Optimization of Synthetic Mooring Systems for Floating Offshore Wind Turbines

As the floating offshore wind industry matures it has become increasingly important for researchers to determine the next generation materials and processes that will allow platforms to be deployed in intermediate (50-85 m) water depths which challenge the feasibility of traditional catenary chain mooring systems and fixed-bottom jacket structures. One such technology, synthetic ropes, has in recent years come to the forefront of this effort. A significant challenge of designing synthetic rope moorings is capturing the complex physics of the materials which exhibit viscoelastic and nonlinear elastic properties. Currently numerical tools for modeling the dynamic behavior of floating offshore wind turbines (FOWTs) are limited to mooring materials that lack these strain-rate dependent properties and have a linear tension-strain response. To address this limitation, a mooring modelling module, MoorDyn, which operates within the popular FOWT design and analysis program, OpenFAST, was modified to allow for nonlinear elastic mooring materials to add additional capabilities in the numerical tools. Simulations from the modified OpenFAST tool were then compared with 1:52-scale test data for a 6-MW FOWT Semi-submersible platform in 55m of water subjected to representative design load cases. A strong correlation between the simulations and test data was observed. In addition to reducing the cost of the mooring systems, synthetic systems can also reduce the footprint compared to a chain catenary system which frees areas around the turbine for other maritime uses such as commercial fishing. Both the mooring systems component cost and footprint are pertinent design criteria that lend themselves naturally to a multi-objective optimization routine. A new approach for efficiently screening the design space for plausible mooring systems that balance component cost and footprint using a multi-objective genetic algorithm is presented. This method uses a tiered-constraint method to avoid performing computationally expensive time-domain simulations of mooring system designs that are infeasible. Performance metrics for assessing the constraints of candidate designs are performed using open-source software such as Mooring Analysis Program (MAP++), OpenFAST and MoorDyn. A case study is presented providing a Pareto-optimal design front for a taut synthetic mooring system of a 6-MW floating offshore wind turbine. As the wind industry develops larger turbines for offshore deployment the problems with stationkeeping systems are exacerbated. While turbines increase in size so do the loads on the turbine. Meanwhile the offshore sites available for leasing in the intermediate water depth are still available to developers regardless of turbine and platform size. This complicates the process of designing mooring systems for these larger systems and emphasizes the importance of having a good methodology for automating this process. The final portion of this dissertation presents a method for mapping objectives for a multi-objective genetic algorithm to obtain the relationship between mooring system minimum cost and mooring radius. This work implements and expands on the aforementioned tiered-constraint evaluation scheme. These techniques are used to find the most cost-effective mooring designs for a 15-MW FOWT with a semi-taut mooring system over a range of mooring radii. New components and constraints are added to the system to allow the optimizer to find realistically deployable designs with reasonably accurate cost estimates

Numerical appoaches for loads and motions assessment of floating offshore renewable energy devices moored by means of catenary mooring systems.

