Application of Frequency Domain Methods for Response Based Analysis of Flexible Risers

2017 ◽  
Author(s):  
C. Armstrong ◽  
Y. Drobyshevski ◽  
C. Chin
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Composite Structures ◽  
Full Time ◽  
Threshold Limit ◽  
Frequency Domain Methods ◽  
Non Linear ◽  
Flexible Risers ◽  
The Time Domain ◽  
Extreme Responses

Response Based Analysis (RBA) is an advanced method for the prediction of long term distributions of critical responses in offshore floating systems. For complex non-linear systems such as flexible risers, RBA requires time domain simulations that form the core data to which probabilistic models are applied. Because RBA requires significantly larger amounts of data than traditional short term analysis approaches, running the required number of simulations in the time domain can quickly become unfeasible if the system’s physics being modelled are exceedingly complex. In addition, flexible risers are complex composite structures with highly dynamic, non-linear responses which further limit the feasibility of application of the RBA process to these systems. As an alternative, frequency domain solvers, such as that used in the OrcaFlex software, are potential substitutes for portions of datasets due to their processing times being significantly faster than time domain solvers. A comparison of extreme responses generated by frequency and time domain solvers was performed over the duration of two storms. An upper threshold limit for the frequency domain’s accuracy was found by comparing the differences of the two solver’s responses as the storm progressed; where the differences became too large the threshold limit was set. For environmental conditions smaller than this threshold, the frequency domain solver may provide a quicker method for predicting the riser responses. Conditions that exceed this threshold require full time domain analysis for accurate responses to be generated. Limitations of the frequency domain solvers include their reduced ability to deal with non-linear mechanics such as bending/curvature responses. As a result, curvature component results from the frequency domain are limited in their direct usability, especially when exposed to more extreme metocean conditions and locations along the riser that are subject to larger curvature (generally where risers are connected to structures with greater stiffness). Although these limitations exist, the frequency domain solver may still provide reasonable insight into metocean conditions that potentially cause extreme responses. A method is proposed for the use of both frequency and time domain simulations in the flexible riser flowline RBA process. Screening, filtering and ‘stitching’ methods utilizing the speed of the frequency domain solver are presented in order to compensate for the time domain’s extensive computation times. The proposed method of stitching, when applied to an example storm history, required 39% of the processing time when using only the time domain solver.

Comparing Frequency and Time Domain Simulations for Geometry Optimization of a Floating Offshore Wind Turbine Mooring System

2018 ◽  
Author(s):  
Ajit C. Pillai ◽  
Philipp R. Thies ◽  
Lars Johanning
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Pareto Front ◽  
Geometry Optimization ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Mooring System ◽  
Efficient Design ◽  
Frequency Domain Methods ◽  
The Time Domain

This paper explores geometry optimization of an offshore wind turbine’s mooring system considering the minimization of the material cost and the cumulative fatigue damage. A comparison of time domain simulations against frequency domain simulations is made to explore the suitability of these methods to the design process. The efficient design options, the Pareto front, from the frequency domain study are also re-evaluated using time domain simulations and compared against the time domain Pareto front. Both the time and frequency domain results show optimal results utilizing similar design philosophies, however, the frequency domain methods severely under predict the fatigue loads in the mooring system and incorrectly class infeasible solutions as feasible. The frequency domain is therefore not suitable for optimization use without some external means of applying engineering constraints. Furthermore, re-evaluation of the frequency domain solutions provides guidance to the uncertainty and the necessary design fatigue factors required if implementing frequency domain methods in design.

NON LINEAR ANALYSIS OF SHIP MOTIONS AND LOADS IN LARGE AMPLITUDE WAVES

2021 ◽  
Vol 153 (A2) ◽  
Author(s):  
G Mortola ◽  
A Incecik ◽  
O Turan ◽  
S.E. Hirdaris
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Large Amplitude ◽  
Linear Time ◽  
Ship Motions ◽  
Wave Loads ◽  
Time Step ◽  
Strip Theory ◽  
Non Linear ◽  
The Time Domain

A non linear time domain formulation for ship motions and wave loads is presented and applied to the S175 containership. The paper describes the mathematical formulations and assumptions, with particular attention to the calculation of the hydrodynamic force in the time domain. In this formulation all the forces involved are non linear and time dependent. Hydrodynamic forces are calculated in the frequency domain and related to the time domain solution for each time step. Restoring and exciting forces are evaluated directly in time domain in a way of the hull wetted surface. The results are compared with linear strip theory and linear three dimensional Green function frequency domain seakeeping methodologies with the intent of validation. The comparison shows a satisfactory agreement in the range of small amplitude motions. A first approach to large amplitude motion analysis displays the importance of incorporating the non linear behaviour of motions and loads in the solution of the seakeeping problem.

