CALM Buoy Research Articles

Coupled CFD-FEM simulation of hydrodynamic responses of a CALM buoy

In this paper, the Finite-Analytic Navier-Stokes (FANS) code is coupled with an in-house finite-element code to study the dynamic interaction between a floating buoy and its mooring system. Hydrodynamic loads on the buoy are predicted with the FANS module, in which Large Eddy Simulation (LES) is used as the turbulence model. The mooring lines are modeled based on a slender body theory. Their dynamic responses are simulated with a nonlinear finite element module, MOORING3D. The two modules are coupled by transferring the forces and displacements of the buoy and its mooring system at their connections through an interface module. A free-decay model test was used to calibrate the coupled method. In addition, to investigate the capability of the present coupled method, numerical simulations of two degree-of-freedom vortex-induced motion of a CALM buoy in uniform currents were performed. With the study it can be verified that accurate predictions of the motion responses and tension responses of the CALM buoy system can be made with the coupling CFD-FEM method.

An Analytical Solution of Wave Exciting Loads on CALM Buoy with Skirt

Linearized potential wave theory is applied to calculate the wave exciting loads on a CALM buoy in water of finite depth. The solution is based on the domain decomposition method and the unknown constants in the velocity potentials are determined by matched eigenfunction expansions. A comparison of the analytical solution with published experimental results on a vertical truncated cylinder is performed as part of the validation process. The effects of the disk on the wave exciting forces are discussed.

The new wave energy converter WaveCat: Concept and laboratory tests

For wave energy to become a fully-fledged renewable, efficient and reliable Wave Energy Converters (WECs) must be developed. The objectives of this article are to present WaveCat, a recently patented WEC, and its proof of concept by means of an experimental campaign in a large wave tank. WaveCat is a floating WEC whose principle of operation is oblique overtopping; designed for offshore deployment (in 50–100 m of water), it has two significant advantages: minimum (if at all) impact on the shoreline, and access to a greater resource than nearshore or shoreline WECs. It consists of two hulls, like a catamaran (hence its name); unlike a catamaran, however, these hulls are not parallel but converging. Using a single-point mooring to a CALM buoy, the bows of WaveCat are held to sea, so incident waves propagate into the space between the hulls. Eventually, wave crests overtop the inner hull sides, and overtopping water is collected in reservoirs at a level higher than the (outer) sea level. As the water is drained back to sea, it drives turbine-generator groups. The freeboard and draught, as well as the angle between the hulls, can be varied depending on the sea state. After preliminary tests with a fixed model of WaveCat in a wave flume, which constituted the first step in the development of the WaveCat patent, in this work a floating model was tested in a large wave tank. In addition to serving as a proof of concept of the WaveCat model, this experimental campaign allowed to gather data that will be used to calibrate and validate a numerical model with which to optimise the design. In addition, it was found in the tests that the overtopping rates (and, therefore, the power performance) greatly depended on the angle between hulls, so that the possibility of varying this angle (as contemplated in the patent) should indeed be incorporated into the prototype.

Vertical bending moments and axial forces in a floating marine hose-string

The primary mechanisms by which vertical bending moments and axial forces are induced in a floating hose-string attached to a CALM buoy are quantitatively assessed by means of model tests and analytical study. It is found that the dynamic axial forces on the hose-string are determined by the push-pull action exerted on the string by the surge response of the buoy in waves. If the surge characteristics of the buoy are known, dynamic axial forces along the hose-string may be estimated using the empirical model presented. The dynamic vertical bending moment on the hose at the buoy mostly depends on the heave response of the buoy in waves. Good estimates of the vertical bending at the buoy may be obtained using the analytical model presented.