| Being a new-type mooring system applied for deep waters, synthetic fibre ropesare employed in the taut-wire mooring system as main part of mooring lines. Underthe complicated ocean environment, the nonlinear mechanical properties of thesynthetic fiber ropes including the time-dependent behaviors (creep-recoverybehaviors), special cyclic tension-elongation relationship, dynamic stiffness andfatigue propeties et al. closely relate to the performance and safety of the system. It isof vital importance and significant to obtain deeply knowledge of these properties forthe deepwater application of the taut-wire mooring system. Based on the profoundinsight and forward looking knowledge of the key characteristics of the taut-wiremooring system, the creep-recovery behaviors, cyclic properties of the fiber ropes andthe dynamic response and fatigue life of the taut-wire mooring system have beenstudied systematically through theoretical and experimental methods. The mainresearch works and creative achievements are as follows:Firstly, in order to study the cyclic mechanical properties of the fiber ropes, anew-type horizontal model experimental system which can perform the cyclic loadingon the synthetic fiber ropes is designed and set up, which is a first one in China. Itshould be noted that, in the international field, the model experimental system hasthree highlights:(1) A new measurement system combined with the axletree, magnentand wire transducer, which have self-protected function, has been designed and set upto measure the actual elongation of the fiber ropes;(2) A new water cycling system isdesigned and set up to simulate the water environment in which the fiber ropes fullyimmersed during the engineering application;(3) the laboratory model experimentalsystem combined with the machinery and servo-controlled machines has theadvantages of convenient installtion and transportation, low costs and energy et al.Secondly, the mechanical properties of synthetic fiber ropes under cyclic loadingincluding the tension-elongation relationship and hysteresis properties, the evolutionof the dynamic stiffness and residual strain, are studied in detail on the model experimental system. The influence of the mean load and strain amplitude is analyzedand discussed in detail. Besides, the resemblance rule suitable for the modelexperiment of dynamic stiffness for the fiber ropes is induced based on the dimensionanalysis methods and is verified by the comparison of experimental results of ropeswith different diameters. The dynamic stiffness measured in this paper is comparedwith the international empirical expressions. Based on the deep knowledge of thecyclic propeties of the fiber ropes, a new empirical expression considering load cyclesis proposed, which is invariant with the rope size and arrangement. The newexpression has not only retained the considering factors in the traditional empiricalexpressions but also improved them. It is of vital importance to the engineeringapplications.Thirdly, a new constitutive model which can describe quantitatively bothviscoelastic and viscoplastic behaviors of the aramid and polyester fiber ropes isproposed. Detailed methods for identifying the model parameters are proposed, whichcan be applied to any component of the fiber rope such as the fiber, yarn, sub-ropeand rope. In order to examine the feasibility and precision of the model, theviscoelastic and viscoplastic strains are calculated and compared with experimentaland other numerical simulation results of polyester and aramid yarns in the literaturesof Chailleux and Davies. It is observed that there is a better agreement between theresults based on the present model and experimental data. The present model not onlyeliminate the physical irrational results caused by the parameterD ppreviouslynoticed by Chailleux and Davies, but also can be capable of describe quantitativelythe deformation properties of fiber ropes mentioned by Flory.Forthly, a constitutive model combined with Schapery’s theory and Owen’stheory is proposed to describe quantitatively the cyclic properties of synthetic fiberropes. The detailed parameter identification methods and procedure are presented. Inorder to verify the model, the numerical results are compared with modelexperimental results of polyester and aramid fiber ropes. The dynamic stiffnessevolution results are compared with the empirical expressions and the experimental results, and they correspond well with each other; the calculating hysteresis loops arecompared with the model experimental results; it is indicated that the model can beable to simulate the physical phenomenon of the fiber ropes under cyclic loading. Thepresent model is proposed based on the transformation mechanism and loadinghistory of the fiber ropes, and it is the true “time domain†constitutive modelmentioned by Fran ois and Davies. For one thing, it is the first model which can takeinto account the time-dependent behaviors of synthetic fiber ropes under cyclicloading; for another, it is a practical tool for the detailed dynamic analysis ofdeepwater mooring system under a certain sea state.Finally, A FPSO and its taut-wire mooring system are employed and acomprehensive mooring analysis and fatigue analysis are performed based on the timedomain dynamic theory, the rainflow counting methods and Palmgren-Miner linearaccumulation methods. The analysis considers three factors including the pretension,dynamic stiffness and T-N curve. Besides, the detailed fatigue analysis of the mooringsystem with one line damage is carried out. The analysis proves that one-line failurehas remarkable decreasing effects on the fatigue lives of the mooring system.One-line failure changes the mooring mode and thus increases significantly thetension of others especially the adjacent lines. At present, there are still difficulties inthe inspection of mooring lines in deep waters. Therefore, the quantitative descriptionof the effect of one-failure to the fatigue damage and fatigue life of taut-wire mooringsystem is of vital importance. |
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