A Consistent Soil Fatigue Framework Based on the Number of Equivalent Cycles

Cyclic soil degradation and hardening affects soil stiffness and strength, and is linked to an increase or decrease in the mean effective confining stress due to void ratio or pore pressure changes. This change of state can be explicitly modeled by using effective stress methods, or implicitly modeled using total stress methods. In the latter, this is achieved by using empirical functions based on the number of loading cycles that are derived from constant-amplitude stress or strain laboratory tests. To suite generalized loading conditions, these functions must be extrapolated to variable-amplitude loading. This falls under the general class of a fatigue-based problem. The main focus of this paper is to present a generalized consistent soil fatigue formulation for soils under cyclic loading. The paper then goes on to discuss the implementation of various cyclic soil degradation and hardening models reported in the literature, and highlights their important underlying assumptions, capabilities and limitations.     

Parametric study of geometrical effects on the degree of bending (DoB) in offshore tubular K-joints under out-of-plane bending loads

In this paper, results of comprehensive finite element (FE) analyses carried out on 81 generated models of tubular K-joints are presented. Results of FE models were verified against experimental data and parametric equations. The fatigue life of a tubular joint, which is commonly found in offshore industry, is not only dependent on the value of hot-spot stress (HSS), but is also significantly influenced by the through-the-thickness stress distribution characterized by the degree of bending (DoB). Data extracted from FE analyses was used to study the effects of geometrical parameters on the DoB values in tubular K-joints subjected to two types of out-of-plane bending (OPB) moment loads. The determination of DoB values in a tubular joint is essential for improving the accuracy of fatigue life estimation using the stress-life (S–N) method and particularly for predicting the fatigue crack growth based on the fracture mechanics (FM) approach. Geometrically parametric investigation was followed by a set of nonlinear regression analyses to develop DoB parametric formulas for the fatigue analysis of K-joints under OPB loadings.     

Fatigue reliability analysis in time domain for inspection strategy of fixed offshore structures

The paper describes a methodology for estimation of reliability of fixed offshore structures with respect to fatigue and extreme stress. The failure criteria for fatigue are formulated using fracture mechanics principle and that for extreme stress using API Code. Advanced Second Moment method is used to find the reliability index. The total life of the structure is divided into a set of stationary sea-states, occurring during storms and described by directional power spectrum. The method has been illustrated through application to a typical jacket platform. Usefulness of the methodology for planning in-service inspections has been highlighted.     

引水隧洞工程中热应力对围岩表层稳定性的影响分析

岩石中的热应力作用,是岩石热学问题中的一个重要研究内容。热应力由温度变化造成,温度的周期性变化引起热应力的周期性变化,热应力周期性作用导致岩石的疲劳破坏。以雅砻江锦屏水电站深埋引水隧洞为例,分析隧洞围岩温度场的分布特征,根据热应力交变作用特点,提出岩石的疲劳破坏判据,并利用这个判据分析热应力作用下隧洞围岩表层的破坏特点。     

Modeling touchdown point oscillation and its relationship with fatigue response of steel catenary risers

The riser-seabed interaction resulting in a trench formed in the touchdown zone (TDZ) of steel catenary risers (SCR) has a significant influence on accumulated fatigue damage. Several studies have used different trench modeling approaches to investigate the trench effect on fatigue. However, contradictory observations have been reported with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue. In this study, the significance of trench geometry in fatigue damage evaluation was investigated. Using analytical and numerical approaches, a meaningful relationship was observed between the trench slope in different zones and the peak fatigue damage. A new set of rules was proposed for the qualitative assessment of the overall trend of trench effect on the variation of fatigue damage. The proposed assessment rules were validated by performing comprehensive numerical fatigue analysis. A comparison with samples of published experimental and numerical studies was also completed. It was observed that depending on the direction of the low-frequency vessel excursions, the peak fatigue damage may increase towards the near offsets and decrease towards the far vessel offset. This implied that the case dependency of the trench effect on fatigue response in different geographical locations with various environmental loads was a potential source for the contradictory results reported in previously published studies.     

