Dynamic Response Of Steel Catenary Riser With Touch Down Zone

Riser is a kind of slender structure connected the seabed to platform, and mainly used for drilling and oil & gas transportation. Among all riser types, steel catenary riser (SCR) is more and more used in kinds of platforms with the advantages of easy installation, low cost and compatibility.Although SCR has many advantages, the vortex-induced vibration (VIV) and the interaction between seabed and riser become critical to the fatigue damage due to its particular configuration. Therefore, the research on the SCR VIV and the interaction between riser and seabed has important academic significance and engineering application value.For the steel catenary riser, the truncated model is used commonly on the current research of pipe-soil interaction and vortex induced vibration, which can not estimate the fatigue failure of the touch down zone ground contact part of riser accurately. In this paper, the method of dynamic response analysis of SCR-soil interaction and vortex induced vibration are carried outThe main research work in this paper as follows:(l)The research progress of pipe-soil interaction and vortex induced vibration and the models used in the above research are summarized and discussed. The complex environment load on riser system are reached and a reasonable calculation method of environment load on riser system is proposed.(2)The model of interaction between soil static and dynamic analysis of pipe are given The interaction model between pipe and soil in static and dynamic response analysis are given. Basing on the beam-spring model, the program of vertical static tube prepared initial embedding seabed analysis is developed to research the influence of the seabed surface stiffness, shear stiffness gradient and riser wall thickness riser on the shape of touch down zone. The research results show that:Vertical pipe buried in the soil of the region increases with SuO decreased, while with increasing the thickness of the tube wall is vertical, less affected by Sug. Basing on the initial groove model proposed by Abudery, the effect of maximum depth of maximum suction factor, soil separation factor, pipe trench on the dynamic response of dynamic riser system are researched, the results show that:the vertical maximum touch down zone tube bending moment increases with pipe soil separation,maximum suction factor and maximum value increasing trench.(3)This paper has presented the VIV experiment of SCR. In the experiment, the dynamic characteristics of the riser studied both in sag bend and flow line with different current speeds and seabed. The following results are shown.The amplitudes of strain and response frequencice increases with the current speed increased both in CF and IL, while there will be more than one response frequency participated in the VIV motion with a higher current speed. The seabeds have influence on the response frequencies of riser. The sag bend is mainly on first-order and second-order vibration appears for flow-line. At lower current speed the strain both in sag bend and flow-line is low and the disparity among them is little, while at higher current speed the strain of riser for sand and mixed seabeds is much higher than that for clay seabed. The damage of riser may appears in sag bend for clay seabed, however, the flow-line for the sand seabed. It suggest that the influence of seabed should be included for engineering design.(4)The fundamental equations of structural dynamic characteristics are established. The effects of the velocity of internal and external fluids and the truncated model on the natural frequency of riser are discussed. The results show that the natural frequency of the riser increases with the velocity of external fluids but decreases with the velocity of internal fluids. The model truncated or not has no influence on the natural frequency of riser. Distinctive from the pretension riser, the catenary riser has different vibration modes on in-plane and out-plane. The first four ranks of vibration modes are given and the results show that the amplitude of vibration mode is not constant and the value is high near to the high axial force. The vortex induced vibration of out-plane has greater effect on the ground contact part(5)Based on the user-defined element, the wake oscillator model is adopted to simulate the vortex induced vibration of riser. According to the proposed method of pipe-soil interaction, the dynamic response of riser in the ground contact part that caused by vortex induced vibration is calculated. The model is built based on the experimental data and its feasibility is verified by comparison between the simulation and experiment results of the riser under uniform fluids. The Responses of vortex induced vibration of the downstream and transverse coupling are analyzed.