Specific Wave Frequencies Research Articles

Design and stability analysis of a new six-floater oscillating water column-based floating offshore wind turbine platform

The operational efficiency and lifespan of Floating Offshore Wind Turbines (FOWTs) are adversely impacted by the inherent platform motions and undesired vibrations induced by wind and wave loads. To effectively address these effects, the control of specific structural motions is of utmost importance, with platform pitch and yaw identified as the primary Degrees Of Freedom (DOF) that require attention. This study proposes a novel utilization of Oscillating Water Columns (OWCs) as a reliable and viable solution to mitigate platform pitch and yaw motions, thereby significantly enhancing the efficiency and reducing fatigue in wind turbines. This article aims to evaluate the impact resulting from integrating OWCs within each discrete floater of a Six-Floater platform. By considering different combinations of OWCs, a comprehensive analysis of the Response Amplitude Operators (RAOs) associated with pitch and yaw motions is presented. The primary objective is to identify the most efficient arrangements of OWCs and determine suitable combinations that effectively stabilize platform pitch and yaw motions. The empirical results substantiate that specific OWC configurations exhibit notable dampening effects on both pitch and yaw motions, particularly within specific wave frequency intervals. Consequently, it can be inferred that the integration and adequate operation of OWCs facilitate a substantial improvement in the stabilization of multi-floater platforms.

A CLOSED-FORM SOLUTION FOR INTERACTIONS BETWEEN WAVES AND AN ARRAY OF FISH NET CAGES

Understanding the multi-cage interaction mechanism in fish cage arrays and the hydrodynamic behaviour of cage structures will help engineers explore the feasibility of offshore fish farming. In this study, a semi-analytical method is developed to simulate the wave response of an array of cylindrical fish cages with arbitrary layouts and dimensions, including the wave interference effects among the multiple cages. By eigenfunction expansions, the general solution of the present research problem is expressed in the form of a Fourier-Bessel series, and the corresponding particular solution is obtained by introducing relevant boundary conditions and the least-squared method. The simulation results show that significant wave effects occur at the top of the cages, and the dynamic response of each cage has obvious phase differences. Parametric studies on the wave forces show that the wave force is zero at a specific wave frequency and the wave forces are decreased under high porosity nets.

Mean Drift Wave Forces on Arrays of Bodies Surrounded by Thin Porous Surfaces

A semi-analytical solution is presented for the evaluation of mean drift wave forces on arrays of structures composed of impermeable vertical cylindrical bodies and surrounded by thin, porous surfaces. The porous cell is assumed to have fine pores so that a linear pressure drop can be considered. The mean drift wave forces on the array are determined via two principally different approaches: the momentum conservation theory and the direct integration of all pressure contributions upon the bodies’ instantaneous wetted surfaces. A solution method that solves the corresponding diffraction problem following the multiple scattering approach is proposed to account for the hydrodynamic interaction phenomena between the waves and the bodies. Numerical results are presented which illustrate the consistency of each of the two approaches in accurately evaluating the mean wave drift forces on several examined array configurations. It is shown that the thin, porous surface significantly reduces the drift forces on the array of bodies, whereas at specific wave frequencies, local enhancements of the forces’ values are noted due to the amplified wave field between the members of the array.

Analytical solutions of hydroelastic interactions between waves and submerged open-net fish cage modeled as a porous cylindrical thin shell

In this paper, a new semi-analytical solution is proposed to describe the interactions between ocean waves and a flexible cylindrical net cage that is submerged at different depths below the mean water level. The flexible net is treated as a thin perforated shell, and its deformation is governed by the membrane vibration equation of cylindrical shells. The small-amplitude wave theory is adopted to simulate the wavefield, while the flow passing through the cage is described by the porous medium theory. The numerical results exhibit significant wave responses of the net cage, including the distribution properties of wave surfaces, dynamic pressure drops on the net interface, and net structure displacements. Furthermore, the influences of several important design parameters on the hydrodynamic action imposed on the net cage are revealed by parametric studies. The present studies conclude that the significant wave impact is mainly concentrated on the free water surface, and increasing the porosity and flexibility of the net can alleviate wave scattering and the hydrodynamic actions. In addition, at specific wave frequencies, the horizontal wave force acting on the cage will vanish. These findings should be useful to engineers who are designing offshore fish cage systems.

