Reliability of Fracture Mechanics Approach to Fatigue

2009 ◽  
Author(s):  
Mario L. Macia ◽  
Jaime Buitrago ◽  
Wan Kan ◽  
Barron Bichon ◽  
Jonathan Moody ◽  
...  
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Safety Factor ◽  
Important Variable ◽  
Probability Of Detection ◽  
Inspection System ◽  
Acceptance Criteria ◽  
Fatigue Design ◽  
Critical Components ◽  
Selection Of

Current fatigue design of fracture-critical components, such as tendons and risers, requires dual fatigue life criteria to be satisfied. The S-N approach includes a safety factor (SF) of 10 on the life of the component, while the fracture mechanics (FM) approach includes a safety factor of 5 on the life through-thickness of an acceptable initial flaw. FM provides critical initial flaw sizes such that suitability of the selected NDE methods and weld acceptance criteria can be established. This paper pertains to a comparative fatigue life reliability study between those two approaches. The objective is to develop a rationale for the selection of a safety factor on fatigue life to use in FM calculations. A reliability-based methodology is proposed and implemented. The SFs for FM are obtained by targeting the reliability obtained in fatigue designs based on historically proven S-N damage approach. Random variables entering both approaches were characterized and a number of weld design cases devised to obtain reliabilities. One important variable is the distribution of initial flaw sizes. For this study, flaw distributions were developed from actual inspection records, accounting for the effects of probability of detection and sizing accuracy of the inspection system, as well as the flaw acceptance criteria during fabrication. Comparisons of reliabilities obtained for designs by both approaches for various quality S-N curves, stress spectra, pipe sizes, and initial flaw sizes indicate that there is ample scope to modify downward the current FM safety factor. However, given the limited scope of this study, it is recommended to asses the FM SF using reliability analysis on a project-specific basis.

Optimizing Probe Active Aperture for Phased Array Weld Inspections

2021 ◽  
Vol 79 (8) ◽  
pp. 797-804
Author(s):  
Anmol Birring
Keyword(s):  
Fracture Mechanics ◽  
Ultrasonic Testing ◽  
Phased Array ◽  
Pressure Vessels ◽  
Acceptance Criteria ◽  
Nondestructive Technique ◽  
Cross Sectional ◽  
Area Of Interest ◽  
Array Technology ◽  
Selection Of

Phased array ultrasonic testing (PAUT) has become a popular nondestructive technique for weld inspections in piping, pressure vessels, and other components such as turbines. This technique can be used both in manual and automated modes. PAUT is more attractive than conventional angle-beam ultrasonic testing (UT), as it sweeps the beam through a range of angles and presents a cross-sectional image of the area of interest. Other displays are also available depending on the software. Unlike traditional A-scan instruments, which require the reconstruction of B- and C-scan images from raster scanning, a phased array image is much simpler to produce from line scans and easier to interpret. Engineering codes have incorporated phased array technology and provide steps for standardization, scanning, and alternate acceptance criteria based on fracture mechanics. The basis of fracture mechanics is accurate defect sizing. There is, however, no guidance in codes and standards on the selection and setup of phased array probes for accurate sizing. Just like conventional probes, phased array probes have a beam spread that depends on the probe’s active aperture and frequency. Smaller phased array probes, when used for thicker sections, result in poor focusing, large beam spread, and poor discontinuity definition. This means low resolution and oversizing. Accurate sizing for fracture mechanics acceptance criteria requires probes with high resolution. In this paper, guidance is provided for the selection of phased array probes and setup parameters to improve resolution, definition, and sizing of discontinuities.

Fatigue Safety Factors for Deepwater Risers

2002 ◽  
Author(s):  
B. Stahl ◽  
H. Banon
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Safety Factor ◽  
Damage Index ◽  
Rational Approach ◽  
Fatigue Reliability ◽  
Safety Factors ◽  
Fatigue Design ◽  
Uncertainty Parameters ◽  
Deepwater Risers

Fatigue life is governed by a number of variables that are highly uncertain. The safety factor on fatigue life is used in a deterministic way to account for the estimated fatigue damage uncertainty. High uncertainties lead to high fatigue safety factors, and vice versa. Evaluation of the uncertainties in the variables governing fatigue design provides a grip on what the safety factor should be. This paper addresses riser fatigue using a fatigue reliability model that is relatively simple but still captures the important elements of the fatigue problem. The bias and uncertainty of stress range are extremely important parameters in design against fatigue. This is due not only to the fact that these parameters are highly uncertain, but also to the fact that they are greatly amplified in the fatigue damage equation by the ‘slope’ m of the S-N curve. The Palmgren-Miner fatigue damage index and the intercept value of the S-N curve are additional important variables in fatigue design. A model for combining wave-induced and vortex-induced vibration (VIV) is introduced together with the best available data and reference to industry work in this technology area. A recently completed joint industry project on riser reliability provides good calibration points for the critical fatigue reliability variables. Reliability and sensitivity studies are performed to demonstrate the effect of the uncertainty parameters. An approach to selecting deterministic fatigue design factors that yield specified reliability targets is developed and illustrated. The study provides a rational approach to selecting safety factors for design of deepwater risers, taking into account both wave and VIV-induced fatigue damage.

