Treatment of Low Stresses in Fatigue Design of Girth Welds Under Spectrum Loading

2008 ◽  
Author(s):  
Stephen J. Maddox
Keyword(s):  
Fatigue Tests ◽  
Test Results ◽  
Welded Steel ◽  
Steel Pipes ◽  
Fatigue Design ◽  
Conflicting Evidence ◽  
Miner’S Rule ◽  
Miner's Rule ◽  
Girth Welds ◽  
Spectrum Loading

Although the use of Miner’s rule for calculating the cumulative damage due to spectrum loading is well established, there is conflicting evidence as to how stresses below the constant amplitude fatigue limit (CAFL) should be assessed. They are known to be damaging under spectrum loading, but the extent of that damage is not clear. Since such stress levels potentially contribute the majority of the fatigue damage from wave motion and vortex-induced vibration in risers, this issue is highly significant both economically and with respect to safety. Therefore, fatigue tests were performed on strip specimens cut from girth-welded steel pipes under constant and variable amplitude loading to investigate this issue. The test results were assessed on the basis of Miner’s rule used in conjunction with various methods of extrapolating the S-N curve beyond the CAFL to establish the most suitable for correctly representing the damaging effect of stress ranges below it. A fracture mechanics fatigue crack growth analysis was also performed to calculate the fatigue lives expected under the test loading spectrum.

The Fatigue Behavior of Steel Structures under Random Loading

2008 ◽  
Vol 378-379 ◽  
pp. 3-16
Author(s):  
Henning Agerskov
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Test ◽  
Steel Structures ◽  
Fatigue Tests ◽  
Test Series ◽  
Test Results ◽  
Random Loading ◽  
Mechanics Analysis ◽  
Miner’S Rule ◽  
Miner's Rule

Fatigue damage accumulation in steel structures under random loading has been studied in a number of investigations at the Technical University of Denmark. The fatigue life of welded joints has been determined both experimentally and from a fracture mechanics analysis. In the experimental part of the investigation, fatigue test series with a total of 540 fatigue tests have been carried through on various types of welded plate test specimens and full-scale offshore tubular joints. The materials that have been used are either conventional structural steel or high-strength steel. The fatigue tests and the fracture mechanics analyses have been carried out using load histories, which are realistic in relation to the types of structures studied, i.e. primarily bridges, offshore structures and chimneys. In general, the test series carried through show a significant difference between constant amplitude and variable amplitude fatigue test results. Both the fracture mechanics analysis and the fatigue test results indicate that Miner’s rule, which is normally used in the design against fatigue in steel structures, may give results, which are unconservative, and that the validity of the results obtained from Miner’s rule will depend on the distribution of the load history in tension and compression.

Fatigue Testing of Full Scale Girth Welded Pipes Under Variable Amplitude Loading

2012 ◽  
Author(s):  
Y.-H. Zhang ◽  
S. J. Maddox
Keyword(s):  
Fatigue Limit ◽  
Fatigue Damage ◽  
Fatigue Testing ◽  
Full Scale ◽  
Variable Amplitude Loading ◽  
Variable Amplitude ◽  
Fatigue Design ◽  
Miner’S Rule ◽  
Miner's Rule ◽  
Loading Spectrum

In the fatigue design of steel catenary risers there are concerns regarding the fatigue damage to girth welds from low stresses, below the constant amplitude fatigue limit, in the loading spectrum and the validity of Miner’s cumulative damage rule. In both cases there is increasing evidence that current design methods can be non-conservative. These fundamental issues were addressed in a recent JIP. A key feature was development of the resonance fatigue testing rigs to enable them to test full-scale pipes under variable amplitude loading. Such tests were performed under a loading spectrum representative of that experienced by some risers, with many tests lasting over 100 million cycles to investigate the fatigue damage due to small stresses as well as the validity of Miner’s rule. However, the resonance rigs are only capable of producing spectrum loading by gradually increasing or decreasing the applied load, whereas more ‘spiky’ random load sequences may be relevant in practice. Therefore the programme also included fatigue tests in conventional testing machines on strip specimens cut from pipes to compare the two types of loading sequence. This paper presents the results of these tests, conclusions drawn and recommendations for changes to current fatigue design guidance for girth welded pipes regarding the definition of the fatigue limit, allowance for the damaging effect of low stresses and the validity of Miner’s rule.

