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The purpose of this thesis is the development of an integrated system for the identification and optimization of the performance of Moored Floating Breakwaters (MFB). This optimization is achieved, during their operation, through the modification of the total upstretched length of the mooring lines and by taking as inputs the accelerations of the MFB and the current wave frequency. The feasibility of developing the system is demonstrated numerically through the modelling of the performance of the MFB, which is based on the numerical models. The first model is related to 3D hydrodynamic analysis of the floating body, while the second one includes the static-dynamic analysis of the mooring lines. The stiffness and the drag damping of the mooring lines are calculated according to equations that are derived in this thesis. The results demonstrate that the modification of the length of the mooring lines with a simultaneous modification of the draft of the floating body results to the change of the characteristics of the mooring lines (stiffness-drag damping) and, therefore, it results to the modification of the hydrodynamic behaviour and the effectiveness of the MFB. The proposed system consists of two components fully connected to each other. The first component is related to the identification of the current status of the MFB and is implement numerically through the application of Artificial Neural Networks. The physical problem regarding the status identification of the MFB is expressed as a pattern classification problem. The second component includes the calculation of the new optimum status of the MFB and is implement numerically through the development of an optimization algorithm. The application of the proposed system enables the formation of design curves for the optimization of the hydrodynamic behaviour and the effectiveness of the MFB.

Στόχος της διατριβής είναι η ανάπτυξη ενός ολοκληρωμένου συστήματος αναγνώρισης και βέλτιστης ρύθμισης της συμπεριφοράς και της αποτελεσματικότητας των Αγκυρωμένων Πλωτών Κυματοθραυστών (ΑΠΚ). Η βελτιστοποίηση αυτή επιτυγχάνεται κατά τη διάρκεια της λειτουργίας τους μέσω μεταβολής του ολικού διατεταμένου μήκους των καλωδιώσεων αγκύρωσης και λαμβάνοντας ως δεδομένα τις επιταχύνσεις του ΑΠΚ και την τρέχουσα κυματική συχνότητα. Η δυνατότητα ανάπτυξης του συστήματος καταδεικνύεται αριθμητικά μέσω μοντελοποίησης της συμπεριφοράς του ΑΠΚ, η οποία στηρίζεται στη συζευγμένη εφαρμογή (ανάπτυξη επαναληπτικής διαδικασίας σύζευξης) δύο αριθμητικών μοντέλων. Το πρώτο μοντέλο σχετίζεται με την 3D υδροδυναμική ανάλυση του πλωτού σώματος, ενώ το δεύτερο με τη στατική-δυναμική ανάλυση των καλωδιώσεων. Η δυσκαμψία και η συρτική απόσβεση των καλωδιώσεων υπολογίζονται κατόπιν εξαγωγής εξισώσεων. Τα αποτελέσματα καταδεικνύουν ότι η τροποποίηση του μήκους των καλωδιώσεων με ταυτόχρονη μεταβολή του βυθίσματος, επιφέρει μεταβολή των χαρακτηριστικών των καλωδιώσεων (δυσκαμψία-συρτική απόσβεση) και κατ’ επέκταση της συμπεριφοράς και της αποτελεσματικότητας του ΑΠΚ. Το προτεινόμενο σύστημα αποτελείται…

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The main objective of this research is the development of an innovative system for sophisticated maintenance optimization over the design life for a moored array of floating breakwaters connected with hinges. It needs to be emphasized that the numerical model corresponding to the moored array of floating breakwaters connected with hinges is also innovative and its development consists one of the scopes of the present doctoral thesis. In more details, the aforesaid numerical model includes the coupled modeling of the hydrodynamic behavior of the floating array as well as the mooring system. This modeling has been achieved with the derivation of appropriate equations, which were next included in software. As far as the system proposed herein is concerned, it ensures that the structural integrity and also the risk of the moored array of hinged floating breakwaters, is always within the desirable ranges over its design life. The most important issue, that should be emphasized, is that ensuring the desirable risk can be achieved with the minimum number of possible maintenance interventions (replacements of mooring chains), and thus minimum maintenance cost. The effect of structural aging with time due to corrosion is considered, and thus, risk will be assumed as time-evolved. The risk assessment is achieved by considering both the loading level and the maintenance history of the floating array. The methodology for risk assessment provides the maintenance schedule with the potential of continuous updating based on real-time structural health monitoring data. The current widespread development and use of monitoring devices (sensors) of all kinds strongly encourages the application of such innovative techniques, which are regarded as a current research challenge. It needs to be emphasized that the development and operation of the proposed system does not necessarily require the existence of monitoring data. In more detail, the potential of continuous updating of the risk prediction consists just an extra tool for the cases where monitoring data are available, providing a relevant prediction of higher quality.

