Wave RUN-UP Measurements under very oblique wave incidence

Under the scope of the HYDRALAB+ transnational access project, the so-called RODBreak experiment was conducted in the multidirectional wave basin at the Marienwerden facilities of the Leibniz University Hannover (LUH). A stretch of a rubble-mound breakwater was built in the wave basin with a very gentle slope. Its armour layer was made of Antifer cubes, at the roundhead and adjoining trunk, and of rock, at the rest of the trunk. A set of tests was carried out to extend the range of wave steepness values analysed in wave run-up, overtopping and armour layer stability studies, focusing on oblique extreme wave conditions, with incident wave angles from 40º to 90º. The present study focuses on the analysis of measured wave run-up values obtained in the tests and on their on their variability as well as the influence of the wave obliquity and directional spreading. Keywords: rubble-mound breakwaters; run-up; oblique waves; physical modelling; RODbreak.

CHANNEL CONCENTRATION AND REFLECTIONS FROM DREDGE CHANNELS

Wave reflections from dredge channels are an important consideration for coastal infrastructure. A physical model study of a proposed development for the Port of Townsville demonstrated that channel reflection, and the relatively poorly understood channel concentration are significant processes that need to be considered in coastal developments. The study showed that channel reflection and channel concentration can significantly transform the local waves, resulting in complex multidirectional wave fields and higher design wave conditions.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/XsKsofNZzvQ

Experimental and Numerical Statistics of Storm Wave Forces on a Monopile in Uni- and Multidirectional Seas

Experiments with both uni- and multidirectional wave realizations with a stiff pile subjected to extreme wave forces are considered. Differences in crest heights and force peaks resulting from directional spread waves are analysed. The wave realizations are reproduced numerically in the fully nonlinear wave model OceanWave3D. The numerical reproductions compare well to the experiments. Only for the largest forces significant differences are seen, which is due to a very simple breaking filter applied in OceanWave3D. In the wave spectra, the higher harmonics occur for smaller frequencies than the straight multiples of the peak frequency. Further, the higher harmonics of the multidirectional wave spectra contain less energy. Both effects can be explained by the second order wave theory. Finally, the computed wave kinematics are used to investigate the dynamic response of an offshore wind turbine. The excitation of the first natural frequency is largest for the unidirectional wave realizations, as the higher harmonics are largest for these realizations.

Effect of Wind Loads and Damping on Heading Stability of FPSOs

Tanker vessels used for offshore oil production and storage are kept at station by turret mooring systems, enabling the vessel to weathervane in the direction of dominant environmental loads. The disruption of heading equilibrium for a turret-moored tanker was predicted by experiments and numerical studies. A vessel was observed to lose control in head sea condition, wherein for wavelength from 0.73 < λ/L < 2 (L-ship length) the model drifted to a large angle of 45–60 degrees (Thiagarajan et al. 2013). Previous numerical analyses conducted by the authors identified that this heading drift reduced remarkably in the presence of wind. This finding is confirmed by an experimental study and reported in this paper. A geometrically scaled down version of a turret-moored FPSO at 1:120 scale of a prototype VLCC was tested at the Alfond W2 Wind & Wave Ocean Engineering facility of the University of Maine. This lab is a unique facility equipped with a high-performance wind machine over a multidirectional wave generator, and can create regular or random sea-states with wind speeds up to 7 m/s. The tests reported here were conducted with regular waves under two wind speeds (12 and 25 m/s full scale). It was observed that the presence of an initially bow wind can minimize the heading instability. The reason for this observation is described by analyzing the effect of the wind induced moments on the equilibrium condition. Free-decay tests were also conducted to investigate the contribution of the wind damping to the total damping. Measured results show that in the presence of wind, the damping values are higher than those estimated due to hydrodynamics only. It also has been discussed that this wind induced damping on FPSOs, can result in smaller heading angles. From this study, it is concluded that the wind can play a large role in the station-keeping dynamics of the moored-tankers.

DESIGN OF A POWERFUL AND PORTABLE MULTIDIRECTIONAL WAVEMA

A new multidirectional wave generator with 72 independent paddles has been designed, fabricated and commissioned at the National Research Council labs in Ottawa, Canada. The wet-back piston-mode machine is installed in a new 50 m long by 30 m wide rectangular wave basin, where water depths can be varied over the range from 0 m up to 1.3 m. The new machine is believed to be unique in the world in that it combines the power and stroke required to generate multidirectional spectral wave conditions with significant wave heights exceeding 0.4 m together with the modularity and ease of portability required to move the machine quickly and safely to new positions. The new machine can also be sub-divided to form several shorter machines if desired. The new wave generator features lightweight, composite materials, energy efficient regenerative power supplies, state-of-the-art software and control systems, including capabilities for active wave absorption (reflection compensation), second-order wave generation for improved generation of nonlinear sub- and super-harmonics, side-wall reflection, and more. The design of this new directional wavemaker is described and several of the more innovative features are highlighted in this paper.

Shearlet-Wavelet Regularized Semismooth Newton Iteration for Image Restoration

Image normally has both dots-like and curve structures. But the traditional wavelet or multidirectional wave (ridgelet, contourlet, curvelet, etc.) could only restore one of these structures efficiently so that the restoration results for complex images are unsatisfactory. For the image restoration, this paper adopted a strategy of combined shearlet and wavelet frame and proposed a new restoration method. Theoretically, image sparse representation of dots-like and curve structures could be achieved by shearlet and wavelet, respectively. Under theL1regularization, the two frame-sparse structures could show their respective advantages and efficiently restore the two structures. In order to achieve superlinear convergence, this paper applied semismooth Newton method based on subgradient to solve objective functional without differentiability. Finally, through numerical results, the effectiveness of this strategy was validated, which presented outstanding advantages for any individual frame alone. Some detailed information that could not be restored in individual frame could be clearly demonstrated with this strategy.