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Torgeir Moan - One of the best experts on this subject based on the ideXlab platform.

  • Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower
    Journal of Offshore Mechanics and Arctic Engineering, 2014
    Co-Authors: Marit Irene Kvittem, Torgeir Moan
    Abstract:

    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%.

  • Frequency Versus Time Domain Fatigue Analysis of a Semi-Submersible Wind Turbine Tower
    Volume 9A: Ocean Renewable Energy, 2014
    Co-Authors: Marit Irene Kvittem, Torgeir Moan
    Abstract:

    The current paper addresses a study of a semi-submersible wind turbine, where tower base bending moments and short term tower Fatigue damage was estimated by a frequency Domain method assuming two-dimensional platform motions. Both a rigid structure assumption and a generalised degree of freedom model for including the first flexible mode of the turbine were investigated. First, response wind- and wave loads were 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, non-linear time Domain analyses.In this study a three column, catenary moored semi-submersible with the NREL 5MW turbine mounted on one of the columns was modelled. The time Domain simulation tool used was Simo-Riflex-AeroDyn from Marintek and CeSOS.The frequency Domain method accounting for a flexible turbine gave a good representation of the tower base bending moment histories for the moderate sea states in these analyses, also for the combined wind and wave analyses. The frequency Domain Fatigue damage predictions were underestimated by 0–60%, most likely due to the exponential relationship between damage and stress amplitude.Copyright © 2014 by ASME

  • frequency Domain Fatigue analysis of wide band stationary gaussian processes using a trimodal spectral formulation
    International Journal of Fatigue, 2008
    Co-Authors: Torgeir Moan
    Abstract:

    Abstract In this paper, a procedure for Fatigue analysis of a general wide-band stationary Gaussian process is developed in the frequency Domain using a trimodal spectral formulation, based on a generalization of the principle proposed by Jiao and Moan for predicting bimodal Fatigue damage. The novel method approximates the rainflow cycles with large amplitudes as the sum of the envelopes of the process components. First, the method is derived and the corresponding accuracy is demonstrated for ideal trimodal Gaussian processes whose spectra exhibit peaks at three well separated modes, each assumed to be narrow-banded. Hermite numerical integration method is applied to evaluate the Fatigue damage due to a Rayleigh sum distribution since there is no closed-form solution for a random variable which is the sum of more than two Rayleigh random variables. Then, the method is further developed and applied to general wide-band Gaussian processes by dividing the spectrum into three segments with the same variances and calculating the Fatigue damage in the same way as for the ideal trimodal processes. Based on extensive time series simulated with spectra of wave- and wind-induced structural responses and with generally defined wide-band spectra, the rainflow cycle counting method has been applied to estimate the Fatigue damage in the time Domain in order to check the accuracy of the proposed method and other frequency-Domain methods. It is found that the narrow-band assumption can be very well made for a process with a Vanmarcke’s bandwidth parameter which is less than 0.5. For most of the simulated processes with bandwidth parameters between 0.5 and 0.85, the proposed method slightly overestimates the Fatigue damage on average. If the bandwidth is greater than 0.85, the Fatigue damage obtained by the trimodal formulation may be significantly overestimated in some cases, however spectra with very high bandwidth parameters might be unrealistic. Anyway, it has been shown that the overestimation can be alleviated by using a formulation with more modes for these cases. Moreover, two empirical formulae for wide-band Fatigue damage estimation, one derived by Dirlik and the other proposed by Benasciutti and Tovo, have also been verified to be very accurate but slightly underestimate the Fatigue damage for a wide range of the bandwidth parameters considered in this paper.

Filippo Cianetti - One of the best experts on this subject based on the ideXlab platform.

  • Non-stationarity and non-Gaussianity in Vibration Fatigue
    Sensors and Instrumentation Aircraft Aerospace Energy Harvesting & Dynamic Environments Testing Volume 7, 2020
    Co-Authors: Janko Slavič, Massimiliano Palmieri, Filippo Cianetti, Lorenzo Capponi, Martin Česnik, Miha Boltežar
    Abstract:

