Local Dynamics of Offshore Wind Turbine Jacket Sub-structures

Abstract

The scope of this thesis has been to investigate the existence of out-of-plane vibration of jacket sub-structures for offshore wind turbines. This is done by making the program code JAC (Jacket Analysis Code), which is capable of solving dynamic problems in both time and frequency domain. The code is also able to do damage analysis as it has been a major goal to quantify the change in damage caused by the out-of-plane vibrations and to see how this is influenced by dierent parameters.

It is shown that the out-of-plane vibrations are caused by rotation of the intersection node, between tower and jacket, exciting a global-for-aft and global side-side eigenmode of the jacket structure. The rotation of the intersection node is probably caused by the higher harmonic components of load from the rotor.

The out-of-plane vibration gives a significant contribution to the damage at some parts of the structure. In particular this applies to the face of the jacket perpendicular to the main load direction. However for the analysis performed by the authors, the damage generated by the in-plane stresses is governing.

The infjuence of marine growth, structural damping and incremental time step is investigated. It is shown how these parameters will have significant influence on the damage of the structure.

The analysis is done using a sequential approach where calculations on wind turbine and sub-structure are separated. The thesis shows that this method could highly overestimate the response of out-of-plane vibrations if the structure is exposed to a widely scattered load. This is due to coupling of modes.