Research On Fatigue Damage And Fatigue Reliability Of Steel Catenary Riser

With the gradual decrease of oil and gas resources on the land,the focus of oil and gas exploitation is gradually shifting to the sea.The steel catenary riser(SCR)is a new type of riser which connects the offshore platform and the submarine pipeline,which is extremely vulnerable to fatigue damage.Therefore,the fatigue life prediction method of SCR is very important to the safety of the structure.Because of the marine environment load and the high uncertainty of some design parameters for SCR,the traditional fatigue safety factor based on experience can not meet the requirement of fatigue design.Therefore,it is necessary to analyze the reliability of SCR to ensure the fatigue strength of the riser structure from the angle of reliability.Based on the fatigue damage theory and the L-H regular wave theory,the wave induced fatigue damage of SCR is analyzed.Based on the VIV semi empirical theory,the Vortex Induced Vibration(VIV)analysis and VIV fatigue damage analysis of SCR are carried out with the commercial software Shear7 and Orcaflex.Based on the structural reliability theory,The reliability of the wave induecd fatigue damage and the VIV fatigue damage for SCR is calculated considering the characteristic of them respectively.A method for calculating the fatigue safety factor of the riser which is suitable for different test periods based on the structural reliability theory is given,and is compared with the method in the standard DNV-RP-F204.The main contents are as follows:(1)The study of wave induced fatigue damage for SCR.Based on the fatigue damage theory and the Longuet-Higgins reguler wave theory,the time-domain simulation-rain flow method and regular wave method are used to predict the wave induced fatigue damage of a SCR in the South China Sea,and the reasons for the maximum stress at the top or touch place are explained according to the special configuration of the catenary riser.The efficiency and storage space of the time-domain simulation-rain flow method and the regular wave method are compared.The division of working conditions affecting the accuracy of regular wave method is studied,and the sensitivity of the parameters affecting the wave induced fatigue damage is analyzed.(2)The study of Vortex Induced Vibration fatigue damage for SCR.The calculation method of VIV based on semi empirical theory is studied.The commercial software orcaflex and shear7 are used to predict the VIV fatigue damage of SCR under the currentcondition of a certain sea area in the South China Sea.The potential excitation modes,the main excitation modes and the energy input regions of VIV under single current condition are obtained and analyzed.The excitation modes,maximum displacement,maximum stress and maximum fatigue damage of risers under different current profiles are compared.The VIV fatigue damage under in-plane and out-of-plane current direction are compared,and the influence of spiral plate on VIV fatigue damage are compared and analyzed.(3)Calculation of Combined damage of VIV fatigue and wave induced fatigue.Stress cycle times,average cross zero frequency of wave induced fatigue damage at SCR touch down point are statistically analyzed.The average cross zero frequency of VIV under each current condition is calculated based on the time sharing theory of VIV mode.The combined fatigue damage calculation method recommended by Norway classification society is used to analyze the wave induecd fatigue damage and VIV fatigue damage of SCR.(4)Study on the reliability of SCR fatigue damage.Random variables for calculating wave induced fatigue and VIV fatigue reliability are selected;As to it is very time-consuming to get the standard deviation of wave induced fatigue damage directly from the Taylor series approximation,a simplified expression of fatigue damage is introduced to carry out Reliability analysis of SCR wave induced fatigue.The standard deviation of VIV fatigue damage is directly obtained by Taylor series approximation,and the VIV fatigue reliability of SCR is calculated based on logarithmic normal form.(5)Study on the fatigue safety factor of SCR.On the basis of parameter range study on the design parameters affecting fatigue damage,the fatigue safety factor of riser wave induced and VIV fatigue safety factor are calculated by using the method of the Norway classification society standard DNV-RP-F204.A method is established to obtain the target reliability index of the riser during design life taking the last year of the test period as the failure probability benchmarkthe,on this basis a reliability based method for calculating the safety coefficient of the riser fatigue is proposed.Taking a SCR as an example,the safety factor of wave induced fatigue and VIV fatigue is determined by the proposed method,and the results are in good agreement with the results of DNV-RP-F204 method.