    In vibration Fatigue the frequency contents of dynamic loading and structure’s dynamic response overlap, resulting in amplified stress loads of the structure. Time Domain Fatigue approach does not give a good insight into the underlying mechanics of failure and therefore recently vibration Fatigue in frequency Domain is getting a lot of scientific attention. Gaussianity and stationarity assumptions are applied in frequency-Domain methods for obtaining dynamic structure’s response and frequency-Domain methods for calculating damage accumulation rate. However, in application, the structures are excited with non-Gaussian and non-stationary loads and this study addresses the effects of such dynamic excitation to experimental time-to-failure of a structure. The influence of non-Gaussian, but stationary excitation, is experimentally studied via excitation signals with equal power density spectrum and different values of kurtosis. The non-Gaussianity was found not to significantly change the structure’s time-to-failure and therefore, the study focuses on the non-stationary excitation signals that are also inherently non-Gaussian. The non-stationarity of excitation was achieved by amplitude modulation and significantly shorter times-to-failure were observed when compared to experiments with stationary non-Gaussian excitation. Additionally, the structure’s time-to-failure varied with the rate of the amplitude modulation. To oversee this phenomenon the presented study proposes a non-stationarity index which can be obtained from the excitation time history. The non-stationarity index was experimentally confirmed as a reliable estimator for severity of non-stationary excitation. The non-stationarity index is used to determine if the frequency-Domain methods can safely be applied for time-to-failure calculation.

  • Non-stationarity and non-Gaussianity in Vibration Fatigue
    Sensors and Instrumentation Aircraft Aerospace Energy Harvesting & Dynamic Environments Testing Volume 7, 2019
    Co-Authors: Janko Slavič, Massimiliano Palmieri, Filippo Cianetti, Lorenzo Capponi, Martin Česnik, Miha Boltežar
    Abstract:

    In vibration Fatigue the frequency contents of dynamic loading and structure’s dynamic response overlap, resulting in amplified stress loads of the structure. Time Domain Fatigue approach does not give a good insight into the underlying mechanics of failure and therefore recently vibration Fatigue in frequency Domain is getting a lot of scientific attention. Gaussianity and stationarity assumptions are applied in frequency-Domain methods for obtaining dynamic structure’s response and frequency-Domain methods for calculating damage accumulation rate. However, in application, the structures are excited with non-Gaussian and non-stationary loads and this study addresses the effects of such dynamic excitation to experimental time-to-failure of a structure.

  • Correction formula approach to evaluate Fatigue damage induced by non-Gaussian stress state
    Procedia Structural Integrity, 2018
    Co-Authors: Filippo Cianetti, Massimiliano Palmieri, Claudio Braccesi, Giulia Morettini
    Abstract:

    Abstract In the present paper the authors define an original analytical expression of a corrective coefficient to evaluate Fatigue damage induced by a non-Gaussian stress state affected by high Kurtosis (values higher than 5) and by zero Skewness. This approach starts from a previous activity in which the authors solved an analogous problem but for light non-Gaussian stress states (Kurtosis value less than 5). The proposed procedure assumes to know the Fatigue damage induced by Gaussian equivalent stress state time Domain process. This characteristic allows the proposed procedure to be easily adopted inside the so-called Frequency Domain Fatigue Methods but in parallel with the statistical analysis of the system time Domain response (Kurtosis and Skewness evaluation). Interesting considerations about its applicability will be proposed as concerns the non-Gaussianity and non-Stationarity of the inputs when the system is a flexible component excited in its frequency range.

  • Non-stationarity index in vibration Fatigue: Theoretical and experimental research
    International Journal of Fatigue, 2017
    Co-Authors: Lorenzo Capponi, Filippo Cianetti, Martin Česnik, Janko Slavič, Miha Boltežar
    Abstract:

    Abstract Random vibrations induce damage in structures, especially when they are operating close to their natural frequencies. The stationarity of the input excitation is one of the fundamental assumptions required for frequency-Domain Fatigue-damage theory. However, for real applications, excitation is frequently non-stationary and the identification of this non-stationarity is not easy. This study researches run-tests to identify the index of non-stationarity. Further, using excitation signals with different rates of amplitude-modulated non-stationarity, the index of non-stationarity is experimentally and theoretically researched with regards to the Fatigue life. The experimental research was performed on a flexible structure that was excited close to a natural frequency. The experimental Fatigue life is compared to the theoretical Fatigue life under the stationarity assumption. The analysis of the experimental results reveals a close relation between the identified non-stationarity in the excitation signal and the Fatigue life of the structure. It was found that amplitude-modulated non-stationary excitation results in a significantly shorter Fatigue life if compared to a similar level of stationary excitation.

