarly, the three-parameter Rayleigh-Stokes model and the four-parameter Weibull- Stokes model are derived for the crests, troughs, and heights of non-linear process assuming that the linear variable has a Rayleigh distribution or a Weibull distribution. The model parameters are empirically estimated with the application of the conventional method of moments and the newer method of L-moments. Furthermore, the application of semi-empirical models in extreme analysis and estimation of extreme statistics is discussed. As a main part of this research study, the sensitivity of the model statistics to the variability of the model parameters as well as the variability in the samples is evaluated. In addition, the sample size effects on the performance of parameter estimation methods are studied. Utilizing illustrative examples, the application of semi-empirical probability distributions in the estimation of probability distribution of non-linear random variables is studied. The examples focused on the probability distribution of: wave elevations and wave crests of ocean waves and waves in the area close to an offshore structure, wave run-up over the vertical columns of an offshore structure, and ocean wave power resources. In each example, the performance of the semi-empirical model is compared with appropriate theoretical and empirical distribution models. It is observed that the semi-empirical models are successful in capturing the probability distribution of complex non-linear variables. The semi-empirical models are more flexible than the theoretical models in capturing the probability distribution of data and the models are generally more robust than the commonly used empirical models.

published_or_final_version / Architecture / Master / Master of Landscape Architecture

This thesis deals with the plan of the Dutch government to build an offshore airport outside its territorial waters. Because the airport will be outside territorial waters several problems may arise. Under the Law of the Sea the question is whether such an airport can lawfully be built and what the different conditions are under which it is possible. The Convention on International Civil Aviation is older then the new Law of the Sea Convention and therefore not up to date with the new zones in the sea that have emerged. Air law therefore needs to be interpreted in the light of those new developments. / The first chapter deals with the reasons behind the plan to build such an airport. Thereafter, subsequent chapters discuss the law of the sea, air law, European law and the law of other organizations, which will have an influence on an offshore airport outside the territorial sea. The final chapter deals with plans and examples of other uses of artificial islands, including offshore airports.

[Truncated abstract] The demand for offshore wind turbines is increasing in densely populated areas, such as Europe. These constructions are typically founded on a gravity foundation or a large 'mono pile'. Gravity foundations can only be used at locations where strong soils exist and water depths are limited. Costs associated with a 'mono pile' type foundation contribute to a very large percentage of the total investment costs. This research, therefore, focuses upon a different foundation for offshore wind turbines, namely suction caissons beneath a tripod. This foundation can be used in all kinds of soil types and is cheaper than the 'mono pile' foundation, both in the amount of steel used and installation costs. Cheaper foundations can contribute to a more competitive price for offshore wind energy in comparison with other energy resources. To date, there have been relatively few studies to investigate the behaviour of this type of foundation during the installation process and during operational and ultimate loading for seabed conditions comprising dense sand. Two types of investigations were performed during this research to determine the behaviour of suction caissons beneath a tripod. Firstly, an existing computer program was extended to predict the typical loading conditions for a tripod foundation. Secondly, centrifuge tests on small scale suction caissons were performed to investigate the behaviour during the installation and loading phases. The computer program developed helped to quantify the likely ranges of environmental loading on an offshore wind turbine. For a typical 3 MW wind turbine of 90 m height, the vertical load is low at around 7 MN. During storm conditions the horizontal hydrodynamic load can be in the order of 4 MN. During normal working conditions the horizontal aerodynamic loads can reach 0.4 MN, but can increase to 1.2 MN when the pitch system malfunctions and gusts reach 30 m/s. This aerodynamic load will result in a very large contribution to the overturning moment, due to the high action point of this load. When the wind turbine is placed on top of a tripod, these large moments are counteracted by a push-pull system. ... The development of differential pressure was found to depend on the soil permeability, the extraction speed and a consolidation effect. During cyclic loading no obvious signs of a decrease in resistance were observed. During very fast cyclic loading differential pressures developed, which could increase the drained frictional resistance by approximately 40%. All centrifuge tests results were used to develop methods to predict or back calculate the installation process of suction caissons in sand and layered soil, and the behaviour during tensile and cyclic loading. These methods all use the cone resistance as the main input parameter and predict the force (or required suction) as a function of time, for a given rate of pumping or uplift displacement, in addition to the variation of suction with penetration (or force with uplift displacement). These new methods provide a useful tool in designing a reliable foundation for offshore wind turbines consisting of a tripod arrangement of suction caissons embedded in dense sand.

