Applications of Probabilistic Fracture Mechanics to Offshore Structures

1988 ◽  
Vol 41 (2) ◽  
pp. 61-84 ◽  
Author(s):  
Finn Kirkemo
Keyword(s):  
Fracture Mechanics ◽  
Model Updating ◽  
Fatigue Cracks ◽  
Limit State ◽  
Steel Structures ◽  
Local Stress ◽  
Offshore Structures ◽  
And Performance ◽  
Fatigue Limit State ◽  
Life Predictions

For offshore structures the fatigue limit state is governing the structural dimensions of several members and joint connections. Safety against fatigue failure is achieved through a combination of design requirements and performance of in-service inspections with repair of detected fatigue cracks. A review of uncertainties involved in fatigue life predictions by fracture mechanics is presented with particular reference to steel structures. Sources of uncertainties considered are: environmental conditions, hydrodynamic loading, global structural analysis, local stress calculation at fatigue sensitive points, and fatigue crack growth modeling by fracture mechanics. A probabilistic model using the fracture mechanics in probabilistic form is presented. This model accounts for uncertainties in loading, initial and critical defect sizes, material parameters, and in the uncertainty related to computation of the stress intensity factor. Failure probabilities are computed by first-order reliability methods and sensitivity factors are determined. Model updating based on in-service inspection results is formulated. Uncertainties with respect to detecting a crack and to correctly sizing a crack are included. Experience on application of the analysis method is presented.

Bi-Linear Fatigue and Fracture Approach for Safety Analysis of an Offshore Structure

2014 ◽  
Vol 136 (2) ◽  
Author(s):  
Rizwan A. Khan ◽  
Suhail Ahmad
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Growth Parameters ◽  
Limit State ◽  
Steel Structures ◽  
Published Data ◽  
Offshore Structure ◽  
Structure Degradation

The design of welded structures for the fatigue limit state is normally carried out by means of either linear or bilinear S-N curves, which have been found adequate to predict crack initiation only. To properly assess the effects of the design, fabrication, inspection, and repair strategy for structure degradation due to crack growth, fracture mechanics (FM) models need to be applied. In this paper, alternative S-N and FM formulations of fatigue are investigated. The probabilistic fracture mechanics approach predicts the fatigue life of welded steel structures in the presence of cracks under random spectrum loading. It is based on a recently proposed bi-linear relationship to model fatigue crack growth. Uncertainty modeling, especially on fatigue crack growth parameters, is undertaken with the aid of recently published data in support of the bilinear crack growth relationship. Results pertaining to the fatigue reliability and fatigue crack size evolution are presented using the Monte Carlo simulation technique and the emphasis is placed on a comparison between the linear and bilinear crack growth models. Variations in the system configuration, service life, and coefficients of crack growth laws have been studied on the parametric basis

The Potential for Non-Conservative Results From the Probabilistic Fracture Mechanics Analyses Is Assessed Based on the Collected Observation of Fatigue Cracks in Offshore Steel Structures

2017 ◽  
Author(s):  
Ole Tom Vårdal
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Structural Integrity ◽  
Fatigue Cracks ◽  
Steel Structures ◽  
Lessons Learned ◽  
Probabilistic Methods ◽  
Growth Potential

In structural integrity management, it is essential to know the fatigue crack growth potential. The lessons learned from use of refined fatigue analyses, fracture mechanics and probabilistic methods for platforms in-service are presented. For ageing offshore units of semi-submersible design, the inspection history of more than 20 000 NDT inspections and detection of close to 1000 fatigue cracks, are used in this study. These experience data are used to assess the potential for Non-conservative estimate for the fatigue crack growth potential.

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.

