Maximum Wave Load Cycles on Submerged Rotating Tidal Energy Turbines: Identification of Worst Case Scenarios

Abstract

In recent decades, significant research and development has been invested in techniques that harvest renewable energy from an ocean environment. Starting from offshore wind energy we also see developments in devices that extract energy from sources such as waves and tides. Although a lot can be learned by transferring existing on- and offshore technology, a multitude of new challenges arouse. The offshore wind industry for example has struggled with wave induced loads on their wind turbine foundations structures, which resulted in intense nacelle and rotor vibrations. These vibrations have a significant impact a turbine’s lifetime. The specific problem of oscillating wave loads worsens for tidal energy turbines as no longer only the foundation but also the rotor itself is submerged and directly exposed to wave induced forces. Therefore, the best possible determination of these loads is a key prerequisite for any holistic vibration and fatigue analysis. During operation, the turbine rotor will, depending on the actual tidal current velocity, rotate at different speeds whilst the waves propagate over it. The number of ratios between wave period Tw and rotational period of the rotor Tr is therefore infinite. This raises the question what load changes a revolving rotor blade experiences and which combination of Tw/Tr and wave encounter angle will generate the maximum loads per cycle. This study presents a comprehensive, general approach to identify maximum possible wave induced forces and moments on stationary and turning rotor blades, for any turbine design, position in the water column and orientation in the wave field. The procedure is exemplified for a generic 3-bladed horizontal axis turbine which is fixed to the seabed. The approach identifies loads depending on wave period and height, the period of rotor rotation and the wave encounter angle, utilising diffraction theory. Forces and moments are firstly calculated in a global coordinate system and subsequently transferred into a blade fixed coordinate system. This allows for an examination of the load changes as the blades rotate about the turbine axis. Thus, the worst-case scenarios in terms of load changes for each combination of wave parameters can be identified. Those maximum load cycle events are then combined to transfer functions, for loads and moments respectively. Transfer functions such as these will later allow for a quick identification of maximum load changes and cycle periods, depending on any given environmental condition. This information will help identifying structural loads and immanent fatigue and vibration issues during the actual turbine design process.