Ο κύριος στόχος της παρούσας διατριβής είναι η ανάπτυξη καινοτόμου συστήματος καθορισμού του βέλτιστου σχήματος συντήρησης, καθ’ όλη τη σχεδιασμένη διάρκεια λειτουργίας (χρόνος σχεδιασμού - design life), για την περίπτωση αγκυρωμένης συστοιχίας αρθρωτά συνδεδεμένων πλωτών κυματοθραυστών. Στο σημείο αυτό, κρίνεται σκόπιμο να τονιστεί ότι το αριθμητικό μοντέλο αγκυρωμένης συστοιχίας αρθρωτά συνδεδεμένων πλωτών κυματοθραυστών είναι πρωτότυπο και αναπτύχθηκε στα πλαίσια της παρούσας διατριβής. Αναλυτικότερα, για την ορθή μοντελοποίηση της αγκυρωμένης συστοιχίας αρθρωτά συνδεδεμένων πλωτών κυματοθραυστών, πραγματοποιήθηκε η ταυτόχρονη μοντελοποίηση της συστοιχίας καθώς και του συστήματος αγκύρωσής της. Η μοντελοποίηση αυτή υλοποιήθηκε με την κατάστρωση κατάλληλων εξισώσεων, οι οποίες στη συνέχεια ενσωματώθηκαν σε κώδικα υπολογισμού Η/Υ. Όσον αφορά το προτεινόμενο σύστημα, αυτό διασφαλίζει ότι η δομική ακεραιότητα, και…

▼  Nowadays, the Norwegian Public Roads Administration plans to replace the ferry connections along the western coast of Norway with fixed road connections through the coastal highway E39 project. Due to the necessity of bridging long distances with considerable depths, non-conventional engineering solutions are being developed. To date, the first fjord planned to be cross is Bjørnafjord, located just in the middle of Trondheim and Kristiansand. In order to bridge the fjord distance, three innovative solutions have been developed: a submerged floating tube bridge, a floating pontoon bridge, and finally, a multi-span suspension bridge. It is known that these structures are affected by periodic environmental forces which may cause significant fatigue problems in some structural components, disturbing their service behaviour. The aim of this master thesis is to deeply analyse the cumulative fatigue damage produced in the mooring lines of a proposed solution for crossing Bjørnafjorden as well as to investigate the influence of the implementation of different mooring line scenarios on the damage. A 3D finite element model of the chosen alternative was conducted and verified. The model provided the stress variations in each mooring line which were processed through Excel software using macros performed as well as Matlab scripts in order to obtain the results. Then, a deep fatigue damage assessment was carried out discussing how different tension-history time period calculations affect to the damage, as well as which fatigue evaluation methodology is adequate for the chosen solution. Moreover, the fatigue damage produced on the initial mooring line system configuration was analysed. It was investigated the repercussions of each direction and the contribution of each environmental load on the total damage. Last but not least, it was researched the influence of different water depths and line configurations on the fatigue damage. The results show that the fatigue damage obtained for the adopted solution remains far from the failure threshold with the worst damage produced in the mooring line chain components. The previous results have been computed employing a 1-hour analysis duration through the rainflow counting method. Also, the worst fatigue damage is produced by environmental states propagating in a westerly direction, corresponding to the maximum fetch. Regarding the load contribution in the fatigue damage, it is proved in this case that the current loads stabilise the mooring lines, reducing the fatigue damage. Finally, investigating the effect of various mooring line scenarios on the damage it has been proved that the fatigue damage is increased as the water depth is decreased and as the lateral mooring lines are separated from the initial configuration.