Design of Dynamic High Voltage Cables for Floating Substation

2020 ◽  
Author(s):  
Lucie Guignier ◽  
Riccardo Mariani ◽  
Arthur Cottet-Emard ◽  
Stéphane Toumit ◽  
Thomas Choisnet
Keyword(s):  
Fatigue Limit ◽  
Limit State ◽  
Time Domain Analysis ◽  
Fatigue Properties ◽  
Design Verification ◽  
Coupled Models ◽  
Limit State Design ◽  
Pacific Coast Of Japan ◽  
And Performance ◽  
Fatigue Limit State

Abstract This paper presents the design and performance assessment of 220kV dynamic export cables for a floating substation characterized by a ring-shaped floater known as Damping Pool. The main originality of the design presented is that the cables considered have dry conductors. They are shielded from the water by a longitudinally welded corrugated copper sheath. Similar cables have been operating at lower voltage levels and thus with thinner insulation thicknesses. The export cable configuration has been designed considering environmental conditions representative of both the Central North Sea, Pacific Coast of Japan or the US, in 100m water depth. Ultimate and fatigue limit-state design verification of the configuration are made through nonlinear time-domain analysis using coupled models comprising the floating substation hull, the mooring system and dynamic export cables. Fatigue limit-state design verification is based on the fatigue properties of the cable section, combined with appropriate S-N curves of the armour layers and metallic screen-sheath. Design verifications show that the dynamic export cable configuration proposed could satisfactorily meet the performance requirements for a service life over 25 years, considering proven cable equipment such as a bend stiffener remaining within today’s manufacturer molding capacities.

scholarly journals Influence of Local Microstructure on Stresses, Durability and Fracture Mechanics of Cast Iron Components

2018 ◽  
Vol 925 ◽  
pp. 264-271
Author(s):  
Corinna Thomser ◽  
Jakob Olofsson ◽  
Vitalii Gurevitch
Keyword(s):  
Cast Iron ◽  
Fracture Mechanics ◽  
Process Simulation ◽  
Local Stress ◽  
Casting Process ◽  
Design Stage ◽  
Local Material ◽  
Recent Developments ◽  
And Performance ◽  
Casting Process Simulation

Cast iron components show a large variety of different microstructures in dependence on chemical composition, inoculation and cooling conditions. In conventional static and dynamic calculations as well as in fracture mechanics assessment of cast iron components, the influence of local microstructure on the overall behavior of the component is not considered. Usually one material dataset is applied for the whole material. The paper describes recent developments in the field of the prediction of local microstructure and its correlation to local stress-strain, fatigue durability as well as fracture toughness. The benefit of combining casting process simulation with lifetime predictions and fracture mechanics assessment is shown for selected examples. By integrating casting process simulation, microstructure modelling, local material characterization and load analysis, a simulation based approach for predicting the behavior and performance of cast iron components already during the design stage is enabled. Thus, the local assessment helps designers to assess risks and strive for light weight designs before the casting is made.

A New Recommended Practice for Inspection Planning of Fatigue Cracks in Offshore Structures Based on Probabilistic Methods

2014 ◽  
Author(s):  
Inge Lotsberg ◽  
Gudfinnur Sigurdsson
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Cracks ◽  
Offshore Structures ◽  
Probability Of Detection ◽  
Probabilistic Methods ◽  
Event Analysis ◽  
Inspection Planning ◽  
Standard Design ◽  
Input Parameters ◽  
Service Inspection

During the last 30 years a methodology for planning in-service inspection of fatigue cracks based on probabilistic methods has been developed. Due to the nature of the fatigue phenomena it is well known that minor changes in basic assumptions can have significant influence on the predicted crack growth lives. Calculated fatigue lives are sensitive to input parameters using standard design analysis procedures. Calculated probabilities of fatigue failure using probabilistic methods are even more sensitive to the analysis methodology and to input parameters to the analyses. Fracture mechanics analysis is required for prediction of crack sizes during service life in order to account for probability of detection after an inspection event. Analysis based on fracture mechanics needs to be calibrated to that of fatigue test data or S-N data. Thus, use of these methods for planning inspection requires considerable education and engineering skill. Therefore the industry has asked for guidelines that can be used to derive reliable inspection results using these methods. DNV has during the last years performed a joint industry project on use of probabilistic methods for planning in-service inspection for fatigue cracks in offshore structures. The recommendations from this project are now being included in a DNV Recommended Practice. The main background for this document is presented in this paper.