Effect of traveling wave on the vortex-induced vibration of a long flexible pipe

It is expected that the vortex-induced vibration response of long flexible tubular structures exposed to a shear current will consist of multiple modes of standing and traveling waves simultaneously. The vibration characteristics of traveling wave caused by VIV and its effect on the vibration form for a flexible pipe model are presented in this paper. Two methods are proposed to reveal the characteristics of standing and traveling waves from the perspective of an instantaneous illustration and a time-averaged statistical analysis separately. The first is associated with the adoption of an illustration method depending upon an instantaneous local upper envelope to identify the dominance of the standing or traveling wave. It is found that the in-line and cross-flow wave types show synchronization in that the wave types are mostly changeless or occasionally transform from one type to the other nearly simultaneously. In both the in-line and cross-flow directions, it is evident that the vibration response is not dominated by the standing wave characteristics but can often be characterized by the alternate presence of a traveling wave and a hybrid standing-traveling wave. The second method is associated with the adoption of the significant strain peak concept proposed to estimate the occurrence of the peak strain response and fatigue damage risk. It is found that the strain peak value attributed to in-line static strain accounts for a large proportion of the final equivalent significant strain peak near the pipe end with high reduced velocities, especially when the flow velocity is high. With the occurrence of a hybrid pattern of standing-traveling wave for higher velocities, the distribution of the significant cross-flow strain peak becomes uniform and its corresponding wave shape is apt to be flat.     

A comparative study for the fatigue assessment of a ship structure by use of hot spot stress and structural stress approaches

Recently, a mesh-size insensitive structural stress definition (Battelle structural stress method) has been proposed. This method gives a stress state at a weld toe with a relatively coarse mesh, such as up to 2t (t=plate thickness) or irregular mesh shapes. The structural stress definition is based on the elementary structural mechanics theory and provides an effective measure of a stress state in front of a weld toe. In this present study, a fatigue strength assessment was carried out for a side shell connection of a container vessel using both the hot spot stress and the structural stress methods. A consistent approach to computing extrapolated hot spot stress for design purposes based on converged hot spot stresses is described and current fatigue guidance is evaluated. Fatigue strength predicted by the two methods, i.e. hot spot stress and structural stress approaches, at hot spot locations of a typical ship structure are compared and discussed.     

An innovative block partition and equivalence method of the wave scatter diagram for offshore structural fatigue assessment

The fatigue assessment plays an increasing role for the offshore structural safety. Many fatigue assessment methods have been developed for this purpose. Among those methods, the time domain method is regarded as the most accurate method but less adopted in practice due to time consuming. In order to improve the efficiency of the time domain method, an innovative block partition and equivalence method of the wave scatter diagram is developed for offshore structural fatigue assessment. After the wave scatter diagram is partitioned into several blocks, the newly developed method, involves determination of the equivalent wave height, wave period and occurrence probability of the representative sea states based on modified energy equivalent principle. The equivalent wave period of the representative sea state is calculated via the spectral moment formula in which the equivalent spectral moments of zero and second order are obtained based on the weighted averaging principle. Combining with the determined wave period, the equivalent significant wave height can be determined by reversing the wave spectrum integral formula, where the equivalent wave energy of a divided block of the wave scatter diagram is modified by introducing a factor to compensate the effect of low- and high-amplitude cycles fatigue damage. The equivalent occurrence probability is equal to the summation of the original sea states’ occurrence probability within the divided block. The developed method has the advantage of preserving the stochastic characteristics of the short term sea states within the divided block during determining the representative sea state. At the same time the newly developed method has no limitation on block partition and can be applied on different offshore structure. Two structural models, a fixed mono-pile platform and a floating semi-submersible platform, are demonstrated in the numerical examples. Results indicate that the newly developed method is robust, computationally affordable, and accurate within engineering expectations.     

Fatigue crack growth investigation on offshore pipelines with three-dimensional interacting cracks

Due to high cost of full-scale experimental setup, this study presents a numerical model on fatigue behaviours of offshore pipeline with multiple coplanar cracks under cyclic tensile loadings. The validation on numerical results is made by other researchers' experimental results, and significant parameters affecting fatigue crack growth are studied.