Semi-analytical study on an integrated-system with separated heaving OWC and breakwater: Structure size optimization and gap resonance utilization

To complement the topic on the integration of Oscillating Water Column (OWC) device with breakwater structures, a novel integrated system composed of an offshore heaving OWC device and a floating stationary breakwater with a certain length of gap between each other is proposed. Under the framework of potential flow theory, corresponding theoretical model is established and solved by decomposing the boundary-value problems into one wave scattering component and two radiation components. Apart from the examinations concentrating on the influences from the geometrical configuration of the breakwater, the gap distance between the OWC device and the breakwater is changed in a broad range to identify its effects on the interested hydrodynamic performance. It is found that for a specific wave frequency, when the gap distance is well-tuned against the wave length to satisfy the condition of sloshing mode happening inside the gap, power extraction efficiency is much likely to achieve its maximum extent. In this way, the optimization in terms of gap distance is proved to be effective in enhancing wave power capturing within a wider range of wave conditions.

An analytical, numerical, and experimental study of Rayleigh wave scattering for internal vertical crack evaluation

This paper describes an analytical, numerical, and experimental study of surface wave scattering for an internal vertical crack (SWS-IVC). The analytical model of SWS-IVC is validated through comparison with parametric studies based on dynamic finite element (FE) simulations and experimental measurement. The results of the study allow for the demonstration of crack depth estimation understanding response variation curves, which are the wave response ratio of scattering waves to incident waves according to specific wave frequency and crack geometry. In addition, the study advances the crack evaluation based on the SWS-IVC model leveraging relative wave energy attenuation obtained from arrayed air-coupled MEMS sensors.

Opening the air‐chamber of an oscillating water column spar buoy wave energy converter to avoid parametric resonance

The oscillating-water-column (OWC) spar-buoy is a type of wave energy converter that may exhibit undesirable large roll and pitch amplitudes caused by a dynamic instability induced by parametric resonance. The occurrence of this phenomenon not only reduces the power extraction but significantly increases the structural loads on the buoy, the turbine rotor and on the mooring system. The paper compares the parametric resonance behaviour of two configurations of an OWC spar-buoy using experimental data obtained in a wave flume at a scale of 1:100. The configurations investigated were: (1) closed and (2) fully open-air chamber. The experimental tests covered a wide range of regular and irregular waves, as well as in free decay experiments. Results showed that opening the air chamber reduces the coupling between the buoy and the OWC within, thus shifting the damped natural heave frequency of the system in comparison with the closed chamber configuration. This effect changes parametric resonance characteristics of the two configurations due to the coupling between roll/pitch and heave modes. Moreover, for specific wave frequencies, the occurrence of parametric resonance observed when the chamber is closed do not occur while the air chamber is fully open. These results suggest the possibility of controlling a pressure relief valve installed on top of the device to reduce parametric resonance whenever this dynamic instability is detected.

Local Enhancements of the Mean Drift Wave Force on a Vertical Column Shielded by an Exterior Thin Porous Shell

The wave interaction with a vertical column shielded by an exterior porous shell is studied within the framework of potential flow theory. The structures are fixed rigidly at the sea bottom. The interior cylinder is impermeable, and the exterior shell is slightly porous and thin. Additionally, the exterior shell is assumed to have fine pores, and a linear pressure drop is adopted at the porous geometry. The mean drift wave force on the system is thereby formulated by two alternative ways, based respectively on the direct pressure integration, i.e., the near-field formulation, and the application of the momentum conservation theorem in the fluid domain, i.e., the far-field formulation. The consistency of the two formulations in calculating the mean drift wave force is assessed for the present problem. Numerical results illustrate that the existence of the porous shell can substantially reduce the mean drift wave force on the interior column. It also appears that the far-field formulation consists of a conventional part as well as an additional part caused by the energy dissipation through the porous geometry. The mean drift wave force on the system is dominated by the first part, which resembles that on an impermeable body. Local enhancements of the mean drift wave force are found at some specific wave frequencies at which certain propagation modes of the fluid satisfy a no-flow condition at the porous shell.

