Felice Arena

Weakly nonlinear statistics of high random waves

It is well known that in a Gaussian sea state for an infinitely narrow spectrum the crest height and the trough depth follow the same Rayleigh distribution, because of linearity of the first order Stokes expansion solution. For spectra of finite bandwidth, Boccotti obtained, as a corollary of his first formulation of the theory of quasideterminism (which is exact to the first order in a Stokes expansion), that the crest height and the trough depth still follow asymptotically the Rayleigh law for high waves in Gaussian sea states. In this paper we extend the theory of quasideterminism of Boccotti to the second-order, deriving new wave crest and wave trough distributions that take into account nonlinear effects and are valid for finite bandwidth of the spectrum in deep water. Nonlinear Monte Carlo simulations validate our theoretical predictions and comparisons with experimental data and the recent model of Forristall are finally presented.

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 Statistics and Extreme Waves of Sea Storms

Abstract A stochastic model of sea storms for describing long-term statistics of extreme wave events is presented. The formulation generalizes Boccotti’s equivalent triangular storm model by describing an actual storm history in the form of a generic power law. The latter permits the derivation of analytical solutions for the return periods of extreme wave events and associated statistical properties. Lastly, the relative validity of the new model and its predictions is assessed by analyzing wave measurements retrieved from two NOAA National Oceanographic Data Center (NODC) buoys in the Atlantic and Pacific Oceans.

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.

New perspectives in offshore wind energy

The design of offshore wind turbines is one of the most fascinating challenges in renewable energy. Meeting the objective of increasing power production with reduced installation and maintenance costs requires a multi-disciplinary approach, bringing together expertise in different fields of engineering. The purpose of this theme issue is to offer a broad perspective on some crucial aspects of offshore wind turbines design, discussing the state of the art and presenting recent theoretical and experimental studies.

Nonlinear Crest, Trough, and Wave Height Distributions in Sea States With Double-Peaked Spectra

The peak to trough distributions of nonlinear high sea waves in bimodal sea states in deep water are investigated. The statistical distribution of wave height is first analyzed by considering the Boccottis expression, where the parameters of the distribution are calculated for some bimodal spectra of sea states recorded in the Atlantic Ocean. The nonlinear crest and trough distributions are then obtained, particularizing for two peaked spectra the second-order Fedele and Arena expression, which depends on two parameters. The results have been finally validated by means of Monte Carlo simulations of second-order random waves with bimodal spectra.

A family of narrow-band non-linear stochastic processes for the mechanics of sea waves

A bi-parametric family of non-linear stochastic processes is introduced, to investigate the properties of second-order random processes with a narrow-band spectrum in the mechanics of the sea waves. In particular, the expressions of the probability density function and of the probabilities of exceedance of the absolute maximum and absolute minimum are obtained for this stochastic family. The analytical results are particularized for some processes of basic interest in the mechanics of the sea waves: the free surface displacement, and the fluctuating wave pressure beneath the sea surface.

Seismic analysis of offshore wind turbines on bottom-fixed support structures

This study investigates the seismic response of a horizontal axis wind turbine on two bottom-fixed support structures for transitional water depths (30–60 m), a tripod and a jacket, both resting on pile foundations. Fully coupled, nonlinear time-domain simulations on full system models are carried out under combined wind–wave–earthquake loadings, for different load cases, considering fixed and flexible foundation models. It is shown that earthquake loading may cause a significant increase of stress resultant demands, even for moderate peak ground accelerations, and that fully coupled nonlinear time-domain simulations on full system models are essential to capture relevant information on the moment demand in the rotor blades, which cannot be predicted by analyses on simplified models allowed by existing standards. A comparison with some typical design load cases substantiates the need for an accurate seismic assessment in sites at risk from earthquakes.

Nonlinear High Wave Groups in Bimodal Sea States

Nonlinear effects for wave groups in bimodal sea states are investigated. The linear free surface displacement is obtained by applying the quasi-determinism theory of the highest waves, with the spectrum given by the superposition of two JONSWAP spectra. Linear groups are modeled as the sum of two groups produced by each unimodal spectrum. Nonlinear effects are given as second-order contribution to the linear free surface displacement. The structure of wave groups is then analyzed for some bimodal spectra. The linear component of wave groups shows a strong variation with respect to the classical structure with unimodal spectra, particularly in mixed seas. It is observed that the nonlinearity gives small contributions for swell dominated seas. For wind dominated sea states, the main nonlinear contribution is given by the wind wave spectral components. For mixed sea states, the interaction among the components of the wind wave spectrum and the components of the swells could give a meaningful contribution to the nonlinear profile. The strongest nonlinear effects occur when the two peaks of the spectrum are close to each other. Finally, the results are validated by means of Monte Carlo simulations of nonlinear sea waves.

Small-Scale Field Experiment on Wave Forces on Upright Breakwaters

A small-scale field experiment was conducted on wave forces on upright breakwaters, and the following observations were made. The largest waves in a sea state yield one of two very characteristic time histories of the horizontal force, F, according to whether or not there is overtopping. With a large overtopping, F has a very sharp peak at the instant at which water reaches beyond the top of the wall. Without overtopping, F has a smooth history with its maximum when the water reaches the highest elevation on the wall. The sharp peaks of the wave-force per unit of length at a given section of a breakwater are estimated to have a negligible effect on the force on one caisson, provided that the wave crest makes an angle of only a few degrees with the breakwater. The dimensionless pressure distributions are not very sensitive to the ratio between the Hs of the incident waves and the elevation of the wall above the mean water level. Our field experiment confirms that Goda’s model (GM) is very effective for est...