Giovanni Malara

On Design and Building of a U-OWC Wave Energy Converter in the Mediterranean Sea: A Case Study

Since the nineties, the OWC (Oscillating Water Column) plants were developed at full scale to produce electrical power from ocean waves [1]. A prototype was built into a caisson breakwater of the Sakata Port, in Japan; other plants were built in India, in Scotland at Islay, in Portugal at the Azores. A new plant was built in Mutriku (Spain) recently. A new kind of OWC caisson, named U-OWC or REWEC3, was proposed by Boccotti [2]. With respect to a traditional OWC, a U-OWC plant includes an additional vertical duct, which enables to tune the eigenperiod of the plant to the peak period of the wave pressures acting on the converter-breakwater. In this way, resonance conditions can be reached without phase control devices and the wave pressures into the air pocket are increased in amplitude, amplifying the performance of the plant. In 2012, a full scale U-OWC (REWEC3) breakwater has been designed in Italy, for the harbour of Civitavecchia (the port of Rome – Port Authority of Civitavecchia). Such a breakwater embodies 19 caissons, each including 8 cells, 34m long. The paper disseminates the key issues pertaining the design stage. Further, it describes the main phases of the construction stage. The building of the caisson started in October 2012. The first caisson has been completed at the end of 2012. It is the first device for wave energy in the Mediterranean Sea and one of the biggest in the world.Copyright

Long-Term Modelling of Wave Run-Up and Overtopping during Sea Storms

ABSTRACT Arena, F.; Malara, G.; Barbaro, G.; Romolo, A., and Ghiretti, S., 2013. Long-term modelling of wave run-up and overtopping during sea storms. This paper is concerned with the determination of the return period of a sea storm in which response (run-up or overtopping) of a coastal structure exceeds a fixed threshold. The method is based on long-term statistical analysis of the sea states interacting with the structure and on practical formulas proposed in the past decades for determining response of coastal structures. The proposed methodology accounts for nonstationarity of sea states in time domain by the equivalent triangular storm model, which allows us to determine closed-form solutions of the return period of a storm where maximum significant wave height is larger than a fixed threshold. In this paper the analytical solution is supplemented by a practical application pertaining calculation of the response of a coastal structure. A rubble-mound breakwater is supposed to be placed at a known water depth, and then, starting from offshore significant wave height data, the return period of a given response threshold is calculated.

Nonlinear Random Vibrations of Beams with Fractional Derivative Elements

AbstractThis paper deals with nonlinear random vibrations of a beam comprising a fractional derivative element; the nonlinear term arises from the assumption of moderately large beam displacements. It is shown that the beam response can be determined reliably via an optimal statistical linearization procedure. Specifically, the solution is obtained by utilizing an appropriate iterative representation of the stochastic response spectrum, which involves the linear modes of vibration of the beam. Such a representation allows retaining the nonlinearity in the time-dependent part of the response, which, in turn, is linearized in a stochastic mean square sense. The reliability of the proposed approximate solution is assessed in relation to the results of relevant Monte Carlo simulations. In this regard, a boundary integral method (BIM)–based algorithm is employed, in conjunction with a Newmark integration scheme, for estimating the beam response from spectrum-compatible realizations of the excitation, while acc...

Installing U-OWC devices along Italian coasts

In the last decades, the research has directed its efforts and resources paper is to investigate towards the possibility to incorporate wave energy converters, into the traditional maritime breakwaters to combine classical use with new opportunities and developments (for example, the Green Ports). Since the nineties, the OWC (Oscillating Water Column) plants were developed at full scale to produce electrical power from ocean waves. For instance, a new plant was built in Mutriku (Spain) recently.A new kind of OWC caisson, named U-OWC or REWEC3, which has the advantage to obtain an impressive natural resonance without any device for phase control, has been patented by Boccotti [1]. This new U-OWC device gives performances better than those of a conventional OWC either with small wind waves or with high waves [2,3,4]. The properties of the REWEC3 have been verified with two small-scale field experiments carried out in the natural ocean engineering laboratory NOEL of Reggio Calabria off the eastern coast of the Sicily Channel [5–7]. The aim of the present two sites along Italian coasts for possible installations of REWEC3 devices: i) the port of Civitavecchia (Rome, Italy)in the Tyrrhenian sea; ii) the port of Pantelleria, in the Sicily Channel.Copyright

A Formula for the Calculation of the Longshore Sediment Transport Including Spectral Effects

ABSTRACT Barbaro, G.; Foti, G.; Sicilia, L., and Malara, G., 2014. A formula for the calculation of the longshore sediment transport including spectral effects. The objective of this paper is to propose an expression for the calculation of the longshore sediment transport (LST), including the effect of the spectral shape associated with the free surface displacement. The expression is derived theoretically by using the wave flux approach in conjunction with the spectral representation of the wave field propagating to the coast. The resulting expression possesses the simplicity of implementation of classical LST formulae, with the additional feature of including a dimensionless coefficient encapsulating the spectral shape influence. The reliability of the proposed formula is assessed against the results of pertinent field data. Furthermore, its adequacy is investigated in the light of existing formulae. In this article, we show that the proposed LST formula is capable of reducing the large overestimations associated with the LST calculation and of reducing scattering between calculated and the measured LST rates.

