Structural Fatigue Assessment of Offshore Platform Considering the Effect of Nonlinear Drag Force

2016 ◽  
Author(s):  
Weichen Ding ◽  
Liang Pang
Keyword(s):  
Fatigue Life ◽  
Frequency Domain ◽  
Drag Force ◽  
Time Domain ◽  
Nonlinear Effect ◽  
Nonlinear Term ◽  
Wave Force ◽  
Morison Equation ◽  
Nonlinear Form ◽  
Wave Induced

Fatigue assessment for jacket platforms is an indispensable practical issue. Because of the small-scale leg diameter, these structures are often drag dominated and wave-induced force in these structures can be tackled by using either linear or nonlinear form of Spectral Morison Equation. However, it is really complicated and difficult to incorporate nonlinear form of the Morison Equation to acquire the spectral density of the wave force, which is an important step of fatigue estimation. In this paper, in order to estimate the influence of nonlinear effect in wave force, fatigue assessments containing nonlinear effect for the fixed offshore structure are presented. Firstly, shallow-water jacket model locating at a water depth of 20 m is established and involved in calculation. Besides, for the sake of validating the effectiveness of the nonlinear term, the linear and nonlinear form of wave-induced force spectral densities are calculated by the Morison Equation in frequency domain. Secondly, the fatigue life of the jacket platform is assessed in time domain, where time-history of wave force can be obtained by transforming the linear or nonlinear wave force spectral densities from frequency domain to time domain. After the contrast of acquired fatigue life, the comparative results can indicate that the nonlinear drag force contributes a 14% fatigue damage to the total and the influences of the nonlinear term cannot be ignored for the jacket model.

Hydroelastic Analysis of a 3D Floating Body Considering Uncoupled Flexural and Torsional Vibrations

2018 ◽  
Author(s):  
Debasmit Sengupta ◽  
Ranadev Datta ◽  
Debabrata Sen
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Three Dimensional ◽  
Real Life ◽  
Impulse Response Function ◽  
Floating Body ◽  
Design Stage ◽  
Beam Equation ◽  
Torsional Vibrations ◽  
Wave Induced

A semi analytic three-dimensional time domain method is developed to predict the hydroelastic effect due to wave induced loads on a floating body. The methodology being a semi analytic approach is able to capture real life scenario of bending of a ship like structure on sea taking both flexural and torsional vibrations. A prismatic beam equation with analytically defined modeshapes is taken into consideration to represent the structural response. The elastic deformation is solved using modal superposition technique. The radiation forces for elastic modes are obtained through impulse response function in time domain where frequency domain added mass, damping coefficients and wave exciting forces for the flexible modes are derived from a frequency domain panel method code. The Duhamel integral is employed in order to get the flexural and torsional deflection, velocity. A rectangular barge with zero forward speed is chosen for the analysis. Structural responses, torque, bending moments are calculated to assess the wave induced loads on the floating elastic body. The proposed technique, developed in Fortran, appears to be robust, efficient and computationally less expensive and can be used to predict the wave induced loads on a flexible structure as a first approximation in the initial design stage.

scholarly journals Durability Analysis for Coil Spring Suspension Based on Strain Signal Characterisation

2018 ◽  
Vol 7 (3.17) ◽  
pp. 104
Author(s):  
Chin Chuin Hao ◽  
Shahrum Abdullah ◽  
Ahmad Kamal Ariffin ◽  
Salvinder Singh Karam Singh
Keyword(s):  
Fourier Transform ◽  
Fatigue Life ◽  
Frequency Domain ◽  
Time Domain ◽  
Life Prediction ◽  
Energy Content ◽  
Fatigue Life Prediction ◽  
Coil Spring ◽  
Time Frequency ◽  
Strain Signal

This paper aims to predict the durability of an automobile coil spring by characterising the captured strain data. The load histories collected at coil spring are often presented in time domain but time domain cannot provide sufficient information for fatigue life prediction. The objective of this study was to characterise the strain signal in time domain, frequency domain and time-frequency domain for fatigue life prediction. The signal obtained in time domain was used to predict the fatigue life of the coil spring through Rainflow cycle counting technique and models of strain-life relationships. In frequency domain, fast Fourier transform revealed that the frequency components in the strain signal ranged between 0-5 Hz. The frequencies can be further categorised into two ranges: 0-0.3 Hz and 1-2 Hz. Power spectral density confirmed that the frequencies with high energy content were 0-5 Hz and the total energy content in the signal is 4.0872x103 µɛ2. Short time Fourier transform can identify the local time and frequency properties of the signal but it has a limitation in time-frequency resolutions. Wavelet transform can provide a better time-frequency resolutions and it confirmed that the transients in the time domain had frequency range of 1-2 Hz. In summary, this study revealed different possible approaches of signal processing in fatigue life assessment of automotive components as guidance for the selection of suitable approach based on the type of information needed for the analysis.  