▼ A two-dimensional, nonlinear dynamic analysis is conducted on a moored breakwater configuration to investigate snap loads in mooring lines. Breakwaters are structures used to attenuate or eliminate waves and protect shorelines, harbors, and other natural and man-made marine structures from wave damage. The breakwater in this investigation is modeled both as a point mass and as a rigid body. Both models are subjected to free undamped motions and forced undamped wave motion. Energy is dissipated through the use of a coefficient of restitution applied when a mooring line becomes taut (i.e., reaches its natural length). The mooring line is modeled as an inextensible cable with no axial or bending resistance when slack. Snap loading arises when a mooring line transitions suddenly from a slack condition to a taut condition. The analysis was conducted on a breakwater configured upside down and hanging by two mooring lines. The length of the mooring lines, coefficient of restitution, size and shape of the breakwater, initial position of the breakwater, amplitude of wave forcing, ratio of vertical to horizontal forcing, and frequency of forcing were all varied in the analysis. The results show that the rotations of the rigid body and the wave forcing have a significant role in the analysis, indicating that a rigid-body model for a moored breakwater under wave forcing is the more accurate model. Advisors/Committee Members: Plaut, Raymond H. (committeechair), Rojiani, Kamal B. (committee member), Holzer, Siegfried M. (committee member).

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A method is developed to assess the dynamic stability of a floating tidal energy platform during ballast operations. This method is applied to a tidal energy platform designed by Bluewater Energy Services B.V., showing the sensitivity of this platform’s dynamic stability during ballast operations to tidal current velocity, changes in lightship centre of gravity and changes in the rate of ballasting. The developed method includes dynamic effects, tidal currents and mooring lines. Part of the developed method is a numeric model, written in the MATLAB environment, that simulates the platform motions, in 6 DOF, in time, during ballast operations. The model allows for large amplitude translations and rotations and for a changing mass and waterline area in time. Gravity, buoyancy, mooring line forces and tidal current lift and drag forces are taken into account. The model is analytically verified.

Ship Hydromechanics

Marine Technology

Mechanical, Maritime and Materials Engineering

Advisors/Committee Members: Huijsmans, R.H.M. (mentor), Palm, M. (mentor), De Koning Gans, H.J. (mentor), Keetels, G.H. (mentor).

▼ The Constitution Truss Spar, operated by Anadarko Petroleum Corporation (APC), is located in Green Canyon Block 679 and 680 in a water depth of 1,500 m. It was installed in October of 2006 and has since weathered multiple hurricanes and other storms. The platform is equipped with an Environmental Platform Response Monitoring System (EPRMS) which records real-time motions, environmental parameters and loads. These measurements were used to hind-cast the platform mooring tensions and estimate fatigue damage index accrued over the short life (install to start of study, July 2010) of the platform. The study found that extreme events such as Hurricane Ike (~100 yr storm) accounted for considerably higher fatigue damage index than the total caused by other small storms likely to occur in the 20 year service life of the vessel. It is therefore a recommendation of this study that a design criterion for fatigue damage accrued during extreme events such as 100 yr hurricanes be considered in the design of station keeping systems in a similar manner to the guidelines found in API RP 2T (2010) for design of tension leg platforms. Advisors/Committee Members: Zhang, Jun (advisor), Mercier, Richard (committee member), DiMarco, Steven (committee member).

▼ The design of a mooring system for a floating structure is a significant challenge; the choice of line structure and layout determine highly non-linear hydrodynamic behaviors that, in turn, influence the dynamics of the whole system. The difficulty is particularly acute for Self-Reacting Point Absorber Wave Energy Converters (SRPA WEC) as these machines rely on their movements to extract useful power from wave motions and the mooring must constrain the SRPA WEC motion without detracting from power production. In this thesis this topic has been addressed in an innovative way and new ideas on how these devices should be moored were investigated. As part of the study, an optimization routine was implemented to investigate the optimal mooring design and its characteristics. In this process, different challenges were faced. To evaluate the different mooring configurations, a high fidelity representation of the system hydrodynamics is necessary which captures the non-linearities of the system. Unfortunately, high-fidelity modeling tends to be very computationally expensive, and for this reason previous studies based mooring design largely relies on simplified representations that only reflect part of the mooring design space since some physical and hydrodynamic properties are dropped. In this work, we present how a full hydrodynamic time domain simulation can be utilized within a Metamodel-Based Optimization to better evaluate a wider range of mooring configurations spanning the breadth of the full design space. The method uses a Metamodel, defined in terms of the mooring physical parameters, to cover the majority of the optimization process a high fidelity model is used to establish the Metamodel in a pre-processing stage. The method was applied to a case study of a two-body heaving SRPA WEC. Survivability constrains where introduce into the model using a new statistical approach which reduces the execution time, and allowed the optimization routine. The analysis results lead to the conclusion that for SRPA WEC the mooring loads have a significant impact on how the body reacts with the waves, affecting both the energy that enter the system as well as the energy that is extracted as power. This implies that, in some cases, the mooring lines need to be considered in early stages of the designs as opposed to an afterthought, as is typically done. Results indicate that an optimal mooring design can result in a 26% increase in total annual power production. In addition, the mooring lines impact on mitigating parasitic pitch and roll were analyzed. It was established that in regular waves, the mooring lines can reduce the parametric excitations and improve the power extraction up to 56% for a particular sea state. By applying a computationally efficient iterative design approach to a device's mooring, parasitic motions and suboptimal device operation can be reduced, ultimately making WECs a more competitive source of energy. Advisors/Committee Members: Crawford, Curran (supervisor), Buckham, Bradley Jason (supervisor).

