A Fast Calculation Procedure for Fatigue Reliability Estimates of Offshore Structures

2003 ◽  
Author(s):  
H. Karadeniz
Keyword(s):  
Spectral Analysis ◽  
Probability Distribution ◽  
Offshore Structures ◽  
Inertia Force ◽  
Stress Process ◽  
Fatigue Reliability ◽  
Uncertainty Modelling ◽  
Reliability Estimates ◽  
Reliability Method ◽  
Stress Spectrum

This paper presents formulations and procedure of a fast and efficient computation of fatigue reliability estimates of offshore structures, which eliminates repetitive execution of spectral analysis procedure so that it is performed only once for all reliability iterations. This is archived by a suitable uncertainty modelling and spectral formulation of the stress process. For this purpose, a new uncertainty variable is defined to represent all uncertainties in the stress spectrum, except those in the damping and inertia force coefficients, thicknesses of marine growths and structural members, which are represented by their own uncertainty variables. Apart from uncertainties in the stress spectrum, a detailed modelling of the fatigue-related uncertainties is presented. Uncertainties in SCF, damage model (S-N line), analytical modelling of the probability distribution of non-narrow banded stress process, long-term probability distribution of sea states and in the reference damage at which failure occurs, are all considered in the group of fatigue-related uncertainties. Formulation of the stress spectrum and stress spectral moments is presented explicitly in the idealized uncertainty space. Then, the failure function of the reliability analysis is expressed in terms of uncertainty variables as being independent of the spectral analysis. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty origins. The procedure presented in the paper is demonstrated by an example jacket type structure and the results are compared with previously calculated results using more sophisticated uncertainty modelling of the stress spectrum.

Uncertainties in Spectral Fatigue Damages and an Efficient Simple-Reliability Calculation of Offshore Structures

2006 ◽  
Author(s):  
H. Karadeniz
Keyword(s):  
Spectral Analysis ◽  
Damage Model ◽  
Water Structure ◽  
Offshore Structures ◽  
Statistical Characteristics ◽  
Random Wave ◽  
Stress Process ◽  
Efficient Computation ◽  
Fatigue Reliability ◽  
Reliability Method

This paper presents uncertainties in spectral fatigue damages of offshore structures firstly. Then, attention is given to the formulation and procedure of a fast and efficient computation of fatigue reliability estimates. Most of uncertainties are embedded in response characteristics of the stress process and the damage-model used. Uncertainties in stress statistical characteristics are associated with the modeling of structures and random wave environment as well as wave loading and the analysis used. Uncertainties arising from degradation of member stiffness, wave-current and water-structure interactions can be considered in the modeling of structures, wave environment and loading. In the fatigue damage, there are additional uncertainties arising from the modeling of damage-mechanism. These uncertainties are due to experimental fatigue data and structural joint configurations. All these uncertainties can be classified into aleatory (naturally inherent) and epistemic (due to lack of knowledge) categories. The second part of the paper is devoted to a fast and efficient computation of fatigue reliability. This algorithm eliminates repetitive execution of spectral analysis procedure. It is performed only once for all reliability iterations. In this technique, a suitable spectral formulation of the stress process is used and a new uncertainty parameter is introduced to represent most of uncertainties in the stress spectrum. A detailed modeling of the fatigue-related uncertainties is presented. The failure function of the reliability analysis is expressed independently of the spectral analysis. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty parameters. The procedure is demonstrated by an example jacket structure and the results are compared with previously available ones.

Spectral Fatigue and Stress Based Reliability Assessment of Offshore Steel Structures

2008 ◽  
Author(s):  
H. Karadeniz
Keyword(s):  
Spectral Analysis ◽  
Limit State ◽  
Steel Structures ◽  
Offshore Structures ◽  
Statistical Characteristics ◽  
Random Wave ◽  
State Function ◽  
Efficient Computation ◽  
Reliability Method ◽  
Stress Spectrum

Having summarized briefly uncertainties in spectral fatigue damages of offshore structures, this paper presents the formulation and procedure of an efficient computation of reliability estimates on basis of fatigue damages and stresses. Most of uncertainties are embedded in response characteristics of the stress process and the damage-model used. Uncertainties in stress statistical characteristics are associated with the modeling of structures, random wave environment, wave loading and the analysis used. In the fatigue damage, additional uncertainties arise from the modeling of damage-mechanism. These uncertainties are due to experimental fatigue data and structural joint configurations. All these uncertainties can be classified into the categories as a) those naturally inherent (aleatory) and b) those due to lack of knowledge (epistemic). The second part of the paper is devoted to a fast and efficient computation of the fatigue reliability. This algorithm prevents repetitive execution of spectral analysis procedure during the reliability iteration. In this technique, a suitable formulation of the stress spectrum is used with a model uncertainty parameter representing most of uncertainties in the stress spectrum. The failure function of the reliability analysis is expressed independently of the spectral analysis. For the stress based reliability calculation the mean stress-amplitude of the stochastic stress variation is used to define a limit state function. The related uncertainties are the same as those aforementioned. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty parameters. The procedure is demonstrated by an example jacket structure. The third part of the paper explains the inverse reliability method to determine some parameters, which may be deterministic or probabilistic, under required reliability constraints.

scholarly journals Probabilistic Assessment of Degree of Bending in Tubular X-Joints of Offshore Structures Subjected to Bending Loads

