The Dynamic Response Analysis And Experimental Study Of Free Span Submarine Pipelines

As a subdivision of the lifeline engineering, submarine pipelines are an important component of the offshore oil exploiting system. Due to landform, scouring, construction etc., there often occurs a free span to submarine pipelines. With the appearance of a free span pipeline, its loading types and stress state will be changed. Especially when ocean currents pass through it, a periodical vortex shedding is often produced and induces vibration of the free span pipeline. This type of vibration contributes a lot to the fatigue failure of a free span pipeline. Most of the previous study on a free span pipeline merely considered the vortex shedding caused by external fluids. Based on the pervious research, this paper makes a relatively systematic theoretical and experimental study on the dynamic response of a free span pipeline under the combined effect of internal and external fluids.Firstly, according to the Hamilton theorem, this paper derives the vibration differential equation of a free span submarine pipeline. The allowable scouring length considering the effect of inner flow on a free span pipeline is got and the relationship between them is also acquired from numerical studies. Secondly, this paper adopts the finite element method to solve the vibration differential equation and explores the relationships of the resonant amplitude and vibration frequency of a free span pipeline with its internal and external flow velocity. Furthermore, the comparison on the allowable scouring length and the dynamic response between in-line direction and cross-flow direction is investigated in this paper. Some useful conclusions are drawn from above investigation.Besides the theoretic analysis discussed above, a physical model experiment of free span submarine pipelines is carried out. In this experiment, pipeline model is designed according to certain similarities. Dynamic response of free spans conveying internal flow of different length was investigated under different wave heights, current velocity and their combinations. Results of the experiment can prove the theoretical analysis and some conclusions are obtained as well: The frequency of a free span is usually affected by effect of internal fluids; The cross-flow resonant amplitude is larger than in-line resonant amplitude when the vortex shedding induced by ocean currents appears, but the participation of waves will change this phenomenon sometimes; The response of a free span pipeline is affected much by its length, so in practice, the span length must be kept away from its allowable scouring length to avoid the resonance and maintain its safety.