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.

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.

Fatigue and Fracture Reliability of Articulated Tower Joint Under Random Loading

2009 ◽  
Author(s):  
Mohd Moonis Zaheer ◽  
Nazrul Islam
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Damage ◽  
Offshore Structures ◽  
Probability Of Failure ◽  
Random Wave ◽  
Shear Stresses ◽  
Random Loading ◽  
Sea Bed ◽  
Fatigue Damage Accumulation ◽  
Reliability Method

An articulated tower is one of the compliant offshore structures connected to the sea bed through a universal joint. In the random sea environment, this joint is subjected to reversal of shear stresses, which makes it susceptible to fatigue damage. In this study, fatigue damage accumulation in articulated joint under random loading is studied. The dynamic analysis of the tower has been carried out for twelve simulated sea states under random wave alone and random wave with wind. Nonlinearities due variable submergence, instantaneous position of the tower and hydrodynamic loading have been taken into account in the derivation of equation of motion. Fatigue life of the joint has been determined by S-N curve and fracture mechanics (F-M) approaches. Advanced First Order Reliability Method (FORM) and Monte Carlo Simulation method have been used for the reliability estimation. The results of the analyses are presented in terms of probability of failure and reliability indices. Sensitivity analysis is carried out to study the effect and participation of various random variables on the joint reliability. Most probable point (MPP) or design points have been located on the failure surface. Important parametric studies have been carried out which yield important information for reliability based design. The results of the study indicate that Miner’s rule, which is generally used in the design against fatigue in steel structures, yields a conservative estimate of probability of failure as compared to the fracture mechanics approach.

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.

Seismic Analysis of a Double-Hinged Articulated Offshore Tower

2008 ◽  
Author(s):  
Syed Danish Hasan ◽  
Nazrul Islam ◽  
Khalid Moin
Keyword(s):  
Seismic Analysis ◽  
Offshore Structures ◽  
Random Wave ◽  
Wave Forces ◽  
Offshore Structure ◽  
Concentrated Mass ◽  
Energy Principle ◽  
Rotational Angle ◽  
Sea Bed ◽  
Time Histories

The response of offshore structures under seismic excitation in deep water conditions is an extremely complex phenomenon. Under such harsh environmental conditions, special offshore structures called articulated structures are feasible owing to reduced structural weight. Whereas, conventional offshore structure requires huge physical dimensions to meet the desired strength and stability criteria, therefore, are uneconomical. Articulated offshore towers are among the compliant offshore structures. These structures consist of a ballast chamber near the bottom hinge and a buoyancy chamber just below the mean sea level, imparting controlled movement against the environmental loads (wave, currents, and wind/earthquake). The present study deals with the seismic compliance of a double-hinged articulated offshore tower to three real earthquakes by solving the governing equations of motion in time domain using Newmark’s-β technique. For this purpose Elcentro 1940, Taft 1952 and Northridge 1994 earthquake time histories are considered. The tower is modeled as an upright flexible pendulum supported to the sea-bed by a mass-less rotational spring of zero stiffness while the top of it rigidly supports a deck in the air (a concentrated mass above water level). The computation of seismic and hydrodynamic loads are performed by dividing the tower into finite elements with masses lumped at the nodes. The earthquake response is carried out by random vibration analysis, in which, seismic excitations are assumed to be a broadband stationary process. Effects of horizontal ground motions are considered in the present study. Monte Carlo simulation technique is used to model long crested random wave forces. Effect of sea-bed shaking on hydrodynamic modeling is considered. The dynamic equation of motion is formulated using Lagrangian approach, which is based on energy principle. Nonlinearities due to variable submergence and buoyancy, added mass associated with the geometrical non-linearities of the system are considered. The results are expressed in the form of time-histories and PSDFs of deck displacement, rotational angle, base and hinge shear, and the bending moment. The outcome of the response establishes that seismic sea environment is an important design consideration for successful performance of hinges, particularly, if these structures are situated in seismically active zones of the world’s ocean.

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.

Experimental Study of Slow-Drift Ship Motions in Shallow Water Random Waves

2011 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Trygve Kristiansen
Keyword(s):  
Spectral Analysis ◽  
Shallow Water ◽  
Transfer Functions ◽  
Second Order ◽  
Ship Motions ◽  
Random Wave ◽  
Random Waves ◽  
Low Frequencies ◽  
Wave Basin ◽  
Drift Forces

Slowly varying motions and drift forces of a large moored ship in random waves at 35m water depth are investigated by an experimental wave basin study in scale 1:50. A simple horizontal mooring set-up is used. A second-order wave correction is applied to minimize “parasitic” long waves. The effect on the ship motion from the correction is clearly seen, although less in random wave spectra than in pure bi-chromatic waves. Empirical quadratic transfer functions (QTFs) of the surge drift force are found by use of cross-bi-spectral analysis, in two different spectra have been obtained. The QTF levels increase significantly with lower wave frequencies (except at the diagonal), which is special for finite and shallow water. Furthermore, the QTF levels frequencies at low frequencies increase significantly out from the QTF diagonal. Thus Newman’s approximation should preferrably not be used in these cases. Using the LF waves as a direct excitation in a “linear” ship force analysis gives random records that compare reasonably well with those from the cross-bi-spectral analysis. This confirms the idea that the drift forces in shallow water are closely correlated to the second-order potential, and thereby by the second-order LF waves.

scholarly journals Comparison of Extreme Surface Elevation for Linear and Nonlinear Random Wave Theory for Offshore Structures

2018 ◽  
Vol 203 ◽  
pp. 01021
Author(s):  
Nurul 'Azizah Mukhlas ◽  
Noor Irza Mohd Zaki ◽  
Mohd Khairi Abu Husain ◽  
Gholamhossein Najafian
Keyword(s):  
Probabilistic Approach ◽  
Wave Theory ◽  
Offshore Structures ◽  
Random Wave ◽  
Linear Wave ◽  
Random Waves ◽  
Sea State ◽  
Higher Order Terms ◽  
Structural Responses ◽  
Nonlinear Random Wave

For offshore structural design, the load due to wind-generated random waves is usually the most important source of loading. While these structures can be designed by exposing them to extreme regular waves (100-year design wave), it is much more satisfactory to use a probabilistic approach to account for the inherent randomness of the wave loading. This method allows the statistical properties of the loads and structural responses to be determined, which is essential for the risk-based assessment of these structures. It has been recognized that the simplest wave generation is by using linear random wave theory. However, there is some limitation on its application as some of the nonlinearities cannot be explained when higher order terms are excluded and lead to underestimating of 100-year wave height. In this paper, the contribution of nonlinearities based on the second order wave theory was considered and being tested at a variety of sea state condition from low, moderate to high. Hence, it was proven that the contribution of nonlinearities gives significant impact the prediction of 100-year wave's design as it provides a higher prediction compared to linear wave theory.

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