Complicated Mechanical Behavior Of Synthetic Fiber Mooring Ropes And Its Effects On The Dynamic Response Of The Taut-wire Mooring System For Deep Waters

Station-keeping of deep water floating structures is a key technology to exploit ocean resource.As oil and gas exploration moving into deep and ultra-deep waters,utilizing taut-wire mooring systems becomes a prefer operation for ocean engineers.However,synthetic fiber ropes which are different from steel wire ropes and chains show complicated mechanical behaviors.Hence performing experiments and developing constitutive models of synthetic fiber ropes are essentially significant for theoretical developments and engineering applications.Besides,different mechanical behaviors will largely affect the dynamic responses of mooring systems.Therefore,investigating the effects of mechanical behaviors on response of mooring systems is of great concern in the oil and gas industry.To ensure the safe operation of mooring systems,in the present work,the mechanical behavior of synthetic fiber ropes and its effects on the dynamic response of mooring systems were respectively investigated and analyzed.Firstly,to investigate mechanical behaviors of synthetic fiber ropes,an experimental system that can simulate the practical working condition of mooring ropes was developed.By utilizing this specially designed experimental system,the detailed testing procedure and techniques which includes eye-splice termination technology were proposed.Based on this technology above,the dynamic stiffness evolution of damaged fiber ropes,creep and creep-rupture behavior of HMPE ropes,fatigue resistence behavior of HMPE ropes and evaluating bedding-in condition of synthetic fiber ropes were respectively investigated as follows.To investigate the dynamic stiffness evolution of damaged fiber ropes,systematic tests were performed and meaningful results were obtained.It is observed that the initial damage value,mean load,strain amplitude and loading cycles affect the dynamic stiffness.Under the framework of the dimensional analysis,an empirical expression that takes into account the damage effect,mean load,strain amplitude and loading cycles was proposed and the results showed that the comparison between the measured data and empirical expression is a generally good agreement.Besides,the dynamic stiffness obtained from damaged small size ropes can be easily extrapolated to large size ones.Creep and creep-rupture experiments of HMPE samples were performed and the results showed that the creep-rupture curve shows three stages and an empirical expression that is able to describe the creep rate of the second stage was proposed to estimate the creep strain of HMPE ropes.Besides,an empirical expression that can describe the creep lifetimes of HMPE ropes was also proposed based on the current experimental results.To understand the fatigue evolution of HMPE ropes,experimental investigations of HMPE ropes were systematically performed.The results including tension-time curves,strain-time curves,tension-strain curves and dynamics stiffness evolution curves were firstly respectively showed to describe the mechanical behaviors of samples under long-term cyclic loading.In addition,the average failure strain of samples is 7%.Furthermore,an empirical expression that accounts for the relation of mean load,loading amplitude and fatigue life was firstly proposed to describe the damage evolution of HMPE ropes during cyclic loading.Different bedding-in procedures were proposed by separate guidance notes of mooring fiber rope and bedding-in procedure affects the mechanical behavior of fiber ropes.Therefore,the slope of the static tension-strain curve was utilized to quantitatively describe the bedding-in condition of samples.Based on this method,systematical experiments were performed and an empirical expression that can quantitatively describe the bedding-in effects was proposed.This equation makes the experimental results of samples under the different bedding-in procedure of mooring rope codes can be comparable.Furthermore,reasonable bedding-in procedure was proposed considering rules that loading range should not be too large and bedding-in time should not be too long.It should be noted that it is of benefit to understand effects of loading history on rope mechanical behaviors.Secondly,a creep-rupture model was developed within the framework of thermodynamics to investigate the creep and damage behaviors of synthetic fiber ropes.Methods for identifying the model parameters were proposed in detail,which can be applied to any component of synthetic fiber ropes such as the fiber,yarn,strand,sub-rope and rope.The present work also demonstrates that the creep-rupture model is capable of utilizing a small quantity of short-term experimental data to obtain the model parameters,and then effectively characterizing the whole creep-rupture response of synthetic fiber ropes at any loading levels.Overall,the predicted results from the present model agree well with the experimental data.Furthermore,to more accurately describe the mechanical behavior of fiber ropes,especially to describe the mechanical behavior under cyclic loading levels,a viscoelastic,viscoplastic and damage coupled model was proposed.Besides,this new model was currently validated by new creep-rupture data of HMPE strands.Finally,based on the cognition of mooring ropes,a new hybrid mooring line was proposed to address the station-keeping of a floating platform in ultra-deepwater areas.Besides,to illustrate the feasibility of chain-HMPE-polyester-chain mooring line,dynamic analysis and fatigue analysis of this mooring line were performed and their results were contrasted with the results of chain-polyester-chain mooring line,respectively.By these contrasts,results show that using of new-type hybrid mooring lines in ultra-deepwater mooring can significantly reduce platform offset and the new-type taut-wire mooring system is suitable and has good fatigue resistance.However,it should pay attention to design suitable stiffness of mooring lines by changing the length ratio of HMPE rope to polyester rope to avoid too large loading range.These cognitions will provide significant references for the application of the deepwater and ultra-deepwater taut-wire mooring systems with new hybrid mooring lines.