The Research Of Characteristics And Factors Of Low Period Motions Of Tension Leg Platform

As the development of marine oil and gas continues to advance into deep water,Tension Leg Platform(TLP),which is suitable for deep-sea operation,is receiving more and more attention.TLP,which is limited by tendons,semi-submerges in the ocean,interacting with fluids.The energy of fluids transfers to TLP through pontoons and risers.The energy transfer results in the six degrees of freedom harmonic motion---surge,sway,heave,roll,pitch,yaw of TLP,and the motion of riser.Especially if a riser is under setting down and recovery operations or severe sea conditions,the complicated motion of free-hanging riser caused by the ocean environment loads have a significant impact on the safety of riser.The motion of TLP and TLP's free-hanging riser are divided into three frequency ranges,specifically,low frequency(LW),wave frequency(WF)and high frequency(HW)in the frequency domain,corresponding to high period motions,wave period motions and low period motions.The low period motions of TLP and TLP's free-hanging riser are closely related to structural resonance and fatigue.They have a major threat to the stable operation of TLP.It is of great significance to control the low period motions in a reasonable range for the safe service of TLP.The low period motions of TLP in the same period order are mainly composed of two parts:first,the roll,pitch and heave motion generated by the interaction between platform body and waves;second,the motion generated by the interaction between free-hanging riser and fluids(wave and flow),including wave induced motion(WIM)and flow induced motion(FIM).This dissertation focuses on the characteristics and factors of low period motions of a TLP and TLP's free-hanging riser model.Therefore,the numerical and experimental prediction models are developed.The important parameters in prediction models serve as influencing factors in analyzing parameters' effect on the low period motions.Based on the influence laws and the sensitive low period motions,the control regime of design parameters is generated to guide the design of TLP and riser.In the design of TLP,the parameters of TLP and riser are determined within the feasible range by bypassing the control region,so that the TLP naturally has the motion performance that meets the requirements for low period motions.The purpose of this dissertation is to achieve the feedforward control of the low period motions of TLP.In the research on the low period motions of TLP,the numerical prediction models of natural rolling period and effective height of green water are developed to investigate the characteristics of natural rolling period and air gap which are closely related to low period motions.The numerical prediction models are built upon stability theory and statistical process control theory respectively with simplified motion system,based on roll/pitch and heave.The influence of important parameters in the prediction models on natural rolling period and air gap is analyzed.Natural rolling period is sensitive to platform weight and roll damping;Initial static air gap and sea conditions have a great impact on air gap.Based on the influence laws and the sensitive low period motions,the control regime of TLP's parameters is generated to guide the design of TLP.In the research on the low period motions of free-hanging riser,the experimental framework and prediction models of riser's low period motions response model are generated by free-hanging riser simplified model,as functions of wave load and flow load in the frequency domain.The influence of loads input conditions and boundary layer separation position controlled by PTC on the response characteristics of the low period motions of riser model is investigated.The dominant-load-regime map and magnitude response RAO are obtained by the response characteristics in the frequency domain and KC domain.Loads input conditions,and boundary layer separation controlled by the type and position of PTC have an effect on low period motions.Load types have a great influence on low period motions.For free-hanging riser model,water flow dominates small magnitude motion in the low frequency range;water waves dominate large magnitude motion in the high frequency range.Based on the influence laws and the sensitive low period motions,the control regime of riser's parameters is generated to guide the design of risers.This dissertation aims at the low period motions of TLP and TLP's free-hanging riser caused by non-linear effect.In addition,experimental and numerical models are developed to predict the response characteristics of the low period motions of TLP and TLP's free-hanging riser.The influence of influencing factors,such as TLP's size,weight,damping and load conditions,boundary layer separation,on the response characteristics of the low period motions of TLP is investigated.The influence laws and the sensitive low period motions are developed into multilevel control regimes,which are functions of TLP and riser's parameters,to guide the design of TLP and TLP's free-hanging riser.Therefore,this would provide a potential approach to feedforward control of TLP's low period motions in a reasonable range to avoid the sensitive low period motions occurrence,and make the whole system much better,so as to eliminate the threat of sensitive low period motions to the safe operation of TLP.