Hydrodynamic interaction among multiple columns in front of a vertical wall

The berthing or mooring facilities in ports and/or harbors are usually in the form of a group of vertical columns. These structures are arranged in close proximity to the long vertical harbor wall. Accurate prediction of wave force on such structures is of importance, yet, challenging, as both interferences among cylinders as well as between cylinders and the wall must be correct. In this work, a novel analytical model has been developed to investigate the hydrodynamic characteristics of such structures within the framework of linear wave theory. The original problem is considered to be equivalent to that of two symmetrical arrays of cylinders in open water; both are exposed to the ambient incident waves. The original array of cylinders is labelled as real cylinders hereafter, and the second array as image cylinders. The assumption of full wave reflection on the wall is essential by using this treatment. Due to the symmetry, simple relationships between the diffracted waves from the real and image cylinders are found, which reduces the number of unknowns associated with the velocity potential by half. After solving the velocity potential, the wave forces as well as the surface elevation are evaluated. Two different solutions have been developed for the calculation of mean drift wave force. One is based on the direct pressure integration and the other is based on the application of momentum conservation theorem in a limited fluid volume surrounding a certain cylinder in the array. Then, the systematic analysis is performed for a line array of equally-spaced cylinders parallel to a vertical wall. Largely amplified wave forces are found at specific wave frequencies, which occur with the appearance of pronounced wave runup around the cylinders. Furthermore, the relative contribution of the first-order and the mean drift wave forces to the total wave force is examined.

Wave Radiation by A Submerged Ring Plate in Water of Finite Depth

A plate submerged at a certain depth underneath the sea surface has been proposed as a structure type for different purposes, including motion response reduction, wave control, and wave energy harvesting. In the present study, the three-dimensional wave radiation problem is investigated in the context of the linear potential theory for a submerged ring plate in isolation or attached to a floating column as an appendage. In the latter case, the ring plate is attached at a certain distance above the column bottom. The structure is assumed to undergo a heave motion. An analytical model is developed to solve the wave radiation problem via the eigenfunction expansion method in association with the region-matching technique. With the velocity potential being available, the hydrodynamic coefficients, such as added mass and radiation damping, are obtained through the direct pressure integration. An alternative solution of radiation damping has also been developed in this study, in which the radiation damping is related to the Kochin function in the wave radiation problem. After validating the present model, numerical analysis is performed in detail to assess the influence of various plate parameters, such as the plate size and submergence depth. It is noted that the additional added mass due to the attached ring plate is larger than that when the plate is in isolation. Meanwhile, the radiation damping of the column for the heave motion can vanish at a specific wave frequency by attaching a ring plate, corresponding to a condition that there exist no progressive waves in the exterior region.

Buoy Analysis in a Point-Absorber Wave Energy Converter

In this paper, a single-body point absorber system is analyzed to enhance its power absorption performance. The wave energy converter consists of a single floating body coupled to a direct-drive power takeoff system placed on the seabed. The geometry of a cylindrical buoy with large draft is modified, obtaining a particular geometry that is used to enhance the power absorption of the wave converter at a given site and at a finite depth. A numerical analysis tool (NEMOH) is used to obtain the buoy's frequency-dependent hydrostatic parameters; in addition, the buoy's dimensions are parameterized to tune the natural frequency of the oscillating system toward the frequency of dominant incident waves, thus enhancing wave power absorption for a specific wave frequency range. Furthermore, the damping influence of the power takeoff system on the performance of the wave energy converter is also considered.

A discrete-modules-based frequency domain hydroelasticity method for floating structures in inhomogeneous sea conditions

Based on the three-dimensional (3D) potential theory and finite element method (FEM), this paper proposes a new numerical method for hydroelastic predictions of floating structures in inhomogeneous seabed and wave field conditions. The continuous floating structure is first discretized into rigid modules connected by elastic beams. The motion equations of the entire floating structure are established according to the six degrees of freedom (6DOF) motions of each module by coupling the hydrodynamics of the modules with the structural stiffness matrix of the elastic beams in the frequency domain. By applying different wave excitation forces onto different modules, this discrete-modules-based method then uniquely realizes application of various wave excitation forces onto different modules of the structures in inhomogeneous waves. The hydroelastic responses of a plate and a Wigley hull under an even and uneven seabed using the proposed method are verified against the results from the published model tests and the conventional 3D hydroelastic method. Finally, the effects of inhomogeneous waves on the distributions of the bending moment, shear force and vertical displacements of the freely floating plate are investigated. The results show that the inhomogeneity of waves may induce about 2∼3 times increase of the force responses in a specific wave frequency.