Quadratic Transfer Function

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Mingyue Liu - One of the best experts on this subject based on the ideXlab platform.

  • Second-order responses of a conceptual semi-submersible 10 MW wind turbine using full Quadratic Transfer Functions
    Renewable Energy, 2020
    Co-Authors: Qun Cao, Xiaoxian Guo, Longfei Xiao, Mingyue Liu
    Abstract:

    Abstract Second-order components of the wave loads on floating wind turbines may induce severe resonance at the natural frequencies of the structures and result in fatigue damage. A conceptual semi-submersible platform for the DTU 10 MW Wind Turbine was introduced and simulated by FAST to obtain the second-order responses under steady winds with collinear random waves. The dynamic responses were calculated according to three models, labeled as “1st”, “1st + the full Quadratic Transfer Function (QTF)”, and “1st + Newman approximation”. The average and wave-frequency responses are similar based on three models. With a larger natural frequency, the pitch resonant responses differed more significantly between the Newman model and the full QTF model, as compared to the surge resonant responses. Thus, the method using Newman approximation is not accurate for second-order calculation. A high-frequency response appeared in the tower-top shear force spectrum only when the full QTF was utilized. Sensitivity studies were conducted with various wind speeds, significant wave heights, wave periods, and wave directions. The wind load significantly affected the pitch motion and tower-top shear force and Functioned as aerodynamic damping in low-frequency and high-frequency ranges. The second-order responses exhibited Quadratic relationships with the significant wave height.

Arvid Naess - One of the best experts on this subject based on the ideXlab platform.

  • The Effect of Springing Response of a TLP on Tether Fatigue
    Journal of Offshore Mechanics and Arctic Engineering, 1996
    Co-Authors: Arvid Naess
    Abstract:

    The purpose of this paper is to investigate the effect on estimated fatigue damage of TLP tethers of the method used to model the springing response. In particular, the goal has been to look into the consequence for long-term fatigue calculation of modeling the springing response as a second-order, sum-frequency process as opposed to assuming that the springing response is Gaussian. It is shown that with a standard engineering approach to the calculation of long-term fatigue damage, this effect is in fact marginal. However, the deviation between the numerical estimates of the Quadratic Transfer Function describing the springing response as provided by different computer codes is found to produce estimates of the long-term fatigue that exhibit substantial variability.

  • SECOND-ORDER, SUM-FREQUENCY RESPONSE STATISTICS OF TETHERED PLATFORMS IN RANDOM WAVES
    Applied Ocean Research, 1992
    Co-Authors: Arvid Naess, G.m. Ness
    Abstract:

    Abstract The second-order, sum-frequency forces and motions of tension leg platforms in random seas are studied. The work is based on the assumption that these forces and motions can be characterized by a Quadratic Transfer Function. A representation theorem for the second-order, sum-frequency force and response process is derived. This is used for calculating the statistical properties, including the probability density Functions, of the forces and motions.

Xiaoxian Guo - One of the best experts on this subject based on the ideXlab platform.

  • Second-order resonant motions of a deep-draft semi-submersible under extreme irregular wave excitation
    Ocean Engineering, 2020
    Co-Authors: Xiaoxian Guo, Wenyang Duan
    Abstract:

    Abstract Deep-draft semi-submersible platforms may present significant second-order resonant motions in severe sea states. Evaluating the slow-drift motions of such floating systems is important from the initial stages of their designs considering not only the offset requirement and mooring line load but also the air-gap responses beneath the lower deck. Theories and corresponding applications for these second-order resonant motions have been investigated for decades, but the problem has not been solved completely considering the requirements of a model that is sufficiently accurate and expedient. In this study, high-quality numerical simulations and model tests were conducted. Numerical calculations were carried out with the applications of full Quadratic Transfer Function (QTF) matrices by means of commercial boundary element method (BEM) code. Comparisons show perfect agreement between the wave-frequency components obtained from both the model tests and the numerical calculations. Furthermore, the results also indicate that the second-order resonant motions can be to some extent estimated numerically by the second-order potential theory and that deviations can still be observed compared with those motions obtained by the model tests.