Sour Environmentally Assisted Fatigue of Welded SCR Materials: Post-Weld Finishing Treatment Evaluation

2010 ◽  
Author(s):  
Philippe P. Darcis ◽  
Israel Marines-Garcia ◽  
Stephen J. Hudak ◽  
Mariano Armengol ◽  
Hector M. Quintanilla
Keyword(s):  
Oil And Gas ◽  
Fatigue Tests ◽  
Oil And Gas Development ◽  
Intermediate Scale ◽  
Fatigue Design ◽  
X65 Steel ◽  
Girth Welds ◽  
Welded Pipe ◽  
Welding Procedure ◽  
Brine Environment

The current work aims to point out the influence of sour brine environment on the fatigue resistance of welded SMLS (seamless) steel pipe used for design and fabrication of risers for oil and gas development. A C-Mn steel X65 pipe 10.75″ (273.1 mm) outside diameter (OD) and 25.4 mm wall thickness (WT) was chosen for this program. The Welding Procedure designed for girth welds manufacturing involved the use of Lincoln STT-GMAW™ process for the root pass and SAW process with twin wire configuration for the fill and cap passes. This welding procedure presents a special post-weld finishing treatment, which consist in flapping the inner and outer weld overfills to produce a flush profile between weld metal and outer/inner pipe surfaces. The experimental approach focused on quantifying the effect of H2S using a sour brine environment. For this purpose, intermediate-scale fatigue data in the sour brine environment, using full thickness’ strip specimens extracted from the welded SMLS (seamless) pipe, have been generated. Intermediate-scale fatigue tests in air have also been obtained to provide a baseline for comparison with the sour brine data. Those results have been compared with full-scale fatigue tests in air environment. Finally, results were statistically analyzed to determine which standard fatigue design curves best represent the measured S-N fatigue endurance in air and sour brine environments. Results were also compared with available literature and results on other seamless’ welded pipe of the same API 5L, Grade X65 steel in comparable environments.

A First Investigation on Cumulative Fatigue Life for a Type 304-L Stainless Steel Used for Pressure Water Reactor

2009 ◽  
Author(s):  
A. Fissolo ◽  
J. M. Stelmaszyk
Keyword(s):  
Stainless Steel ◽  
Fatigue Life ◽  
Fatigue Tests ◽  
Operating Conditions ◽  
Damage Effect ◽  
Final Conclusion ◽  
Variable Loading ◽  
Miner’S Rule ◽  
Miner's Rule ◽  
Type 304

In order to estimate the crack initiation damage, and also the water leakage conditions on PWR pipes, uniaxial fatigue curves are often used. They were deduced from strain or stress load control tests using normalised cylindrical specimens. However, severe thermo-mechanical loading fluctuations are observed in operating conditions. Components may also be submitted to transient loadings. The purpose of the present work is to start investigation on the fatigue life with a variable loading, in order to examine cumulative damage effect in fatigue. In this frame, multilevel strain controlled fatigue tests have been performed on a Type 304-L stainless steel (elaborated in accordance with the RCC-M specifications). The experimental results show that linear Miner’s rule is not verified in our conditions. When the strains are applied in a decreasing order (High-Low strain sequence), the summation of cycle ratios is smaller than unity, whatever the number of applied levels, whereas this summation is higher than one for an increasing order (Low-High strain sequence). A loading sequence effect is clearly evidenced. Different cumulative fatigue damage theories, proposed in literature, have been also tested. Some of them have been given better estimation than the Miner’s rule. That is the case of the so-called “Hybrid Theory” proposed and tested before by Bui Quoc on a Type 304-L steel. Extension of a model proposed by S. Taheri would seem also promising. At this stage, final conclusion cannot be yet deduced, additional investigations are needed.

