Analysis of Nonlinear Dynamic Response of Mooring Lines in Deepwater

2010 ◽  
Author(s):  
Su-xia Zhang ◽  
You-gang Tang ◽  
Xi-jun Liu
Keyword(s):  
Nonlinear Propagation ◽  
Mooring Line ◽  
Stress And Strain ◽  
Dynamic Tension ◽  
Mooring Lines ◽  
Nonlinear Dynamic Response ◽  
Motion Equations ◽  
Double Period ◽  
Nonlinear Motion ◽  
Fixed End

Nonlinear motion equations of cable is reduced with the assuming of linear constitutive relation, and simplified further according to propagation characters of stress wave. The loading acting on quasi-static cables are analyzed and the detailed expressions are given. The displacement, stress and strain in mooring line in deep water are calculated in this paper, and the nonlinear propagation characters of stress and strain, the motion characters and the effect of drag force reduced by fluid on the dynamic tension in cable are analyzed respectively. The results show that, the stress in cable propagating from exciting end to fixed end and there is a difference in phase. At the same time, the stain in cable different point is also different. At the point with maximum tension, the normal motion is double-period and the tangential motion is quasi-periodic. The tension in cable is effected by tangential and normal drag.

Full Scale Data Comparison for the Horn Mountain Spar Mooring Line Tensions During Hurricane Isidore

2005 ◽  
Author(s):  
Arcandra Tahar ◽  
Lyle Finn ◽  
Pierre Liagre ◽  
John Halkyard
Keyword(s):  
Excellent Agreement ◽  
Coulomb Friction ◽  
Field Measurements ◽  
Analytical Models ◽  
Mooring Line ◽  
Conservative Estimate ◽  
Dynamic Tension ◽  
Mooring Lines ◽  
Data Comparison ◽  
Scale Data

The Horn Mountain Production Spar was installed in 5,400 feet of water in June 2002. This was the deepest floating production unit at that time. A comprehensive instrumentation program was initiated to measure spar and riser responses (Edwards et al, DOT 2003), while motion comparisons were presented on previous publication (Halkyard et al, OMAE 2004). The present paper discusses the results of these measurements and compares with analytical predictions of spar mooring tension during hurricane Isidore in September 2002. Particular attention has been placed on the importance of Coulomb friction between wire chain and the fairlead bearing to the dynamic tension of mooring lines. Mooring tensions were measured at chain jack location (inboard tension), while analytical models computed those tensions at the fairlead location (outboard tension). Our conclusion is that there is excellent agreement between field measurements and computed tensions at the chain jacks when fairlead friction is included, and when the vessel motions are accurately predicted. Ignoring fairlead friction results in a slightly conservative estimate for the tension at the chain jack. This has been the standard practice in all spar designs to date.

scholarly journals Numerical Modelling of Seabed Impact Effects on Chain and Small Diameter Mooring Cables

2018 ◽  
Author(s):  
Chee Meng Low
Keyword(s):  
Numerical Modelling ◽  
Reaction Force ◽  
Small Diameter ◽  
Mooring Line ◽  
Force Model ◽  
Dynamic Tension ◽  
Commercial Software ◽  
Mooring Lines ◽  
Root Cause ◽  
Code Validation

Catenary mooring lines experience liftoff from and grounding on the seabed when undergoing large dynamic motions. Numerical line mooring models account for this interaction using various seabed models and it is known that the action of liftoff and grounding may lead to large dynamic tension fluctuations. These fluctuations may be spurious due to the inability of discretised mooring models to adequately account for the effect of the seabed on the mooring line. In this work, the root cause and conditions that lead to the production of the large dynamic tension fluctuations is determined. The effect of line discretisation and seabed model on the tensionfluctuations is investigated using the widely used spring-mattress approach and a modified seabed reaction force model. An in-house mooring code was developed to perform these investigations. For code validation and benchmarking, and to illustrate the existence of the tension fluctuations problem due to nodal grounding inexisting mooring line simulation codes, comparisons are made to a commercial software.

Dynamic Response of the Fiber Metal Laminated (FML) Plate with Interfacial Damage in Unstable Temperature Field

2014 ◽  
Vol 490-491 ◽  
pp. 403-411
Author(s):  
Yi Ming Fu ◽  
Xue Fei Shao
Keyword(s):  
Dynamic Response ◽  
Nonlinear Dynamic ◽  
Constitutive Relations ◽  
Thin Layers ◽  
Interfacial Damage ◽  
Nonlinear Dynamic Response ◽  
Motion Equations ◽  
Nonlinear Motion ◽  
Fiber Metal ◽  
Unstable Temperature

During the past decades, increasing requirement in aircraft for high-performance, lightweight structures have caused strong interests on the development of fiber-metal laminates (FMLs), which are manufractured from thin layers of glass fibre reinforced composite and alluminium alloy. In this paper, the nonlinear dynamic response problem of the FML plate subjected to unstable temperature with interfacial damage is analyzed. Based on the weak bonded theory, the interfacial constitutive relations of the FML are constructed. According to the Hamiltons variance principle, the nonlinear motion equations of the FML with interfacial damages subjected to the unstable thermal field are obtained. And then, the finite difference, Newmark-and the iteration method are applied to solve the nonlinear motion equations. In the numerical examples, the effects of the interface damage, the amplitude and frequency of imposed loads and the temperature fields on the nonlinear dynamic response of the FML plates are investigated. And in conclusion, the effects of various type of temperature on the nonlinear dynamic response of FML plate are different obviously.