  • Random multiaxial Fatigue: A comparative analysis among selected frequency and time Domain Fatigue evaluation methods
    International Journal of Fatigue, 2015
    Co-Authors: Claudio Braccesi, Filippo Cianetti, G. Lori, D. Pioli
    Abstract:

    Abstract Fatigue analysis of mechanical components subjected to random loads has been recently upgraded through several developments of calculation procedures, with the scope to support the designer within the loading condition numerical simulation. Under such scenario, the frequency Domain approach is characterized by interesting features, which support its adoption in alternative or in conjunction with the classic time-Domain approach, especially when the frequency Domain is applied for the individuation of the component critical locations. The major goal of this paper consists of an overview about the strength and weaknesses of frequency approach with respect to the time Domain one by comparing the reference time Domain methods with their frequency Domain translation. A significant test case development will be shown, representing a classic automotive one (chassis validation). Promising results of the frequency method application will be presented, encouraging its adoption on large scale.

Miha Boltežar - One of the best experts on this subject based on the ideXlab platform.

  • Non-stationarity and non-Gaussianity in Vibration Fatigue
    Sensors and Instrumentation Aircraft Aerospace Energy Harvesting & Dynamic Environments Testing Volume 7, 2020
    Co-Authors: Janko Slavič, Massimiliano Palmieri, Filippo Cianetti, Lorenzo Capponi, Martin Česnik, Miha Boltežar
    Abstract:

    In vibration Fatigue the frequency contents of dynamic loading and structure’s dynamic response overlap, resulting in amplified stress loads of the structure. Time Domain Fatigue approach does not give a good insight into the underlying mechanics of failure and therefore recently vibration Fatigue in frequency Domain is getting a lot of scientific attention. Gaussianity and stationarity assumptions are applied in frequency-Domain methods for obtaining dynamic structure’s response and frequency-Domain methods for calculating damage accumulation rate. However, in application, the structures are excited with non-Gaussian and non-stationary loads and this study addresses the effects of such dynamic excitation to experimental time-to-failure of a structure. The influence of non-Gaussian, but stationary excitation, is experimentally studied via excitation signals with equal power density spectrum and different values of kurtosis. The non-Gaussianity was found not to significantly change the structure’s time-to-failure and therefore, the study focuses on the non-stationary excitation signals that are also inherently non-Gaussian. The non-stationarity of excitation was achieved by amplitude modulation and significantly shorter times-to-failure were observed when compared to experiments with stationary non-Gaussian excitation. Additionally, the structure’s time-to-failure varied with the rate of the amplitude modulation. To oversee this phenomenon the presented study proposes a non-stationarity index which can be obtained from the excitation time history. The non-stationarity index was experimentally confirmed as a reliable estimator for severity of non-stationary excitation. The non-stationarity index is used to determine if the frequency-Domain methods can safely be applied for time-to-failure calculation.

  • Non-stationarity and non-Gaussianity in Vibration Fatigue
    Sensors and Instrumentation Aircraft Aerospace Energy Harvesting & Dynamic Environments Testing Volume 7, 2019
    Co-Authors: Janko Slavič, Massimiliano Palmieri, Filippo Cianetti, Lorenzo Capponi, Martin Česnik, Miha Boltežar
    Abstract:

    In vibration Fatigue the frequency contents of dynamic loading and structure’s dynamic response overlap, resulting in amplified stress loads of the structure. Time Domain Fatigue approach does not give a good insight into the underlying mechanics of failure and therefore recently vibration Fatigue in frequency Domain is getting a lot of scientific attention. Gaussianity and stationarity assumptions are applied in frequency-Domain methods for obtaining dynamic structure’s response and frequency-Domain methods for calculating damage accumulation rate. However, in application, the structures are excited with non-Gaussian and non-stationary loads and this study addresses the effects of such dynamic excitation to experimental time-to-failure of a structure.

  • Non-stationarity index in vibration Fatigue: Theoretical and experimental research
    International Journal of Fatigue, 2017
    Co-Authors: Lorenzo Capponi, Filippo Cianetti, Martin Česnik, Janko Slavič, Miha Boltežar
    Abstract:

    Abstract Random vibrations induce damage in structures, especially when they are operating close to their natural frequencies. The stationarity of the input excitation is one of the fundamental assumptions required for frequency-Domain Fatigue-damage theory. However, for real applications, excitation is frequently non-stationary and the identification of this non-stationarity is not easy. This study researches run-tests to identify the index of non-stationarity. Further, using excitation signals with different rates of amplitude-modulated non-stationarity, the index of non-stationarity is experimentally and theoretically researched with regards to the Fatigue life. The experimental research was performed on a flexible structure that was excited close to a natural frequency. The experimental Fatigue life is compared to the theoretical Fatigue life under the stationarity assumption. The analysis of the experimental results reveals a close relation between the identified non-stationarity in the excitation signal and the Fatigue life of the structure. It was found that amplitude-modulated non-stationary excitation results in a significantly shorter Fatigue life if compared to a similar level of stationary excitation.

Guttorm Grytoyr - One of the best experts on this subject based on the ideXlab platform.

  • Methodology for Time Domain Fatigue Life Assessment of Risers and Umbilicals
    29th International Conference on Ocean Offshore and Arctic Engineering: Volume 5 Parts A and B, 2010
    Co-Authors: Oddrun Steinkjer, Nils Sødahl, Guttorm Grytoyr
    Abstract:

    Risers and umbilicals are exposed to dynamic loading from waves and floater motions. These structures are known to have a pronounced non-linear response characteristic. Non-linear time-Domain finite element analyses is in general required to give an adequate description of the non-linearities involved. Analyses of a large number of short-term environmental conditions considering stochastic wave loading are required to give a representative description of the long-term Fatigue loading on the structure. Short term Fatigue damage is established by means of rain-flow cycle (RFC) counting in each stationary short-term condition. It is has been experienced that significant statistical uncertainties can be present in the short-term Fatigue damage estimates. This is because the accumulated Fatigue damage in a stationary condition normally has significant contribution from the largest stress cycles in the realisation. Selection of proper simulation length is hence essential to obtain reliable Fatigue life estimates. Applicable codes and standards for risers and umbilicals provide Design Fatigue Factors (DFF) to secure adequate safety against failure due to wave induced Fatigue. The total uncertainty in the calculated Fatigue damage comes from various sources and the DFFs in e.g. DNV-OS-F201 “Dynamic Risers” and API-RP-2RD corresponds to a certain uncertainty level in the Fatigue damage estimate. A recommended target value for the statistical uncertainty of the Fatigue damage estimates is given with basis in these design codes. The objective of this paper is to give a description of a methodology recommended for time Domain Fatigue assessment. Special focus will be on the importance of adequate simulation time for predicting the short-term Fatigue damage and selection of the short-term seastates in the scatter diagram. Statistical uncertainty is one source that the analyst actually can influence by selecting proper analysis methodology. A statistical uncertainty meassure can be used to evaluate the robustness in the estimated Fatigue life. Assessment of statistical uncertainty in Fatigue damage estimate is demonstrated by case studies. The Fatigue assessment methodology discussed in this paper, will be described in an update of DNV-RP-F204 “Riser Fatigue” 2010.Copyright © 2010 by ASME

Marit Irene Kvittem - One of the best experts on this subject based on the ideXlab platform.

  • Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower
    Journal of Offshore Mechanics and Arctic Engineering, 2014
    Co-Authors: Marit Irene Kvittem, Torgeir Moan
    Abstract:

    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%.

  • Frequency Versus Time Domain Fatigue Analysis of a Semi-Submersible Wind Turbine Tower
    Volume 9A: Ocean Renewable Energy, 2014
    Co-Authors: Marit Irene Kvittem, Torgeir Moan
    Abstract:

    The current paper addresses a study of a semi-submersible wind turbine, where tower base bending moments and short term tower Fatigue damage was estimated by a frequency Domain method assuming two-dimensional platform motions. Both a rigid structure assumption and a generalised degree of freedom model for including the first flexible mode of the turbine were investigated. First, response wind- and wave loads were 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, non-linear time Domain analyses.In this study a three column, catenary moored semi-submersible with the NREL 5MW turbine mounted on one of the columns was modelled. The time Domain simulation tool used was Simo-Riflex-AeroDyn from Marintek and CeSOS.The frequency Domain method accounting for a flexible turbine gave a good representation of the tower base bending moment histories for the moderate sea states in these analyses, also for the combined wind and wave analyses. The frequency Domain Fatigue damage predictions were underestimated by 0–60%, most likely due to the exponential relationship between damage and stress amplitude.Copyright © 2014 by ASME