Fatigue Design and Analysis of Offshore Pipelines and Risers Subjected to Waves and Currents

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Fatigue Damage ◽  
Damage Accumulation ◽  
Coupling Effect ◽  
Limit State ◽  
Ultimate Limit State ◽  
Offshore Pipelines ◽  
Fatigue Design ◽  
Fatigue Damage Accumulation ◽  
Waves And Currents ◽  
Wave Induced

The industry consensus would appear that the effect of currents on wave-induced fatigue damage accumulation is assumed as insignificant and can be ignored. Only when dealing with stability, ultimate limit state design, and vortex-induced vibration (VIV), is the recommended industry practice to consider both currents and waves simultaneously, except for fatigue design. This paper presents a study on how environmental loads should be considered in terms of currents and waves for the fatigue life design of offshore pipelines and risers. The study is intended as a spur to redress the misapprehension by focusing on the coupling effect of direct waves and currents in the context of fatigue damage assessment. It is demonstrated unequivocally that waves and currents cannot be decoupled for fatigue design assessments. Wave-induced fatigue with the inclusion of currents is manifested twofold, not only the increased mean stress correction effect but also higher total damage accumulation due to elevated stress ranges. The practice of using wave histograms while ignoring currents is shown to result in an unacceptable nonconservative fatigue design. Both effects should be accounted for in the engineering assessment. A first-order correction factor involving the ratio of current and wave velocities is introduced to evaluating the environmental load coupling effect. It is recognized that fatigue associated specifically with VIV phenomena is well understood and documented elsewhere, its discussion is thus out with the aims of this paper.

Fatigue Design and Analysis of Offshore Pipelines and Risers Subjected to Waves and Currents

2016 ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Fatigue Damage ◽  
Damage Accumulation ◽  
Coupling Effect ◽  
Limit State ◽  
Ultimate Limit State ◽  
Offshore Pipelines ◽  
Fatigue Design ◽  
Fatigue Damage Accumulation ◽  
Waves And Currents ◽  
Wave Induced

The industry consensus would appear that the effect of currents on wave induced fatigue damage accumulation is assumed as insignificant and can be ignored. Only when dealing with stability, ultimate limit state design and vortex induced vibration, the recommended industry practice is to consider both currents and waves simultaneously, but except for fatigue design. This paper presents a study on how environmental loads should be considered in terms of currents and waves for the fatigue life design of offshore pipelines and risers. The study is intended as a spur to redress the misapprehension by focusing on the coupling effect of direct waves and currents in the context of fatigue damage assessment. It is demonstrated unequivocally that waves and currents cannot be decoupled for fatigue design assessments. Wave induced fatigue with the inclusion of currents is manifested twofold, not only the increased mean stress correction effect but also higher total damage accumulation due to elevated stress ranges. The practice of using wave histograms while ignoring currents is shown to result in an unacceptable non-conservative fatigue design. Both effects should be accounted for in the engineering assessment. A first order correction factor involving the ratio of current and wave velocities is introduced to evaluating the environmental load coupling effect. It is recognized that fatigue associated specifically with VIV phenomena is well understood and documented elsewhere, its discussion is thus outwith the aims of this paper.

Methods for calculation and experimental evaluation of nucleation and development of the fatigue damages in tin based babbitt and babbitt lining of plain bearings

2021 ◽  
Vol 87 (11) ◽  
pp. 43-54
Author(s):  
M. V. Zernin ◽  
A. V. Matyuhin ◽  
N. N. Rybkin
Keyword(s):  
Fatigue Damage ◽  
Damage Accumulation ◽  
Limit State ◽  
Complex Stress State ◽  
Material Structure ◽  
Damage Development ◽  
Rotating Shaft ◽  
Antifriction Materials ◽  
Fatigue Damage Accumulation ◽  
Microdamage Accumulation

