Numerical Investigation Of Coupling Effect Of Vibration And Local Scour Around Submarine Pipeline Under Wave Action

Local scour below offshore pipeline is induced by the current and wave in ocean environment. Local scour leads to free span of the pipeline. Vortex shedding can form behind the free span pipeline which leads to vortex induced vibration. The pipeline may breakdown by fatigue damage induced by vortex induced vibration. Free span leaded by local scour and vibration of the pipeline will endangers the safety of pipeline. Once the pipeline breakdown, it is not only great economic loss but also serious environmental disaster.So far, the numerical studies of local scour around submarine pipeline are limited to the investigation of local scour around pipeline under stead flow with the pipeline fixed. No publication is seen for the coupling numerical investigation of wave motion, vibrating pipeline and local scour. Therefore, it is necessary to establish a numerical model that can solve the problem of local scour around vibrating pipeline. In this paper a numerical model that considered the coupling of wave motion, sand movement and pipeline vibration is established. Moreover, local scour around vibrating pipeline in waves is investigated by using this numerical model.In this paper, the second chapter established a flow model to describe the motion of fluid flow. The problem of flow over multiple circular cylinders is investigated by using the model. For flow over twin circular cylinders, it can be seen from the numerical results that the lift forces on the circular cylinders and vortex shedding frequencies have abrupt change at GI D= 0.9 and G/D= 2.6. It is found that different flow patterns lead to the change of flow forces and vortex shedding frequencies. Four flow patterns are found with the increase of the gap ration G/D. For flow over circular cylinder with multiple control rods, it can be seen that the flow forces on the main cylinder can almost totally be suppressed by the method of arranging six control rods around the main cylinder.The third chapter established a flow-pipeline motion coupling model. Vortex induced vibration of a circular cylinder with six control rods is investigated. From the numerical results, it can be seen that the vibration amplitude of the circular cylinders can be reduced great compared with one single circular cylinder.In the fourth chapter, a numerical model about local scour around fixed pipeline under wave action is established. The dynamic mesh technique is used to deal wave free surface and moving sea bed. Wave is no longer treated as simple oscillatory flow. From the numerical results it can be seen that the incident wave height and period have great effect on the local scour around pipeline. The scour profile is different for pipeline installed on a slope seabed comparing with the pipeline installed on flat seabed.At last, the wave-sand-pipeline coupling model is developed. Both the suspended load transport rate and the bed load transport rate are considered. It can be seen from the numerical results that the maximum scour depth for vibrating pipeline is almost two times that a stationary pipeline. The scour depth, lift wave force and vibration response are large when the reduced velocity falls in between 2.05 and 3.28.