Multiple Axial Fatigue of Pressure Armors in Flexible Risers

2011 ◽  
Author(s):  
Naiquan Ye ◽  
Svein Sævik
Keyword(s):  
Oil And Gas ◽  
Fatigue Behavior ◽  
Equivalent Stress ◽  
Local Stress ◽  
External Pressure ◽  
Mean Stress ◽  
Stress Range ◽  
Stress Components ◽  
Flexible Risers ◽  
Axial Fatigue

The design of flexible risers has been challenged by the exploration of oil and gas goes into ever-deep regions as in Mexico Gulf and West Africa. Comparing to the fatigue analysis of the tensile armors which have been extensively investigated in recent years, much less effort has been devoted in the fatigue of pressure armor. The fatigue of the pressure armor is much more complicated than the tensile armors. For the tensile armor, the longitudinal stress along the helix path dominates the fatigue behavior, while for the pressure armor, more stress components will play together to affect its fatigue. If the fatigue of the tensile armor can be characterized as a uni-axial fatigue phenomenon, the fatigue of the pressure armor will be a typical multi-axial problem. The stress components in the pressure armor consist of contribution from the following sources: stress in the hoop direction due to internal/external pressure, stress in the radial direction due to the pressure and contact pressure from the tensile armor layer, stresses caused by the ovalization when the riser is bent, and local stresses due to local bending (nub/valley contact). The friction between the nub and valley interface is reflected in the local stress components as well. A Finite Element (FE) based computer program BFLEX developed by MARINTEK for the stress analysis of flexible risers are capable of calculating the complicate stress components of the pressure armors. In order to perform fatigue damage calculation for the pressure armor, mean stress and stress range must be computed based on these stress components. Mean stress correction becomes very important due to large mean stress experienced by the pressure armor. There are several ways to make use of these stress components to derive the mean stress and stress range. Equivalent stress models and critical plane models are the main models to address the general feature of the multi-axial fatigue. The application of these models on the fatigue of the pressure armor of the flexible risers will be discussed in this paper. The best suited model will be suggested based on the specific stress components in the pressure armor.

The Effect of Mean Stress on the Fatigue Behavior of Woven-Roving Glass Fiber-Reinforced Polyester Subjected to Torsional Moments

2005 ◽  
Vol 127 (3) ◽  
pp. 301-309 ◽  
Author(s):  
Mohamed N. A. Nasr ◽  
M. N. Abouelwafa ◽  
A. Gomaa ◽  
A. Hamdy ◽  
E. Morsi
Keyword(s):  
Glass Fiber ◽  
Detrimental Effect ◽  
Fatigue Behavior ◽  
Local Stress ◽  
Mean Stress ◽  
Glass Fiber Reinforced ◽  
Stress Components ◽  
Amplitude Component ◽  
The Mean ◽  
Stress Ratios

The effect of torsional mean stress on the fatigue behavior of glass fiber-reinforced polyester (GFRP) is studied by testing thin-walled, woven-roving tubular specimens with two fiber orientations, [±45°]2s and [0,90°]2s, at negative stress ratios (R),R=−1,−0.75,−0.5,−0.25, 0. The [±45°]2s specimens were found to have higher fatigue strength than the [0,90°]2s specimens at all stress ratios. This is attributed to the difference in local stress components, the [±45°]2s specimens being subjected to tension-compression local stress components, while the [0,90°]2s specimens being subjected to pure local shear stress. For the studied stress ratios; the mean stress component had a detrimental effect on the amplitude component for the [±45°]2s specimens; while it was ineffective for the [0,90°]2s specimens in a certain region in the mean-amplitude diagram, region (1), then it had a detrimental effect in the rest of the diagram, region (2). The S–N curves for positive stress ratios were extrapolated from those for negative stress ratios, which were found experimentally, for the [0,90°]2s specimens. The positive stress ratio points, having the same local stress state as the negative ones, showed an acceptable behavior tending to decrease the amplitude component for the same life.

Alternative Stress Models With Focus on Full FE Model for Flexible Risers

2016 ◽  
Author(s):  
Lidong Wang ◽  
Svein Sævik ◽  
Naiquan Ye ◽  
Qianjin Yue ◽  
Zhixun Yang ◽  
...  
Keyword(s):  
Stress State ◽  
Pressure Wave ◽  
Numerical Models ◽  
External Pressure ◽  
Fe Model ◽  
Stress Components ◽  
Stress Prediction ◽  
Flexible Risers ◽  
Longitudinal Slip ◽  
Stress Models

Flexible risers are exposed to complicated loading conditions including internal/external pressure, wave and current loads, as well as floater movements. The combination of these three loads may lead to instability of the tensile armor wire in both longitudinal and transverse directions, which may result in different stress state based on different numerical models in BFLEX software. Simplified models have been implemented to calculate the stress in tensile armor wires with moderate accuracy, with only considering the longitudinal slip of wires. In order to investigate how the transverse slip will influence the stress state, a full finite element (FullFE) modeling option has been implemented recently to capture the interaction between different layers more precisely thus giving more accurate stress prediction. Different stress components will be compared with already existing simplified approaches. In addition, geodesic and loxodromic slip assumptions will be applied to study the slip behavior of the tensile armor layers.