Fatigue damage to babbitt layers of plain bearings is often manifested during operation. The goal of the study is to develop a model for accumulation of the fatigue damage and destruction of antifriction materials and layers of plain bearings. A generalized fatigue diagram of tin-based babbitts including the main stages of fatigue damage and a diagram of the fatigue damage development in the antifriction layer of plain bearings are presented. The generalized model of V. V. Bolotin for damage accumulation and destruction is modified with regard to antifriction materials containing rather large structural elements. An explicit (direct) modeling of damage processes appeared possible for such materials. The model describes dissipated accumulation of microcracks (interpreted as destruction of the elements of the material structure), initiation and development of a system of short cracks, initiation and development of macro-cracks up to the limit state of the object. The model suggests discretization of the volume into sections with constant levels of complex stress state and discretization of the time axis into the intervals (blocks of loading cycles). The problem of identifying the parameters of a multistage model of the fatigue damage accumulation in the alloy is solved proceeding from the analysis of the results of testing babbitt specimens. We used the simplest optimization procedure, i.e., the method of deformable polyhedron. The parameters of the power function in the dependence of the rate of microdamage accumulation on the level of stresses are obtained. The parameters of the initiation and development of the crack system in the babbitt layer are obtained from the analysis of experimental results of studying steel-babbitt samples. The problem of calculating the durability of antifriction babbitt layers required the development of a new software. The program is examined by comparing calculated and experimental values of the durability of fatigue-tested bearing specimens forced against a rotating shaft by varying cyclic load. The calculated values of the durability match the experimental which confirms the performance of the calculated model.

Wave and Current Induced Cumulative Fatigue Damage Assessment for Offshore Pipeline on Free-Span

2008 ◽  
Author(s):  
Ali Valipour ◽  
Mehrdad Zoroufi ◽  
Abbas Yeganeh Bakhtiary ◽  
Reza Valipour
Keyword(s):  
Fatigue Damage ◽  
Element Model ◽  
Major Influence ◽  
Spectral Amplitude ◽  
Linear Wave ◽  
Offshore Pipelines ◽  
Drag Forces ◽  
Waves And Currents ◽  
Cumulative Fatigue Damage ◽  
Wave Induced

Suspending pipelines between two points over an uneven seafloor is called free-spanning which induces more vortices behind the pipe especially when the pipelines are exposed to the hydrodynamic effects of waves and currents. These influences cause a significant increase in the dynamic acting drag forces. In fact, the oscillatory acting drag forces, as a result of changes in the net pressure and velocity filed around the pipeline, increase the probability of fatigue damage. Since the fatigue life of offshore pipelines has a major influence on the useful life of a project, studying it for offshore pipelines laid on uneven sea beds has become very important. Both the consequence of vortex shedding as a result of wave and current interaction with the pipeline, and the selected suitable approach in assessing fatigue damage has made the design process sophisticated. In the present study, it is attempted to employ a proper finite element model with the capability of coupling the dynamic drag forces induced by wave and current conditions with estimating cumulative fatigue damage methods. The wave profile is simulated using Non-deterministic Spectral Amplitude in Gaussian sea state and using linear wave formulation to calculate wave -induced forces. Moreover, the current force is estimated by using the conventional Morison Equation as well as employing a new approach of estimating current -induced dynamic drag forces obtained by a numerical model that includes the VIV effects. Finally cumulative fatigue damage as a result of acting those forces is investigated for various conditions of laying pipeline on the seabed.

Fatigue damage accumulation around notches

2004 ◽  
Vol 46 (6) ◽  
pp. 309-313
Author(s):  
Yutaka Iino ◽  
Hideo Yano
Keyword(s):  
Fatigue Damage ◽  
Damage Accumulation ◽  
Fatigue Damage Accumulation

Different Fatigue Dynamics Under Statistically and Spectrally Similar Deterministic and Stochastic Excitations

2013 ◽  
Vol 81 (4) ◽  
Author(s):  
Son Hai Nguyen ◽  
Mike Falco ◽  
Ming Liu ◽  
David Chelidze
Keyword(s):  
Crack Growth ◽  
Fatigue Damage ◽  
Damage Accumulation ◽  
Growth Dynamics ◽  
Experimental System ◽  
Counting Method ◽  
Random Loading ◽  
Stochastic Excitations ◽  
Fatigue Damage Accumulation ◽  
System Coupling

Estimating and tracking crack growth dynamics is essential for fatigue failure prediction. A new experimental system—coupling structural and crack growth dynamics—was used to show fatigue damage accumulation is different under chaotic (i.e., deterministic) and stochastic (i.e., random) loading, even when both excitations possess the same spectral and statistical signatures. Furthermore, the conventional rain-flow counting method considerably overestimates damage in case of chaotic forcing. Important nonlinear loading characteristics, which can explain the observed discrepancies, are identified and suggested to be included as loading parameters in new macroscopic fatigue models.

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