Fatigue damage induced by nonGaussian bimodal wave loading in mooring lines

A method is developed for the static and dynamic analysis of flexible risers and pipelines in the offshore environment under conditions of arbitrarily large motions due to wave loading and vessel movements. A mixed finite element formulation is adopted where the axial force is independently interpolated and only combined with the corresponding axial displacements via a Lagrangian constraint. An advantage of the resulting hybrid beam element is that it may be applied to offshore components varying from mooring lines or cables to pipelines with finite bending stiffnesses. Results are presented for the motions and forces on a flexible riser connecting a tanker to a subsea tower and also on a combined flexible riser and subsea support buoy structure which is part of a floating offshore production system.

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Fatigue Damage Estimation for Mooring lines of Spar Platform Using System Identification Method

Journal of Ocean Engineering and Technology ◽

10.5574/ksoe.2016.30.3.161 ◽

2016 ◽

Vol 30 (3) ◽

pp. 161-168

Author(s):

Yong-Gyun Kim ◽

Yooil Kim ◽

Byoung-Hoon Kim

Keyword(s):

System Identification ◽

Fatigue Damage ◽

Damage Estimation ◽

Spar Platform ◽

Mooring Lines ◽

System Identification Method ◽

Identification Method

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Effects of Mooring Line Design Parameters on the Line Dynamics and Fatigue Response of Subsea Mooring Lines

Volume 1: Offshore Technology ◽

10.1115/omae2018-78716 ◽

2018 ◽

Author(s):

Hamid Sedghi ◽

Mehrdad Kimiaei

Keyword(s):

Dynamic Analysis ◽

Fatigue Damage ◽

Dynamic Responses ◽

Statistical Characteristics ◽

Design Parameters ◽

Mooring Line ◽

Engineering Practice ◽

Mooring Lines ◽

Fatigue Design ◽

Line Design

Dynamic characteristics of mooring lines play an important role in overall structural response and fatigue design of mooring systems. Full dynamic analysis including line dynamics is a vital part of fatigue design process although in time domain it needs excessive computational efforts. For fatigue analysis of mooring lines where hundreds of different environmental loads have to be checked, alternative analysis approach such as quasi-dynamic analysis with implicit inclusion of the line dynamic effects are used widely in engineering practice. This paper presents the results of series of case studies on the effects of various mooring line design parameters on the line dynamics as well as the mooring line dynamic fatigue response. Various mooring line composition types (all chain and chain-polyester-chain) used in different mooring configurations (catenary, semi-taut and taut) with variable range of mooring line pretensions connected to a floater in shallow and deep water depths are studied. Ratios of fatigue damage results between dynamic and quasi-dynamic results as well as the relation between fatigue damage and statistical characteristics of the line dynamic responses for different line configurations and load cases are investigated in detail.

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Simplifying Methods for Fatigue Analysis of Risers

Volume 3: Structures, Safety, and Reliability ◽

10.1115/omae2019-95386 ◽

2019 ◽

Cited By ~ 1

Author(s):

Luiz O. C. M. Pereira ◽

Paulo M. Videiro ◽

Luís V. S. Sagrilo

Keyword(s):

Fatigue Damage ◽

Computing Time ◽

Fatigue Analysis ◽

State Matrix ◽

Sea State ◽

Mooring Lines ◽

Steel Catenary Riser ◽

Advantages And Disadvantages ◽

Surface Method ◽

Early Design Stages

Abstract This work presents methods for reducing computational costs to estimate fatigue damage in riser structures by simplifying the sea state matrix to be solved in time domain by finite element analyses. Two different methods are proposed: The Equivalent Damage Method (EDM), which is a lump block method with an innovative formulation using vessel linearized response to determine significant sea states; and the Response Surface Method (RSM), which uses results from a few sea states to estimate results for other intermediary scenarios required for the fatigue analysis. Basic assumptions and guidance on how to apply the proposed methods are explained through the text. The methods are used to solve an example of a Steel Catenary Riser (SCR) connected to a Semi-Submersible vessel, resulting in a reduction of almost 80% of the required computing time. Results show accurate values for damage estimates at the top of the riser, and limitations at the touch down zone. Advantages and disadvantages of each method are discussed in detail, demonstrating that they can be used with discretion at early design stages to estimate fatigue damage of risers, umbilicals and mooring lines in general.

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VIM Simulation Method on a Cylindrical Floating Structure

Volume 1: Offshore Technology; Offshore Geotechnics ◽

10.1115/omae2016-54307 ◽

2016 ◽

Cited By ~ 1

Author(s):

Toshifumi Fujiwara

Keyword(s):

Fatigue Damage ◽

Low Frequency ◽

Safety Evaluation ◽

Simulation Method ◽

Offshore Structures ◽

Dominant Factor ◽

Floating Structure ◽

Mooring Lines ◽

Wake Oscillator ◽

Low Frequency Motion

A cylindrical floating structure can basically experience Vortex-induced Motion (VIM) in strong current. Since the VIM on the structure with long term low-frequency motion causes fatigue damage of the structure’s mooring lines and risers, precise VIM assessment is needed for the safety evaluation of them. In the standard of the International Organization for Standardization ISO19901-7, ‘Specific requirements on stationkeeping systems for floating offshore structures and mobile offshore units’, for instance, a concrete method of assessing VIM displacement is not represented in the standard document, though the requirement on the VIM demands to do the assessment on the basis of proper ways. Then in this paper, a VIM simulation method on a floating structure with circular cylinder form, that is, for example a Spar, a MPSO (Mono-column type floating Production Storage and Offloading) and so on, is shown using the wake-oscillator model. Transverse VIM is only treated since it is dominant factor on the fatigue damage of the mooring lines and risers. The assessment quality of the simulation method on the transverse VIM of floaters in current is confirmed by model test data.

