Hydrodynamic Performance of Heave Plates on Floating Offshore Wind Turbine Platforms

Offshore wind energy plays an important role in marine renewable energy. Onshore wind production has increased over the last few decades. However, it is limited by space, noise and aesthetic concerns. Offshore wind farms can be deployed far from the coast taking advantage of stronger and steadier winds. Currently, most of the offshore wind farms are installed in shallow waters (30 m or less) using fixed foundation types. However most of the wind resources are located in waters deeper than 60 m, where fixed foundations are no longer viable. This problem can be addressed by installing floating offshore wind turbines (FOWTs). Heave plates play an important role in the hydrodynamic behavior of FOWT structures. They provide additional added mass and damping that improves the global performance of the wind turbine. Some prototype designs, e.g. Windfloat [Roddier et ah, 2009], use heave plates to stabilize the platform in pitch thus improving the operating conditions of the wind turbine. Experimental research is the main means to study the hydrodynamic characteristics of heave plates. Scaled model tests can provide a good understanding of the behavior of floating platforms for a reasonable economic investment. This reduces risks and helps to optimize the design of the full scale platform. In this thesis, experimental results for added mass and damping coefficients of a disk and column configuration were investigated. First, experimental results for a circular disk and column configuration were compared at two model scales 1:20 and 1:80. The 1:20 scaled model experiments were performed at the Technical University of Madrid wave basin that is 100m long, 3.8m wide and 2.2m deep and results were reported in [Lopez P. and Souto I., 2015]. This study presents 1:80 scaled model test results from experiments conducted at the University of Maine’s Marine Ocean & Offshore Research (MOOR) facility. The platform consists of a circular heave plate of 0.25m diameter and 1.25mm thickness, attached to a column of 0.088 m diameter and 0.3 m height. Experiments were performed at three different amplitude of oscillations and at eight different frequencies. The effects on the hydrodynamic coefficients of different shapes and thicknesses of the heave plates were also studied. Added mass and damping were compared for an hexagonal heave plate of 0.137m edge length and a circular heave plate of 0.25m diameter. Also, the effects of thickness on the hydrodynamic coefficients were investigated by comparing two circular heave plates of 1.3 mm and 4.3 mm thickness. Finally, the interaction between an oscillating heave plate and a wave field was investigated, followed by a discussion of how the added mass and damping coefficients are affected by the wave field