Time-Domain Analysis of the TALOS Wave Energy Converter using different Computational Tools

This paper focuses on the preliminary time-domain analysis of a multimode Wave Energy Converter (WEC), the so-called TALOS WEC, by deploying two different computational tools. The device consists of an internal sphere attached to its floater with springs and dampers, and power is captured through the sphere’s motions relatively to the floater. The equation of motion is formed based on the Cummins formulation using different calculation approaches for the convolution terms in the two tools. A comparative study, initially, is conducted assuming rigid connection of the sphere with the floater. Next, by enabling the sphere to oscillate in heave, as well as in both heave and surge, the device’s performance for one and two operational modes is assesse

Virtual Sea-Drifting Experiments between the Island of Cyprus and the Surrounding Mainland in the Early Prehistoric Eastern Mediterranean

Seaborne movement underpins frontier research in prehistoric archaeology, including water-crossings in the context of human dispersals, and island colonisation. Yet, it also controls the degree of interaction between locations, which in turn is essential for investigating the properties of maritime networks. The onset of the Holocene (circa 12,000 years ago) is a critical period for understanding the origins of early visitors/inhabitants to the island of Cyprus in the Eastern Mediterranean in connection with the spread of Neolithic cultures in the region. The research undertaken in this work exemplifies the synergies between archaeology, physical sciences and geomatics, towards providing novel insights on the feasibility of drift-induced seaborne movement and the corresponding trip duration between Cyprus and coastal regions on the surrounding mainland. The overarching objective is to support archaeological inquiry regarding the possible origins of these visitors/inhabitants—Anatolia and/or the Levant being two suggested origins

Development and application of an aero-hydro-servo-elastic coupling framework for analysis of floating offshore wind turbines

In order to enhance simulation capabilities of existing numerical tools for the design of floating offshore wind turbines (FOWTs), this study has developed and implemented a coupling framework (F2A) that is capable of predicting nonlinear dynamics of a FOWT subjected to wind, wave and current loadings. F2A integrates all the advantages of FAST in efficiently examining aero-servo-elastic effects with all the numerical capabilities of AQWA (e. g. nonlinear hydrodynamics, mooring dynamics and material nonlinearity) for the dynamic analysis of a FOWT. The verification of F2A is carried out by comparing it with OpenFAST through the case study of a 5 MW wind turbine supported by the OC3-Hywind spar platform. The results show excellent agreements between F2A and OpenFAST in predicting dynamic responses of the blades, tower, platform and station-keeping system under both steady and turbulent winds combined with wave conditions. This implies that the simulation capabilities of FAST are well implemented within AQWA. Further advantages and capabilities of F2A in examining the dynamics of a FOWT are investigated via a case study of a multi-body platform concept connected by flexible elements. Some unique phenomena can only be observed from the results obtained using F2A as opposed to conventional tools. The results indicate that the newly-developed F2A coupling framework can be used for the analysis of FOWTs and it has been released to the public

Hydrodynamic response and produced power of a combined structure consisting of a spar and heaving typewave energy converters

During the past years, researchers have studied both numerically and experimentally multibody wave-wind combined energy structures supporting wind turbines and different types of Wave Energy Converters (WECs); rigid body hydrodynamic assumptions have been adopted so far for the development of their numerical models and the assessment of their produced power. In the present paper a numerical model that is based on the use of generalized modes addressing wave-structure interaction effects for the case of a multibody wave-wind combined structure is developed and presented. Afterwards, the developed numerical model is used for the assessment of the hydrodynamic response and the prediction of the produced power of different possible configurations of the updated WindWEC concept which consists of a spar supporting a wind turbine and one, two, three or four heaving type WEC buoys. The combined effects of the center-to-center distance of the WEC and spar platform, the number of the WECs and the grid configuration of spar and WECs on the hydrodynamic interaction between the different floating bodies, spar and WEC buoys, and consequently on their response and wave power production are examined for regular and irregular waves. Strong hydrodynamic interaction effects exist for small distance between spar and WECs that result to the decrease of the produced power. Power matrices of the updated WindWEC concept are presented for all examined configurations with different number of WECs. Moreover, the annual produced power of the updatedWindWEC in two sites is estimated and presented. The generalized modes analysis presented in this paper is generic and can be used for the early stage assessment of wave-wind combined energy structures with low computational cost. The updated WindWEC can be used in sea sites with different environmental characteristics while extracting valuable amount of wave power

VD-PQ; A Velocity-Dependent Viscous Damping Model for Wave-Structure Interaction Analysis

