Statistical distribution of free water surface over a mild bed slope for extreme wavefields

This paper examines the probability density function (PDF) of free water surface elevations in coastal areas. The functional form and properties of PDFs of extreme storms propagating over a mildly sloping bathymetry are investigated. This is facilitated through comparisons between experimental measurements and a wide range of probability models; the latter including both analytical and empirical distributions. The incident wave conditions correspond to realistic storm spectra (JONSWAP) and have been simulated as long random timeseries of 60-hour duration. The length of the records is sufficient to provide an accurate description of distribution tails. Six sea-states with varying offshore steepness have been generated and measured at different cross-shore locations. The cross-shore evolution of the wavefield initially leads to the development of nonlinear harmonics, both at low and high frequencies, and a broadening of the wave spectrum. This is enhanced by wave breaking particularly at shallower water depths or steeper sea-states. These result in rapid deviations from Gaussian theory with respect to the PDFs of surface elevations. Available models are generally successful in capturing nonlinear evolution arising at a second-order of wave steepness but cannot model the probability structure once a significant proportion of waves are breaking. In comparing the deviations between experimental data and model predictions, the best performing model is identified

EFFECTS OF NONLINEARITY ON THE FORMULATION OF THE CREST ELEVATION AND THE CREST WIDTH OF EXTREME WAVES IN RANDOM SEAS

This work highlights the effects of nonlinearity on extreme events that rise during directional focused and random wave simulations in finite water depths. For this purpose, the fully nonlinear model “HOS-Ocean” by Ducrozet et al. (2016) [1] is considered and compared with linear simulations. Large waves are formed through a series of linear and nonlinear focused and random simulations. The effect of water depth and directionality is considered by applying different directional spreadings relative to a range of short crested to long crested sea states. The results offer transparency in understanding the physics underlying such extreme events. It is shown that “walls of water” are also formulated in finite water depth, as is the case for deep water [2]. Indeed, the sea-state becomes more long-crested during the formulation of the extreme events in both focused and random simulations; all highly dependant on the directionality of the wavefield. The above is highlighted in the substantially increased crest width measured in the nonlinear simulations despite the decreased maximum crest elevations. Copyright © 2022 by ASME

Hydrodynamics and near trapping effects in arrays of multiple elliptical cylinders in waves

The purpose of the present study is the investigation of the hydrodynamic interactions induced by arrays of elliptical cylinders subjected to regular waves. The solution methodology employs linear potential theory and is based on pure analytic considerations. The interaction phenomena are approached using the Mathieu functions addition theorem which converts elliptical harmonics from one elliptic coordinate system to a remote elliptic coordinate system. The followed approach resembles the ‘direct’ solution methodology which is used for the analytical solution of the diffraction problem by arrays of circular cylinders. The ‘direct’ approach allows, among others, the construction of a linear matrix equation for the calculation of the expansion coefficients of the diffraction component(s) which accordingly is used to trace the wavenumber(s) under which trapped modes may be stimulated to induce wave trapping in the array, and reduction of the energy radiated to the far-field. The numerous computations which are performed, specifically verify that any array of elliptical cylinders, in accord with arrays of circular cylinders, is potentially a wave trapping configuration, which is connected mainly i) with sharp amplifications in all modes of hydrodynamic loading (both forces and moments) and ii) strong free-surface elevations in the liquid regions between the cylinders and on the cylinders’ surfaces accordingly. © 2018 Elsevier Lt

The importance of accurate calculation of the nonlinear extreme wave kinematics in the design of offshore platforms

The purpose of this work is to highlight the importance of the calculation of the nonlinear extreme wave kinematics appropriate for load calculations of the elements of a platform in deep water. Numerical calculations have been undertaken using a fully nonlinear three-dimensional wave model capable of accurately representing a realistic distribution of wave energy in both the frequency and the directional domains (Bateman, Swan, and Taylor 2001, Bateman, Swan, and Taylor 2003). The purpose of these calculations is to provide what is essentially an exact benchmark against which to assess the accuracy of the commonly applied design wave solutions; linear and 2nd order random waves and nonlinear steady wave solutions. The platform, which is used for the analysis, is a compliant tower set-up in deep water. The load calculations were made by applying the well-known Morison’s equation on all the members of the structure. Through the static analysis, the differences of the results are obvious in the case were geometric nonlinearity is taken into account in the analysis. In order to complete these analyses a well-known software for static and dynamic analysis of structures was used (SAP2000). The results have shown that the fact that none of the common design methods describes accurately the particle velocities, lead not only to a misestimate of the applied loads on the structure investigated but also to important deviations from the predicted displacements of the basic elements of the structure. © 2015 Taylor & Francis Group, London