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O desenvolvimento de pesquisas relativas ao comportamento das estruturas offshore são de fundamental importância para as atividades de exploração de petróleo a fim de permitir uma análise mais precisa do desempenho do sistema de ancoragem. Neste sentido, o presente trabalho tem como objetivo analisar o comportamento das linhas de ancoragem submetidas a esforços de tração bem como avaliar e quantificar a atenuação das cargas ao longo do trecho enterrado determinando assim as forças que efetivamente chegam à estaca torpedo. Inicialmente foram projetados e construídos os equipamentos, em modelo reduzido, para a realização dos ensaios experimentais de carregamento estático conduzidos na escala de 1:40. As argilas empregadas nos ensaios foram produzidas em laboratório com o objetivo de obter um solo com propriedades físicas similares à da argila típica do leito marinho, a partir da mistura caulim, bentonita e água, sendo definida a composição em massa seca de 85% de caulim, 15% de bentonita e teores de 100% de umidade para a mistura denominada como argila A, e 120% de umidade para a mistura referida como argila B. Após a caracterização geotécnica concluiu-se que a argila B se assemelha às condições de campo. A força que chega ao ponto de ancoragem, equivalente ao que seria o ponto de fixação da amarra ao torpedo, foi medida experimentalmente através de uma célula de carga para duas componentes (vertical e horizontal) instalada no piso do tanque. A catenária invertida formada pela amarra foi registrada em alguns pontos não visíveis através de um sensor por ultrassom. O carregamento da linha de ancoragem foi produzido para diferentes ângulos de incidência, em ciclos de carga e descarga. Os resultados mostram que, mesmo em uma abordagem estática, flutuações de carga na linha de ancoragem não são inteiramente transmitidas ao ponto de ancoragem (torpedo), sendo parcialmente absorvidas ao longo do trecho enterrado da amarra. A atenuação de carga na argila A foi de 24%, enquanto que a atenuação apresentada na argila B variou entre 14 %, 6% e tendendo a 0 nos ensaios realizados respectivamente nas alturas de solo de 72 cm, 48 cm e 24 cm. Verificou-se também que a qualidade dos critérios de similaridade inicialmente adotados foi satisfatória.

The research development on the behavior of offshore structures is fundamentaly important for the exploration of oil in order to allow a more precise analysis of the performance of offshore mooring systems. In this sense, the present work aims to analyze the behavior of embedded mooring lines subjected to tensile stresses and to evaluate and quantify the attenuation of loads along the stretch buried thus determining the forces that effectively reach the torpedo pile. Initially, the equipment were designed and built, in a reduced model, so the experimental tests of conducted static load on the scale of 1:40 could be conducted. The clays used in the tests were produced in the laboratory in order to obtain a soil with similar physical properties to the typical clay seabed, from mixing…

Advisors/Committee Members: Schnaid, Fernando, Rocha, Marcelo Maia.