Capítulo 5 confidencialActualmente se están desarrollando tecnologías para la captación de energías renovables marinas basadas en plataformas flotantes, como las energías eólica marina, undimotriz y mareomotriz, con el fin de lograr un coste de la energía competitivo. El impacto económico del sistema de fondeo es significativo dentro del coste total de dichos despliegues y se están realizando grandes esfuerzos para optimizar los diseños. Con el fin de obtener diseños económicamente eficientes, es muy conveniente que el sistema de fondeo se considere con suficiente precisión desde las primeras etapas del desarrollo tecnológico de las tecnologías correspondientes. El análisis de los sistemas de fondeo en las primeras etapas generalmente requiere un equilibrio entre métodos rápidos de análisis y una precisión suficiente para llevar a cabo análisis de sensibilidad de múltiples variables. Aunque los enfoques más precisos se basan en el método de elementos finitos no lineales en el dominio del tiempo, éstos pueden resultar en altos costes computacionales.En la etapa preliminar de esta tesis se han analizado y comparado los enfoques numéricos más utilizados para estimar las cargas en las líneas de amarre. Se muestra que se pueden realizar análisis de optimización de fondeos para convertidores de energía de olas basados en su dinámica vertical considerando la rigidez horizontal que introduce el sistema de fondeo en la estructura flotante, siempre que las pretensiones consideradas sean leves. Asimismo, se verifica que estimaciones precisas de las tensiones de línea requieren que se tengan en cuenta las fuerzas de inercia y viscosas del fluido en dichas líneas.Posteriormente se ha desarrollado un modelo acoplado de estructura flotante y fondeo basado en el enfoque `lumped mass¿. Éste ha sido validado con resultados de ensayos experimentales en tanque de olas de una boya cilíndrica flotante amarrada a través de tres líneas en catenaria, proporcionados por TECNALIA. Se ha confirmado que las diferencias encontradas entre los resultados del modelo numérico y los del modelo físico se han producido principalmente por incertidumbre en las estimaciones de las fuerzas hidrodinámicas en la estructura flotante, en lugar de en el modelo de las líneas de fondeo, basado en el método de `lumped mass¿ que se ha verificado que es muy preciso. Asimismo, la fricción con el lecho marino influye significativamente en la tensión de las líneas con movimientos transversales, y los modelos de fricción deben seleccionarse y ajustarse cuidadosamente.Con el fin de permitir estimaciones rápidas de la tensión de la línea, se ha propuesto una linealización del modelo acoplado de estructura y fondeo y se ha desarrollado un enfoque en el dominio de la frecuencia. Se ha verificado con los correspondientes resultados del modelo en el dominio del tiempo ya validado, ambos aplicados a un convertidor de energía undimotriz flotante amarrado con tres líneas en catenaria. Los resultados obtenidos en condiciones operativas han sido satisfactorios, permitiendo a su vez el análisis modal del sistema acoplado

Numerical appoaches for loads and motions assessment of floating offshore renewable energy devices moored by means of catenary mooring systems.

Capítulo 5 confidencialActualmente se están desarrollando tecnologías para la captación de energías renovables marinas basadas en plataformas flotantes, como las energías eólica marina, undimotriz y mareomotriz, con el fin de lograr un coste de la energía competitivo. El impacto económico del sistema de fondeo es significativo dentro del coste total de dichos despliegues y se están realizando grandes esfuerzos para optimizar los diseños. Con el fin de obtener diseños económicamente eficientes, es muy conveniente que el sistema de fondeo se considere con suficiente precisión desde las primeras etapas del desarrollo tecnológico de las tecnologías correspondientes. El análisis de los sistemas de fondeo en las primeras etapas generalmente requiere un equilibrio entre métodos rápidos de análisis y una precisión suficiente para llevar a cabo análisis de sensibilidad de múltiples variables. Aunque los enfoques más precisos se basan en el método de elementos finitos no lineales en el dominio del tiempo, éstos pueden resultar en altos costes computacionales.En la etapa preliminar de esta tesis se han analizado y comparado los enfoques numéricos más utilizados para estimar las cargas en las líneas de amarre. Se muestra que se pueden realizar análisis de optimización de fondeos para convertidores de energía de olas basados en su dinámica vertical considerando la rigidez horizontal que introduce el sistema de fondeo en la estructura flotante, siempre que las pretensiones consideradas sean leves. Asimismo, se verifica que estimaciones precisas de las tensiones de línea requieren que se tengan en cuenta las fuerzas de inercia y viscosas del fluido en dichas líneas.Posteriormente se ha desarrollado un modelo acoplado de estructura flotante y fondeo basado en el enfoque `lumped mass¿. Éste ha sido validado con resultados de ensayos experimentales en tanque de olas de una boya cilíndrica flotante amarrada a través de tres líneas en catenaria, proporcionados por TECNALIA. Se ha confirmado que las diferencias encontradas entre los resultados del modelo numérico y los del modelo físico se han producido principalmente por incertidumbre en las estimaciones de las fuerzas hidrodinámicas en la estructura flotante, en lugar de en el modelo de las líneas de fondeo, basado en el método de `lumped mass¿ que se ha verificado que es muy preciso. Asimismo, la fricción con el lecho marino influye significativamente en la tensión de las líneas con movimientos transversales, y los modelos de fricción deben seleccionarse y ajustarse cuidadosamente.Con el fin de permitir estimaciones rápidas de la tensión de la línea, se ha propuesto una linealización del modelo acoplado de estructura y fondeo y se ha desarrollado un enfoque en el dominio de la frecuencia. Se ha verificado con los correspondientes resultados del modelo en el dominio del tiempo ya validado, ambos aplicados a un convertidor de energía undimotriz flotante amarrado con tres líneas en catenaria. Los resultados obtenidos en condiciones operativas han sido satisfactorios, permitiendo a su vez el análisis modal del sistema acoplado