Observations on the Interaction of High Mean Stress and Type II Hot Corrosion on the Fatigue Behavior of a Nickel Base Superalloy

1985 ◽  
Vol 107 (1) ◽  
pp. 220-224 ◽  
Author(s):  
J. M. Allen ◽  
G. A. Whitlow
Keyword(s):  
Stress Corrosion ◽  
Maximum Stress ◽  
Rupture Strength ◽  
Fatigue Behavior ◽  
Nickel Base Superalloy ◽  
Chloride Salt ◽  
Mean Stress ◽  
R Ratio ◽  
Nickel Base ◽  
Udimet 720

A study measuring the effects of a molten sulfate/chloride salt on the creep/fatigue behavior of a nickel base turbine blade superalloy, Udimet 720, at 1300°F (704°C) is described. Cyclic stress–cycles to failure (S-N) curves were generated at high mean stress levels, with mean stress, maximum stress, or the ratio of minimum to maximum stress (R ratio) held constant. In salt, it was found that when maximum stress is above the yield, with the cyclic component 20 percent of the maximum, failure occurs by stress corrosion fatigue in orders of magnitude less time than for corresponding loading conditions in air. It is significant, from a failure analysis point of view, that fatigue fracture is intergranular in these circumstances. Similar fatigue behavior may be expected for other nickel base alloys, however, at substantially lower maximum stresses in as much as Udimet 720 exhibits superior short time rupture strength, i.e., resistance to this form of stress corrosion, over the other blade alloys evaluated in this environment.

Experimental and Numerical Investigation of High-Temperature Multi-Axial Fatigue

2021 ◽  
Author(s):  
Harish Ramesh Babu ◽  
Marco Böcker ◽  
Mario Raddatz ◽  
Sebastian Henkel ◽  
Horst Biermann ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Biaxial Loading ◽  
Complex Geometry ◽  
Low Cycle Fatigue ◽  
Axial Stress ◽  
Equivalent Stress ◽  
Turbine Blades ◽  
Damage Development ◽  
Simulation Results ◽  
Axial Fatigue

Abstract Gas turbines and aircraft engines are dominated by cyclic operating modes with fatigue-related loads. This may result in the acceleration of damage development on the components. Critical components of turbine blades and discs are exposed to cyclic thermal and mechanical multi-axial fatigue. In the current work, planar-biaxial Low-Cycle-Fatigue tests are conducted using cruciform specimens at different test temperatures. The influence on the deformation and lifetime behaviour of the nickel-base disk alloy IN718 is investigated at selected cyclic proportional loading cases. The calculation of the stress and strain distribution of the cruciform specimens from the experimental data is difficult to obtain due to complex geometry and temperature gradients. Therefore, there is a need for Finite Element Simulations. A viscoplastic material model is considered to simulate the material behaviour subjected to uniaxial and the selected planar-biaxial loading conditions. At first, uniaxial simulation results are compared with the uniaxial experiment results for both batches of IN718. Then, the same material parameters are used for simulating the biaxial loading cases. The prediction of FE simulation results is in good agreement with the experimental LCF test for proportional loadings. The equivalent stress amplitude results of the biaxial simulation are compared with the uniaxial results. Furthermore, the lifetime is calculated from the simulation and by using Crossland and Sines multi-axial stress-based approaches. The Crossland model predicts fatigue life significantly better than the Sines model. Finally, the simulated lifetime results are compared with the experimental lifetime

STUDY ON FATIGUE BEHAVIOR OF STRENGTHENED NON-LOAD-CARRYING CRUCIFORM WELDED JOINTS USING CARBON FIBER SHEETS

2012 ◽  
Vol 12 (01) ◽  
pp. 179-194 ◽  
Author(s):  
TAO CHEN ◽  
QIAN-QIAN YU ◽  
XIANG-LIN GU ◽  
XIAO-LING ZHAO
Keyword(s):  
Stress Concentration ◽  
Carbon Fiber ◽  
Welded Joints ◽  
Failure Modes ◽  
Fatigue Behavior ◽  
Local Stress ◽  
Fatigue Loading ◽  
Cfrp Sheets ◽  
Load Carrying ◽  
Weld Toe

This paper reports an experimental study on the use of carbon fiber-reinforced polymer (CFRP) sheets to strengthen non-load-carrying cruciform welded joints subjected to fatigue loading. Failure modes and corresponding fatigue lives were recorded during tests. Scatter of test results was observed. Thereafter, a series of numerical analyses were performed to study the effects of weld toe radius, the number of CFRP layers and Young's modulus of reinforced materials on local stress concentration at a weld toe. It was found that fatigue life of such welded connections can be enhanced because of the reduction of stress concentration caused by CFRP strengthening. Parametric study indicates that the weld toe radius and the amount of CFRP are the key parameters influencing the stress concentration factors and stress ranges of the joint. Enhancement of modulus for adhesive and CFRP sheets can also be beneficial to the fatigue performance to some extent.

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