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 $H_s$ 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 estimating the positive wave pressures, although it is slightly conservative. However, Goda’s model yields some negative wave forces that prove to be smaller than the largest negative forces measured in the course of the experiment. Boccotti’s model (BM), based on a linear theory with virtual wave heights, proves to be very effective for estimating negative wave pressures.

The design of any wave energy converter involves the determination of relevant statistical data on the wave energy resource oriented to the evaluation of the structural reliability and energy performance of the device. Currently, limited discussions concern the estimation of parameters connected to the energy performance of a device. Thus, this paper proposes a methodology for determining average downtime and average missed energy, which is the energy that is not harvested because of device deactivations during severe sea storms. These quantities are fundamental for evaluating the expected inactivity of a device during a year or during its lifetime and are relevant for assessing the effectiveness of a device working at a certain site. For this purpose, the equivalent power storm method is used for their derivation, starting from concepts pertaining to long-term statistical analysis. The paper shows that the proposed solutions provide reliable estimations via comparison with results obtained by processing long wave data.

A small-scale field experiment was conducted on a U-OWC incorporated into a caisson breakwater at the NOEL laboratory of Reggio Calabria (Italy). The U-Oscillating Water Column (U-OWC) or REWEC (REsonant Wave Energy Converter) is a device belonging to the family of OWCs. Such a device is very innovative, being able to absorb a very high percentage of incoming sea waves energy and to produce electrical power via proper PTO. The focus of the paper has been the analysis of the impact wave loads acting on the modified U-OWC structure during extreme wave events. A total of 250 records of pure wind waves were analyzed to verify the behaviors of wave loads acting on a U-OWC breakwater during operating conditions. The occurrence of both “quasi-standing wave” loads due to non-breaking waves and “impulsive wave loads”, exerted by a wave breaking against the U-OWC model, were observed. Then, Goda’s model was applied to predict the wave pressure distribution on the external wall of the U-OWC pneumatic chamber, and the theoretical results were compared to those obtained via small-scale field experiment.

Measuring the free-surface displacement on a vertical wall of a marine structure is not a trivial problem. In this context, the efficacy of ultrasonic probes is affected by the interaction between the signal emitted by the sensor and the vertical wall, whereas image-based techniques are computationally demanding, especially if long-time series are utilized. Considering these difficulties, this paper proposes a novel approach for measuring the sea surface elevation on vertical breakwaters. The proposed methodology involves the use of pressure measurements and a reconstruction algorithm based on a compressive sensing (CS) technique in conjunction with a generalized harmonic wavelet (GHW) basis. In particular, a constrained CS optimization approach is proposed by utilizing the known values of the free-surface data to reconstruct all other missing data while adhering at the same time to prescribed upper and lower bounds at all time instants. The reliability of the methodology was assessed against field data pertaining to a vertical wall equipped with pressure transducers recorded at the Natural Ocean Engineering Laboratory of Reggio Calabria. It was shown that direct application of an unconstrained GHW-based CS optimization approach yielded physically inconsistent minima and maxima values; thus, it was inadequate for reliably reconstructing the free surface. These drawbacks were removed by the constrained GHW-based CS. Furthermore, examination of the reconstructed sea surface profiles in the vicinity of extremely high wave crests or wave troughs showed that they are in agreement with pertinent theoretical data obtained by using the nonlinear quasi-determinism theory.

The analytical solution for the return period R≥2′ (H) of a sea storm having at least two individual waves higher than a fixed level H, is obtained. This solution is based on the application of the Boccotti’s Equivalent Triangular Storm model for the representation of actual storms. One of the corollaries of the solution gives the exact expression for the probability that at least two waves higher than level H are produced during the lifetime of a structure. The previous solution of R≥2′ (H) and the relative probability of exceedance can be effectively applied for the risk analysis of ocean structures. The results obtained show an excellent agreement with those achieved by considering the different approach, proposed by Arena and Pavone (2009), for the calculation of the return period R≥2 (H).