Modeling and Control of Flexible Link Manipulators

Autonomous maritime navigation and offshore operations have gained wide attention with the aim of reducing operational costs and increasing reliability and safety. Offshore operations, such as wind farm inspection, sea farm cleaning, and ship mooring, could be carried out autonomously or semi-autonomously by mounting one or more long-reach robots on the ship/vessel. In addition to offshore applications, long-reach manipulators can be used in many other engineering applications such as construction automation, aerospace industry, and space research. Some applications require the design of long and slender mechanical structures, which possess some degrees of flexibility and deflections because of the material used and the length of the links. The link elasticity causes deflection leading to problems in precise position control of the end-effector. So, it is necessary to compensate for the deflection of the long-reach arm to fully utilize the long-reach lightweight flexible manipulators. This thesis aims at presenting a unified understanding of modeling, control, and application of long-reach flexible manipulators. State-of-the-art dynamic modeling techniques and control schemes of the flexible link manipulators (FLMs) are discussed along with their merits, limitations, and challenges. The kinematics and dynamics of a planar multi-link flexible manipulator are presented. The effects of robot configuration and payload on the mode shapes and eigenfrequencies of the flexible links are discussed. A method to estimate and compensate for the static deflection of the multi-link flexible manipulators under gravity is proposed and experimentally validated. The redundant degree of freedom of the planar multi-link flexible manipulator is exploited to minimize vibrations. The application of a long-reach arm in autonomous mooring operation based on sensor fusion using camera and light detection and ranging (LiDAR) data is proposed.publishedVersio

Offshore floating vertical axis wind turbines, dynamics modelling state of the art. Part II: Mooring line and structural dynamics

The need to exploit enhanced wind resources far offshore as well as in deep waters requires the use of floating support structures to become economically viable. The conventional three-bladed horizontal axis wind turbine may not continue to be the optimal design for floating applications. Therefore it is important to assess alternative concepts in this context that may be more suitable. Vertical axis wind turbines (VAWTs) are a promising concept, and it is important to first understand the coupled and relatively complex dynamics of floating VAWTs to assess their technical feasibility. As part of this task, a series of articles have been developed to present a comprehensive literature review covering the various areas of engineering expertise required to understand the coupled dynamics involved in floating VAWTs. This second article focuses on the modelling of mooring systems and structural behaviour of floating VAWTs, discussing various mathematical models and their suitability within the context of developing a model of coupled dynamics for. Emphasis is placed on computational aspects of model selection and development as computational efficiency is an important aspect during preliminary design stages. This paper has been written both for researchers new to this research area, outlining underlying theory whilst providing a comprehensive review of the latest work, and for experts in this area, providing a comprehensive list of the relevant references where the details of modelling approaches may be found

High-fidelity computational modelling of fluid–structure interaction for moored floating bodies

