Numerical Analysis Of Underwater Suspended Pipe With Crack Flaws

As one of the five most important transport manners, pipelines have been used all over the world. Also, oil and gas pipelines have been being the aorta of the world economy. With the development of economy, pipelines become a important part of lifeline works in every country. But, at the same time that they bring us convenience and fast transport, they also induce many safety accidents which cause huge losses for aging of pipe, cracks, etc. These accidents seriously affect the development of economy, safety of society and property, also, the ecological environment.In china, the oil pipelines across the river account for a considerable proportion among long distance pipelines. Most of the oil pipelines laid early have run about 30 years, and still shoulder the important tasks. Further, the parts across the river are laid with the method of excavation, part of them do not reach the design depth, coupled with the erosion of water and fetching soil of people, some pipelines has been bared. Under the eroding of water, such pipelines are easy to be suspended. For there being no constraint of soil, the suspended part is very easy to oscillate under the effect of water wake vortex. If there are defects in the suspended part of the pipeline, its pressure capacity will decrease. Coupled with the action of vortex, it is easy to failure suddently, which will affect normal production, pollute envorinment and cause huge losses.Aiming at above problems, numerical simulation of water flowing around a suspended pipe is done. On the basis of that, the maximum stress and fatigue cycles of suspended pipe with cracks are studied. Main content of the paper is as follows1,Flow characteristic and theory under high Reynolds numberIn a certain velocity, relief frequency eddy behind the suspended pipe can be close to the natural frequency channels. Thus similar resonance effect vertical to the flow appears. It leads the vibration along the flow direction and drag increase. Two directions of vibration can produce great alternating stress, which will cause the pipeline to fail.2,Numerical analysis of water flowing around pipe under high Reynolds numberAccording to hydromechanics and computation hydromechanics theory, numerical analysis of flow around a pipe under high reynold number is studied. Lift and drag coefficients are monitored. The distribution of eddy contours, pressure distribution, shear stress distribution and vortex shedding natural frequency of immobile pipe are gained. 3,Numerical analysis of viscous flow around a suspended pipe which is resonant transversely in a uniform stream under high Reynolds numberTransverse vibration of suspended pipe is studied. The influence on fatigue life of transverse vortex-induced vibration is far greater than that of flow direction. Therefore, in order to avoid the transverse vortex-induced vibration, lift coefficient, drag coefficient, the distribution of vortex contours and power spectrum of lift coefficient under different frequency ratios are calculated. The frequency ratio is specified when"frequency-locking phenomenon"occurs, further underwater suspended pipe's critical lengthes are calculated under different flow rates.4,Numerical simulation of underwater pipe with a circumferential crack flawComputation model of a suspended pipe with a circumferential crack flaw is established. Combined with support conditions, peak stresses and fatigue cycles of pipe with a circumferential crack flaw under several loads are calculated. Also, the influence of moment, length of the crack, depth of the crack, defect position, internal pressure of the pipe and water load is analyzed on peak stress and fatigue life.5,Numerical simulation of underwater pipe with a longitudinal crack flawComputation model of a suspended pipe with a longitudinal crack flaw is established, combined with support conditions, peak stresses and fatigue cycles of pipe with a circumferential crack flaw under several loads are calculated. Also, the influence of moment, length of the crack, depth of the crack, internal pressure of the pipe and water load is analyzed on peak stress and fatigue life.6,Numerical simulation of underwater pipe with two crack flawsComputation model of a suspended pipe with two crack flaws is established. The influence of relative position of the defects is studied when they are in the region that affecting each other. After comparative analysis, the result shows that the pipeline with two flaws is more dangerous than that with one flaw. If there is only one defect in the pipeline, the circumfetential flaw is more harmful.