Time Domain Fatigue Analysis of the Pin for Offshore Bridges Considering the Nonlinear Effect of Sliding Connections

2017 ◽  
Author(s):  
Wenbin Dong ◽  
Ingar Scherf ◽  
Gudfinnur Sigurdsson
Keyword(s):  
Fatigue Life ◽  
Friction Coefficient ◽  
Frequency Domain ◽  
Time Domain ◽  
Stochastic Analysis ◽  
Fatigue Analysis ◽  
Fatigue Assessment ◽  
Nonlinear Spring ◽  
Linear Frequency ◽  
Nonlinear Time Domain

A bridge between platforms needs to operate safely and continuously over its lifecycle. This paper focuses on the fatigue assessment of the bridge pin connection due to relative movements between platforms. A nonlinear time domain stochastic fatigue analysis of the pin connection in a bridge in the North Sea using a combined model of the jacket platforms and the interconnecting bridge is presented. The fatigue life is compared to the fatigue life from a linear frequency domain stochastic analysis. The facility has been in operation for more than 40 years and the operator requested an update of the inspection plans for the bridge. An RBI analysis has been done according to [1] based on fatigue results from wind gusts and relative movements. Regarding the fatigue assessment due to relative movements there are uncertainties related to selection of the friction coefficient. It was assessed that a friction coefficient of 0.4 is slightly conservative in this case. The fatigue life of the pin was calculated based on a linear frequency domain stochastic analysis, assuming that the bridge was fixed at both ends and this was considered reasonable conservative for fatigue estimation. Efforts have been made in the study presented here to assess the conservatism through a nonlinear time domain stochastic fatigue analysis. The sliding connections of the bridge are simulated by nonlinear springs. The effects of assuming different friction coefficients and different nonlinear spring models for a certain friction coefficient on the fatigue damage of the pin are investigated by a sensitivity study. The fatigue lives of the pin thus computed for a series of short-term sea states for the different assumptions for the friction coefficient and the nonlinear spring model are then compared to the result from a corresponding frequency domain approach.

scholarly journals Fatigue Life Prediction of a Drag Link by Using Finite Element Method

2010 ◽  
Author(s):  
Barıs¸ Koca ◽  
Bu¨lent Ekici
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Fatigue Life ◽  
Frequency Domain ◽  
Time Domain ◽  
Life Analysis ◽  
Fatigue Life Analysis ◽  
The Time Domain ◽  
Drag Link ◽  
Element Method

The focus of this study is to find fatigue behavior and fatigue life of a drag link in the different road and loading conditions. Finite element method was used for fatigue analysis and fatigue life of the drag link was predicted. Firstly, the historical changes in the concept of the fatigue and fatigue life calculation methods were explained in the chapter one and two. Factor affecting the fatigue performance was explained. Stress and strain based fatigue analysis methods were described clearly. Finally, fatigue life analysis in the frequency domain which is a new method relative to the others was explained. Then, two different steering drag links of a midibus were examined and fatigue life calculations of these two drag links were made. The fatigue life analysis in the time domain of the drag links were made in the static steering conditions and the results were compared with the test results made by the vendor of the drag links. After that, the drag link which has a greater fatigue life than the other was selected, the road loads were taken from another test report which was made by using the same drag link and the fatigue life of the drag link was computed by using the finite element method in the time domain. Finally, the same road loads were converted in the frequency domain and the fatigue life analysis of the same drag link were made in the frequency domain. The results from the time domain and the frequency domain were compared and the advantages of the fatigue life analysis in the frequency domain were expressed.

On the Use of the White-Noise Approximation for Modelling the Slow-Drifts of a FOWT: An Example Using FAST

2018 ◽  
Author(s):  
Alexandre N. Simos ◽  
Lucas Henrique Souza do Carmo ◽  
Ewerton Carlos Camargo
Keyword(s):  
White Noise ◽  
Frequency Domain ◽  
Time Domain ◽  
Significant Loss ◽  
Second Order ◽  
Preliminary Design ◽  
Original Formulation ◽  
Slow Drift ◽  
Wave Induced

When designing the mooring system of a floating unit, performing extensive time-domain simulations in several sea conditions is common practice. For this, the second-order wave induced forces, expressed by QTF matrices, are most often precomputed in frequency domain diffraction codes. However, the computation of the full QTFs is quite demanding and it is also not uncommon for the designer to be in doubt as to the frequency limits and resolution required for their construction. Among the approximations that can be used to ease this burden, the most well-known is Newman’s approximation, which performs quite well as long as the natural periods of drift and the water depth are sufficiently large. The white noise approach, on the other hand, leads to an approximation of a different kind. Taking advantage of the fact that the slow-drift response is narrow-banded, it approximates the second-order force spectrum where it contributes the most, and in a way that is independent of the natural periods and depth. However, its original formulation, based on the force spectra, is certainly more convenient in frequency domain. This article presents an easy way to make use of the white noise approach in time domain simulations. For this, the well-known OC4 semi-submersible FOWT is taken as a case-study. Simulations in different wave conditions are performed with the software FAST using both, the original full QTFs and new ones, simplified according to the principle of the white noise approximation. It is shown that, with the latter, the simulations can be performed without significant loss of accuracy, indicating that the white-noise approach indeed is an interesting option for preliminary design stages.

