# The influence of a rotational turbine model on the mooring loads of a tidal energy converter

The influence of a rotational turbine model on the mooring loads of a tidal energy converter

2016-08-17

In a world with an increasing energy demand and consciousness of the long-term effects of the use of fossil fuels, offshore industries are continuously searching for new sources of energy. Marine tidal currents, which are a consequence of the gravitational forces exerted by the Moon and the rotation of the Earth, have a theoretical potential up to 120 GW. At some of these sites generating energy with the use of tidal energy converters may be economically viable. At the moment the trend tends to go to floating tidal energy converters, since they are easier to install and maintain. To be cost-competitive, more research needs to be done. The goal of this research is how a more sophisticated turbine model influences the mooring loads of a tidal energy converter. Between Texel and Den Helder in the Marsdiep a BlueTEC has been installed. An artist impression of the BlueTEC is shown below. The floating platform is a complex coupled system. The forces exerted by the turbine on the platform influence its motion behavior, but the motion behavior of the platform influences the loading on the turbine on its turn by a changing inflow velocity and angle. At the same time a control system ensures the power output of the turbine by adjusting the rotational speed of the turbine to the optimum. The horizontal motions of the floating platform are restrained by the mooring system, although also the vertical motions are influenced by the presence of a mooring system. While the mooring system influences the motion behavior of the tidal energy converter, this works also the other way around, the motions of the floater influencing the motions of the mooring system. This leads to the inflow velocity at the turbine being dependent on the current, waves and the velocity of the floater, all motions being coupled to each other. Due to non-linearity’s this coupling can only be taken into account in the time-domain. An easy way to take the coupling of the hydrodynamic forces working on the turbine into account on the motions of the floater is by applying an actuator disk model. The basic idea is that the turbine rotor is modelled as a disk with an equal area. Another model that can be applied is a quasi-static blade element momentum model. In this model the tip speed ratio dependent coefficients are taken into account. Also the generator torque working on the structure that is applied by the generator to keep the tip speed ratio of the turbine at the optimum is taken into account. This leads to asymmetrical loading of the mooring lines due to an out of plane moment of the generator torque. A certain base case is formulated and an analysis is done for the mooring loads in this base case. The largest difference in the mooring loads are seen for the maximum fairlead tensions and in the fatigue in the mooring lines. The actuator disk underestimates the loads in the mooring lines, especially for the fatigue lifetime of the lines. A parametric sensitivity study is performed, from this is concluded that these effects are the largest in case of a low current velocity with high waves and a high mooring stiffness. It must be noted that for the BlueTEC the mooring system is displacement driven.

http://resolver.tudelft.nl/uuid:98cd6569-c60b-4c5c-aab1-4211f4ce7a8e

Embargo date2026-08-17

Student theses

Document typemaster thesis

(c) 2016 Zwennis, R.M.

file embargo until 2026-08-16 |