An Approach for Reliability-Based Fatigue Design of Welded Joints on Aluminum High-Speed Vessels

2006 ◽  
Vol 50 (01) ◽  
pp. 85-98
Author(s):  
Matthew Collette ◽  
Atilla Incecik
Keyword(s):  
High Speed ◽  
Hot Spot ◽  
Fatigue Cracks ◽  
Limit State ◽  
Design Standards ◽  
State Equations ◽  
Fatigue Design ◽  
Highly Nonlinear ◽  
Input Variables ◽  
Fatigue Limit State

Fatigue cracks are an ongoing problem for aluminum high-speed vessels, and preventing fatigue cracks caused by wave loading is expected to be a significant challenge for future aluminum high-speed ferries and military vessels. To aid in this effort, a hot-spot fatigue design approach using first-order reliability methods (FORM) is constructed. Two different limit state functions are investigated, and the accuracy and consistency of the FORM method for the highly nonlinear fatigue limit state equations are evaluated through a comparison with Monte Carlo simulation results. The sensitivity of the resulting safety index to changes in the input variables, and their uncertainties, are presented graphically. The method is compared to existing design standards for four simple structural details.

Do We Need a Safe Excavation Pressure for Dented Pipelines: How Should it Be Defined?

2018 ◽  
Author(s):  
Muntaseer Kainat ◽  
Doug Langer ◽  
Sherif Hassanien
Keyword(s):  
Fracture Mechanics ◽  
Boundary Conditions ◽  
Structural Reliability ◽  
Limit State ◽  
Stress And Strain ◽  
Operating Pressure ◽  
Fitness For Purpose ◽  
Element Analysis ◽  
Overburden Pressure ◽  
Conflicting Objectives

Pipeline operators’ utmost priority is to achieve high safety measures during the lifecycle of pipelines including effective management of integrity threats during excavation and repair processes. A single incident pertaining to a mechanical damage in a gas pipeline has been reported previously which resulted in one fatality and one injury during investigation. Some operators have reported leaking cracks while investigating rock induced dents. Excavation under full operating pressure can lead to changes in boundary conditions and unexpected loads, resulting in failure, injuries, or fatalities. In the meantime, lowering operating pressure during excavation can have a significant impact on production and operational availability. The situation poses two conflicting objectives; namely, maximizing safety and maximizing operational availability. Current pipeline regulations require that operators have to ensure safe working conditions by depressurizing the line to a level that will not cause a failure during the repair process. However, there are no detailed guidelines on how an operator should determine a safe excavation pressure (SEP) level, which could lead to engineering judgment and subjectivity in determining such safety level. While the pipeline industry relies on well-defined fitness for purpose analyses for threats such as crack and corrosion, there is a gap in defining a fitness for purpose for dents and dents associated with stress riser features in order to set an SEP. Stress and strain based assessment of dents can be used in this matter; however, it requires advanced techniques to account for geometric and material nonlinearity. Additionally, loading and unloading scenarios during excavation (e.g. removal of indenter, overburden pressure, etc.) drive a change in the boundary conditions of the pipe that could lead to leakage. Nevertheless, crack initiation or presence within a dent should be considered, which requires the incorporation of crack geometry and application of fracture mechanics in assessing a safe excavation pressure. Recently, there have been advancements in stress and strain based finite element analysis (FEA) of dents coupled with structural reliability analysis that can be utilized to assess SEP. This paper presents a reliability-based approach to determine a safe excavation pressure for dented liquid pipelines. The approach employs nonlinear FEA to model dents interacting with crack features coupled with uncertainties associated with pipe properties and in-line-inspection information. A fracture mechanics-based limit state is formulated to estimate the probability of failure of dents associated with cracks at different levels of operating pressure during excavation. The application of the developed approach is demonstrated through examples within limited scope. Recommended enhancements and future developments of the proposed approach are also discussed.

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