M. T. Rahmati

A JOINT NUMERICAL AND EXPERIMENTAL STUDY OF A SURGING POINT ABSORBING WAVE ENERGY CONVERTER (WRASPA)

This paper presents the findings from using several commercial computational fluid dynamics codes in a joint numerical and experimental project to simulate WRASPA, a new wave energy converter (WEC) device. A series of fully 3D non-linear simulations of WRASPA are presented. Three commercial codes STAR-CCM, CFX and FLOW-3D are considered for simulating the WRASPA device and final results are presented based on the use of Flow-3D. Results are validated by comparison to experimental data obtained from small scale tank tests undertaken at Lancaster University (LU). The primary aim of the project is to use numerical simulation to optimize the collector geometry for power production over a range of likely wave climates. A secondary aim is to evaluate the ability of commercial codes to simulate rigid body motion in linear and non-linear wave climates in order to choose the optimal code with respect to compute speed and ease of problem setup. Issues relating to the ability of a code in terms of numerical dissipation of waves, wave absorption, wave breaking, grid generation and moving bodies will all be discussed. The findings of this paper serve as a basis for an informed choice of commercial package for such simulations. However the capability of these commercial codes is increasing with every new release.Copyright

Control systems for Wraspa

The paper discusses the need for a wave energy converter (WEC) to sense and respond to its environment in order to survive and to produce its maximum useful output. Such systems are described for Wraspa, a WEC being developed at Lancaster University and first reported at ICCEP in 2007. The main control system that continually monitors and optimises the power-take-off is termed “Stepwise Control” and seeks to continually adjust the damping force applied to the collector to suit the wave force that drives it. The complete instrumentation and control system that will be needed is considered briefly, including the above PTO control system; direction sensing and heading control; tide level compensation; condition monitoring and provisions for access and maintenance.

Experimental and Numerical Study of the Bending Behavior of a Flexible Riser Model

Unbonded flexible risers have become the main means of extracting hydro carbonates from deep waters. So, understanding the complex structural integrity of flexible risers has become a crucial issue for the offshore industry. In this paper, an experimental test and a detailed finite element analyses were carried out on a scaled down model of a flexible riser pipe in order to understand its bending behavior. The model used consists of four layers which include two cylindrical polycarbonate tubes and two steel helical layers. One helical layer, wounded around the pipe assembly, represents the carcass layer in an actual flexible riser whilst the other represents the riser tendon armour layers. The model was subjected to a three point bending load in order to study its bending-curvature behavior. The test data was then compared with the FE numerical results which predicted a similar nonlinear trend but under predicts the strain at tendon layers.Copyright

Small-Scale FE Modelling for the Analysis of Flexible Risers

Flexible risers which are used for transporting oil and gas between the seabed and surface in ultra-deep waters have a very complex internal structure. Therefore, accurate modeling of their behaviour is a great challenge for the oil and gas industry. Constitutive laws based on beam models which allow the large-scale dynamics of pipes to be related to the behaviour of its internal components can be used for multi-scale analysis of flexible risers. An integral part of these models is the small-scale FE model chosen and the detailed implementation of the boundary conditions. The small scale FE analyses are typically carried out on models of up to a few meters length. The computational requirements of these methods limit their applications for only multi-scale structural analysis based on a sequential approach. For nested multi-scale approaches (i.e. the so called FE2 method) and for multi-scale multi-physic analyses, e.g. fluid structure interaction modeling of flexible risers, more efficient methods are required. The emphasis of the present work is on a highly efficient small-scale modelling method for flexible risers. By applying periodic boundary conditions, only a small fraction of a flexible pipe is used for detailed analysis. The computational model is firstly described. Then, the capability of the method in capturing the detailed nonlinear effects and the great advantage in terms of significant CPU time saving entailed by this method are demonstrated. For proof of concept the approach is applied on a simplified 3-layer pipe made of inner and outer polymer layers and an intermediate armour layer made of 40 steel tendons.Copyright

Optimum Power Capture of a New Wave Energy Converter in Irregular Waves

This paper presents the optimum power capture of a new point-absorber wave energy converter, in irregular waves. A stepwise control system for the wave energy converter (WEC) is developed. The control system is used to efficiently extract power from irregular waves where amplitudes vary from wave to wave. The Bretschneider spectrum is used in the experiment and the device is ‘tuned’ to the peak period of the sea state. It is shown that this WEC has a reasonable capture width in irregular waves. However, the optimum mean power depends on the wave spectrum, the shape of the collector body, its freeboard and the device pivot depth.Copyright

Numerical hydrodynamic modelling of a pitching wave energy converter

Two computational methodologies – computational fluid dynamics (CFD) and the numerical modelling using linear potential theory based boundary element method (BEM) are compared against experimental measurements of the motion response of a pitching wave energy converter. CFD is considered as relatively rigorous approach offering non-linear incorporation of viscous and vortex phenomenon and capturing of the flow turbulence to some extent, whereas numerical approach of the BEM relies upon the linear frequency domain hydrodynamic calculations that can be further used for the time-domain analysis offering robust preliminary design analysis. This paper reports results from both approaches and concludes upon the comparison of numerical and experimental findings.

Numerical and experimental study of a point absorbing wave energy converter in regular waves

This paper describes recent numerical modelling of a new wave energy converter WRASPA (Wave-driven, Resonant, Arcuate action, Surging Power-Absorber). The presented results are based on a range of regular incident waves. Also the experimental setup is outlined to illustrate the brief design principles of the device. A comparison of experimental and numerical results is performed as part of the validation process. The interaction of waves with WRASPA device has been simulated using a commercial code Flow-3D as one of the primary objectives of this project. The ability of the method to compute a range of linear waves and coupled motion of the device is discussed. The codes ability to deal with wave propagation and interaction with rigid moving structures is evaluated.

An Inverse Approach For Airfoil Design

Inverse design methods directly compute geometry for specified design parameters such as surface pressure or velocity, which is related to the performance of an airfoil (or a blade) geometry. These methods replace the time consuming iterative procedure of direct methods in which a large number of different blade shapes are designed and analysed to find the one which creates the surface velocity or pressure distribution closest to the desired one. In this paper a viscous inverse method for airfoil design is described. The inverse design approach computes an airfoil shape based on the target surface pressure distribution. The re-design of an airfoil, starting from an initial arbitrary profile in subsonic flow regimes, demonstrates the merits and robustness of this approach.

Investigating a Power-Obsorber Wave Energy Converter

This paper presents the assessment of the optimum performance of a wave energy converter. In this device which is hinged at the seabed, wave forces act on the face of a collector body, carried on an arm that rotates about a fixed horizontal axis. The collector body oscillates at about the frequency of the ocean swell generating high power from this relatively small and economical device. The performance of the device is investigated using numerical hydrodynamic analysis and the wave tank experiment for a model at a nominal scale of 1/100. Also the optimum mean power output of the device in irregular wave climates is assessed. A first-order method is used to calculate the mean power spectrum of the device in waves of dissimilar spectra. It is shown that the choice of tuning period has a major effect on the power absorbed, hence on the power output.Copyright