On Tower Top Axial Acceleration and Drivetrain Responses in a Spar-Type Floating Wind Turbine

Common industrial practice for designing floating wind turbines is to set an operational limit for the tower-top axial acceleration, normally in the range of 0.2-0.3g, which is typically understood to be related to the safety of turbine components. This paper investigates the rationality of the tower-top acceleration limit by evaluating the correlation between acceleration and drivetrain responses. A 5 MW reference drivetrain is selected and modelled on a spar-type floating wind turbine in 320 m water depth. A range of environmental conditions are selected based on the long-term distribution of wind speed, significant wave height, and peak period from hindcast data for the Northern North Sea. For each condition, global analysis using an aero-hydro-servo-elastic tool is carried out for six one-hour realizations. The global analysis results provide useful information on their own regarding the correlation between environmental condition and tower top acceleration, and correlation between tower top acceleration and other responses of interest which are used as input in a decoupled analysis approach. The load effects and motions from the global analysis are applied on a detailed drivetrain model in a multi-body system (MBS) analysis tool. The local responses on bearings are then obtained from MBS analysis and post-processed for the correlation study. Although the maximum acceleration provides a good indication of the wave-induced loads, it is not seen to be a good predictor for significant fatigue damage on the main bearings in this case. ∗Address all correspondence to this author. INTRODUCTION Floating offshore wind turbines (FWTs) hold great promise for harvesting the wind power resource in relatively deep water (>50 m). In order to realize this potential in commercial floating wind parks, reductions in the levelized cost of produced electricity are needed. The development of rational design criteria and operational limits will allow for more efficient yet safe floating offshore wind turbines. At present, there is a common practice in the industry to set a limit for the maximum axial acceleration on the tower-top in the range of 0.2g-0.3g, even though this is not explicitly specified in the design codes. This limit has important consequences for platform design [1] as well as wind turbine control strategies [2]. An earlier study by authors [3] evaluated the correlation of the tower top axial acceleration and drivetrain load effects for a bottom-fixed offshore wind turbine. The maximum axial acceleration in the monopile offshore wind turbine, which was below 0.1g, was found to be primarily a function of the tower motion, and the drivetrain design drivers such as axial force, bending moment and the torque were not necessarily correlated with the tower motion. Here, we examine the rationality of the nacelle axial acceleration limit with respect to the drivetrain responses of a floating wind turbine. Based on the results of a previous study [4] of several 5 MW FWT designs (a TLP, two semi-submersibles and a spar), the spar was chosen for further examination due to its relatively large motion, axial force, and nacelle (or tower top) axial (i.e. along the Proceedings of the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering OMAE2017 June 25-30, 2017, Trondheim, Norway