Research Of Free Spanning Submarine Pipeline Under Effects Of Near-fault Ground Motions

With the development of modern industry, the contradiction between supply and demand of oil and gas is aggravating gradually. As the most efficient, economical and reliable transportation way of oil and gas, submarine pipeline has became an important lifeline of the industrialized process and also an important part of mining system and storage system. However, with the oil spill accident frequently, more and more attention have been focus on the security and reliability of submarine pipeline. Especially our country located on the seismically active region, the destruction of submarine pipeline by earthquake is unable to estimate and compensate and it is essential to set up earthquake mechanism of submarine pipeline. There is lots of research on the response of submarine pipeline under the earthquake motion, but too limited research on the free spanning submarine pipeline under effects of near-fault ground motions. So this article focuses on the response of free spanning submarine pipeline under effects of near-fault ground motion.At present the distance from fracture surface of 20m is regarded as main near fault region. Near-fault earthquake is significantly different from the far-fault earthquake in its unique effect of velocity pulse, a large amplitude pulse will cause permanent damage to the structure at the beginning, especially will cause more damage to the structure with long period.Based on the existing research on the near-fault earthquake, CHI-CHI earthquake ground motions(In September 21,1999, the magnitude is 7.6) with forward directivity, fling-step effects and far-fault effects were selected as seismic input, the hydrodynamic force acting on the pipeline are derived by Morrison equitation in the form of added mass in the pipeline. Analyze different nonlinear response of free spanning subsea pipelines by using the finite element software ANSYS. A numerical analysis model of free spanning submarine pipelines were established by using the finite element software ANSYS. The model took account of nonlinear constitutional relationships of pipe steel and soil. The earthquake time history analysis of the free spanning submarine pipeline subjected to the three seismic effects of pipeline with the horizontal direction of uniform input was performed. The results show that nonlinear material models should be introduced. Based on this, different earthquake intensities and different periods were considered when the nonlinear dynamic response of the pipeline were performed, and made a comparative analysis of the pipe under three earthquake effects. Through the comparative analysis, the influence of near-fault effects on the free spanning submarine pipelines was more obvious than the far-fault effects, especially do more damage to the pipe with long period.Due to the real earthquake input are dimensions, in order to find out the most unfavorable earthquake input directions to the free spanning submarine pipeline. Six different directions were selected as the seismic input directions. At first, the dynamic response of pipelines subjected to three different earthquake effects were comparative analysis. The results indicated that the response of pipeline was basically the same. Further comparison of input ground motion in different directions shows that the response of pipe has respectively the minimum and maximum response when the earthquake input along the X axis and Y axis directions. At last the responses of pipe with earthquake input along different angels on different planes were compared.