Nonlinear Analysis Of Free Spanning Submarine Pipelines Subjected To Spatially Varying Earthquake Ground Motions

With the marine exploration and production of oil and gas submarine pipelines as the most important way to collect and transport offshore hydrate play an important role in the producing, refining, and storing of oil and gas and are regarded as the lifelines of offshore oil and gas fields. The security and integrity of submarine pipelines become the issue of marine resource development. The Bohai Sea is a seismically active region in China. The combination of seismic load and working loads is the critical case in the design of the submarine pipelines in the area. Moreover, due to the anisotropy and inhomogeneity of geologic structure the seismic ground motions can vary significantly in magnitude, phase angle and frequency along pipelines routes. The distinction of ground motions in different stations is a key factor that affects the seismic response of the submarine pipelines. It is of great significance to study the dynamic response and failure mechanism of submarine pipelines subjected to spatially varying earthquake ground motions and other loads.An improved method for simulating multiple-station ground motions was presented. The method proposed by modifying the random phase angle can synthesize multi-station earthquake ground motions satisfying seismic propagation mechanism. The synthetic methods for non-stationary multi-station earthquake ground motions are introduced under the conditions of known phase-difference spectrum and unknown phase-difference spectrum respectively. Further a method for synthesizing the time histories of multi-station ground motions on the base of actual earthquake records is derived. The proposed method is applied to simulate the multi-station ground motions recorded from the Northridge earthquake and the ChiChi earthquake. The results indicate that the multi-station ground motions synthesized by the proposed method accord with propagating characteristics of the real earthquake wave. Therefore, the method can be used for multi-support excitation analysis of large scale structures.A three-dimensional (3D) finite element model of buried submarine pipelines with free span subjected to spatially variable ground motions was established. The model took account of nonlinear constitutional relationships of pipe steel and soil, and geometry nonlinearity. The motion equations of the pipeline were derived and nonlinear multi-support input time-history analysis was performed. The results show that nonlinear material models related to soil and pipe should be introduced in the multi-support input seismic analysis of submarine pipelines. The large displacement should be considered in nonlinear seismic analysis for long-span submarine pipelines. Furthermore, seismic response of submarine pipelines was compared with different input method such as the multi-station input, traveling wave input and identical input. Some factors including pipe length, free span length, outside diameter of the pipe, wall thickness of the pipe, concrete coating thickness, and seabed slope 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.FE analysis of submarine pipelines with free span under complex loads was performed with the aid of a pipe element considering internal pressure effect. The nonlinear material models of the pipe and the soil as well as the large displacement effect were also considered. The spatially varying earthquake ground motions, the internal pressure and the thermal loading were imposed on FE model. Firstly, validation of the pipe element was carried out. Then, the effects of internal pressure, thermal loading and the interaction between pressure and temperature on the multi-support input response of the submarine pipelines were studied.Model tests of free spanning submarine pipeline were performed on an underwater shaking table in the Sate Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. Hydrodynamic forces imposed on the span of submarine pipeline due to different direction excitations are studied in detail. The pipe-water interaction was simulated using a 3D FE model and the numerical results were compared with the experimental results.Based on Morison equation and Wake model two improved hydrodynamic force models considering the effects of seismic exciting directions were derived. Finite element method on the base of improved Morison equation and improved Wake model were employed to analyze the dynamic response of free spanning submarine pipelines subjected to earthquakes. FE models were established to simulate the above experimental conditions. The numerical results considering sine wave inputs and simulated El Centro earthquake inputs were obtained. Then a conclusion can be drawn that the improved hydrodynamic force models could satisfactorily predict the hydrodynamic force 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 subjected to spatially varying earthquake ground motions were studied.