For the analysis and design of coastal and offshore structures, viscous loads represent one of the most influential parameters that dominate their response. Very commonly, the potential flow theory is used for identifying the excitation wave loads, while the viscous damping loads are taken into consideration as distributed drag type loads and/or as linear and quadratic damping loads approximated with the use of motion decay curves of the structure in specific degrees of freedom. In the present paper, is developed and proposed a numerical analysis method for addressing wave-structure interaction effects through a velocity-dependent viscous damping model. Results derived by a computational fluid dynamics model are coupled with a model that uses the boundary element method for the estimation of the viscous damping loads iteratively in every time-step of the analysis. The computational fluid dynamics model solves the Navier–Stokes equations considering incompressible flow, while the second model solves the modified Cummins Equation of motion of the structure in the time domain. Details about the development of the coupling method and the velocity-dependent viscous damping (VD-PQ) are presented. The coupling between the different models is realized through a dynamic-link library. The proposed coupling method is applied for the case of a wave energy converter. Results derived with the use of the developed numerical analysis method are compared against experimental data and relevant numerical analysis predictions. The importance of considering the instantaneous velocity of the structure in estimating the viscous damping loads is demonstrated. The proposed numerical analysis method for estimating the viscous damping loads provides good accuracy compared to experimental data and, at the same time, low computational cost

Power performance and dynamic response of the WLC wave energy converter based on hydroelastic analysis

In the present paper, a novel Wave Energy Converter (WEC) named hereafter as Water Level Carpet (WLC) is introduced. Its dynamic response and performance are evaluated based on hydroelastic analysis and are presented for both regular and irregular waves. WLC consists of four floating modules inter-connected flexibly in two directions with hinges and Power Take-Off (PTO) mechanisms with known damping characteristics. The dynamic response (e.g. axial load at hinges, motions) and the estimation of the produced power of the WLC are evaluated through linear hydroelastic analysis in frequency domain with the use of a radiation/diffraction 3D hydroelastic model considering the effect of both the flexibility of the WLC as well as the damping forces associated with the energy extraction by the PTO mechanisms. The results that are obtained demonstrate the relationship between the produced power and the dynamic response of the WLC as well as the relationship between the damping coefficients of the PTO with the axial loads of WLC's hinges. The produced power of WLC obtains its maximum value for wave frequencies close to the resonance of the generalized degrees of freedom irrespectively of the direction that the waves have. Moreover, the results that are obtained emphasize the importance of the hydroelastic analysis for the assessment of the performance of this type of WEC

Vd-pq; a velocity-dependent viscous damping model for wave-structure interaction analysis

For the analysis and design of coastal and offshore structures, viscous loads represent one of the most influential parameters that dominate their response. Very commonly, the potential flow theory is used for identifying the excitation wave loads, while the viscous damping loads are taken into consideration as distributed drag type loads and/or as linear and quadratic damping loads approximated with the use of motion decay curves of the structure in specific degrees of freedom. In the present paper, is developed and proposed a numerical analysis method for addressing wave-structure interaction effects through a velocity-dependent viscous damping model. Results derived by a computational fluid dynamics model are coupled with a model that uses the boundary element method for the estimation of the viscous damping loads iteratively in every time-step of the analysis. The computational fluid dynamics model solves the Navier–Stokes equations considering incompressible flow, while the second model solves the modified Cummins Equation of motion of the structure in the time domain. Details about the development of the coupling method and the velocity-dependent viscous damping (VD-PQ) are presented. The coupling between the different models is realized through a dynamic-link library. The proposed coupling method is applied for the case of a wave energy converter. Results derived with the use of the developed numerical analysis method are compared against experimental data and relevant numerical analysis predictions. The importance of considering the instantaneous velocity of the structure in estimating the viscous damping loads is demonstrated. The proposed numerical analysis method for estimating the viscous damping loads provides good accuracy compared to experimental data and, at the same time, low computational cost

Effects of misaligned wave and wind action on the response of the combined concept WindWEC

In the present paper, the effects of misaligned wave and wind action on the dynamic response of the WindWEC combined concept are evaluated and presented. WindWEC is a recently proposed combined wind and wave energy system; a hybrid offshore energy system that consists of: (a) a 5MW floating wind turbine supported by a spar-type substructure (e.g. Hywind), a Wave Energy Converter (WEC) that is of heaving buoy type (e.g. Wavestar), (c) a structural arm that connects the spar with the WEC and (d) a common mooring system. Hybrid offshore platforms are combining wave and wind energy systems and are designed in order to gain the possible synergy effects and reduce the cost of generated electrical power while increasing the quality of delivered power to grids. During the lifetime of a combined concept, wave and wind can be misaligned which may affect the dynamic response and as a result the functionality of it. In particular, for asymmetric configurations, the misalignment of the wave and wind may result in unexpected behaviour and significant effects that may reduce the produced power. For the case of the WindWEC concept, the relative motion of the spar platform and WEC buoy results to the produced power. In this paper, the dynamic response and power production of the buoy type WEC and wind turbine are examined for different loading conditions where the wave and wind are misaligned. Integrated/coupled aero-hydro-servo-elastic time-domain dynamic simulations considering multi-body analyses are applied. The motion, structural and tension responses as well as power production are examined. The misalignment of wave and wind results to higher loads that affect the mooring line system and motion responses of the spar. It is found that the produced power of wind turbine is not significantly affected