The importance of accurate calculation of the nonlinear extreme wave kinematics in the design of offshore platforms

The purpose of this work is to highlight the importance of the calculation of the nonlinear extreme wave kinematics appropriate for load calculations of the elements of a platform in deep water. Numerical calculations have been undertaken using a fully nonlinear three-dimensional wave model capable of accurately representing a realistic distribution of wave energy in both the frequency and the directional domains (Bateman, Swan, & Taylor 2001, Bateman, Swan, & Taylor 2003). The purpose of these calculations is to provide what is essentially an exact benchmark against which to assess the accuracy of the commonly applied design wave solutions; linear and 2nd order random waves and nonlinear steady wave solutions. The platform, which is used for the analysis, is a compliant tower set-up in deep water. The load calculations were made by applying the well-known Morison’s equation on all the members of the structure. Through the static analysis, the differences of the results are obvious in the case were geometric nonlinearity is taken into account in the analysis. In order to complete these analyses a well-known software for static and dynamic analysis of structures was used (SAP2000). The results have shown that the fact that none of the common design methods describes accurately the particle velocities, lead not only to a misestimate of the applied loads on the structure investigated but also to important deviations from the predicted displacements of the basic elements of the structure. © 2015 Taylor and Francis Group

Hydrodynamics and ellipsoidal harmonics

This paper presents the theoretical basis for treating hydrodynamic problems in which the use of ellipsoidal harmonics is necessary. Those include the water wave diffraction problem, the wave resistance problem and the added mass - attraction force calculation of an ellipsoid moving steadily close to a rigid wall. All these problems are tackled using Miloh's theorem for the ultimate image singularity system of ellipsoidal harmonics. © 2018 Author(s)

The effect of nonlinear wave-structure and soil-structure interactions in the design of an offshore structure

This work highlights the importance of the consideration of nonlinearities in the analysis and design of deep water platforms. The nonlinearities concern both the calculation of the wave loading and the structural analysis of the platform. More specifically, numerical calculations have been undertaken using a fully nonlinear three-dimensional wave model capable of accurately representing a realistic distribution of wave energy in both frequency and direction. The purpose of these calculations is to provide what is essentially a benchmark against which to assess the accuracy of the commonly applied design wave solutions; linear and 2nd order random waves and linear and nonlinear steady wave solutions. On the other hand, in the field of structural analysis, three cases are considered for comparison, linear analysis, nonlinear analysis with the nonlinearity limited to the modelling of the soil and finally, geometrically nonlinear analysis with nonlinear modelling of the soil. The calculations are performed using the well-known structural analysis software SAP2000, enhanced with an interface that was developed in order to calculate the forces due to wave loading and apply them on the structural members. The example treated in the context of the present paper is a compliant tower, set-up in deep water. The results show that for the specific environmental conditions and for the structure examined herein, the consideration of the nonlinear particle kinematics in the wave loading and the consideration of the structural nonlinearities leads to significant differences with respect to the common design methods in both the displacements and stresses of the structure. Moreover, irrespectively of the adopted wave theory, the structural nonlinear analyses lead to significant discrepancies from the linear ones. © 2016 Elsevier Lt

Beach Erosion and Consequential Impacts Due to the Presence of Harbours in Sandy Beaches in Greece and Cyprus

Harbours with protruding breakwaters that are built on coasts cause alterations to the wave field, which attacks the coastline adjacent to the structures. As a result, sandy beaches often start eroding. Examples from Greece and Cyprus are presented, veryfying that the causes of the phenomenon, which are (i) the obstruction of longshore sand movement, and (ii) the longshore sand transport from the wave-action zone to the calm wave-shelter zone triggered by wave diffraction, work almost always simultaneously and none can be underestimated. A large discussion is held internationally indicating that although the construction of harbours is meant to enhance tourism and local economy, it very often leads to environmental and socio-economic failure that are analysed in the present paper. In addition, the paper discusses the methods of beach protection in such cases and gives guidelines for following effective practices in Greece. © 2015 Springer International Publishing Switzerland

Prediction of wave transmission coefficient using neural networks and experimental measurements

In the present paper, an application of composite modeling is presented, for the estimation of wave transmission through flushing culverts. Specifically, experimental measurements are used and an Artificial Neural Network (ANN) is structured for the prediction of the transmission coefficient. Measurements were obtained from physical model tests that have been conducted in 2D and 3D experimental facilities, at the Laboratory of Harbour Works, at National Technical University of Athens. The derived ANN, despite the limited available experimental data, showed satisfactory behaviour and may be used as an alternative tool for a first estimation of the transmission coefficient through flushing culverts in harbour design in combination with proposed empirical equations or numerical models. © 2015 Taylor and Francis Group