A Comparison of Time Domain and Frequency Domain Approaches for the Fully Coupled Analysis of Deepwater Floating Systems

2006 ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Coupled Analysis ◽  
Integration Scheme ◽  
Global Coordinate System ◽  
Mooring Lines ◽  
Frequency Domain Methods ◽  
The Time Domain ◽  
Fully Coupled ◽  
Fully Coupled Analysis

The recognition of the need for a fully coupled analysis of deepwater floating production systems has led to the research and development of several coupled analysis tools in recent years. Barring a handful of exceptions, these tools and available commercial packages are invariably in the time domain. This has resulted in a much better understanding and confidence in time domain coupled analysis, but less so for the frequency domain approach. In this paper, the viability of frequency domain coupled analysis is explored by performing a systematic comparison of time and frequency domain methods using computer programs developed in-house. In both methods, a global coordinate system is employed where the vessel is modeled with six degrees-of-freedom, while the mooring lines and risers are discretized as lumped masses connected by extensional and rotational springs. Coupling between the vessel and the mooring lines is achieved by stiff springs, and the influence of inertia and damping from the lines are directly accounted for without the need for prior assumptions. First and second order wave forces generated from a random environment are applied on the vessel, as well as drag and inertia loading on the lines. For the time domain simulation, the Wilson-theta implicit integration scheme is employed to permit the use of relatively large time steps. The frequency domain analysis is highly efficient despite being formulated in global coordinates, owing to the banded characteristics of the mass, damping and stiffness matrices. The nonlinear drag forces are stochastically linearized iteratively. As both the time and frequency domain models of the coupled system are identical, a consistent assessment of the error induced by stochastic linearization can be made.

Comparison of Time and Frequency Domain Simulations of an Offshore Floating Wind Turbine

2011 ◽  
Author(s):  
Maxime Philippe ◽  
Aure´lien Babarit ◽  
Pierre Ferrant
Keyword(s):  
Wind Turbine ◽  
Frequency Domain ◽  
Time Domain ◽  
Degrees Of Freedom ◽  
Domain Model ◽  
Floating Platform ◽  
Floating Wind Turbine ◽  
Non Linear ◽  
Offshore Floating Wind Turbine ◽  
The Time Domain

Time domain simulations of an offshore floating wind turbine have been performed. Hydrodynamic impulse responses of the floating platform are calculated with linear hydrodynamic simulation tool ACHIL3D. A user defined module for the wind turbine design code FAST has been developed to calculate hydrodynamic and mooring loads on the structure. Resolution of the movements of the system is done with FAST. Simulation results in time domain are compared with frequency domain results. In the frequency domain model, the whole system is linearized. In the time domain model, the wind turbine model is not linearized. A good agreement between time and frequency domain calculations is observed, even for the pitch motion. Furthermore we observe a non linearity in the response of sway, roll and yaw degrees of freedom around 0.3 rad.s-1. The effect of viscous damping on the movements of the floating wind turbine system has been studied with the time domain model, and a non linear hydrostatic and Froude-Krylov load model has been developed. Effects of these non linear terms are shown.

scholarly journals Filters, Waves and Spectra

Econometrics ◽  
2018 ◽  
Vol 6 (3) ◽  
pp. 35 ◽  
Author(s):  
D. Pollock
Keyword(s):  
Frequency Response ◽  
Frequency Domain ◽  
Time Domain ◽  
Econometric Analysis ◽  
Frequency Domain Methods ◽  
The Time Domain ◽  
Filtering Techniques

Econometric analysis requires filtering techniques that are adapted to cater to data sequences that are short and that have strong trends. Whereas the economists have tended to conduct their analyses in the time domain, the engineers have emphasised the frequency domain. This paper places its emphasis in the frequency domain; and it shows how the frequency-domain methods can be adapted to cater to short trended sequences. Working in the frequency domain allows an unrestricted choice to be made of the frequency response of a filter. It also requires that the data should be free of trends. Methods for extracting the trends prior to filtering and for restoring them thereafter are described.

scholarly journals Modeling the Gas price data Using Periodogram and Fourier Series Analysis: استخدام التحليل في مجال التردد لنمذجة بيانات مبيعات الغاز الطبيعي في الولايات المتحدة الأمريكية بواسطة شكل الدورة وتحليل سلسلة فورير

2019 ◽  
Vol 3 (1) ◽  
Author(s):  
Safia Abdullah Al Fadhel, Amal Al-Ser Al-khadir, Obeid Mahmo
Keyword(s):  
Fourier Series ◽  
Frequency Domain ◽  
Time Domain ◽  
Domain Analysis ◽  
The United States ◽  
Frequency Domain Method ◽  
Series Analysis ◽  
Frequency Domain Methods ◽  
The Time Domain ◽  
Fourier Series Analysis