An Investigation of the Fatigue Performance of Riser Girth Welds

2008 ◽  
Vol 130 (1) ◽  
Author(s):  
Stephen J. Maddox ◽  
Julian B. Speck ◽  
G. Reza Razmjoo
Keyword(s):  
Oil And Gas ◽  
Welding Process ◽  
Fatigue Loading ◽  
Fatigue Tests ◽  
Fatigue Performance ◽  
Steel Catenary Riser ◽  
Gas Recovery ◽  
Fatigue Design ◽  
Pipeline Welding ◽  
Girth Welds

Increasing deep-water oil and gas recovery has highlighted the need for high integrity, high fatigue performance girth welds in steel catenary riser systems. Such systems include girth welds made from one side. However, the widely used fatigue design classification, UK Class F2, for such welds is not well founded, but probably overconservative for pipeline welds. In an attempt to justify upgrading current fatigue design classifications and providing a better basis for design, fatigue tests were performed on a range of girth-welded pipes produced by pipeline welding contractors. This paper presents the results of those tests and their evaluation in terms of the factors that influence the fatigue performance of girth welds, including welding process, welding position, backing system, joint alignment, weld quality, specimen type, and fatigue loading conditions. Conclusions are drawn regarding the scope for adopting higher design classifications and the conditions that must be met to justify them.

Statistical Analysis of Fatigue Test Data

2017 ◽  
Author(s):  
Carol Johnston
Keyword(s):  
Statistical Analysis ◽  
Fatigue Strength ◽  
Cyclic Loading ◽  
Fatigue Test ◽  
Test Data ◽  
Fatigue Tests ◽  
Fatigue Performance ◽  
Test Results ◽  
Girth Welds ◽  
New Procedures

The offshore environment contains many sources of cyclic loading. Standard design S-N curves, such as those in DNVGL-RP-C203, are usually assigned to ensure a particular design life can be achieved for a particular set of anticipated loading conditions. Girth welds are often the ‘weak link’ in terms of fatigue strength and so it is important to show that girth welds made using new procedures for new projects that are intended to be used in fatigue sensitive risers or flowlines do indeed have the required fatigue performance. Alternatively, designers of new subsea connectors, used for example in tendons for tension leg platforms, mooring applications or well-heads which will experience cyclic loading in service, also wish to verify the fatigue performance of their new designs. Often operators require contractors to carry out resonance fatigue tests on representative girth welds in order to show that girth welds made using new procedures qualify to the required design S-N curve. Operators and contractors must then interpret the results, which is not necessarily straightforward if the fatigue lives are lower than expected. Many factors influence a component’s fatigue strength so there is usually scatter in results obtained when a number of fatigue tests are carried out on real, production standard components. This scatter means that it is important first to carry out the right number of tests in order to obtain a reasonable understanding of the component’s fatigue strength, and then to interpret the fatigue test results properly. A working knowledge of statistics is necessary for both specifying the test programme and interpreting the test results and there is often confusion over various aspects of test specification and interpretation. This paper describes relevant statistical concepts in a way that is accessible to non-experts and that can be used, practically, by designers. The paper illustrates the statistical analysis of test data with examples of the ‘target life’ approach (that is now included in BS7608:2014 + A1) and the equivalent approach in DNVGL-RP-C203, which uses the stress modification factor. It gives practical examples to designers of a pragmatic method that can be used when specifying test programmes and interpreting the results obtained from tests carried out during qualification programmes, which for example, aim to determine whether girth welds made using a new procedure qualify to a particular design curve. It will help designers who are tasked with specifying test programmes to choose a reasonable number of test specimens and stress ranges, and to understand the outcome when results have been obtained.

An Investigation of the Fatigue Performance of Riser Girth Welds

2006 ◽  
Author(s):  
Stephen J. Maddox ◽  
Julian B. Speck ◽  
G. Reza Razmjoo
Keyword(s):  
Oil And Gas ◽  
Welding Process ◽  
Fatigue Loading ◽  
Fatigue Tests ◽  
Fatigue Performance ◽  
Steel Catenary Riser ◽  
Gas Recovery ◽  
Fatigue Design ◽  
Pipeline Welding ◽  
Girth Welds

Increasing deep-water oil and gas recovery has highlighted the need for high integrity, high fatigue performance girth welds in steel catenary riser systems. Such systems include girth welds made from one side. However, the widely used fatigue design classification, UK Class F2, for such welds is not well founded, but probably over-conservative for pipeline welds. In an attempt to justify upgrading current fatigue design classifications and providing a better basis for design, fatigue tests were performed on a range of girth-welded pipes produced by pipeline welding contractors. This paper presents the results of those tests and their evaluation in terms of the factors that influence the fatigue performance of girth welds, including welding process, welding position, backing system, joint alignment, weld quality, specimen type and fatigue loading conditions. Conclusions are drawn regarding the scope for adopting higher design classifications and the conditions that must be met to justify them.

