Seismic Response Analysis Of Free Spanning Submarine Pipelines Based On Hydrodynamic Force Model Suitable For Earthquake

As an important component of the offshore oil and gas strorage and transportation system, submarine pipelines are playing significant roles in the offshore exploitation and regarded as the lifelines of offshore oil and gas fields. The security production of the offshore oil and gas fields depends largely on the safe operation of the submarine pipelines. Therefore, the structural integrity and reliability of submarine pipelines are paid more and more attention in practice. The potential seismic risk is very severe for submarine pipelines installed in seismically active regions such as Bohai Bay in China and West offshore in USA. It is of necessity that submarine pipelines laid in these areas be designed to resist earthquake ground motions. Since the characteristics of earthquake compared with wave and current are short duration, ample frequency content and high magnitude, interaction between pipeline and surrounded fluid under earthquake differs from that under wave and current. It is complicated and time-consuming to calculated seismic response of submarine pipeline on the base of interacting model considering water and structure coupling. Thereby, it is an alternate to study on the hydrodynamic force models to simulate and simplify interacting model between pipeline and water subject to earthquakes. Recently, it has been recognized that the spatial variation of seismic ground motion has an important effect on the dynamic response of submarine pipeline. It is of great significance to research the dynamic response of submarine pipelines subjected to spatially varying earthquake ground motions.The hydrodynamic forces on the span of submarine pipeline which is surrounded by sea water had been paid much attention by many researchers. Model tests of a free spanning rigid submarine pipe subjected to earthquake were carried out on an underwater shaking table in the Sate Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. The fluid field adjacent to the pipe and hydrodynamic pressure on the surface of the pipe were measured. Based on Morison's equation, the drag coefficient CD and the inertial coefficient Cm related to earthquakes were obtained by the least square method. The effects of Re (Reynolds number), Kc (Keulegan-Carpenter number), water depth d and the gap ratio e/D on CD and CM were discussed. The experimental results indicate that the seismic input direction has important influence on the characteristic of hydrodynamic forces imposed on pipe. The hydrodynamic force exerted on pipe is negligible under horizontal seismic input. Re and Kc have remarkable effect on the hydrodynamic forces imposed on pipeline under earthquakes. Compared with waves and currents high frequency contents of seismic ground motions were abundant, which resulted in significant effect on the hydrodynamic force of pipe. The rigid pipe-water interaction was simulated using a three-dimensional (3D) finite element model and the numerical results were compared with the experimental results.Using the underwater shaking table, model tests of a free spanning flexible submarine pipe subjected to earthquake were conducted considering the relative movement between pipe and water. Based on the joint elastic and gravity similitudes, the model scale design is performed. The hydrodynamic force coefficients under different input directions were obtained on the base of least square method as well. The influence of Re, Kc, water depth d and the gap ratio e/D on the hydrodynamic force coefficients was analysed. According to the experimental results, the coupling action of the hydrodynamic forces occurred. Hydrodynamic forces exsited both in-line and cross-flow directions under a certain sesimic input direction. Re and Kc are two primay parameters which influence the hydrodynamic force coefficients. The flexible pipe-water interaction was simulated using a 3D FE model and the numerical results were compared with the experimental results. The results show that numerical simulation can satisfactorily agree with the pipe movement and the characteristics of the hydrodynamic forces.Utilizing non-linear least square method, the variation of hydrodynamic force coefficients with Re and Kc was fitted by regression analysis and the quantitative expressions were obtained for rigid pipeline and flexible pipeline, respectively. Then, hydrodynamic force models for free spanning submarine pipeline suitable for earthquake were established. A conclusion can be drawn that the improved hydrodynamic force model could satisfactorily predict the hydrodynamic forces on the free span of submarine pipelines due to earthquakes. Finally, the effects of different hydrodynamic force models on the seismic response of submarine pipeline with free span were studied.A spectral-representation-based simulation algorithm was presented to simulate non-stationary multi-point seismic ground motion time histories compatible with target power spectrum by using prescribed non-stationary cross-spectral density matrix. The results indicate that the multi-point ground motions synthesized by the proposed method can be directly used as an input for the dynamic seismic analysis of large scale structures such as bridges, lifelines, dams, oil and gas pipelines etc. A 3D FE model of submarine pipeline with free span considering hydrodynamic force coupling induced by different seismic input directions subjected to spatially variable ground motions was established. The motion equations of the pipeline were derived and nonlinear multi-support input time-history analysis was performed. The effect of hydrodynamic force action method on seismic response of pipe was analysed. Seismic response of submarine pipelines was compared with different input method such as the multi-station input and identical input. The effect of nonlinear constitutional relationships of pipe steel and soil, and geometry nonlinearity was investigated. Furthermore, geometry factors were studied in the sensitivity analysis. The numerical results display that the spatial variation of ground motions can significantly increase the seismic response of submarine pipelines. Meanwhile, the other factors also influence on the response of the submarine pipelines under multiple-station earthquake ground motions to some extents.