A Control Strategy for PTO Systems in a U-OWC Device

This paper deals with the implementation of a control algorithm for a turbine equipped into a U-Oscillating water column (U-OWC) plant. The open literature has largely ignored the problem of coupling the U-OWC behaviour to the turbine behaviour with the objective of maximizing the energy harvesting. Therefore, this paper has the objective of assessing the reliability of a Maximum Power Point Tracking (MPPT) algorithm.The paper uses a numerical algorithm for estimating the energy produced by a U-OWC in real seas. In this regard, the computations rely on long time domain analyses that allow testing a variety of environmental conditions.The MPPT is proved adequate for controlling the turbine behavior in conjunction with U-OWCs. In this context, it is shown that a reference parameter, used for determining the reference rotational speed, can be the significant wave height.Copyright

U-Oscillating Water Column in Random Waves: Modelling and Performances

This paper deals with the analytical modelling of an U-Oscillating Water Column (U-OWC). It is shown that this device can be adequately described by a nonlinear equation of motion including hydrodynamic memory effects. The excitation of the system, the added mass and the retardation function are derived by approximating the solution of a pertinent initial boundary value problem via eigen-function expansions of the (linear) velocity potential.Next, the performance of the system is investigated in random waves by relying on Monte Carlo simulations. The excitation of the system is synthesized from a given power spectral density. Then, the nonlinear equation of motion is numerically integrated. Pertinent statistical measures are estimated for assessing the efficiency of the U-OWC in exploiting sea wave energy. In this regard, the parameters show that the device can absorb most part of the incident wave energy. Further, the device can work in safe conditions even in quite rough sea states.Copyright

An Analytical Approach for the Calculation of Random Wave Forces on Submerged Tunnels

This paper deals with the random forces produced by high ocean waves on submerged horizontal circular cylinders. Arena [1] obtained the analytical solution of the random wave field for two dimensional waves by extending the classical Ogilvie solution [2,3] to the case of random waves. In this paper, the wave force acting on the cylinder is investigated and the Froude Krylov force [4], on the ideal water cylinder, is calculated from the random incident wave field. Both forces represent a Gaussian random process of time. The diffraction coefficient of the wave force is obtained as quotient between the standard deviations of the force on the solid cylinder and of the Froude Krylov force. It is found that the diffraction coefficient of the horizontal force Cdo is equal to the Cdv of the vertical force. Finally, it is shown that, given that a very large wave force occurs on the cylinder, it may be calculated, in time domain, starting from the Froude Krylov force. It is then shown that this result is due to the fact that the frequency spectrum of the force acting on the cylinder is nearly identical to that of the Froude-Krylov force.Copyright

Modelling of Sea Storms Associated With Energy Harvesters: Downtime and Energy Losses

The design of an energy harvester involves achieving the two following objectives: to install a safe structure with a reasonable safety margin; and to install an effective device which is able to capture energy in a variety of environmental conditions. In this context, the long-term modelling of the environmental variables plays a crucial role.In the context of wave energy harvesters, the occurrence of sea storms is a critical element in the design process. Indeed, its identification is required for determining extreme loads as well as controlled de-activations of the device for preserving the mechanical components into the device.Considering these issues, the paper proposes an analysis of the wave climate oriented to the determination of the downtime and of the energy losses. Specifically, the paper provides expressions: for calculating the average deactivation time of a wave energy device, given that it must be deactivated if the significant wave height is larger than a certain threshold; and for calculating the energy “lost” (as it is not absorbed by the device) during a storm in which the maximum wave height is larger than the mentioned threshold.The paper shows that closed-form expressions can be obtained by relying on the Equivalent Triangular Storm (ETS) model and that the adequacy of the estimations improves for larger values of the significant wave height threshold.Copyright

A Small-Scale Field Experiment on Random Froude-Krylov Force on a Rectangular Structure

This paper investigates random Froude-Krylov (FK) force on a rectangular structure. It is a key parameter in the design process of some maritime structures. Indeed, the exciting force on a large floating body is commonly determined by a contribution due to the incident wave field (FK) and by a contribution due to the diffraction of sea waves.The work is based on results of a small-scale field experiment at NOEL (Natural Ocean Engineering Laboratory) in Reggio Calabria, Italy. First, field experiment is described, with characteristics of the selected sea states. Then, FK forces are analytically derived in the context of linear random waves. Frequency spectrum of the FK force is derived and it is discussed the occurrence of zeros in frequency domain.Extreme FK forces are determined by Quasi-Determinism theory. The theory enables to derive the analytical expression of the FK force when a large wave (either a large crest height or a large crest-to-trough wave height) occurs at any given point of the wave field, in a fixed time instant. Time domain representation allows investigating the wave force and extreme wave pressure. It is shown that the wave force is highly width dependent in time domain. Further, time histories are not quasi-impulsive. This characteristic is well-rendered in large structures (large with respect to the dominant wave length), where the wave group crossing gives rise to a time history “protraction” in time domain.In the last part of the paper theoretical results are supplemented by comparison with experimental data.© 2012 ASME