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STRUCTURAL BEHAVIOUR OF FULLY COUPLED SPAR–MOORING SYSTEM UNDER EXTREME WAVE LOADING

Journal of Civil Engineering and Management ◽

10.3846/13923730.2013.801899 ◽

2014 ◽

Vol 19 (Supplement_1) ◽

pp. S69-S77 ◽

Cited By ~ 9

Author(s):

A. B. M. Saiful Islam ◽

Mohammed Jameel ◽

Suhail Ahmad ◽

Mohd Zamin Jumaat ◽

V. John Kurian

Keyword(s):

Response Time ◽

Deep Water ◽

Mooring Line ◽

Mooring System ◽

Wave Loading ◽

Extreme Wave ◽

Spar Platform ◽

Mooring Lines ◽

Time Histories ◽

Fully Coupled

Floating spar platform has been proven to be an economical and efficient type of offshore oil and gas exploration structure in deep and ultra-deep seas. Associated nonlinearities, coupled action, damping effect and extreme sea environments may modify its structural responses. In this study, fully coupled spar–mooring system is modelled integrating mooring lines with the cylindrical spar hull. Rigid beam element simulates large cylindrical spar hull and catenary mooring lines are configured by hybrid beam elements. Nonlinear finite element analysis is performed under extreme wave loading at severe deep sea. Morison's equation has been used to calculate the wave forces. Spar responses and mooring line tensions have been evaluated. Though the maximum mooring line tensions are larger at severe sea-state, it becomes regular after one hour of wave loading. The response time histories in surge, heave, pitch and the maximum mooring tension gradually decreases even after attaining steady state. It is because of damping due to heavier and longer mooring lines in coupled spar–mooring system under deep water conditions. The relatively lesser values of response time histories in surge, heave, pitch and the maximum mooring tension under extreme wave loading shows the suitability of a spar platform for deep water harsh and uncertain environmental conditions.

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Research on Ship Structural Fatigue Damage Under Nonlinear Wave Bending Moment

Volume 3A: Structures, Safety and Reliability ◽

10.1115/omae2017-62328 ◽

2017 ◽

Author(s):

Jingxia Yue ◽

Lihua Peng ◽

Wengang Mao ◽

Chi Zhang ◽

Wei Dong ◽

...

Keyword(s):

Numerical Analysis ◽

Frequency Response ◽

Fatigue Damage ◽

Nonlinear Wave ◽

Model Test ◽

Bending Moment ◽

Irregular Waves ◽

Wave Loading ◽

Ship Structures ◽

Structural Fatigue

Loads acting on ship structures are complex and randomly over time and the nonlinear effect caused by wave loading is one of the research focus. The linear and nonlinear vertical wave bending moment (VBM) in different speeds and sea states and their effects on ship structural fatigue strength were investigated for a flat container with high ratio of width to depth. The VBM under the linear regular waves and irregular waves were calculated based on the three dimension (3D) potential theory. The considered nonlinear wave loading was caused by sea pressure near the mean free surface as well as the geometric nonlinearity. Hydrodynamic calculations in regular wave were presented to figure out the frequency response function (FRF) of VBM in the mid-ship section. Irregular waves were verified to obtain the VBM history in 4 sea states. What’s more, VBMs from a segmented elastic model test were obtained to investigate the influence of nonlinearity. On the basis of the wave loadings obtained from simulation and test, the hotspot stress histories under irregular waves were deduced in time domain by using the beam theory. Fatigue cumulative damage per hour under several random sea states were obtained on the basis of the rain-flow counting and S-N curve. Based on the fatigue damage from the numerical analysis and model test, it is believed that speeds and significant wave height have a positive correlation with the fatigue damage of ship structures. A good agreement was obtained between the numerical analysis values and the low frequency part of the test and the nonlinear analysis in the simulation could offer reasonable prediction for the fatigue damage caused by the wave frequency response. Also shown as the test result, fully nonlinearities have a great contribution to the fatigue damage.

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Numerical Simulation of Dynamic Response of Submerged Floating Tunnel under Regular Wave Conditions

Shock and Vibration ◽

10.1155/2022/4940091 ◽

2022 ◽

Vol 2022 ◽

pp. 1-15

Author(s):

Wenlong Luo ◽

Bo Huang ◽

Yao Tang ◽

Hao Ding ◽

Ke Li ◽

...

Keyword(s):

Dynamic Response ◽

Weight Ratio ◽

Wave Period ◽

Hydrodynamic Performance ◽

Regular Wave ◽

Wave Loading ◽

Mooring Lines ◽

Vibration Period ◽

Submerged Floating Tunnel ◽

Submergence Depth

A submerged floating tunnel (SFT) is considered an innovative alternative to conventional bridges and underground or immersed tunnels for passing through deep water. Assessment of hydrodynamic performance of SFT under regular wave loading is one of the important factors in the design of SFT structure. In this paper, a theoretical hydrodynamic model is developed to describe the coupled dynamic response of an SFT and mooring lines under regular waves. In this model, wave-induced hydrodynamic loads are estimated by the Morison equation for a moving object, and the simplified governing differential equation of the tunnel with mooring cables is solved using the fourth-order Runge–Kutta and Adams numerical method. The numerical results are successfully validated by direct comparison against published experimental data. On this basis, the effects of the parameters such as the cable length, buoyancy-weight ratio, wave period, wave steepness, and water/submergence depth on the dynamic response of the SFT under wave loading are studied. The results show that tunnel motions and cable tensions grow with wave height and period and decrease with submergence depth. The resonance of the tunnel will be triggered when the wave period is close to its natural vibration period, and the estimation formula of wave period corresponding to tunnel resonance is proposed in this paper.

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