Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
This thesis dene a limit for when hydroelasticity is necessary to include in an analysis of a large oating semi-submersible wind turbine platform in waves. The thesis also includes a description of how to include hydroelasticity in the design of such a structure.
A simple analysis studying two two-dimensional beams' hydroelastic behaviour in waves is also conducted, observing resonance, large deformations and stresses in the vicinity of the rst elastic natural frequency.
Hydroelasticity concerns the combined uid-structure interaction for oating exible structures in waves. In a hydroelastic analysis the uid forces and structural deformations are coupled to account for dynamic and kinematic eects. In this thesis the analysed structure is assumed to be beam-like and Euler beam theory is used. The hydrodynamic forces are determined using a linearised Morison's equation. The hydroelastic response is performed in the frequency domain using a modal analysis and it is modelled in a self-developed model using Matlab.
Most of the concepts and prototypes of oating wind turbines of today have one turbine installed on a oater and the structure is assumed to be rigid. When modelling a structure as exible, elastic responses is observed around the elastic natural frequencies.
The analysis has been performed on two beams with dierent lengths and stiness' to observe a hydroelastic behavior: 1) when the rst wet elastic natural frequency is about four times the peak frequency of the sea spectra and 2) when the rst wet elastic natural frequency is almost within the sea spectra.
It has been found that if the rst wet elastic natural frequency of the structure is higher than about 2-5 times than the wave frequency in regular waves or about ve times the peak frequency, a quasi-static assumption is reliable. If the rst wet elastic natural frequency is less than that, hydroelasticity needs to be considered. The actual limit for a quasi-static/hydroelastic assumption needs to be further investigated.
2014. , 75 p.