scholarly journals Reliability of Mechanised UT Systems to Inspect Girth Welds During Pipeline Construction

1998 ◽  
Author(s):  
Jan A. de Raad
Keyword(s):  
High Speed ◽  
Ultrasonic Inspection ◽  
Probability Of Detection ◽  
Inspection System ◽  
Present System ◽  
Immediate Feedback ◽  
Long Distance ◽  
Acceptance Criteria ◽  
Actual Use ◽  
Girth Welds

As an alternative to radiography, a field-proven mechanized ultrasonic inspection system is discussed. Called Rotoscan, this system has been developed for inspection of girth welds during construction of long-distance pipelines, both on- and offshore. It is characterized by high inspection speed and instant recording of results. Unlike prevailing radiography, it provides immediate feedback to the welders. Recent technical improvements in flaw sizing and recording have allowed the application of rejection/acceptance criteria for weld defects based on fracture mechanics principles. The development and actual use of such modern acceptance criteria, particularly in Canada, supported the introduction of mechanised ultrasonic inspection. World wide applications proved that, contrary to expectations, ultrasonic inspection does not lead to higher weld repair rates than radiography does. Between early 1989 and now, over 5.000 km of pipeline (300.000 welds) were inspected with Rotoscan and its reliability proven. The introduction of colour enhanced transit distance “C-scan mapping”, producing a coherent picture based on the signal’s transit distance, enabled the system to cope with most existing ultrasonic procedures and acceptance criteria, because of its capability to detect and quantify volumetric defects. Moreover, the integrated simultaneous Time Of Flight Diffraction (TOFD) function enables through-thickness sizing of defect. The present system is capable of achieving a high Probability Of Detection (POD) together with a low False Call Rate (FCR). In the meantime, Rotoscan has been qualified in various countries, for different customers and for a variety of weld processes, pipe diameters and wall thicknesses. Because of its features, the now mature system has demonstrated its capabilities also for use on lay barges as an alternative to high-speed radiography.

Development of Design Criteria and NDE Method for HPHT Equipment

2021 ◽  
Author(s):  
Dilip Parth Pathak ◽  
Kishore Padmanabha ◽  
Robert Joseph Wilhelmi ◽  
Luiz Henrique Velloso Coutinho ◽  
Venkata Rao Brahmaji Vampati Veera
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
High Sensitivity ◽  
Flaw Size ◽  
Design Criteria ◽  
Project Design ◽  
Acceptance Criteria ◽  
Inspection Method ◽  
Critical Areas ◽  
Design Life

Abstract Subsea production systems have been using API-specified methods and detection criteria for nondestructive examination (NDE) for equipment up to 15,000-psi rated working pressure. With higher pressure requirements and use of methods for evaluation of the fatigue life of HPHT equipment, existing criteria cannot be sufficient to achieve the desired fatigue life. This paper demonstrates more stringent design criteria and a detection method that was developed to achieve HPHT fatigue life, yet is cost and schedule effective. When fracture mechanics was used for fatigue life estimation, an initial flaw size is stated as a starting point in determining design life. Using API-specified NDE criteria of 1/16-inch detection of surface flaw, project design life was not achieved for certain components without a major redesign and retest. Smaller flaw lengths were preferred in critical areas instead of standard flaw sizes. This created a need for a high-sensitivity penetrant inspection technique to effectively detect this new minimum flaw size in critical areas. Various methods of NDE were considered, and a conclusion was taken in selecting the best inspection method. Fracture mechanics and finite element analysis required a minimum detectable flaw size length of 1/32 inch to meet the project design life without changing equipment technology. By using surface NDE with fluorescent liquid dye penetrant of sensitivity levels 2 and 3, all seeded flaws of 1/32-inch were consistently detected in the validation test coupon, which enabled the use of this stricter criteria for the analysis. Detailed procedures were established, and validation testing results were documented with photographs of detected flaws. Seeded flaw coupons were manufactured for validation of procedures and to train the various facilities that will use these techniques for production equipment. Additionally, procedures and guidelines were provided to inspectors and inspection facilities to ensure proper implementation of the methods. Measurement system analysis for repeatability and reproducibility was conducted at the facilities. This enabled the fatigue design of the HPHT equipment to advance beyond the boundaries of traditional methods and acceptance criteria set by current industry standards. New and tighter acceptance criteria were developed to improve HPHT fatigue life. High-sensitivity penetrant inspection technique, capable of detecting flaw sizes as small as 1/32-inch, was established and implemented. This inspection technique is not common to the oil and gas industry because of the ability of standard methods to readily detect the API-required criteria. The method has improved detection capabilities and has the potential to move toward adopting advanced design methods to address HPHT requirements.

