Frequency domain modelling of a coupled system of floating structure and mooring Lines: An application to a wave energy converter

Floating structures for single offshore renewable energy devices, i.e. wave energy converters, tend to be significantly smaller than those of the traditional offshore industry and the interaction between floater motions and mooring line dynamics become important. Installation sites are generally subject to powerful waves and currents experiencing more dynamically excited motions. Water depths are also lower, ranging generally from 50 m to 200 m and mooring systems are to be designed to assure the station keeping of them while not interfering with the power conversion. However, floater motions may induce large dynamic tensions on mooring lines, making quasistatic analyses inaccurate in terms of design tension while non-linear time domain simulations too time consuming. This paper introduces a numerical model of lumped mass for mooring lines and rigid body motions for the floating structure coupled by means of kinematic relations, and its subsequent linearization, which is solved in the frequency domain. The linearized model is applied to a two-body floating spar type oscillating water column, subject to the 36 most occurrent sea states at the BIMEP site. Its accuracy is verified through a comparison with the equivalent time domain simulation and a review of the results and its limitations are also pointed out

Contribution à la description théorique de la dynamique des processus élémentaires hétérogènes : collisions de l'azote moléculaire et de l'hydrogène atomique avec des surfaces de tungstène

Heterogeneous elementary processes at the gas-solid interface are ofgreat interest in many domains such as heterogeneous catalysis, atmospheric and interstellar media chemistry, spacecraft atmospheric re-entry and plasma-wall interactions description. This thesis focus on the dynamics of nitrogen, N2, non reactive scattering on a tungsten W(100) surface and hydrogen, H2, recombination processes on tungsten surfaces W(100) and W(110). The quasiclassical dynamics of these processes is simulated using potential energy surfaces based on density functional theory calculations. In particular, a multi-adsorbate potential is developed to include surface coverage in the dynamics simulation in order to scrutinize the interplay between both direct abstraction, the so-called Eley-Rideal recombination,and the Hot-Atom recombination process after hyperthermal diffusion on the surfaceLes processus élémentaires hétérogènes à l’interface gaz-solide présentent un intérêt fondamental dans de nombreux domaines tels que la catalyse hétérogène, la chimie atmosphérique et des milieux interstellaires, la rentrée atmosphérique de véhicules spatiaux ou encore la description des interactions plama-paroi. Cette thèse a pour objet l’étude de la dynamique des processus de collision non réactive de l’azote N2 sur une surface de tungstène W(100) et des processus de recombinaison moléculaire de l’hydrogène H2 sur des surfaces de tungstène W(100) et W(110). Leur dynamique quasi classique est simulée au moyen de surfaces d’énergie potentielle préalablement construites à partir de calculs de théorie de la fonctionnelle de la densité. Un potentiel multi-adsorbats est notamment développé pour tenir compte du taux de couverture de surface afin d’étudier la compétition entre la recombinaison directe, de type Eley-Rideal et la recombinaison par « atomes chauds » après diffusion hyperthermique d’un atome sur la surface

A comparison of numerical approaches for the design of mooring systems for wave energy converters

This paper analyses the numerical outcome of applying three different well-known mooring design approaches to a floating wave energy converter, moored by means of four catenary lines. The approaches include: a linearized frequency domain based on a quasistatic model of the mooring lines, a time domain approach coupled with an analytic catenary model of the mooring system, and a fully coupled non-linear time domain approach, considering lines' drag and inertia forces. Simulations have been carried out based on a set of realistic combinations of lines pretension and linear mass, subject to extreme environmental conditions. Obtained results provide realistic cost and performance indicators, presenting a comparison in terms of total mooring mass and required footprint, as well as the design line tension and structure offset. It has been found that lines' viscous forces influence significantly the performance of the structure with high pretensions, i.e., >1.2, while there is acceptable agreement between the modelling approaches with lower pretensions. Line tensions are significantly influenced by drag and inertia forces because of the occurrence of snap loads due to the heaving of the floater. However, the frequency domain approach provides an insight towards the optimal design of the mooring system for preliminary designs

Development of a Test-Bench for the Accurate Positioning of Scintillation Detector Modules for Medical Imaging Applications

When assessing the impact of novel concepts of scintillation block detectors for PET tomography by measuring the performance of single modules, or one module in coincidence with another, it is not trivial to translate the results of these investigations into a real PET scanner, made of several rings of modules. In particular, the real sensitivity, the signal to noise ratio of reconstructed images, and the spatial resolution (especially in the areas far from the center of field of view) among others are very critical to understand how good a PET scanner will perform for a real patient, and they are specific of the fully assembled scanner. Hence, demonstrating that a new idea under development is worth implementing in a real scanner requires more than just a measurement with two modules one in front of the other. For this reason, the set-up described in the present work will permit to reproduce full ring conditions using only a few modules at the price of a very good positioning accuracy.I must express all my gratitude towards one of my colleagues at CERN MME Group, Tommi Mikkola, for his infinite patience and teachings when reviewing the design and drawings of the mechanical parts

Small scale experimental validation of a numerical model of the HarshLab2.0 floating platform coupled with a non-linear lumped mass catenary mooring system

When focusing on mooring system numerical modelling, the efforts are focused on validating models that increase the accuracy and maintain the computation time under reasonable limits. In this paper an approach for modelling the interaction among supporting structure and mooring system is introduced through kinematic relations. The proposed approach has been validated with the experimental wave tank 1:13.6 scaled data of the HarshLab 2.0 platform, a CALM type buoy moored with a three-line catenary system and used as a floating laboratory for materials and corrosion testing, to be installed at BiMEP. The drag forces of the buoy as well as the Morison coefficients of the heave-pitch coupling, induced by the attached structure for ships boat landing, have been identified. Results of the mooring line tensions are validated with imposed displacements of the structure and, subsequently, with coupled simulations of the moored buoy in a set of realistic sea states. Sources of differences on the estimation of line tensions are found to be mainly due to uncertainties of seabed friction forces, a high sensitivity of line tensions to small swaying and a poor pitching performance of the numerical model, very likely due to a very non-linear pitching of the physical model