Fatigue Design of Machine Elements Under Cumulative Damage Effect, Using Bagci’s Fatigue Failure Surface Line

1984 ◽  
Vol 106 (4) ◽  
pp. 466-475
Author(s):  
C. Bagci
Keyword(s):  
Fatigue Life ◽  
Fatigue Strength ◽  
Material Properties ◽  
Cumulative Damage ◽  
Damage Effect ◽  
Stress Effect ◽  
Mean Stress ◽  
Fatigue Design ◽  
Miner’S Rule ◽  
Miner's Rule

A thorough review of the state-of-the-art of determining fatigue life of machine and structural members considering cumulative damage effect under varying stress amplitudes is given. Among the many proposed theories, Miner’s linear damage rule is seen to be as reliable as any other rule alleged to be an improvement for predicting fatigue life under cumulative damage effects. Its simplicity and amenability for easy modification have in fact been the basis for some other theories and used in design codes. In its original form, Miner’s rule, however does not account for fatigue strength reducing factors. Observing fatigue data on the effects of fatigue strength reducing factors, the article offers a modified form of the Miner’s rule to consider the effects of fatigue strength reducing factors, such as the notch, reliability, surface finish, size, and environmental factors. The mean stress effect and material properties are incorporated utilizing Bagci’s mean stress line and the S-N diagram. The safe fatigue life of a component subjected to stresses of varying magnitudes becomes Ns=df/∑i=1s(αi/10zi) where zi=A{B−log(pig/Rf)+log[1−(pi/mi)r]} in the ith block of stress range, Rf being the resultant of fatigue strength reducing factors; A, B, g are parameters defined by material properties, pi is the ratio of the basic alternating stress times the factor of safety (the failure value) to the yield strength of the material, and mi is the slope of the load line in the ith block of loading. Design charts for zi for steel and aluminum alloys for cases with and without basic mean stress for r=4 are given. Numerical examples are included. Therefore, the article offers the most general form of the Miner’s rule for designers’ use for fatigue design considering cumulative damage effect.

Fatigue Testing of Full Scale Girth Welded Pipes Under Variable Amplitude Loading

2014 ◽  
Vol 136 (2) ◽  
Author(s):  
Y.-H. Zhang ◽  
S. J. Maddox
Keyword(s):  
Fatigue Limit ◽  
Fatigue Damage ◽  
Fatigue Testing ◽  
Full Scale ◽  
Variable Amplitude Loading ◽  
Variable Amplitude ◽  
Fatigue Design ◽  
Miner’S Rule ◽  
Miner's Rule ◽  
Loading Spectrum

In the fatigue design of steel catenary risers, there are concerns regarding the fatigue damage to girth welds from low stresses, below the constant amplitude fatigue limit, in the loading spectrum and the validity of Miner's cumulative damage rule. These fundamental issues were addressed in a recent joint-industrial project (JIP). A key feature was development of the resonance fatigue testing rigs to enable them to test full-scale pipes under variable amplitude loading. Such tests were performed under a loading spectrum representative of that experienced by some risers, with many tests lasting over 100 million cycles to investigate the fatigue damage due to small stresses as well as the validity of Miner's rule. However, the resonance rigs are only capable of producing spectrum loading by gradually increasing or decreasing the applied load whereas more “spiky” random load sequences may be relevant in practice. Therefore, the program also included fatigue tests in conventional testing machines on strip specimens cut from pipes to compare the two types of loading sequence. This paper presents the results of these tests, conclusions drawn, and recommendations for changes to current fatigue design guidance for girth welded pipes regarding the definition of the fatigue limit, allowance for the damaging effect of low stresses, and the validity of Miner's rule.

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