ECA Methodology for Fatigue Design of Risers and Flowlines

2007 ◽  
Author(s):  
C. H. Luk ◽  
T. J. Wang
Keyword(s):  
Fracture Mechanics ◽  
Design Method ◽  
Steel Catenary Riser ◽  
Fatigue Design ◽  
Wide Range ◽  
Fatigue And Fracture ◽  
Level 3 ◽  
Fatigue Loads ◽  
Vessel Motion ◽  
Level 2

Engineering Criticality Assessment (ECA) is a procedure based on fracture mechanics that may be used to supplement the traditional S-N approach and determine the flaw acceptance and inspection criteria in fatigue and fracture design of risers and flowlines. A number of design codes provide guidance for this procedure, e.g. BS-7910:2005 [1]. However, more investigations and example studies are still needed to address the design implications for riser and flowline applications. This paper provides a review of the existing ECA methodology, presents a fracture mechanics design method for a wide range of riser and flowline fatigue problems, and shows flaw size results from steel catenary riser (SCR) and flowline (FL) examples. The first example is a deepwater SCR subjected to fatigue loads due to vessel motion and riser VIV. The second example is a subsea flowline subjected to thermal fatigue loads. The effects of crack re-characterization and material plasticity on the Level-2 and Level-3 ECA results of the SCR and flowline examples are illustrated.

The Influencing Factors and Assessment Rule of Fatigue Life of Shaft Based on Product Lifecycle Management

2007 ◽  
Vol 561-565 ◽  
pp. 2253-2256 ◽  
Author(s):  
You Tang Li ◽  
Ping Ma ◽  
Jun Tian Zhao
Keyword(s):  
Fatigue Life ◽  
Influencing Factors ◽  
Axial Stress ◽  
Assessment Method ◽  
Product Lifecycle Management ◽  
Product Lifecycle ◽  
Fatigue Design ◽  
Main Factors ◽  
Set Up ◽  
Lifecycle Management

Product lifecycle management is one of the main developmental aspects of advanced manufacturing technology. Anti-fatigue design is the key content in product lifecycle management. For designing the fatigue life of shaft exactly and determining the assessment method, the influencing factors must be realized roundly. The mechanical model of shaft is set up at first, and then the main factors that affect the fatigue life of shaft is discussed, the interrelations of the main factors and the framework are founded. The assessment equation of fatigue life for shaft is put forward and the influencing coefficient of multi-axial stress to fatigue life is analyzed. The results of this paper will establish the base of anti-fatigue and assessment life of shaft.

Thermal Fatigue and Fracture Mechanics Analysis of Aluminium Alloy

2011 ◽  
Vol 201-203 ◽  
pp. 2476-2480
Author(s):  
Wen Xiao Zhang ◽  
Guo Dong Gao ◽  
Guang Yu Mu
Keyword(s):  
Aluminum Alloy ◽  
Fatigue Life ◽  
Fracture Mechanics ◽  
Thermal Fatigue ◽  
Low Carbon Steel ◽  
Strain Range ◽  
J Integral ◽  
Low Carbon ◽  
Thermal Fatigue Life ◽  
Energy Aspect

The in-phase and out-of-phase thermal fatigue of aluminum alloy were experimentally studied. The fatigue life was evaluated analytically by using the elastic-plastic fracture mechanics method (mainly J integral). The results of experiments and calculations showed that the life of out-of-phase fatigue was longer than that of in-phase fatigue within the same strain range. This is the same as the results of other materials such as medium and low carbon steel. On the other hand, the predicted life was consistent with experimental results. This suggests that J integral as a mechanics parameter for characterizing the thermal fatigue strength of aluminum alloy and the calculation method developed here is efficient. A parameter ΔW was proposed from energy aspect to characterize the capacity of crack propagation. The in-phase thermal fatigue life was the same as the out-of-phase thermal fatigue life for identical ΔW values.

Concrete Fatigue Life Characteristics of the Different Survival Rates in the Lateral Pressure Conditions

2012 ◽  
Vol 600 ◽  
pp. 250-255
Author(s):  
Qiang Cai ◽  
Ji Ming Kong ◽  
Ze Fu Chen
Keyword(s):  
Fatigue Life ◽  
Life Stress ◽  
Failure Modes ◽  
Survival Curve ◽  
Survival Rates ◽  
Life Distribution ◽  
Fatigue Design ◽  
Set Up ◽  
Fatigue Equation ◽  
Two Parameter

Under cyclic loading of concrete structures, fatigue failure is the main failure modes of fatigue, which has become the fatigue design of concrete structure must be considered, then the concrete fatigue studies must clarify the fatigue life of concrete under different survival curve S-N curve. Based on the statistics of the two parameter Weibull distribution theory, obtain the concrete under different survival rates of fatigue life distribution, namely to improve survival, reduce the fatigue life; stress level is reduced, the fatigue life is increased; and has set up more than 50% under different survival rates of concrete fatigue equation.

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