2015 ◽  
Vol 2015 ◽  
pp. 1-12 ◽  
Author(s):  
Hamid Ahmadi ◽  
Amirreza Ghaffari
Keyword(s):  
Probability Distribution ◽  
Goodness Of Fit ◽  
Accurate Determination ◽  
Offshore Structures ◽  
Fatigue Reliability ◽  
Distribution Models ◽  
Estimated Parameters ◽  
Jacket Structures ◽  
Kolmogorov Smirnov ◽  
Bending Loads

Fatigue life of tubular joints in offshore structures is significantly influenced by the degree of bending (DoB). The DoB exhibits considerable scatter calling for greater emphasis in accurate determination of its governing probability distribution which is a key input for the fatigue reliability analysis of a tubular joint. Although the tubular X-joints are commonly found in offshore jacket structures, as far as the authors are aware, no comprehensive research has been carried out on the probability distribution of the DoB in tubular X-joints. In the present paper, results of parametric equations available for the calculation of the DoB have been used to develop probability distribution models for the DoB in the chord member of tubular X-joints subjected to four types of bending loads. Based on a parametric study, a set of samples was prepared and density histograms were generated for these samples using Freedman-Diaconis method. Twelve different probability density functions (PDFs) were fitted to these histograms. In each case, Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, after substituting the values of estimated parameters for each distribution, a set of fully defined PDFs have been proposed for the DoB in tubular X-joints subjected to bending loads.

The Fatigue Reliability Evaluation & Improvement of the Single-Arm Subway Current Collector

2012 ◽  
Vol 215-216 ◽  
pp. 826-831 ◽  
Author(s):  
Yu Chen ◽  
Zhi Ming Liu ◽  
Qiang Li
Keyword(s):  
Fatigue Life ◽  
Calculation Result ◽  
Dynamic Stress ◽  
Current Collector ◽  
Reliability Evaluation ◽  
Fatigue Reliability ◽  
Railway Vehicles ◽  
Load Spectra ◽  
Reliability Method

This study developed a fatigue reliability method for evaluating and improving the key parts on railway vehicles, which was applied to real structures. The study involved a type of single-arm current collector, while its contact shoe often collapsed in operation and needs improvements. The dynamic stress data from the actual line was tested and converted to load spectra based on damage consistency rule, and then the fatigue life of the contact shoe structure was achieved. The calculation result comes to correspond to its operation life. Based on the method, an improving plan for the structure was developed under optimizing algorithms.

Vibration Fatigue Reliability of Lead and Lead-Free Solder Joint by FORM/SORM

2007 ◽  
Vol 345-346 ◽  
pp. 1393-1396
Author(s):  
Ouk Sub Lee ◽  
Man Jae Hur ◽  
Yeon Chang Park ◽  
Dong Hyeok Kim
Keyword(s):  
Solder Joint ◽  
Failure Probability ◽  
Lead Free ◽  
Lead Free Solder ◽  
Fatigue Reliability ◽  
Probability Models ◽  
First Order ◽  
Reliability Method ◽  
Vibration Fatigue ◽  
Free Solder

It is well-known that the vibration significantly affect the life of solder joint. In this paper, the effects of the vibration on the failure probability of the solder joint are studied by using the failure probability models such as the First Order Reliability Method (FORM) and the Second Order Reliability Method (SORM). The accuracies of the results are estimated by a help of the Monte Carlo Simulation (MCS). The reliability of the lead and the lead-free solder joint was also evaluated. The reliability of lead-free solder joint is found to be higher than that of lead solder joint.

System Identification of Jack-Up Platform by Spectral Analysis

2010 ◽  
Author(s):  
X. M. Wang ◽  
C. G. Koh ◽  
T. N. Thanh ◽  
J. Zhang
Keyword(s):  
Spectral Analysis ◽  
System Identification ◽  
Time Domain ◽  
Time History ◽  
Offshore Structures ◽  
Wave Load ◽  
History Of ◽  
Structural Responses ◽  
Jack Up ◽  
Numerical Strategy

For the purpose of structural health monitoring (SHM), it is beneficial to develop a robust and accurate numerical strategy so as to identify key parameters of offshore structures. In this regard, it is difficult to use time-domain methods as the time history of wave load is not available unless output-only methods can be developed. Alternatively, spectral analysis widely used in offshore engineering to predict structural responses due to random wave conditions can be used. Thus the power spectral density (PSD) of structural response may be more appropriate than time history of structural responses in defining the objective (fitness) function for system identification of offshore structures. By minimizing PSD differences between measurements and simulations, the proposed numerical strategy is completely carried out in frequency domain, which can avoid inherent problems rising from random phase angles and unknown initial conditions in time domain. A jack-up platform is studied in the numerical study. A search space reduction method (SSRM) incorporating the use of genetic algorithms (GA) as well as a substructure approach are adopted to improve the accuracy and efficiency of identification. As a result, the stiffness parameters of jack-up legs can be well identified even under fairly noisy conditions.

Efficient Derivation of the Probability Distribution of the Extreme Values of Offshore Structural Response: Comparison of Three Different Methods

2013 ◽  
Author(s):  
M. K. Abu Husain ◽  
N. I. Mohd Zaki ◽  
G. Najafian
Keyword(s):  
Probability Distribution ◽  
Extreme Values ◽  
Structural Response ◽  
Offshore Structures ◽  
Simulation Technique ◽  
Random Wave ◽  
Wave Loading ◽  
Sampling Variability ◽  
Time Simulation ◽  
Extreme Responses

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional (Monte Carlo) time simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated extreme responses (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, three different versions of a more efficient time simulation technique (ETS) are compared by exposing a test structure to sea states of different intensity. The three different versions of the ETS technique take advantage of the good correlation between extreme responses and their corresponding surface elevation extreme values, or quasi-static and dynamic linear extreme responses.

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