Dynamic Analysis Of Top Tensioned Riser With Hydro-pneumatic Tensioner

Top tensioned riser(TTR)is a kind of key equipment serving as the link between the floating platform and the well head at the seabed for oil and gas transportation.In order to compensate the heave motion of floating platform and prevent buckling of TTR caused by low tension,a essential equipment called tensioner is equipped between floating platform and TTR.In the current TTR dynamic analysis process,such as free vibration,forced vibration,parametric instability and so on,tensioner systems are commonly simplified and simulated as a constant vertical tension load or an equivalent linear spring or an equivalent nonlinear spring,which,however,cannot reflect well the the complex tension characteristics of the tensioner.Accurate simulation of the tension variation in the tensioner has become increasingly important,as the water depth increases and the tension and stroke of tensioner become larger.At present,researches about hydro-pneumatic tensioner have been carried out abroad.However,there is still no engineering case of designing,manufacturing and applying hydro-pneumatic tensioner in China.Therefore,it is of great significance to accelerate the research and localization process of hydro-pneumatic tensioner.Based on the above consideration,this paper aims to establish a more accurate tension calculation model of hydro-pneumatic tensioner,and further study the dynamic characteristics of the TTR with hydro-pneumatic tensioner.Firstly,an improved mathematical model of a hydro-pneumatic tensioner system for TTR is derived based on the the structure characteristics of hydraulic pneumatic tensioner.The friction,mass of piston and piston rod,tension loss in hydraulic oil piping,and compressibility of hydraulic oil is taken in consideration in the improved model.Then the proposed detailed model and the conventional simplified model are comparatively studied.The tension characteristic and performance parameters of the hydro-pneumatic tensioner are also analyzed based on the proposed model.Based on the parameter sensitivity analysis,a basic tensioner design guideline is proposed.Subsequently,a dynamic analysis model of a TTR with hydro-pneumatic tensioner is established based on the improved mathematical model of hydro-pneumatic tensioner system.The proposed model is proved with the experimental results.The natural characteristics,forced vibration and parameter vibration of the TTR with hydro-pneumatic tensioner are studied.The influence of tensioner model and performance parameters of tensioner on the natural characteristics,forced vibration and parameter vibration of the TTR are also studied.Finally,a three-dimensional finite element analysis model of TTR with hydro-pneumatic tensioner is proposed,considering the nonlinear stiffness of tensioner,the kinematic relationship of the platform,tensioner and riser system,the actual riser string configuration and the Christmas tree.The proposed model is proved with the experimental results.Base on the proposed model,the influence of different tensioner models,performance parameters of tensioner,the coupling motion of the plptform ant the broken of tensioner on the dynamic response of the TTR are also studied.Results show that the conventional simplified model indeed overestimates the tension of tensioner as it neglects some performance parameters resulting in tension loss,which may lead to a deviation on estimation of the dynamic response,stability,and structural strength of the TTR.In order to obtain more accurate response results of the riser,improved tension calculation models of tensioner should be established and the kinematic relationship between the floating platform,tensioner and the riser should be fully considered.