Research On Safety And Reliability Assessment Of Multi-span Submarine Pipeline

Submarine pipeline is the lifeline of the ocean oilfield, deposit and transport of the oil, gas, water are completed mostly by submarine pipeline, which connects every product parts of the ocean oilfield to each other. The environment what submarine pipeline lies is very curviness, and if the pipe is failure, exploitation stay and environment pollution would make huge loss. Thus the research of the submarine pipeline has been taken great attention by many Scholars at home and abroad.This paper studies the reliability of multi-span submarine pipelines. The emergence of the span changes the load borne by the state and stress of the submarine pipelines, when a multi-span appears, the boundary conditions are much more complex. Among various types of environmental load which submarine pipeline bears, the waves, the current are the most important dynamic loads. The lift and drag force which is alternately generated to effect on the span can result the alternating power when the seawater flows by. The fatigue of vibration and the vortex-induced vibration resonance are the most factors caused spans to be failure, as well as the main problems to be solved in the reliability design, reliability assessment.Aimed on the span of the submarine pipeline, this paper analyses the span formation mechanism and the mechanism of vortex-induced vibration of the submarine pipeline suspended, the scope of multi- span is also defined. The differential vibration equations of the span are established under the roles of waves and currents, and then Matteo Luca Facchinetti's wake oscillator model is combined with the differential vibration equation of the span under the role of the currents to form the integration of differential equations by the pipeline and the fluid coupling vibration equation, and more the boundary conditions of multi-span pipeline are flexible inlay constraints. Based on DNV-RP-F105(2006), this paper studies the fatigue reliability assessment method of the submarine span. According to the Miner cumulative fatigue damage linear theory, and SN curve integrated theory of reliability analysis, the fatigue life of the span which is aroused by the current and wave forces is studied, of which vibration response is obtained by two ways: (1) To consider nonlinear force and vortex-induced vibration, using precise integration method to work out the response of then span vibration. (2) Ignore the interaction of currents and pipelines, using frequency domain analysis to obtain response spectrum of the span vibration.Finally, the safety and reliability assessment system is integrated, and the numerical results of examples show that: the greater the elasticity coefficient of the soil and the rigid coefficient of pipe ends are the longer fatigue life is, and the fatigue life of multi-span submarine pipeline is less than it of the single span. The fatigue life which is deserved by time-domain analysis is much smaller than the ones gained by frequency domain analysis.