scholarly journals Characterization of Wavelet Decomposition Strain Signal Using the K-Mean Clustering Method

2018 ◽  
Vol 7 (3.17) ◽  
pp. 158
Author(s):  
A A. Rahim ◽  
C H. Chin ◽  
S Abdullah ◽  
S S. K. Singh ◽  
M Z. Nuawi ◽  
...  
Keyword(s):  
Fatigue Life ◽  
Frequency Domain ◽  
Root Mean Square ◽  
Time Domain ◽  
Average Distance ◽  
Industrial Area ◽  
Discrete Wavelet ◽  
Mean Square ◽  
Time Frequency ◽  
Strain Signal

This paper aims to study the characterisation of time-frequency domain to analyse the fatigue strain signal due to weaknesses in time domain and frequency domain approaches. The objectives were to determine the behaviour of strain signal, characterise the fatigue life of strain signal and validate the fatigue life in time-frequency domain. The strain signal was obtained using data acquisition devices and strain gauges on two types of road condition including highway and industrial area. The acquired signals were analysed with time domain, frequency domain and time-frequency domain approaches. In time-frequency domain, the signals were decomposed using 4th Daubechies discrete wavelet transform. To validate the effectiveness of time-frequency approach in characterising vibration fatigue signal, fatigue data was clustered by mapping of the data based on the spectrum energy, root-mean-square and fatigue life obtained. The clustering was performed by comparing the centroid values which both data had five clusters as the optimum data clustering with 0.836 average distance to centroid. From this, the relationship between fatigue life, root-mean-square and spectrum energy can be determined and thus a new fatigue life criterion was developed. 

Automotive Component Fatigue Life Estimation by Frequency Domain Approach

2005 ◽  
Vol 297-300 ◽  
pp. 1776-1783 ◽  
Author(s):  
Do Hyun Jung ◽  
Sung In Bae
Keyword(s):  
Fatigue Life ◽  
Frequency Domain ◽  
Time Domain ◽  
Motor Vehicle ◽  
Local Strain ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Life Estimation ◽  
Fatigue Life Estimation

Time domain approach with S-N approach and local strain approach were used for fatigue life estimation. But these days, using PSD (Power Spectral Density) method is highlighted, because of short amount of time in measurement and analysis. Especially, PSD method is useful for analysis of fatigue failure which is caused by vibration damage, also FRF (Frequency Response Function) is useful for efficient prediction of fatigue life when the same product is employing different motor vehicle or test condition. In order to estimate fatigue life of compressor for air conditioning, time domain analysis and frequency domain analysis were performed and the results were compared. As a result, results of analysis in frequency domain and time domain were similar. With this, there is recognition of decreasing the period of measuring and analysis in PSD analysis. Moreover, in case of FRF pursued of a part, using FRF is applicable at fatigue life prediction in different testing condition. There was investigated an analysis method with curtailed analysis period by FRF.

scholarly journals TIME AND FREQUENCY LOADING ANALYSIS OF SUBMARINE PIPELINES

1984 ◽  
Vol 1 (19) ◽  
pp. 183
Author(s):  
H.C. Alexander ◽  
P.L. Allen ◽  
J.L. Warner
Keyword(s):  
Spectral Analysis ◽  
Frequency Domain ◽  
Fatigue Damage ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Wave Force ◽  
Domain Analysis ◽  
Wave Spectra ◽  
Frequency Domain Methods ◽  
Wave Conditions

Fatigue damage to marine pipelines subjected to wave forces is evaluated using time-domain and frequency-domain methods. Spectral techniques are applied to North Sea, Gulf of Mexico and offshore Atlantic Canada wave conditions. Time-domain analysis is applied to wave conditions in the Canadian North Atlantic Ocean. The frequency-domain analysis is performed using spectral and probabilistic techniques suggested by L. Borgman {?.). The pipeline dynamic characteristics are described by classical analytical descriptions. The time-domain analysis computes time histories of wave force loading on the submerged pipeline from actual wave records. The traditional Morison wave force equation is used to obtain the time history of the loading on the pipeline. Empirically determined wave spectra are shown not to produce as good a correlation with the deterministic results as the actual wave spectra. Free spanning submarine pipelines subjected to cyclic surface wave loading accumulate strength reductions leading to failure from material fatigue. The Palmgren-Miner rule for the linear accumulation of fatigue damage is applied to evaluate the time to failure. The American Welding Society X-X stress accumulation curve is applied. The results of the deterministic analysis were compared with those of the more efficient spectral analysis. It is shown that comparable results can be obtained from the spectral analysis provided the actual spectra of the water surface elevation is employed in the spectral analysis.

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