Hydrodynamic Study and Performance Analysis of the OC4-DeepCWind Platform by CFD Method

2020 ◽  
pp. 2050020
Author(s):  
Yang Huang ◽  
Yuan Zhuang ◽  
Decheng Wan
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Center Of Gravity ◽  
Mooring Line ◽  
Time Step ◽  
Mooring Lines ◽  
Frequency Wave ◽  
Nonlinear Motion ◽  
Motion Responses ◽  
Motion Performance

(1) The RAOs of OC4-DeepCWind platform motions are more sensitive to the low-frequency wave than the high-frequency wave. The nonlinear motion responses for platform heave and pitch motions are comparatively remarkable. (2) The pitch motion of OC4-DeepCWind platform is much more apparently influenced by the height of center of gravity (COG) than surge and heave motions. The lower COG height within a suitable range leads to a smaller fluctuation amplitude of platform pitch motion in waves. (3) A large horizontal displacement abruptly occurs to the OC4-DeepCWind platform when one mooring line is failure. The risk of failure for the other mooring lines significantly increases. To better understand the hydrodynamic performance of a floating support platform in various wave environments, a two-phase CFD solver naoe-FOAM-SJTU based on the open source CFD toolbox OpenFOAM is applied to investigate the hydrodynamic characteristics and motion performance of the OC4-DeepCWind platform. Moreover, the restoring force and moment of mooring lines are simulated using the solver in time domain. The studies of grid sensitivity and time step refinement are first conducted to determine an appropriate time step and mesh size. Then hydrodynamic responses of the floater in free-decay tests are analyzed and compared with experimental data, and the motion performance of the platform in regular waves with different parameters is also investigated. In addition, the platform motion responses with one mooring line broken and different heights of center of gravity are explored. It is shown that simulation results have good agreement with published data, and several conclusions can be drawn through the study. The RAOs of platform motions are found to be more sensitive to the low-frequency wave than the high-frequency wave. Nonlinear motion responses are comparatively remarkable in platform heave and pitch motions. Besides, the lower height of center of gravity within a suitable range is benefit to the stability of floating platform. Survival condition with broken mooring line should be paid enough attention to avoid the failure of other mooring lines.

Study on Snap Tension in Mooring Lines of Deepwater Platform

2009 ◽  
Author(s):  
Su-xia Zhang ◽  
You-gang Tang ◽  
Hai-xiao Liu
Keyword(s):  
Floating Body ◽  
Impact Dynamics ◽  
Mooring Line ◽  
Mooring System ◽  
Body System ◽  
External Excitation ◽  
Spar Platform ◽  
Mooring Lines ◽  
Motion Equations ◽  
Maximum Tension

Based on the theory of impact dynamics, the motion equations for a mooring line-floating body system after and before impact loading are established with consideration of the viscoelastic property of mooring lines. The factors that influence the taut-slack conditions of a mooring system are analyzed through classifying the taut-slack regions, which are defined by nondimensional ratios of displacement, frequency and damping of the system. The mooring lines of Jip spar platform is analyzed, and the snap tension characteristics of mooring lines are given, the factors that influence the maximum tension in mooring lines, including the mass of floating body, length of mooring lines, frequency and amplitude of external excitation, and pretension in mooring lines, are also analyzed through computing the dynamic response of system and parametric study. It is shown that the maximum tension increases with increase of the mass of floating body, external excitation and pretension and decreases with increase of the length of mooring lines, also, it is found that the influence of the nondimensional ratio of damping increases with increase of the pretension in mooring lines.

scholarly journals Transient Responses Evaluation of FPSO with Different Failure Scenarios of Mooring Lines

2021 ◽  
Vol 9 (2) ◽  
pp. 103
Author(s):  
Dongsheng Qiao ◽  
Binbin Li ◽  
Jun Yan ◽  
Yu Qin ◽  
Haizhi Liang ◽  
...  
Keyword(s):  
Service Condition ◽  
Mooring Line ◽  
Mooring System ◽  
Transient Effects ◽  
Floating Platform ◽  
Transient Responses ◽  
Mooring Lines ◽  
Performance Deterioration ◽  
Steady State Responses ◽  
Influence Process

During the long-term service condition, the mooring line of the deep-water floating platform may fail due to various reasons, such as overloading caused by an accidental condition or performance deterioration. Therefore, the safety performance under the transient responses process should be evaluated in advance, during the design phase. A series of time-domain numerical simulations for evaluating the performance changes of a Floating Production Storage and Offloading (FPSO) with different broken modes of mooring lines was carried out. The broken conditions include the single mooring line or two mooring lines failure under ipsilateral, opposite, and adjacent sides. The resulting transient and following steady-state responses of the vessel and the mooring line tensions were analyzed, and the corresponding influence mechanism was investigated. The accidental failure of a single or two mooring lines changes the watch circle of the vessel and the tension redistribution of the remaining mooring lines. The results indicated that the failure of mooring lines mainly influences the responses of sway, surge, and yaw, and the change rule is closely related to the stiffness and symmetry of the mooring system. The simulation results could give a profound understanding of the transient-effects influence process of mooring line failure, and the suggestions are given to account for the transient effects in the design of the mooring system.