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

Risers em configuração de catenária podem apresentar nível de tração muito baixo próximos ao fundo do mar. Isso pode ocorrer em algumas condições de lançamento, em que sua estrutura pode se configurar de forma quase vertical. Quando se trata de tubos flexíveis ou cabos umbilicais, a composição interna do riser contém armaduras helicoidais de tração. Essas podem induzir a ocorrência de giro axial quando o tubo é solicitado à tração. Se esse movimento não for permitido, surgirá um momento de torção na estrutura. O baixo nível de tração da configuração de catenária combinado com o momento de torção surgido durante o lançamento do riser pode levar a uma forma de instabilidade estrutural que culmina na formação de um laço. Isso é indesejável uma vez que, se existe o laço, dependendo dos esforços submetidos à linha, é possível que o laço se transforme em uma dobra, danificando a estrutura. O presente trabalho analisa as condições de formação de laços em configurações de catenária. Para isso, foram utilizados critérios de estabilidade aplicados a um modelo de elementos finitos, que leva em conta as não linearidades geométrica e de contato entre o riser e o solo. Foi utilizada uma formulação cinematicamente exata de elemento de viga através de uma descrição lagrangiana atualizada, que pode tratar de forma correta as grandes rotações que são impostas ao riser para induzir o surgimento do momento de torção. É mostrado que uma expressão analítica baseada na Fórmula de Greenhill pode prever o fenômeno com boa concordância com os resultados numéricos, mesmo considerando-se fenômenos como contato unilateral com atrito e correntezas marítimas. Além disso, foi feita uma análise paramétrica para prever a formação do laço para diversas geometrias de catenária, procurando generalizar as conclusões obtidas. / Catenary risers can show a very low tension level close to the seabed. This can occur in some launching conditions, in which the structure can be almost vertical. When dealing with flexible pipes or umbilical cables, their internal composition includes the helical tension armors. These can induce an axial rotation to the riser when it is tensioned. If this movement is constrained, a torsion moment will act upon the structure. The low tension level present in the catenary configuration combined with the torsion moment that occurs during the riser launching can lead to a structural instability, resulting in a loop formation. This is undesirable since, if the loop is present, depending on the loads applied to the riser it is possible that it turns into a kink, causing damage. The present work analyzes the loop formation conditions in catenary riser. For that, stability criteria were applied to a finite element model that takes into account the geometric nonlinearities and also the contact between the riser and the seabed. A kinematically-exact beam formulation was adopted, dealing correctly with the large rotations that are imposed to the riser, in order to induce the torsion moment. It is showed that an analytical expression based on Greenhills formula can predict the phenomenon with a good agreement with numerical results, even including phenomena as frictional unilateral contact and sea currents. Furthermore, a parametric analysis was done to predict the loop formation to some catenary geometries, in order to generalize the obtained conclusions.

O estudo da dinâmica estrutural de risers oceânicos apresenta instigantes desafios aos pesquisadores da área da engenharia de estruturas, uma vez que os meios tradicionais de análises dinâmicas lineares nem sempre se ajustam às suas complexas particularidades. No atual estágio do desenvolvimento científico da área de engenharia de estruturas, a aplicação de técnicas de análise dinâmica não linear, dentro de determinadas hipóteses, mostra-se como uma das alternativas possíveis e viáveis à tradicional análise dinâmica linear. Com vistas a uma nova abordagem do problema, o presente trabalho adota uma metodologia de análise não linear dinâmica de risers oceânicos em configuração de lançamento de catenária, conjugada a uma técnica de processamento de Modelos de Ordem Reduzida para o estudo dos fenômenos dinâmicos manifestados por risers. Trata-se de um método de modelagem local, restrito à região de contato unilateral do riser com o solo, considerado este último um meio viscoelástico. Os resultados da aplicação desta metodologia são demonstrados nos estudos de caso apresentados com comparações com modelos numéricos (Método dos Elementos Finitos) e modelos físicos. / The dynamic study of offshore risers still demands large efforts from structural engineering researchers, since these systems may behave in a way that is not well modeled and understood using simply linear dynamic theories. Nevertheless, the current development stage of non linear dynamic theories gives hope that their use for the analyses of such systems can be of great value, even though, this must be carefully done specially by the analyst. The present work refers to a non linear dynamic methodology application to offshore risers, particularly steel catenary risers, by a technique known as reduced-order modeling, in the study of dynamic phenomena that these structures may present. The model is local, which means that it represents the touch-down zone of the riser-soil system. The soil modeling was presumed to be viscoelastic. The results obtained in case studies are compared with those from numerical (Finite Element Method) and small scale physical models.

Orientador: Celso Kazuyuki Morooka / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica e Instituto de Geociencias / Made available in DSpace on 2018-08-13T09:48:02Z (GMT). No. of bitstreams: 1 Tsukada_RaphaelIssamu_M.pdf: 4706299 bytes, checksum: b217ddb6687b4f89959018a445ff1dee (MD5) Previous issue date: 2009 / Resumo: O presente trabalho tem como objetivo principal caracterizar o comportamento dinâmico de risers em configuração catenária (SCR) sob os efeitos do fenômeno da vibração induzida por vórtices (VIV). Estes estudos foram realizados através de experimentos e simulações numéricas. O experimento foi realizado com modelo de SCR em escala reduzida em um tanque de provas sob várias condições simuladas do ambiente, variando a velocidade do carro dinamométrico, a amplitude dos movimentos impostos no topo do modelo, amplitudes e períodos de ondas. As simulações numéricas foram realizadas usando um modelo de VIV criado para a avaliação de risers rígidos verticais. Modificações para o modelo foram implementadas de forma a considerar a inclinação do riser em relação ao escoamento e os movimentos apresentados pela SCR. Os resultados experimentais e numéricos apresentaram algumas características de comportamento semelhantes aos dos risers utilizados para grandes lâminas da água, verificados por outros pesquisadores, tais como: os altos harmônicos da freqüência de desprendimento de vórtices e o comportamento misto de standing e travelling waves. Uma boa concordância foi observada comparação entre os resultados experimentais e numéricos. / Abstract: The primary objective of this work is to characterize the dynamic behavior of steel catenary risers (SCR) under the effects of the Vortex-Induced Vibration (VIV) phenomenon. This study was conducted applying both experimental and numerical approaches. A SCR model test was performed in a towing tank under several simulated environment condition combinations such as; varying the towing speed, riser top forced oscillation amplitudes and surface wave characteristics. Numerical simulations were computed using an empirical VIV model created for the evaluation of top tensioned vertical rigid risers. Modifications to the model were implemented to allow consideration for the inclination of the riser relative to the fluid flow and the movement of the SCR. The experimental e numerical results present some dynamic behavior characteristics similar to that of deep sea risers found by others researcher mainly high harmonics of the vortex shedding frequency and a mix of standing and traveling wave behavior. Results from the comparison of experimental and numerical results have shown a good agreement. / Mestrado / Explotação / Mestre em Ciências e Engenharia de Petróleo