Design of an active reconfigurable 2R joint

Ponencia presentada en 15th IFToMM World Congress on Mechanism and Machine ScienceThe increasing flexibility requirements in the manufacturing processes has led to the development of novel reconfigurable mechanisms to be implemented in ma-chine heads and tables. This is the case of the reconfigurable parallel manipulators which are also used in a wide variety of applications. These mechanisms include often in their kinematic chains active or reconfigurable joints. In this paper, a 2R active reconfigurable joint is presented. As well as carrying out the kinematic characterization of the joint, a demonstrative prototype has been also built.The authors wish to acknowledge the financial support received from the Spanish Government through the Ministerio de Economía y Competitividad (Project DPI2015-67626-P (MINECO/FEDER, UE)) and the financial support given to the research group through the pro-ject with Ref. IT949-16, given by the Departamento de Educación, Política Lingüística y Cultura of the Regional Government of the Basque Country

Development of a Test-Bench for the Accurate Positioning of Scintillation Detector Modules for Medical Imaging Applications

When assessing the impact of novel concepts of scintillation block detectors for PET tomography by measuring the performance of single modules, or one module in coincidence with another, it is not trivial to translate the results of these investigations into a real PET scanner, made of several rings of modules. In particular, the real sensitivity, the signal to noise ratio of reconstructed images, and the spatial resolution (especially in the areas far from the center of field of view) among others are very critical to understand how good a PET scanner will perform for a real patient, and they are specific of the fully assembled scanner. Hence, demonstrating that a new idea under development is worth implementing in a real scanner requires more than just a measurement with two modules one in front of the other. For this reason, the set-up described in the present work will permit to reproduce full ring conditions using only a few modules at the price of a very good positioning accuracy.I must express all my gratitude towards one of my colleagues at CERN MME Group, Tommi Mikkola, for his infinite patience and teachings when reviewing the design and drawings of the mechanical parts

Numerical Approaches for Loads and Motions Assessment of Floating WECs Moored by Means of Catenary Mooring Systems

Technologies for harvesting offshore renewable energy based on float- ing platforms, such as offshore wind, wave and tidal energies, are currently being developed with the purpose of achieving a competitive cost of energy. The eco- nomic impact of the mooring system is significant within the total cost of such deployments, and large efforts are being carried out to optimize designs. Analysis of mooring systems at early stages generally require a trade-off between quick analysis methods and accuracy to carry out multi-variate sensitivity analyses. Even though the most accurate approaches are based on the non-linear finite ele- ment method in the time domain, these can result in being very time consuming. The most widely used numerical approaches for mooring line load estimates are introduced and discussed in this paper. It is verified that accurate line tension estimates require lines drag and inertia forces to be accounted for. A mooring and floating structure coupled model based on the lumped mass finite element ap- proach is also discussed, and it is confirmed that the differences found in the coupled numerical model are mainly produced by the uncertainty on hydrody- namic force estimates on the floating structure rather than by the lumped mass method. In order to enable quick line tension estimates, a linearization of the structure and mooring coupled model is discussed. It shows accurate results in operational conditions and enables modal analysis of the coupled system

Design and Testing of Two Haptic Devices Based on Reconfigurable 2R Joints

per presents the design and testing of two haptic devices, based on reconfigurable 2R joints: an active 2R spherical mechanism-based joint and a differential gear-based joint. Based on our previous works, in which the design and kinematic analysis of both reconfigurable joints were developed, the experimental setup and the various tasks intended to test the reconfigurability, precision, force feedback system and general performance, are presented herein. Two control modes for the haptic device operation are proposed and studied. The statistical analysis tools and their selection principles are described. The mechanical design of two experimental setups and the main elements are considered in detail. The Robot Operating System nodes and the topics that are used in the software component of the experimental setup are presented and explained. The experimental testing was carried out with a number of participants and the corresponding results were analyzed with the selected statistical tools. A detailed interpretation and discussion on of the results is provided.The authors wish to acknowledge the financial support received from the Spanish Government through the Ministerio de Ciencia e Innovación (Project PID2020-116176GB-I00) financed by MCIN/AEI/10.13039/501100011033, and the support for the research group through Project IT949-16 provided by the Departamento de Educación, Política Lingüística y Cultura from the regional Basque Government

Improving Skills in Mechanism and Machine Science Using GIM Software

The field of education has evolved significantly in recent years as it has incorporated new pedagogical methodologies. Many of these methodologies are designed to encourage students’ participation in the learning process. The traditional role of the student as a passive receiver of content is no longer considered valid. Teaching in mechanical engineering is no stranger to these changes either, where new learning activities have been designed to complement theory-heavy lectures. These activities take place in both physical and virtual laboratories. In case of the latter, the use of the GIM software (developed at the Department of Mechanical Engineering of the University of the Basque Country UPV/EHU, Spain) is a promising option. In this paper, features of the GIM that are most frequently used to support and exemplify the theoretical concepts taught in lectures are described using a case study. In addition, GIM is integrated into different learning activities to show its potential as a tool for learning and self-evaluation.This research was funded by Ministerio de Economía y Competitividad, Spanish Government Project, MINECO/FEDER, UE (grant number DPI2015-67626-P), Departamento de Educación, Política Lingüística y Cultura, Regional Government of the Basque Country (grant number IT949-16) and University of the Basque Country UPV/EHU (grant number PIE2012/14)