Experimental Investigation vs Numerical Simulation of the Dynamic Response of a Moored Floating Structure to Waves

2014 ◽  
Author(s):  
Daniele Dessi ◽  
Sara Siniscalchi Minna
Keyword(s):  
Dynamical Behavior ◽  
Irregular Waves ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Time Histories ◽  
Actual Configuration ◽  
Proper Orthogonal ◽  
Dynamic Wave

A combined numerical/theoretical investigation of a moored floating structure response to incoming waves is presented. The floating structure consists of three bodies, equipped with fenders, joined by elastic cables. The system is also moored to the seabed with eight mooring lines. This corresponds to an actual configuration of a floating structure used as a multipurpose platform for hosting wind-turbines, aquaculture farms or wave-energy converters. The dynamic wave response is investigated with numerical simulations in regular and irregular waves, showing a good agreement with experiments in terms of time histories of pitch, heave and surge motions as well as of the mooring line forces. To highlight the dynamical behavior of this complex configuration, the proper orthogonal decomposition is used for extracting the principal modes by which the moored structure oscillates in waves giving further insights about the way waves excites the structure.

Optimized Mooring Line Simulation Using a Hybrid Method Time Domain Scheme

2014 ◽  
Author(s):  
Niels Hørbye Christiansen ◽  
Per Erlend Torbergsen Voie ◽  
Jan Høgsberg ◽  
Nils Sødahl
Keyword(s):  
Hybrid Method ◽  
Time Domain ◽  
Computation Time ◽  
Mooring Line ◽  
Mooring Lines ◽  
Fem Model ◽  
Input Variables ◽  
The Time Domain ◽  
The Cost ◽  
Short Time

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.

The Anchor-Last Deployment Problem for Inextensible Mooring Lines

1975 ◽  
Vol 97 (3) ◽  
pp. 1046-1052 ◽  
Author(s):  
Robert C. Rupe ◽  
Robert W. Thresher
Keyword(s):  
Numerical Model ◽  
Equations Of Motion ◽  
Simultaneous Equations ◽  
Mooring Line ◽  
Integration Scheme ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Concentrated Masses ◽  
Predictor Corrector ◽  
Lagrange’S Method

A lumped mass numerical model was developed which predicts the dynamic response of an inextensible mooring line during anchor-last deployment. The mooring line was modeled as a series of concentrated masses connected by massless inextensible links. A set of angles was used for displacement coordinates, and Lagrange’s Method was used to derive the equations of motion. The resulting formulation exhibited inertia coupling, which, for the predictor-corrector integration scheme used, required the solution of a set of linear simultaneous equations to determine the acceleration of each lumped mass. For the selected cases studied the results show that the maximum tension in the cable during deployment will not exceed twice the weight of the cable and anchor in water.

Validation of Mooring Simulations (for Mooring Integrity Assessment) With In-Service Tension Measurements

2021 ◽  
Author(s):  
Willemijn Pauw ◽  
Remco Hageman ◽  
Joris van den Berg ◽  
Pieter Aalberts ◽  
Hironori Yamaji ◽  
...  
Keyword(s):  
Numerical Models ◽  
Weather Conditions ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Data Set ◽  
Integrity Assessment ◽  
Motion Data ◽  
Load Cells ◽  
Line Loads

Abstract Integrity of mooring system is of high importance in the offshore industry. In-service assessment of loads in the mooring lines is however very challenging. Direct monitoring of mooring line loads through load cells or inclinometers requires subsea installation work and continuous data transmission. Other solutions based on GPS and motion monitoring have been presented as solutions to overcome these limitations [1]. Monitoring solutions based on GPS and motion data provide good practical benefits, because monitoring can be conducted from accessible area. The procedure relies on accurate numerical models to model the relation between global motions and response of the mooring system. In this paper, validation of this monitoring approach for a single unit will be presented. The unit under consideration is a turret-moored unit operating in Australia. In-service measurements of motions, GPS and line tensions are available. A numerical time-domain model of the mooring system was created. This model was used to simulate mooring line tensions due to measured FPSO motions. Using the measured unit response avoids the uncertainty resulting from a prediction of the hydrodynamic response. Measurements from load cells in various mooring lines are available. These measurements were compared against the results obtained from the simulations for validation of the approach. Three different periods, comprising a total of five weeks of data, were examined in more detail. Two periods are mild weather conditions with different dominant wave directions. The third period features heavy weather conditions. In this paper, the data set and numerical model are presented. A comparison between the measured and numerically calculated mooring line forces will be presented. Differences between the calculated and measured forces are examined. This validation study has shown that in-service monitoring of mooring line loads through GPS and motion data provides a new opportunity for mooring integrity assessment with reduced monitoring system complexity.

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