The development and implementation process of a complete numerical framework for high-fidelity Fluid–Structure Interaction (FSI) simulations of moored floating bodies using Computational Fluid Dynamics (CFD) with the Finite Element Method (FEM) is presented here. For this purpose, the following three main aspects are coupled together: Two-Phase Flow (TPF), Multibody Dynamics (MBD), and mooring dynamics. The fluid–structure problem is two-way and fully partitioned, allowing for high modularity of the coupling and computational efficiency. The Arbitrary Lagrangian–Eulerian (ALE) formulation is used for describing the motion of the mesh-conforming fluid–solid interface, and mesh deformation is achieved with linear elastostatics. Mooring dynamics is performed using gradient deficient Absolute Nodal Coordinate Formulation (ANCF) elements with a two-way mooring–structure coupling and a one-way fluid–mooring coupling. Hydrodynamic loads are applied accurately along mooring cables using the solution of the fluid velocity provided by the TPF solver. For this purpose, fluid mesh elements containing cable nodes that do not conform to the fluid mesh are located with a computationally efficient particle-localisation algorithm. As it is common for partitioned FSI simulations of solids moving within a relatively dense fluid to experience unconditional instability from the added mass effect in CFD, a non-iterative stabilisation scheme is developed here. This is achieved with an accurate and dynamic estimation of the added mass for arbitrarily shaped structures that is then applied as a penalty term to the equations of motion of the solid. It is shown that this stabilisation scheme ensures stability of FSI simulations that are otherwise prone to strong added mass effect without affecting the expected response of structures significantly, even when using fully partitioned fluid–structure coupling schemes. Thorough verification and validation for all aspects of the FSI framework ultimately show that the produced numerical results are in good agreement with experimental data and other inherently stable numerical models, even when complex nonlinear events occur such as vortices forming around sharp corners or extreme wave loads and overtopping on moving structures. It is also shown that the mooring dynamics model can successfully reproduce nonlinearities from high frequency fairlead motions and hydrodynamic loads. The large-scale 3D simulation of a floating semi-submersible structure moored with three catenary lines ties all the models and tools developed here together and shows the capability of the high-fidelity FSI framework to model complex systems robustly and accurately

ISWEC toward the sea - Development, Optimization and Testing of the Device Control Architecture

The work performed in this thesis is part of the ISWEC project. This is a floating device devoted to the conversion of the kinetic energy owned by the sea waves. The passage between a technology readiness level (TRL) of 4 up to a TRL of 6 is covered. The existing numerical model has been revised, validated and upgraded. The experimental data used come both from previous collected data and both from the one gathered during a MARINET founded project in Hydraulics and Maritime Research Center (HMRC) in Cork, Ireland (2014). During the last year also the data coming from the full scale experiments in Pantelleria, Italy, (2015) has been processed. The design and implementation of the device Supervisory Control And Data Acquisition system has been a relevant part of the doctorate activities. Several power harvesting control strategies for the ISWEC have been investigated and their productivity for the Pantelleria installation site computed. A comparison is presented. Some preliminary results of the 2015 experimental campaign are presented and a first comparison with the data obtained with the numerical models has been carried on

Numerical modelling tool for multibody wave energy converters

Numerical models of wave energy converters (WECs) have been successfully used since the 1970s to understand a device’s characteristics and improve its performance before advancing to costlier, higher-risk stages of development such as tank testing and sea trials. In the last decade several software packages have become available to the industry specifically for time-domain multibody WEC modelling using potential flow theory. One of these tools is InWave, developed by Innosea, which is based on a reduced-coordinate multibody dynamics solver. However, one of the main challenges in developing a WEC modelling tool is the fact that the wave energy sector has not yet converged on a particular technology and there are many different designs currently in development, featuring a wide range of working principles. This thesis presents a novel WEC modelling tool: InWave-HOTINT, which uses a third-party multibody dynamics code (HOTINT) based on a redundant coordinate multibody dynamics method. This approach enables InWave-HOTINT to model a much wider range of mechanical topologies - including WECs featuring closed mechanical loops, multi- DoF PTOs and net mooring systems. The thesis describes the development and verification of the tool, as well as a demonstration of some of the new capabilities via a model based on the Albatern S12 WEC