Introduction  Wind turbine generator systems (WTGS) are prone to various types of failures due to the harsh operating environment. For the main bearing, a central component of the transmission system, it is difficult to detect and evaluate its early failure, and offshore operations are restricted by limited weather windows. How to accurately evaluate the operating conditions of the main bearings of offshore units has become a major difficulty for the industry.

  Method  The study focused on the operation condition of the main bearing of an offshore direct-drive generator with a capacity of 7 MW. The transmission process of the wind wheel load in the transmission chain was deduced by the theoretical formula, and the radial load and axial load on the main bearing were obtained. Through the finite element calculation and analysis of the main bearing, the load distribution within the bearing raceway was obtained, which was mutually verified with the theoretical derivation, and the position of the vibration monitoring point was determined preliminarily.

  Result  Finally, according to the position of the bearing measuring point, the vibration monitoring is carried out in the WTGS site, and a clear time-domain vibration curve is obtained. The vibration monitoring results such as the effective value of the vibration of the main bearing, the response frequency of the impact signal, and the acceleration envelope characteristics are analyzed. Combined with the detection results of the grease composition inside the bearing, the damage degree of main bearing's specific components is qualitatively judged.

  Conclusion  This study has identified the measuring point position of the main bearing of multi-megawatt direct-drive offshore WTGS, and accurately assessed the operating condition of the main bearing, which can provide technical support for design and maintenance personnel.