Excitation Forces on Point Absorbers Exposed to High Order Non-linear Waves

Viuff, Thomas Hansen ; Andersen, Morten Thøtt

4. term

Structural and Civil Engineering, Master

2013

2013-06-10

63 pages

Wave energy devices are often located in intermediate or shallow water close to the free water surface where the wave exposure is extreme. The structures typically consist of large thin curved plates, spherical shells, or cylindrical structures which are sensitive to pressure loads. The lack of proper methods to calculate design pressure distributions has led to structural failures such as buckling in the shells in wave energy prototypes. As a step towards understanding the complex loading from high order non-linear waves, this paper presents a practical approach to estimate wave excitation forces accounting for both nonlinearity and diffraction effects. The method is validated by laboratory experiments using a hemispherical point absorber with a 6-axis force transducer, but the technique is believed to be applicable for most types of submerged or semisubmerged floating devices. The applied method is based on calculation of a peak force coefficient defined as experimentally measured forces on the structure divided by forces estimated by the chosen theoretical method. Methods used include an integration of the undisturbed wave pressure over the surface of the structure, corresponding to the Froude-Krylov force, and a numerical solution to linear potential diffraction theory. Since the two methods have mutual limitations in describing higher order waves and diffraction, a combination of the two will be introduced.

WEC, hemispherical point absorber, wave loads, wave regimes, non-linear diffraction

Download
Download
Download
View record in AAU Student Projects