Hybrid wind wave systems combine offshore wind turbines with wave energy on a shared platform. These systems optimize power production at a single location by harnessing both the wind and the waves. Wave energy is currently at an earlier development stage than offshore wind. Research in this area is focused in wave energy converters being used for platform motion suppression of floating offshore wind turbines. Wave energy converters can passively shelter offshore wind turbines from waves and can also be actively controlled to reduce the system loads. Additionally, a small amount of supplemental power may be generated, which can be used for offshore wind turbine local power needs. There may be future benefits to these hybrid systems, but at this stage wave energy may increase the project cost and risk of offshore wind turbines. Hybrid wind wave system research and development is discussed, with a focus on floating offshore wind turbines. Additionally, two ocean demonstration scale hybrid wind and wave systems are discussed as case studies: the Poseidon Wave and Wind system and the W2Power system. Hybrid wind wave systems show potential to be part of the future of offshore wind energy.

The research problem discussed in this paper is of relevance to floating offshore wind turbine design, where heave plates are attached to the columns of a semi-submersible in order to improve vertical plane stability and the power output. Because of the shallow draft of these structures, the heave plates are proximal to the water surface. When subject to vertical plane motions the flow around a plate is altered by the presence of the free surface, resulting in changes in added mass and damping forces. In this paper, we present the experimental results for the added mass and damping coefficients for circular heave plates attached to a column, when oscillating in heave in the presence of oncoming waves. The results tend to indicate that applying the hydrodynamic coefficients obtained from still water experiments for a structure moving in waves may only be an approximation. For different relative phases of the wave and the motion, large variations could occur. We define a modified Keulegan—Carpenter ($KC$) number that depends on the relative amplitude of motion with respect to the wave. With this definition, the added mass and damping values are seen to be closer to the still water trends. However, at lower $KC$ values, the added mass coefficients could differ by 30%, which can affect natural frequency estimates. Thus, caution needs to be exerted in the selection of hydrodynamic coefficients for heave plates oscillating in proximity to the free surface.

Heave plates are structural components of a floating structure whose primary purpose is to increase damping and added mass in heave or pitch direction. If multiple plates are stacked along a common axis, then the relative diameters and spacing in between are important factors affecting the performance. In the present study, hydrodynamic coefficients of single, double and triple disks are experimentally investigated in different arrangements as Uniform, Concave and Convex edge shapes. It is observed that each disk in a multiple arrangement with lower diameter ratio have better hydrodynamic performance compared to the cases with higher diameter ratio when the spacing is small compared to the mean diameter. Some hydrodynamic aspects affecting floating structure design are also discussed in this paper.

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water, where snap loading may occur more frequently. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used, and the tension force at the fairlead was measured by tension gages. Moreover, the line geometry, touchdown point speed, and mooring line velocity were derived from image processing techniques. Surge motions at fairlead were driven from a programmable wavemaker. Regular surge motions with different frequencies and pretensions were tested in this system in order to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, the mooring line keeps a typical catenary shape, and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed, and it cyclically goes through the stage of semitaut and fully taut. We successfully reproduced a snap event in the laboratory scale, and the resulting mooring line damping can considerably increase in this manner. Two criteria for snap event were examined, and both of them were verified by the experiments.