Stochastic dynamic load effect and fatigue damage analysis of drivetrains in land-based and TLP, spar and semi-submersible floating wind turbines

Abstract

This paper deals with the feasibility of using a 5 MW drivetrain which is designed for a land-based turbine, on floating wind turbines. Four types of floating support structures are investigated: spar, TLP and two semi-submersibles. The fatigue damage of mechanical components inside the gearbox and main bearings is compared for different environmental conditions, ranging from cut-in to cutout wind speeds. For floating wind turbines, representative wave conditions are also considered.All wind turbines are ensured to follow similar power curves, but differences in the control system (integral to different concepts) are allowed. A de-coupled analysis approach is employed for the drivetrain response analysis. First, an aero-hydro-servo-elastic code is employed for the global analysis. Next, motions, moments and forces from the global analysis are applied on the gearbox multi body model and the loads on gears and bearings are obtained. The results suggest that the main bearings sustain more damage in floating wind turbines than on land-based. The highest main bearing damage is observed for the spar floating wind turbine. The large wave induced axial load on the main shaft is found to be the primary reason of this high damage in the spar wind turbine. Apart from the main bearings -which are located on the main shaft outside the gearbox -other bearings and gears inside the gearbox hold damages in floating wind turbines equal or even less than in the land-based turbine. It is emphasized that the results presented in this study are based on a drivetrain with two main bearings, which considerably reduces the non-torque loads on

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

“…As in previous work [4], a chain mooring system with delta lines and clump weights was applied to approximate the mooring system stiffness described in [6]. The wind turbine specification is provided in Table 1, while the overall characteristics of the floating platform are summarized in Table 2.…”

“…The INP-A bearing carries the radial load and the axial force is supported by INP-B. This is a four-point drivetrain configuration, which significantly reduces the non-torque loadings that enter the gearbox [4,11].…”

“…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 rotor shaft) acceleration.…”

“…This article tries to answer this questions by evaluating correlation and physical relationship between the maximum axial acceleration at the tower top and the load effects in the drivetrain. In an earlier work by Nejad et al [4], it was shown that the main bearing(s) are most affected by the motions and loads on FWTs compared to land-based turbines. It was also found that the wave-induced motions are responsible for the increase in damage in the main bearings.…”

“…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…”