Adi Kurniawan

Modelling and Simulation of a Floating Oscillating Water Column

This article describes the computation of hydrodynamic parameters, modelling, and simulation of a floating oscillating water column wave energy device. The frequency-domain hydrodynamic parameters are computed using a three-dimensional panel method. Parameters related to wave radiation due to applied air pressure inside the air chamber are evaluated from reciprocity relations, without having to solve explicitly for the radiation potential due to the applied pressure. The dynamics of the whole system, including nonlinear air compressibility and relief valve characteristics, is modelled using bond graph as a tool. Both time- and linear frequency-domain models are described and selected simulation results, especially the converted power, are presented.Copyright

Optimal Geometries for Wave Absorbers Oscillating About a Fixed Axis

This paper deals with the optimization of wave absorbers oscillating about a fixed submerged horizontal axis. A multiobjective optimization algorithm is employed to search for the optimal geometries where two cost criteria are used as the objective functions to be minimized. The two cost criteria are the ratios, integrated over a specified frequency range, of the submerged surface area to the maximum absorbed power, and of the maximum reaction force to the maximum absorbed power. Geometric configurations with uniform simple cross-sectional shapes, viz. line, circle, and elliptical sections, are considered. For each configuration, the body dimensions and submergence, as well as the submergence of the rotation axis, are the variables to be optimized. It is found that most of the optimal geometries have their rotation axes close to the sea bottom and their bodies close to the free surface. The optimal size of the geometries varies depending on the selected wave frequency range, but the optimal cross-sectional dimensions are generally less than one third of the water depth when optimized over a uniform distribution of wave frequencies from 0.4 to 1.3 rad/s. Among the cross sections considered, the elliptical one performs best.

Wave energy devices with compressible volumes

We present an analysis of wave energy devices with air-filled compressible submerged volumes, where variability of volume is achieved by means of a horizontal surface free to move up and down relative to the body. An analysis of bodies without power take-off (PTO) systems is first presented to demonstrate the positive effects a compressible volume could have on the body response. Subsequently, two compressible device variations are analysed. In the first variation, the compressible volume is connected to a fixed volume via an air turbine for PTO. In the second variation, a water column separates the compressible volume from another volume, which is fitted with an air turbine open to the atmosphere. Both floating and bottom-fixed, axisymmetric, configurations are considered, and linear analysis is employed throughout. Advantages and disadvantages of each device are examined in detail. Some configurations with displaced volumes less than 2000 m3 and with constant turbine coefficients are shown to be capable of achieving 80% of the theoretical maximum absorbed power over a wave period range of about 4 s.

Power Absorption Measures and Comparisons of Selected Wave Energy Converters

In this study, a selection of Wave Energy Converters (WECs) with different working principle is considered. It comprises a heaving device reacting against the seabed, a heaving self-reacting two-bodies device, a pitching device, and a floating OWC device. They are inspired by concepts which are currently under development. For each of these concepts, a numerical Wave To Wire (W2W) model is derived. Numerical estimates of the energy delivery which one can expect are derived using these numerical models on a selection of wave site along the European coast. This selection of wave site is thought to be representative with levels of mean annual wave power from 15 to 88 kW/m. Using these results, the performance of each WEC is assessed not only in terms of yearly energy output, but also in terms of yearly absorbed energy/displacement, yearly absorbed energy/wetted surface, and yearly absorbed energy per unit significant Power Take Off force. By comparing these criteria, one gets a better idea of the advantages and drawbacks of each of the studied concepts.Copyright

Wave energy absorption by a floating air bag

A floating air bag, ballasted in water, expands and contracts as it heaves under wave action. Connecting the bag to a secondary volume via a turbine transforms the bag into a device capable of generating useful energy from the waves. Small-scale measurements of the device reveal some interesting properties, which are successfully predicted numerically. Owing to its compressibility, the device can have a heave resonance period longer than that of a rigid device of the same shape and size, without any phase control. Furthermore, varying the amount of air in the bag is found to change its shape and hence its dynamic response, while varying the turbine damping or the air volume ratio changes the dynamic response without changing the shape.

A Numerical Analysis of the Response and Air Gap Demand for Semi-Submersibles

This paper examines the effects of viscous damping on the prediction of air gap demand for semi-submersibles. This is illustrated by a case study of a typical six-column, double pontoon semi-submersible in irregular waves. Linear responses of the platform and free-surface elevations at selected locations are computed using WAMIT, in which viscous damping effects are included through the use of a linear damping matrix. The spectra of the platform’s response and of the relative free-surface elevation at the selected locations, as well as the probability distributions of air gap minima obtained using different damping matrices are compared. It is demonstrated that correct modelling of viscous damping is important for an accurate prediction of air gap demand of a floating platform, especially if the peak frequency of the wave spectrum is close to the heave, roll, and pitch natural frequencies of the platform.Copyright

Wave energy absorption by a submerged air bag connected to a rigid float

A new wave energy device features a submerged ballasted air bag connected at the top to a rigid float. Under wave action, the bag expands and contracts, creating a reciprocating air flow through a turbine between the bag and another volume housed within the float. Laboratory measurements are generally in good agreement with numerical predictions. Both show that the trajectory of possible combinations of pressure and elevation at which the device is in static equilibrium takes the shape of an S. This means that statically the device can have three different draughts, and correspondingly three different bag shapes, for the same pressure. The behaviour in waves depends on where the mean pressure-elevation condition is on the static trajectory. The captured power is highest for a mean condition on the middle section.