Time domain analysis procedures for fatigue assessment of a semi-submersible wind turbine

Kvittem, Marit Irene; Moan, Torgeir

doi:10.1016/j.marstruc.2014.10.009

Cited by 94 publications

(41 citation statements)

References 16 publications

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“…This was observed in a study of the effect of misaligned wind and waves for different concepts [12] and in a long-term fatigue assessment for the semisubmersible used in the current paper [13], Frequency domain models for the tower base bending moments were developed and compared to results obtained by fully coupled nonlinear TD simulations. First, a model including forces induced by platform rigid body motions was developed.…”

Section: Frequency Versus Time Domain Fatigue Analysis Of a Semisubmementioning

confidence: 73%

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Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower

Kvittem

Moan

2014

Journal of Offshore Mechanics and Arctic Engineering

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The current paper deals with a study of a semisubmersible wind turbine (WT), where short-term tower base bending moments and tower fatigue damage were estimated by a frequency domain (FD) method. Both a rigid structure assumption and a generalized degree-of-freedom (DOF) model for including the first flexible mode of the turbine tower were investigated. First, response to wind and wave loads was considered separately, then superposition was used to find the response to combined wind and wave loading. The bending moments and fatigue damage obtained by these methods were compared to results from a fully coupled, nonlinear time domain (TD) analysis. In this study a three column, catenary moored semisubmersible with the NREL 5 MW turbine mounted on one of the columns was modeled. The model was inspired by the WindFloat concept. The TD simulation tool used was Simo-Riflex-AeroDyn from Marintek and CeSOS. The FD method gave a good representation of the tower base bending moment histories for wave-only analyses, for the moderate sea states considered in these analyses. With the assumption that the structure is completely rigid, bending moments were underestimated, but including excitation of the elastic tower and blades, improved the results. The wind-induced low-frequency bending moments were not captured very well, which presumably comes from a combination of nonlinear effects being lost in the linearization of the thrust force and that the aerodynamic damping model was derived for a fixed turbine. Nevertheless, standard deviations of the bending moments were still reasonable. The FD model captured the combined wind and wave analyses quite well when a generalized coordinates model for wind excitation of the first bending mode of the turbine was included. The FD fatigue damage predictions were underestimated by 0–60%, corresponding to discrepancies in standard deviations of stress in the order of 0–20%.

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Section: Frequency Versus Time Domain Fatigue Analysis Of a Semisubmementioning

confidence: 73%

“…In a recent fatigue study of this platform [13] no significant excitation of higher modes than the first bending mode of the turbine (in this context defined as tower and blades) was observed.…”

Section: Dynamic Response Of the Flexible Turbinementioning

confidence: 74%

Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower

Kvittem

Moan

2014

Journal of Offshore Mechanics and Arctic Engineering

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“…here was similar, but not identical, to the generic WindFloat specification [22]. be found in [24]. A long-term fatigue analysis of this concept is also presented in [25].…”

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confidence: 70%

“…be found in [24]. A long-term fatigue analysis of this concept is also presented in [25]. The second semi-submersible concept was the OC4 DeepCWind semi-submersible, as described 84 in detail by Robertson et al [26].…”

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

et al. 2015

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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

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“…4 shows the obtained 84 combinations of U w , H s , and T p , as well as the maximum acceleration from 6 one-hour global analyses of each condition as a function of the wind speed. The choice of one-hour simulations is a compromise between 10 minutes (typically in the wind industry) and 3 hours (offshore), and has been shown to be reasonable [10]. As shown in Fig.…”

Section: Methodology Dynamic Response Analysis and Environmental Conditmentioning

confidence: 99%

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

Nejad

Bachynski

Moan

2017

Volume 9: Offshore Geotechnics; Torgeir Moan Honoring Symposium

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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 ...

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Time domain analysis procedures for fatigue assessment of a semi-submersible wind turbine

Cited by 94 publications

References 16 publications

Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower

Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower

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

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

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