Dynamics of Offshore Wind Turbine Towers Onboard a Jack-Up Vessel

Abstract

Offshore wind turbines are usually transported upright on board a jack-up vessel, supported by a sea fastening system. The structural assessment of the towers during transportation and installation is currently done using a local hull and grillage model, assuming that the tower dynamics are decoupled from the jack-up. As the offshore wind turbine towers continue to increase in size, their natural frequencies approach those of the jack-up. This may result in dynamic interaction between the towers and the jack-up. This introduces uncertainty in the assessment of flow-induced vibrations (such as vortex-induced vibrations and interference galloping) of the towers. The question therefore is whether such a local model remains valid for larger towers.

This thesis investigates how the dynamic response of the tower group on board the jack-up vessel differs between a global jack-up model and local hull and grillage model. A computationally efficient finite element model of the jack-up-tower system was developed. This was achieved by combining a Craig-Bampton reduced hull model with beam representations of the legs and towers. The resulting model was shown to accurately capture both global and local dynamics. A local model was created that consisted of a foundation stiffness matrix, representing the local hull and grillage, and beam representations of the towers. These models were used to compare the dynamic behaviour of the towers between the two modelling approaches. In addition a reduced-order CFD method was developed to estimate the vibration amplitudes of the towers subjected to flow-induced vibrations. The method utilizes forced vibration CFD and is based on the principle of energy balance.

Structural analyses showed that the jack-up dynamics can significantly influence the dynamic behaviour of the tower group. This happens when the natural frequencies of the jack-up and the towers are sufficiently close, which is a relatively wide range. This interaction changes the shapes and frequencies of the tower-bending modes. Furthermore, it reduces the tower response. In addition, a global model introduces a significantly lower dynamic stiffness at the tower bases resulting in a different base response compared to a local model. A global model also show increased coupling between the towers and a larger sensitivity to the forcing direction. These effects are not captured by a local hull and grillage model.

The proposed CFD method was successfully implemented and demonstrated for a single vibration mode. Although the predictions are uncertain due to limitations in the simplified CFD model, it is shown that the energy-balance-based approach can, in principle, be applied to a group of offshore wind turbine towers.

It is concluded that a local tower model is only suitable when the global jack-up modes do not interfere with the tower-bending modes. When this interaction occurs, a global model is needed to accurately capture the dynamics of the tower group.