:   This paper takes into account the application of the Periodogram and the Fourier Series Analysis. It is one of the non-parametric methods of Frequency domain analysis or spectral analysis of time series using Gas sales data in the United States of America from 1993-2014. In order to achieve these objectives، the data were obtained and then the Periodogram and the Fourier series methods were used to analyze the data. Based on the analysis، the cycle of variability within the period under study was 135 months، and a high Accuracy data model was estimated for the Fourier series which included trend، seasonal and error components. The RMSE، MASE and MAE standards were used to confirm the efficiency of the model and the model was then used to predict gas sales for six months، and we have 90% 95% confidence intervals for predictions. In addition، a time domain analysis was provided for the data series using Bok Jenkins method to obtain the appropriate ARMA model and to generate Predictions. Finally، a comparison was made between the accuracy measures of the time domain and frequency domain methods The frequency domain method competed with the time domain and the slight difference in efficiency.

Problem of Uniqueness of Non-Linear Joint Parameter Identification by Force-State Mapping Without Knowing the Joint Model

2005 ◽  
Author(s):  
J. H. Wang ◽  
H. Y. Huang
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Important Method ◽  
Joint Force ◽  
Non Linear ◽  
Uniqueness Of The Solution ◽  
Data Points ◽  
The Time Domain ◽  
Frequency Domain Approach ◽  
The Mathematical Model

The technique of Force-State Mapping is an important method to identify the parameters of non-linear joints. There are two approaches with the concept of Force-State Mapping, i.e., the time domain and frequency domain approaches. The advantages of the frequency domain approach compared to time domain approach are (1) the number of data points for identification can be drastically reduced; (2) it is possible to select arbitrarily the data in frequency domain for identification. In this work, an example is given to show that the above conclusion (i.e., the advantages of the frequency domain approach) is false. The result identified by the frequency domain approach is data and model dependent. One obtains different parameters when different data or model are used for identification. The reason for the non-uniqueness of the solution is that the joint force can’t be determined completely by a few discrete data in the frequency domain provided that the mathematical model of the joint is not exactly known in advance.

Investigation of Transient Thermal Analysis Computational Efficiency Improvements via Frequency Domain Methods

2012 ◽  
Author(s):  
Gregory A. Banyay ◽  
John C. Brigham ◽  
Evgenii Rudnyi
Keyword(s):  
Thermal Analysis ◽  
Frequency Domain ◽  
Time Domain ◽  
Model Order Reduction ◽  
Order Reduction ◽  
Model Order ◽  
Frequency Domain Methods ◽  
Thermal Transient ◽  
The Time Domain ◽  
Transient Thermal

During the operation of a Nuclear Steam Supply System (NSSS), the possibility exists for certain thermal transients to occur in the Reactor Coolant System (RCS). These transients exhibit some amount of periodicity in terms of temperature versus time. The current method of solving for temperature or thermal-mechanical stress states in the nuclear pressure vessel industry is by solving the governing equations in the time domain. For some analytical situations, significant computational savings could be realized by solving the thermal transient problem in the frequency domain. That is, the time, memory, and disk space required to solve the analysis is much less in the frequency domain than in the time domain. Two frequency domain methods are discussed in this paper. First, a Laplace-based model order reduction approach is applied to a reactor vessel component subjected to a representative thermal transient. Second, the feasibility of a Fourier-based spectral approach is discussed. For transient thermal analysis, it is shown that by employing model order reduction, significant computational savings can be realized with insignificant compromise in the accuracy of results.

scholarly journals THE INFLUENCE OF FORWARD SPEED AND NONLINEARITIES ON THE DYNAMIC BEHAVIOUR OF A CONTAINER SHIP IN REGULAR WAVES

2021 ◽  
Vol 153 (A2) ◽  
Author(s):  
A Chapchap ◽  
D A Hudson ◽  
P Temarel ◽  
T M Ahmed ◽  
S E Hirdaris
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Source Distribution ◽  
Impulse Response Functions ◽  
Regular Waves ◽  
Nonlinear Method ◽  
Frequency Domain Methods ◽  
Nonlinear Potential ◽  
The Mean ◽  
The Time Domain

The aim of this paper is to compare the heave and pitch motions for the S175 containership, travelling in head regular waves, obtained from frequency domain linear and time domain partly nonlinear potential flow analyses. The frequency domain methods comprise the pulsating and the translating, pulsating Green’s function methods, with the relevant source distribution over the mean wetted surface of the hull. The time domain method uses the radiation and diffraction potentials related to the mean wetted surface, implemented using Impulse Response Functions (IRF), whilst the incident wave and restoring actions are evaluated on the instantaneous wetted surface. The calculations are carried out for a range of Froude numbers, and in the case of the partly nonlinear method for different wave steepness values. Comparisons are made with available experimental measurements. The discussion focuses on the necessity for a nonlinear approach for predicting the radiation potential and the possible numerical methods for its formulation.

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