  • Second-order responses of a conceptual semi-submersible 10 MW wind turbine using full Quadratic Transfer Functions
    Renewable Energy, 2020
    Co-Authors: Qun Cao, Xiaoxian Guo, Longfei Xiao, Mingyue Liu
    Abstract:

    Abstract Second-order components of the wave loads on floating wind turbines may induce severe resonance at the natural frequencies of the structures and result in fatigue damage. A conceptual semi-submersible platform for the DTU 10 MW Wind Turbine was introduced and simulated by FAST to obtain the second-order responses under steady winds with collinear random waves. The dynamic responses were calculated according to three models, labeled as “1st”, “1st + the full Quadratic Transfer Function (QTF)”, and “1st + Newman approximation”. The average and wave-frequency responses are similar based on three models. With a larger natural frequency, the pitch resonant responses differed more significantly between the Newman model and the full QTF model, as compared to the surge resonant responses. Thus, the method using Newman approximation is not accurate for second-order calculation. A high-frequency response appeared in the tower-top shear force spectrum only when the full QTF was utilized. Sensitivity studies were conducted with various wind speeds, significant wave heights, wave periods, and wave directions. The wind load significantly affected the pitch motion and tower-top shear force and Functioned as aerodynamic damping in low-frequency and high-frequency ranges. The second-order responses exhibited Quadratic relationships with the significant wave height.

Qun Cao - One of the best experts on this subject based on the ideXlab platform.

  • Second-order responses of a conceptual semi-submersible 10 MW wind turbine using full Quadratic Transfer Functions
    Renewable Energy, 2020
    Co-Authors: Qun Cao, Xiaoxian Guo, Longfei Xiao, Mingyue Liu
    Abstract:

    Abstract Second-order components of the wave loads on floating wind turbines may induce severe resonance at the natural frequencies of the structures and result in fatigue damage. A conceptual semi-submersible platform for the DTU 10 MW Wind Turbine was introduced and simulated by FAST to obtain the second-order responses under steady winds with collinear random waves. The dynamic responses were calculated according to three models, labeled as “1st”, “1st + the full Quadratic Transfer Function (QTF)”, and “1st + Newman approximation”. The average and wave-frequency responses are similar based on three models. With a larger natural frequency, the pitch resonant responses differed more significantly between the Newman model and the full QTF model, as compared to the surge resonant responses. Thus, the method using Newman approximation is not accurate for second-order calculation. A high-frequency response appeared in the tower-top shear force spectrum only when the full QTF was utilized. Sensitivity studies were conducted with various wind speeds, significant wave heights, wave periods, and wave directions. The wind load significantly affected the pitch motion and tower-top shear force and Functioned as aerodynamic damping in low-frequency and high-frequency ranges. The second-order responses exhibited Quadratic relationships with the significant wave height.

Xiao-bo Chen - One of the best experts on this subject based on the ideXlab platform.

  • Approximation of Second-Order Low-Frequency Wave Loading in Multi-Directional Waves
    29th International Conference on Ocean Offshore and Arctic Engineering: Volume 3, 2010
    Co-Authors: F. Rezende, Xiao-bo Chen
    Abstract:

    Further to the studies by Chen & Rezende (OMAE2009) on the Quadratic Transfer Function (QTF) of low-frequency wave loading in which the QTF is developed by the series expansion associated with the difference-frequency up to the order-Δω2 , new formulations have been developed in order to take into account the effect of interactions between waves of different headings. It provides a novel method to evaluate the low-frequency second-order wave loads in a more accurate than usual order-Δω approximation (often called Newman approximation) and more efficient way comparing to the computation of complete QTF in multi-directional waves. New developments including numerical results of different components of QTF are presented here. Furthermore, the time-series reconstruction of excitation loads by quadruple sums in the motion simulation of mooring systems is analyzed and a new efficient and accurate scheme using only a triple sum is demonstrated.Copyright © 2010 by ASME

  • Second-Order Wave Loads on a LNG Carrier in Multi-Directional Waves
    Volume 1: Offshore Technology, 2008
    Co-Authors: Mathieu Renaud, Fla´via Rezende, Olaf Waals, Xiao-bo Chen, Radboud Van Dijk
    Abstract:

    Due to the installation of LNG terminals moored in proximity to the coast, the wave kinematics in shallow water and the consequence on the behavior of those terminals have recently became a major concern of the offshore industry. One key issue is the accurate simulation of the low-frequency motions of LNG carriers, specially the surge, for which the vessel presents low damping, in order to perform the design of the mooring system. The present paper focuses on the effect of wave directionality on second-order slow-drift loads and the related response of the vessel. The paper describes results of model tests in regular cross waves — monochromatic but coming from two directions separated by 90 degrees, as well as bichromatic cross waves. The new “middle field” formulation extended to the case of cross waves, is used to compute the wave drift loads and low-frequency Quadratic Transfer Function (QTF). The results are compared with those from the model tests.Copyright © 2008 by ASME