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A crescente utilização de plataformas flutuantes na atividade de exploração de petróleo vem exigindo desenvolvimento de estudos relacionados a novas técnicas de execução e metodologias de projetos de fundações em ambiente marinho. A presente pesquisa é continuação do trabalho desenvolvido por Rocha (2014) e tem como objetivo: estudar a atenuação de cargas dinâmicas no trecho da linha de ancoragem embutida no solo, através de experimentos em modelos reduzidos com solos argilosos. Os ensaios foram realizados em laboratório a partir de um sistema de carregamento dinâmico capaz de produzir vibração com faixas de aceleração e frequência desejadas. Para atingir os objetivos da pesquisa foram abordados os conceitos da carga dinâmica, reologia e tixotropia. Foram realizados ensaios em solos argilosos com teores de umidade em torno de 120%, obtidos a partir de misturas no laboratório de caulim (85% em massa seca), bentonita (15% em massa seca) e água, com a finalidade de obter um solo argiloso com propriedades geotécnicas similares às dos solos offshore prospectados pela Petrobras. Os ensaios de mini palheta e extração de umidade ao longo da profundidade permitiram concluir que os valores da resistência não drenada da argila crescem com profundidade, enquanto o teor de umidade decresce, sendo que a resistência ao cisalhamento não-drenada guarda similaridade em relação às condições de campo. Na realização dos ensaios de carregamentos estáticos e dinâmicos realizados com variação do ângulo de referência de 0º a 55º foram utilizadas duas células de cargas para medir as forças aplicadas no touch down point e no ponto de ancoragem. Os resultados obtidos nesses ensaios mostraram atenuação de carga devido à força de reação (atrito) gerada pelo solo na interface solo-corrente, quando a massa do solo envolvente é sujeita a grandes deformações. Observou-se que a magnitude de atenuação estática varia de 12,9 a 18,3% e depende da variação de profundidade de embutimento da estaca (comprimento da linha de ancoragem), do ângulo de referência, da resistência não drenada da argila e do nível de força de pré-tração aplicada, enquanto que no ensaio de carregamento dinâmico verificou-se que os valores de atenuação normalizada de cargas dinâmicas situam-se na faixa de 24 a 26% e são levemente influenciados pela variação da força de pré-tração e frequência de vibração devido à parcela viscosa da argila.

The increasing use of floating platforms for oil exploration activity has required development of studies related to new techniques and methodologies for foundation design in the marine environment. This research is the continuation of the work of Rocha (2014) and aims: to study the attenuation of dynamic loads in buried mooring lines through experiments in reduced scale models in clay deposits. The tests were performed in the laboratory from a system of dynamic loading that is able to produce vibration within desirable ranges of acceleration and frequency. The concepts of dynamic loading, rheology and thixotropy are discussed in order of…

Advisors/Committee Members: Schnaid, Fernando, Rocha, Marcelo Maia.

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O presente trabalho vem propor um estudo numérico qualitativo de um modelo hidro-elástico para simulação das vibrações induzidas por vórtices (VIV) em cabos submersos. O modelo é composto de um sistema acoplado de equações que descrevem tanto a estrutura quanto o fluido. A estrutura é modelada através de um oscilador elástico clássico e discretizada através de um método de massas concentradas. O fluido é considerado através de osciladores discretos baseados em modelos fenomenológicos com equação de Van der Pol ou Ginzburg-Landau. O acoplamento entre os dois ocorre na direção transversal ao escoamento local através da força de sustentação. O estudo foi feito para verificar quais tipos de comportamentos de resposta podem ser obtidos através do uso dos modelos fenomenológicos e, também, avaliar o potencial de tais modelos no ciclo de projeto na indústria atual, com foco na indústria petrolífera. Esta adequabilidade ao uso em projeto leva em conta, em primeiro lugar, a qualidade das respostas, mas também leva em conta a viabilidade da execução de simulações numéricas em tempo apropriado. Assim, um modelo numérico foi desenvolvido para permitir a integração, no domínio do tempo, deste sistema hidro- elástico. Esta integração é feita através de um método explícito de Euler e permite tratar não-linearidades dos osciladores elástico e fluido. Um conjunto de situações foi simulado numericamente, incluindo configurações verticais e também configurações em catenária. As simulações indicam alto grau de acoplamento entre os osciladores, o que leva a ressonância entre ambos na maior parte dos casos. Com relação ao comportamento do oscilador estrutural em si, foi possível observar fenômenos interessantes tais como lock-in e travelling waves, além do terceiro harmônico da vibração transversal detectado em alguns experimentos reportados em literatura. Com relação à adequabilidade do uso do modelo em projeto, considera-se que isto seja possível dada a relativa rapidez das simulações e aos resultados promissores que ainda podem ser melhorados.

The present work proposes a qualitative numerical study of a hydro-elastic model in order to simulate vortex-induced vibrations in submerged cables. The model is composed of a coupled system of equations describing the structure and the fluid. The structure is modeled through a classical elastic oscillator and discretized using a lumped mass approach. The fluid is modeled through discrete oscillators based on phenomenological models using Van der Pol or Ginzburg- Landau type equations. The coupling between these two oscillators is carried by the lift force. The study was carried in order to verify which response behavior can be obtained by using such phenomenological models and also, to evaluate their potential to be used in the offshore industry. In order to accomplish that, the model must provide reliable …

Advisors/Committee Members: Martins, Clóvis de Arruda.