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

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

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

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

Requirements and Solutions for Motion Limb Assistance of COVID-19 Patients

COVID-19 patients are strongly affected in terms of limb motion when imbedded during the acute phase of the infection, but also during the course of recovery therapies. Peculiarities are investigated for design requirements for medical devices in limb motion assistance for those patients. Solutions are analyzed from existing medical devices to outline open issues to provide guidelines for the proper adaption or for new designs supporting patients against COVID-19 effects. Examples are reported from authors’ activities with cable driven assisting devices

Modeling and Parametric Analysis of Quasi-Translational Parallel Continuum Manipulators

Translational Parallel Manipulators proved to be effective mechanisms in different application fields, from industries to haptic devices. The introduction of intrinsic flexibility within these mechanisms looks promising at increasing the safety of robots that are adopted in collaborative work-spaces. This paper focuses on the analysis of different types of Parallel Continuum Manipulator to find the better geometric structure to achieve quasi-translational motions. Therefore, the goal is to look for new flexible architectures that could be used instead of mechanisms composed by rigid links to improve safety in factories, in alignment with the United Nations Sustainable Development Goal 9: build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation