In this paper some non-linear effects for the mechanics of sea wave groups with large waves are i... more In this paper some non-linear effects for the mechanics of sea wave groups with large waves are investigated, either for waves in an undisturbed field or for waves in front of a vertical wall. To the first-order in a Stokes expansion, Boccotti's quasi-determinism theory enables us to foresee the mechanics of wave groups, either in undisturbed or in diffracted fields, when a large wave occurs. The first formulation of this theory shows the random group mechanics when a large crest height occurs ('New wave'); the second theory formulation gives the random group mechanics when a large crest-to-trough wave height occurs. The quasi-determinism theory in both formulations, for undisturbed fields, was extended recently to the second-order by the author. In this paper the procedure to derive the second-order solution is analyzed and is applied to random wave groups in front of a vertical wall. The non-linear effects are then investigated in space-time domain, and it is obtained ...
This paper deals with the problem of examining th e nonlinear dynamic of a UOscillating Water Col... more This paper deals with the problem of examining th e nonlinear dynamic of a UOscillating Water Column (U-OWC) Wave Energy Conver ter. The U-OWC dynamic response is governed by a set of non-linear differential equ ations. In the paper, an approximate linear solution is sought by using the technique of statis tical linearization. The linearization scheme is implemented by identifying a surrogate linear sy stem equivalent to the nonlinear one in a mean-square sense. In this context, frequency-domai n analyses of the U-OWC response are readily implemented via standard linear input-outpu t relationship. Comparisons between the nonlinear response computed via numerical simulatio ns and by the approximate one assess the reliability of the method. The proposed approac h is applied to a small-scale U-OWC model installed in the Natural Engineering Laborato ry (NOEL) in Reggio Calabria, Italy. Sommario. Il lavoro affronta il problema del calcolo della r isposta di un assorbitore di energia del ...
Output-only methods are widely used to characterize the dynamic behavior of very diverse structur... more Output-only methods are widely used to characterize the dynamic behavior of very diverse structures. However, their application to floating structures may be limited due to their strong nonlinear behavior. Therefore, since there is very little experience on the application of these experimental tools to these very peculiar structures, it is very important to develop studies, either based on numerical simulations or on real experimental data, to better understand their potential and limitations. In an initial phase, the use of numerical simulations permits a better control of all the involved variables. In this work, the Covariance-driven Stochastic Subspace Identification (SSI-COV) algorithm is applied to numerically simulated data of two different solutions to Floating Offshore Wind Turbines (FOWT) and for its capability of tracking the rigid body motion modal properties and susceptibility to different modeling restrictions and environmental conditions tested. The feasibility of ap...
Abstract A mathematical model for U-Oscillating Water Column (U-OWC) wave energy converters is de... more Abstract A mathematical model for U-Oscillating Water Column (U-OWC) wave energy converters is developed and tested. The model is based on the linear irrotational wave theory but is able to treat different nonlinearities related to the physical processes involved in the U-OWC dynamics. These nonlinearities include those due to not small oscillations of the free surface in the air chamber and to the air transformation, as well as those related to the characteristics of the air turbine. An approach to include into the model the flow resistances encountered by the flow within the U-OWC is also presented. The model was tested using both laboratory and field experimental data. The latter were obtained using a physical model of a U-OWC installed on the coast. The results show that the model accurately computes the hydrodynamic quantities involved in the U-OWC dynamics. An application of the mathematical model to a real scale U-OWC subject to random waves compatible with a JONSWAP spectrum is presented. The nonlinear part of this model can be incorporated in other linear models of oscillating water column devices already appeared in the literature (H. Martins-Rivas and C.C. Mei, Journal Fluid Mech. 2009).
A novel statistical linearization technique is developed for computing stationary response statis... more A novel statistical linearization technique is developed for computing stationary response statistics of randomly excited coupled bending-torsional beams resting on non-linear elastic supports. The key point of the proposed technique consists in representing the non-linear coupled response in terms of constrained linear modes. The resulting set of non-linear equations governing the modal amplitudes is then replaced by an equivalent linear one via a classical statistical error minimization procedure, which provides algebraic non-linear equations for the second-order statistics of the beam response, readily solved by a simple iterative scheme. Data from Monte Carlo simulations, generated by a pertinent boundary integral method in conjunction with a Newmark numerical integration scheme, are used as benchmark solutions to check accuracy and reliability of the proposed statistical linearization technique.
Abstract The probability distribution of the nonlinear, up to the second order, crest height on a... more Abstract The probability distribution of the nonlinear, up to the second order, crest height on a vertical wall is determined under the assumption of finite spectral bandwidth, finite water depth and long-crested waves. The distribution is derived by relying on the Quasi-Deterministic representation of the free surface elevation on the vertical wall. The theoretical results are compared against experimental data obtained by utilizing a compressive sensing algorithm for reconstructing the free surface elevation on the wall. The reconstruction is pursued by starting from recorded wave pressure time histories obtained by utilizing a sequence of pressure transducers located at various levels. The comparison demonstrates an excellent agreement between the proposed distribution and the experimental data, while, notably, the deviation of the crest height distribution from the Rayleigh one is considerable.
This document describes design and realization of a small-scale field experiment on a 1:30 model ... more This document describes design and realization of a small-scale field experiment on a 1:30 model of spar floating support structure for offshore wind turbines. The aim of the experiment is to investigate the dynamic behaviour of the floating wind turbine under extreme wave and parked rotor conditions. The experiment has been going on in the Natural Ocean Engineering Laboratory of Reggio Calabria (Italy). In this article, all the stages of the experimental activity are presented, and some results are shown in terms of motions and response amplitude operators. Finally, a comparison with corresponding results obtained using ANSYS AQWA software package is shown, and conclusions are drawn. The presented experimental set-up seems promising to test offshore floating structures for marine renewable energy at a relatively large scale in the Natural Ocean Engineering Laboratory field site.
The objective of this paper is to investigate the reliability of the mathematical model used for ... more The objective of this paper is to investigate the reliability of the mathematical model used for estimating the U-Oscillating Water Column (U-OWC) response in the time domain. This device is a wave energy harvester belonging to the family of Oscillating Water Columns. It comprises a water column, an air chamber with a self-rectifying turbine and a vertical U-shaped duct connecting the water column to the open wave field. The model used for describing the water column oscillations is based on the application of the unsteady Bernoulli equation for a real fluid and involves the determination of energy losses occurring over the entire duct length. The reliability of this mathematical model is assessed against experimental data. For this purpose, a U-OWC experimental model is installed in a benign natural basin, the Natural Ocean Engineering Laboratory NOEL. The experimental model is exposed to waves stochastically characterized. Thus, its response is described by relevant statistics. The paper shows that the current mathematical model may yield relevant overestimations of the system response. This relates to the approximate representation of the head losses, which are based on the Darcy-Weisbach formula traditionally employed in the analyses of steady flows. For overcoming these limitations, the paper proposes the utilization of the instantaneous acceleration-based model, which includes unsteady head losses proportional to the water column inertia. In this context, the model shows a general good agreement with the experimental measurements and demonstrate a superior capability of predicting the U-OWC response in all the processed records.
The mass of wave records taken off the beach at Reggio Calabria enables us to verify a few recent... more The mass of wave records taken off the beach at Reggio Calabria enables us to verify a few recent theories on the wind generated waves. In particular, in line with the quasi-determinism theory, we find that the highest waves of each sea state exhibit a well defined period which is related to the autocovariance function. Also the recent concept of indeterminacy of the continuos spectrum is confirmed. The effects of this indeterminacy prove to be more relevant when the line spectrum has two peaks nearly equivalent to each other.
1 ABSTRACT The statistical properties of some narrow-band second-order processes in the mechan- i... more 1 ABSTRACT The statistical properties of some narrow-band second-order processes in the mechan- ics of sea waves in front of a vertical wall are investigated. Arena & Fedele (2002) have shown that the asymmetry between the crest and the trough distributions on a vertical wall is markedly greater than the asymmetry crest-trough in an undisturbed field, either for the free surface displacement or for the fluctuating wave pressure at a fixed depth. In this paper the narrow-band second-order wave force and overturning moment on the vertical wall are derived by analytical integration of the wave pressure. It is obtained that for a fixed threshold of the probability of exceedance, if kd is smaller than 1.38 (be- ing k the wave number and d the bottom depth), positive peaks of the wave force proc- ess (that occur with the wave crests of free surface displacement on the wall) are greater than absolute value of negative peaks (that occur with the wave trough). The wave force is a quasi-symmetric process for kd=1.38: the non-linearities are weak and both the positive peak distribution and the negative peak (in absolute value) distri- bution are given by the Rayleigh law with good approximation. Finally it is noteworthy that for kd>1.38 we find that the wave troughs of surface dis- placement at wall produce negative peaks of the wave force that are (in absolute value) greater than positive peaks produced by wave crests. The predictions are in good agreement with the data of a small-scale field experiment (Arena, 1995; Boccotti, 2000).
Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizz... more Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizzazione di impianti per la produzione di energia elettrica da onde di mare, in tre aree campione localizzate lungo le coste italiane. Nello specifico, sono state considerate le localita di Porto Pischera (Sardegna sud-ovest), Pantelleria e Genova. Per ciascuna localita sono stati eleborati i dati di moto ondoso, sia al largo sia sotto costa, al fine di valutare sia le onde estreme sia l'energia associata al moto ondoso. Le aree campione sono state individuate lungo tratti di costa che risultino di interesse per la realizzazione di cassoni portuali di tipo REWEC3. In ciascuna di tali aree sono stati elaborati dati di moto ondoso per intervalli temporali superiori ai 10 anni al fine di effettuare la stima dei livelli ondosi estremi, al largo. I dati di modo ondoso ed i livelli estremi sono stai poi propagati sotto costa, al fine di determinare i livelli di altezza significativi "locali". I livelli estremi di moto ondoso sono stati, quindi, utilizzati per il predimensionamento di cassoni per la produzione di energia dal moto ondoso per i quali sono state condotte le verifiche di stabilita globale dei cassoni. I livelli di moto ondoso "locali", in termini di altezza significativa, periodi e direzione dominante delle onde, sono stati utilizzati per la stima dell'energia del moto ondoso incidente. Infine si e stimato il potenziale energetico reale, in termini di produzione di energia ottenibile con una diga realizzata con cassoni di tipo REWEC3 nelle localita assegnate.
Abstract This paper presents an approach for estimating the long-term statistics of random wave c... more Abstract This paper presents an approach for estimating the long-term statistics of random wave crests occurring over a certain space–time domain. Such a problem is relevant in a number of marine engineering applications, as classical analyses based, exclusively, on time domain approaches underestimate wave crest amplitudes associated with a given return period. The return period of a certain wave crest is derived by combining the Equivalent Power Storm model, used in long-term statistical analyses, with the Euler Characteristic (EC) of an excursion set concept, recently applied to the study of sea wave statistics. In this regard, the paper shows that by computing the average EC, the probability distribution of the wave crests can be derived. Specifically, an explicit solution valid for finite crest thresholds can be derived by an approximation of the EC. Thus, removing the limitation of actual solutions, which are valid only for extremely large wave crests. In addition, return period of nonlinear wave crests are derived. In this context, Forristall distribution is adopted and introduced on a heuristic basis in the EC framework. The EC approximation and the reliability of the nonlinear crest distribution is assessed against Monte Carlo data by comparing distributions of maximum wave crests both in a sea state and in a sea storm. Then, return value estimations are discussed for a number of cases.
Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizz... more Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizzazione di impianti per la produzione di energia elettrica da onde di mare, in tre aree campione localizzate lungo le coste italiane. Nello specifico, sono state considerate le localita di Porto Pischera (Sardegna sud-ovest), Pantelleria e Genova. Per ciascuna localita sono stati eleborati i dati di moto ondoso, sia al largo sia sotto costa, al fine di valutare sia le onde estreme sia l'energia associata al moto ondoso. Le aree campione sono state individuate lungo tratti di costa che risultino di interesse per la realizzazione di cassoni portuali di tipo REWEC3. In ciascuna di tali aree sono stati elaborati dati di moto ondoso per intervalli temporali superiori ai 10 anni al fine di effettuare la stima dei livelli ondosi estremi, al largo. I dati di modo ondoso ed i livelli estremi sono stai poi propagati sotto costa, al fine di determinare i livelli di altezza significativi "locali". I livelli estremi di moto ondoso sono stati, quindi, utilizzati per il predimensionamento di cassoni per la produzione di energia dal moto ondoso per i quali sono state condotte le verifiche di stabilita globale dei cassoni. I livelli di moto ondoso "locali", in termini di altezza significativa, periodi e direzione dominante delle onde, sono stati utilizzati per la stima dell'energia del moto ondoso incidente. Infine si e stimato il potenziale energetico reale, in termini di produzione di energia ottenibile con una diga realizzata con cassoni di tipo REWEC3 nelle localita assegnate.
The analytical solution for the return period R(C) of a sea storm in which the maximum non-linear... more The analytical solution for the return period R(C) of a sea storm in which the maximum non-linear crest height exceeds a fixed threshold C is obtained, by applying the ‘Equivalent Triangular Storm’ model. Furthermore, the probability P that maximum non-linear crest height in the lifetime L exceeds a fixed threshold is derived from R(C) solution, assuming that the occurrences of storms with highest crest larger than C is given by a Poisson process. Both R(C) and probability P, for fixed values of L, are calculated for some locations and results are compared with predictions obtained with Krogstad model. It is found that narrow-band second-order approach is slightly conservative, with respect to the more general condition of crest distribution for second-order three-dimensional waves. Finally it is shown that Krogstad model tends to overpredict extreme crests and this overprediction tends to decrease as larger L is.
In this paper some non-linear effects for the mechanics of sea wave groups with large waves are i... more In this paper some non-linear effects for the mechanics of sea wave groups with large waves are investigated, either for waves in an undisturbed field or for waves in front of a vertical wall. To the first-order in a Stokes expansion, Boccotti's quasi-determinism theory enables us to foresee the mechanics of wave groups, either in undisturbed or in diffracted fields, when a large wave occurs. The first formulation of this theory shows the random group mechanics when a large crest height occurs ('New wave'); the second theory formulation gives the random group mechanics when a large crest-to-trough wave height occurs. The quasi-determinism theory in both formulations, for undisturbed fields, was extended recently to the second-order by the author. In this paper the procedure to derive the second-order solution is analyzed and is applied to random wave groups in front of a vertical wall. The non-linear effects are then investigated in space-time domain, and it is obtained ...
This paper deals with the problem of examining th e nonlinear dynamic of a UOscillating Water Col... more This paper deals with the problem of examining th e nonlinear dynamic of a UOscillating Water Column (U-OWC) Wave Energy Conver ter. The U-OWC dynamic response is governed by a set of non-linear differential equ ations. In the paper, an approximate linear solution is sought by using the technique of statis tical linearization. The linearization scheme is implemented by identifying a surrogate linear sy stem equivalent to the nonlinear one in a mean-square sense. In this context, frequency-domai n analyses of the U-OWC response are readily implemented via standard linear input-outpu t relationship. Comparisons between the nonlinear response computed via numerical simulatio ns and by the approximate one assess the reliability of the method. The proposed approac h is applied to a small-scale U-OWC model installed in the Natural Engineering Laborato ry (NOEL) in Reggio Calabria, Italy. Sommario. Il lavoro affronta il problema del calcolo della r isposta di un assorbitore di energia del ...
Output-only methods are widely used to characterize the dynamic behavior of very diverse structur... more Output-only methods are widely used to characterize the dynamic behavior of very diverse structures. However, their application to floating structures may be limited due to their strong nonlinear behavior. Therefore, since there is very little experience on the application of these experimental tools to these very peculiar structures, it is very important to develop studies, either based on numerical simulations or on real experimental data, to better understand their potential and limitations. In an initial phase, the use of numerical simulations permits a better control of all the involved variables. In this work, the Covariance-driven Stochastic Subspace Identification (SSI-COV) algorithm is applied to numerically simulated data of two different solutions to Floating Offshore Wind Turbines (FOWT) and for its capability of tracking the rigid body motion modal properties and susceptibility to different modeling restrictions and environmental conditions tested. The feasibility of ap...
Abstract A mathematical model for U-Oscillating Water Column (U-OWC) wave energy converters is de... more Abstract A mathematical model for U-Oscillating Water Column (U-OWC) wave energy converters is developed and tested. The model is based on the linear irrotational wave theory but is able to treat different nonlinearities related to the physical processes involved in the U-OWC dynamics. These nonlinearities include those due to not small oscillations of the free surface in the air chamber and to the air transformation, as well as those related to the characteristics of the air turbine. An approach to include into the model the flow resistances encountered by the flow within the U-OWC is also presented. The model was tested using both laboratory and field experimental data. The latter were obtained using a physical model of a U-OWC installed on the coast. The results show that the model accurately computes the hydrodynamic quantities involved in the U-OWC dynamics. An application of the mathematical model to a real scale U-OWC subject to random waves compatible with a JONSWAP spectrum is presented. The nonlinear part of this model can be incorporated in other linear models of oscillating water column devices already appeared in the literature (H. Martins-Rivas and C.C. Mei, Journal Fluid Mech. 2009).
A novel statistical linearization technique is developed for computing stationary response statis... more A novel statistical linearization technique is developed for computing stationary response statistics of randomly excited coupled bending-torsional beams resting on non-linear elastic supports. The key point of the proposed technique consists in representing the non-linear coupled response in terms of constrained linear modes. The resulting set of non-linear equations governing the modal amplitudes is then replaced by an equivalent linear one via a classical statistical error minimization procedure, which provides algebraic non-linear equations for the second-order statistics of the beam response, readily solved by a simple iterative scheme. Data from Monte Carlo simulations, generated by a pertinent boundary integral method in conjunction with a Newmark numerical integration scheme, are used as benchmark solutions to check accuracy and reliability of the proposed statistical linearization technique.
Abstract The probability distribution of the nonlinear, up to the second order, crest height on a... more Abstract The probability distribution of the nonlinear, up to the second order, crest height on a vertical wall is determined under the assumption of finite spectral bandwidth, finite water depth and long-crested waves. The distribution is derived by relying on the Quasi-Deterministic representation of the free surface elevation on the vertical wall. The theoretical results are compared against experimental data obtained by utilizing a compressive sensing algorithm for reconstructing the free surface elevation on the wall. The reconstruction is pursued by starting from recorded wave pressure time histories obtained by utilizing a sequence of pressure transducers located at various levels. The comparison demonstrates an excellent agreement between the proposed distribution and the experimental data, while, notably, the deviation of the crest height distribution from the Rayleigh one is considerable.
This document describes design and realization of a small-scale field experiment on a 1:30 model ... more This document describes design and realization of a small-scale field experiment on a 1:30 model of spar floating support structure for offshore wind turbines. The aim of the experiment is to investigate the dynamic behaviour of the floating wind turbine under extreme wave and parked rotor conditions. The experiment has been going on in the Natural Ocean Engineering Laboratory of Reggio Calabria (Italy). In this article, all the stages of the experimental activity are presented, and some results are shown in terms of motions and response amplitude operators. Finally, a comparison with corresponding results obtained using ANSYS AQWA software package is shown, and conclusions are drawn. The presented experimental set-up seems promising to test offshore floating structures for marine renewable energy at a relatively large scale in the Natural Ocean Engineering Laboratory field site.
The objective of this paper is to investigate the reliability of the mathematical model used for ... more The objective of this paper is to investigate the reliability of the mathematical model used for estimating the U-Oscillating Water Column (U-OWC) response in the time domain. This device is a wave energy harvester belonging to the family of Oscillating Water Columns. It comprises a water column, an air chamber with a self-rectifying turbine and a vertical U-shaped duct connecting the water column to the open wave field. The model used for describing the water column oscillations is based on the application of the unsteady Bernoulli equation for a real fluid and involves the determination of energy losses occurring over the entire duct length. The reliability of this mathematical model is assessed against experimental data. For this purpose, a U-OWC experimental model is installed in a benign natural basin, the Natural Ocean Engineering Laboratory NOEL. The experimental model is exposed to waves stochastically characterized. Thus, its response is described by relevant statistics. The paper shows that the current mathematical model may yield relevant overestimations of the system response. This relates to the approximate representation of the head losses, which are based on the Darcy-Weisbach formula traditionally employed in the analyses of steady flows. For overcoming these limitations, the paper proposes the utilization of the instantaneous acceleration-based model, which includes unsteady head losses proportional to the water column inertia. In this context, the model shows a general good agreement with the experimental measurements and demonstrate a superior capability of predicting the U-OWC response in all the processed records.
The mass of wave records taken off the beach at Reggio Calabria enables us to verify a few recent... more The mass of wave records taken off the beach at Reggio Calabria enables us to verify a few recent theories on the wind generated waves. In particular, in line with the quasi-determinism theory, we find that the highest waves of each sea state exhibit a well defined period which is related to the autocovariance function. Also the recent concept of indeterminacy of the continuos spectrum is confirmed. The effects of this indeterminacy prove to be more relevant when the line spectrum has two peaks nearly equivalent to each other.
1 ABSTRACT The statistical properties of some narrow-band second-order processes in the mechan- i... more 1 ABSTRACT The statistical properties of some narrow-band second-order processes in the mechan- ics of sea waves in front of a vertical wall are investigated. Arena & Fedele (2002) have shown that the asymmetry between the crest and the trough distributions on a vertical wall is markedly greater than the asymmetry crest-trough in an undisturbed field, either for the free surface displacement or for the fluctuating wave pressure at a fixed depth. In this paper the narrow-band second-order wave force and overturning moment on the vertical wall are derived by analytical integration of the wave pressure. It is obtained that for a fixed threshold of the probability of exceedance, if kd is smaller than 1.38 (be- ing k the wave number and d the bottom depth), positive peaks of the wave force proc- ess (that occur with the wave crests of free surface displacement on the wall) are greater than absolute value of negative peaks (that occur with the wave trough). The wave force is a quasi-symmetric process for kd=1.38: the non-linearities are weak and both the positive peak distribution and the negative peak (in absolute value) distri- bution are given by the Rayleigh law with good approximation. Finally it is noteworthy that for kd>1.38 we find that the wave troughs of surface dis- placement at wall produce negative peaks of the wave force that are (in absolute value) greater than positive peaks produced by wave crests. The predictions are in good agreement with the data of a small-scale field experiment (Arena, 1995; Boccotti, 2000).
Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizz... more Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizzazione di impianti per la produzione di energia elettrica da onde di mare, in tre aree campione localizzate lungo le coste italiane. Nello specifico, sono state considerate le localita di Porto Pischera (Sardegna sud-ovest), Pantelleria e Genova. Per ciascuna localita sono stati eleborati i dati di moto ondoso, sia al largo sia sotto costa, al fine di valutare sia le onde estreme sia l'energia associata al moto ondoso. Le aree campione sono state individuate lungo tratti di costa che risultino di interesse per la realizzazione di cassoni portuali di tipo REWEC3. In ciascuna di tali aree sono stati elaborati dati di moto ondoso per intervalli temporali superiori ai 10 anni al fine di effettuare la stima dei livelli ondosi estremi, al largo. I dati di modo ondoso ed i livelli estremi sono stai poi propagati sotto costa, al fine di determinare i livelli di altezza significativi "locali". I livelli estremi di moto ondoso sono stati, quindi, utilizzati per il predimensionamento di cassoni per la produzione di energia dal moto ondoso per i quali sono state condotte le verifiche di stabilita globale dei cassoni. I livelli di moto ondoso "locali", in termini di altezza significativa, periodi e direzione dominante delle onde, sono stati utilizzati per la stima dell'energia del moto ondoso incidente. Infine si e stimato il potenziale energetico reale, in termini di produzione di energia ottenibile con una diga realizzata con cassoni di tipo REWEC3 nelle localita assegnate.
Abstract This paper presents an approach for estimating the long-term statistics of random wave c... more Abstract This paper presents an approach for estimating the long-term statistics of random wave crests occurring over a certain space–time domain. Such a problem is relevant in a number of marine engineering applications, as classical analyses based, exclusively, on time domain approaches underestimate wave crest amplitudes associated with a given return period. The return period of a certain wave crest is derived by combining the Equivalent Power Storm model, used in long-term statistical analyses, with the Euler Characteristic (EC) of an excursion set concept, recently applied to the study of sea wave statistics. In this regard, the paper shows that by computing the average EC, the probability distribution of the wave crests can be derived. Specifically, an explicit solution valid for finite crest thresholds can be derived by an approximation of the EC. Thus, removing the limitation of actual solutions, which are valid only for extremely large wave crests. In addition, return period of nonlinear wave crests are derived. In this context, Forristall distribution is adopted and introduced on a heuristic basis in the EC framework. The EC approximation and the reliability of the nonlinear crest distribution is assessed against Monte Carlo data by comparing distributions of maximum wave crests both in a sea state and in a sea storm. Then, return value estimations are discussed for a number of cases.
Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizz... more Oggetto del presente progetto e l'analisi dei livelli di moto ondoso finalizzata alla realizzazione di impianti per la produzione di energia elettrica da onde di mare, in tre aree campione localizzate lungo le coste italiane. Nello specifico, sono state considerate le localita di Porto Pischera (Sardegna sud-ovest), Pantelleria e Genova. Per ciascuna localita sono stati eleborati i dati di moto ondoso, sia al largo sia sotto costa, al fine di valutare sia le onde estreme sia l'energia associata al moto ondoso. Le aree campione sono state individuate lungo tratti di costa che risultino di interesse per la realizzazione di cassoni portuali di tipo REWEC3. In ciascuna di tali aree sono stati elaborati dati di moto ondoso per intervalli temporali superiori ai 10 anni al fine di effettuare la stima dei livelli ondosi estremi, al largo. I dati di modo ondoso ed i livelli estremi sono stai poi propagati sotto costa, al fine di determinare i livelli di altezza significativi "locali". I livelli estremi di moto ondoso sono stati, quindi, utilizzati per il predimensionamento di cassoni per la produzione di energia dal moto ondoso per i quali sono state condotte le verifiche di stabilita globale dei cassoni. I livelli di moto ondoso "locali", in termini di altezza significativa, periodi e direzione dominante delle onde, sono stati utilizzati per la stima dell'energia del moto ondoso incidente. Infine si e stimato il potenziale energetico reale, in termini di produzione di energia ottenibile con una diga realizzata con cassoni di tipo REWEC3 nelle localita assegnate.
The analytical solution for the return period R(C) of a sea storm in which the maximum non-linear... more The analytical solution for the return period R(C) of a sea storm in which the maximum non-linear crest height exceeds a fixed threshold C is obtained, by applying the ‘Equivalent Triangular Storm’ model. Furthermore, the probability P that maximum non-linear crest height in the lifetime L exceeds a fixed threshold is derived from R(C) solution, assuming that the occurrences of storms with highest crest larger than C is given by a Poisson process. Both R(C) and probability P, for fixed values of L, are calculated for some locations and results are compared with predictions obtained with Krogstad model. It is found that narrow-band second-order approach is slightly conservative, with respect to the more general condition of crest distribution for second-order three-dimensional waves. Finally it is shown that Krogstad model tends to overpredict extreme crests and this overprediction tends to decrease as larger L is.
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