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

Abstract

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. INTRODUCTIONFloating 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 ...

Design optimization of the OC3 phase IV floating spar-buoy, based on global limit states

“…• For the horizontal nacelle acceleration, the more conservative value of maximum 0.2 g is selected. This leaves some safety margin for higher values in case that the ranking of criticality of the DLCs changes during the optimization, as even up to 0.3 g is mentioned as common operational limit [38,42].…”

“…Furthermore, the targets for inclination and acceleration are very closely approached. Comparing the results with the maximum values obtained with the original OC3 spar-buoy FOWT system, it can be observed, at first, that not all constraints are initially fulfilled: the maximum achieved horizontal nacelle acceleration of the original OC3 floating spar-buoy system exceeds with 2.338 m/s 2 the specified upper limit of 1.962 m/s 2 ; however, the value still lies below 0.3 g, which can as well be found in some literature as operational limit [38,42]. Furthermore, it becomes apparent that both inclination angle and nacelle acceleration are significantly improved with respect to the specified objectives, meaning that the horizontal nacelle acceleration is now within the specified limit, but very close to it, and the maximum total system inclination angle is significantly enlarged -compared to the original maximum angle of 4.9 • -, but as well still below the specified operational value of 10.0 • .…”

“…Due to the fact that the nacelle contains sensitive components -such as gearbox, generator, and bearings -its motion has to be restricted, as otherwise, when exceeding certain acceleration limits, the turbine has to stop operation. The common operational limit for the maximum allowable nacelle acceleration is 20% -30% of the gravitational acceleration (g) [38,42,43], which corresponds to an acceleration of around 1.962 m/s 2 to 2.943 m/s 2 . The final tolerated acceleration highly depends on the specific turbine.…”

Development of a Framework for Wind Turbine Design and Optimization

“…Thus, the diameter and height of the spar-buoy column are constrained, as presented in Table 2, while the optimization itself focuses on the global system performance, using the maximum values of system inclination, nacelle acceleration, and floater translation for defining the objective functions. For inclination and acceleration it is aimed to reach a certain target value, which corresponds to the maximum allowable value for a wind turbine system during operation [64][65][66][67][68], while the translational motion is to be minimized in general. The specific constraints of the optimization problem are presented in Table 2.…”

A fully integrated optimization framework for designing a complex geometry offshore wind turbine spar-type floating support structure

“…for system rotational stability reasons a maximum total inclination angle of 10.0 • is allowed in operational conditions (Leimeister et al, 2020c;Katsouris and Marina, 2016;Kolios et al, 2015;Huijs et al, 2013);8 Floating offshore wind project manager at a leading company in offshore industry, personal communication, 6 February 2020 9 https://www.lkabminerals.com/en/industry-uses/offshore-energy/offshore-wind-structures/ (Accessed: 7 June 2020) 10 https://www.lkabminerals.com/de/floating-offshore-wind-2018/ (Accessed: 7 June 2020) 11 https://www.lkabminerals.com/wp-content/uploads/2019/02/MagnaDense-SDS-12-06INT-19-03.pdf (Accessed: 5 February 2020) 12 https://www.alibaba.com/showroom/sandstone-price-per-ton.html (Accessed: 5 February 2020) 13 https://german.alibaba.com/product-detail/magnadense-heavy-concrete-172429386.html (Accessed: 5 February 2020) 14 https://spendonhome.com/structural-steel-fabrication-cost/ (Accessed: 5 February 2020) 2. the total horizontal acceleration at the tower top: due to sensitive components in the nacelle and to prevent any issues with the lubrication, the nacelle accelerationcorresponding to the acceleration at the tower top -is limited, depending on the specific wind turbine, to a maximum of 0.2 to 0.3 times the gravitational acceleration constant (Nejad et al, 2017;Huijs et al, 2013;Suzuki et al, 2011); herein the lower value of 1.962 m/s 2 is used following a conservative approach (Leimeister et al, 2020c); as well as 3. the mean translational motion of the floating system: based on experience, the static translational displacement of a (non TLP-type) floating offshore wind turbine system, corresponding to the mean of the translational motion, is restricted to 0.2 times the water depth (320.0 m in the case of the OC3 phase IV spar-buoy floating system), and hence to 64.0 m in this application (Leimeister et al, 2020c).…”