Learning objectives

In this case study you will be presented to how eigen period study can be conducted on offshore wind turbines. Upon completion of this case study, you will know:

• basics of how to run eigen period study on a wind turbine,
• basics of how structural parts affects the eigen period of wind turbine system.

Introduction

Three cases are presented:

1. Turbine only attached to a fixed point.
2. Turbine and tower only, where tower is fixed.
3. Turbine on fixed jacket.

The eigen period analysis is arranged such that the first 5 steps the blades are static (not rotating), then in the 10 last steps the system is dynamic (blades are rotating). Eigen periods are considered for both these parts. The aim is to show that eigen periods changes depending on static or dynamic conditions. Analyses are conducted with a wind velocity of 7 m/s.

Turbine only

In this case, only the turbine itself is considered. Result files are found in the folder Analysis > Turbine.

The turbine is fixed at the back end of the shaft. The figure below shows the eigen period and -shape where the upper blade is bending backwards (turbine-only_01eigen2.avz). Eigen period for static part is 2.8 s, and about 2.6 s for dynamic.

Turbine with tower

The same analysis is conducted on the same turbine when the tower is included. Result files are found in the folder Analysis > Turbine.

The figure above shows the eigen period and -shape where the upper blade is bending backwards (turbine-tower_01eigen2.avz). Eigen period for static part is 2.89 s, and about 2.7 s for dynamic.

Turbine on fixed jacket

A jacket, fixed to the seabed, is placed beneath the tower. Results are found in the folder Analysis > Turbine.

The figure above shows the eigen period and -shape where the upper blade is bending backwards (turbine-jacket_01eigen2.avz). Eigen period for static part is 2.99 s, and about 2.9 s for dynamic.

Result summary

The eigen periods for the different wind turbine configurations are presented in the figure below.

Comparing the three cases one can see the eigen period is sensitive to stiffness of the system. From the first case with only the turbine, to the one with jacket, the eigen period is increasing.