Numerical Simulation Study Of Trench Forming Induced By Underwater Moving Jet

With the development of ocean oil and gas industry,subsea pipelines and cables are widely used because of high transport efficiency,low cost and less loss.To protect them from physical damage,such as the activities of ocean engineering,fishing,wave action and geological disaster,the pipelines and cables are usually buried enough depth below seabed.Subsea trencher is a kind of trenching equipment,it is adaptable to environment and has strong trenching capability.Jet trencher is applied in many projects on account of simple geometrical structure and effective trenching capability.However,during the moving trenching process,the sediment is disturbed by jet flow and settles in the back of the trench.It forms siltation in the trench and reduces the effective depth of trench.In this paper,sediment scour problems were simulated and analysed by numerical method.When the scour attained the dynamic balance state,the expressions of the main parameters and profiles of trench were derived.Then the trench profiles with different scour parameters can be predicted.This method can be used to design the jet system of the trench.The preliminary estimate method of trenching in the application of engineering was first introduced.The trench profiles were presupposed for simplification.The stress upon the surface of the trench was compared with the critical shear stress.Then the ability of trench was evaluated.In this mothod,the scour profiles could not be obtained clearly.And the volume of sediment due to siltation in the trench couldn't be estimated.Eulerian model and Lagrangian model were introduced to simulate the two-dimensional single jet fixed scour problem.The flow flied characteristics of fluid phase and sediment phase were observed.And the trench profiles obtained by numerical method were in good agreement with the experimental data.Eulerian model was chosen to be the main method to simulate the scour problem.Firstly,the modified model of “numerical dispersion” in sediment phase was proposed in order to eliminate the wavy interface and unrealistic backfill and to simulate the solid feature of sediment phase correctly.When the slope angle of trench exceeded the internal friction angle,a disturbing force was considered to force the sediment phase in this high-slope section to move which was called slope correction model.These modified models efficiently and accurately simulated the two-dimensional sediment scour generated by fixed jet.Then,the two-dimensional moving jet scour problems were simulated by the modified models.The flow flied characteristics of fluid phase and sediment phase were observed.When the scour attained the dynamic balance state,the trench profiles were found to be similar with the experiment data.The dimensionless data of trench profiles fell on the same curve,called self-similar curve of trench profile.And with the expressions of the main parameters of trench,the trench profiles in different jet condition could be estimated.At last,in the simulation of three-dimensional single moving jet scour problem,the evolution of trench and sedimentation was studied.The trench profiles at lateral section and symmetric plane were obtained.And the self-similar curves of trench were obtained after the data of trench was normalized.The characteristic parameters of trench were derived.