Time-domain Motion Response Of Tension Leg Platform Considering The Coupled Vibration Of Tethers

As a new type of Marine floating system,tension-leg platform has attracted more and more attention from the international community due to its motion performance.At the same time,the domestic also set off the research climax of the deep-water engineering technology.It is of great strategic significance for China to break the monopoly of offshore deep-water technology and promote the progress of Marine science and technology.The research on the motions of TLP in complex Marine environment has been one of the important topics in the field of Marine engineering.Although the semi-rigid and semi-compliant structure enables TLP to effectively avoid the platform resonance phenomenon caused by common frequencies in the ocean,sum frequency and difference frequency forces may cause the platform occur in the plane of springing,ringing and slow drift phenomenons,which affect the movement characteristics of the platform and the fatigue life of tether.Given the randomness and complexity of Marine environment,the dynamic responses and mooring characteristics of TLP in waves are greatly simplified.Based on this situation,TLP mathematical model in this paper is fully considered the coupling of platform motions,the coupling of tether vibrations and the coupling between platform and tethers.The main research object of this paper is TLP.And the 3-D time-domain coupled motion responses of TLP under the action of wave and current are studied.In order to analyze the motions of TLP in detail,this paper will be divided into three parts:the time domain response of platform in waves,the vibration responses of tethers and the uncoupled and coupled motion responses of platform and tethers.The key to solving the time-domain response of platform in waves is to realize the precise and stable numerical calculation of 3-D time-domain Green functions.Based on the properties of Bessel function and ordinary differential equation satisfied by time-domain Green function and its space derivatives,the least squares support vector machine(LS-SVM)theory is firstly proposed to solve this differential equation.LS-SVM algorithm can effectively improve the accuracy and stability of solving time-domain Green function and its space derivatives.Based on the above calculation method of time-domain Green function,the motion response programs of platform in waves are compiled.Furthermore,the influence of wave viscosity on the force and motion of the platform is discussed.In the study of TLP movements when tethers are inclined,the tethers are simplified into a massless linear spring structure,and the restorative force matrix is established taking into account the influence of tethers.At the same time,the coupled effects of horizontal offset and vertical setdown of the platform are considered.By studying the motions of TLP when tethers are in different inclination angles,the influence of horizontal offset on the platform is obtained,and the influence of the coupled effect of horizontal offset and vertical setdown on the TLP motions is further studied.Under the action of platform,wave and ocean current,a tether vibration model is established based on Kirchhoff s hypothesis and Hamilton principle,which takes into account the coupled effect of axial and transverse vibrations of tethers.Besides the influence of wave frequency movements,the static offset and setdown of the platform are also considered.The nonlinear vibration equations of tethers are analyzed by Galerkin method.The axial and transverse vibrations of tethers under the excitation of different parameters are calculated.The vibration rules and causes of axial and transverse vibrations of tether are summarized,and the main factors affecting the fatigue life of tether are given.When the coupled motion responses between the platform and tethers are studied,the interaction forces between the platform and tethers are used to establish the coupled TLP model by combining the motion equations of platform and the vibration equations of tethers.The coupled effect of platform motions and vibrations of tether are considered in the model.The coupled effects are discussed from the motion responses,horizontal offset and vertical setdown of TLP.In order to study the influence of second-order wave forces on the motion responses of TLP,the motion equations of TLP simultaneously takes into account the influence of the mean drift force,second-order difference frequencies force and second-order sum frequencies force of irregular waves.Based on the natural periods of the platform,the influence of second-order wave forces on the movements of platform and tethers are discussed.In conclusion,starting from different factors affecting the dynamic responses of TLP,such as wave direction,water depth,pretension,sea current velocity,static offset and second order force,the hydrodynamic performances of TLP are calculated.External load is considered from regular wave,random wave to combined wave-current.This study is carried out step by step and takes the integration of the hydrodynamic model of the platform,the vibration model of the tether,the uncoupled model and the coupled model of TLP.At the same time,the coupled effects of platform movements,the coupled effects of tether vibrations and the coupled effects of offset and setdown on the coupled TLP model are analyzed The dynamic responses of TLP in Marine environment are studied comprehensively,so as to provide theoretical basis to master the motion performance of TLP systematically.