Study On Cavitating Water Jet Used For Laying Pipelines And Trenching In Seabed

With the increasing intensity of the exploitation of the seabed resources,the future of our country's submarine pipelines and cable laying and other projects will increase day by day.Comparing to the conventional mechanical trenching,hydraulic jet excavation has more advantages,such as simple structure,less pollution,high trenching efficiency,and so on.The conventional hydraulic jet trenching machine uses conical nozzles to form continuous jets,which uses the dynamic pressure of the jets to do the trenching.However when the soil strength becomes high,the erosion effect of the jet will be worse,and the trenching efficiency will be reduced.The nozzle is the key component of the hydraulic jet trenching machine.Therefore,it is very important to study the jet with better erosion ability and the corresponding nozzle structure in order to improve the erosion efficiency.On the basis of a thorough investigation of the relevant theory and technical application of water jet,a cavitating jet with better erosion capability is first proposed to be applied to submarine trenching and buried pipe project.It is feasible to use the cavitating jet in shallow sea seabed to dig trenches.Some conical nozzle,swirling jet nozzle andcontraction-expansion cavitating jet nozzles used for testing are designed on the base of analysis.The flow fields of the designed nozzles are numerically simulated with Fluent software.The results show that under the simulated sea bottom condition,the jet produced by the contraction-expansion nozzle is best of the three.The continueous jet of conical nozzle has advantage over the large effective erosion distance,but the impact area is too small.The impact area of the contraction-expansion nozzle is the largest of the three under the large jetting distance.The swirling jet has advantage in impact area,but the jet velocity is too small.Simulation results show that the contraction-expansion nozzle can be helpful in production of cavitation in the jet,and nozzle structure parameters?throat length Ld,diffusion length Le,diffusion angle??have been selected by the simulation:Ld=3 d₀,Le=4 d₀,?=30°.Based on numerical simulation,the conical nozzle,the swirling jet nozzle and contraction-expansion nozzle have been designed,and comparison erosion tests conducted on sand samples which are specially made in light of Bohai offshore seabed strength with the shear strength30kPa,50kPa and 80kPa.The experimental results show that,under the field hydraulic parameters,the erosion effect caused by contraction-expansion nozzle jet is best of the three,and the optimized structure parameters of the contraction-expansion nozzle are in agreement with the results of numerical simulation.Compared to conical nozzle,contraction-expansion nozzle jetting 50kPa sample at standoff distance from 6.6 to 20 d₀ can lead to increase in erosion volume by 894%.Scale-up contrast tests with big nozzle against the 80kPa sand samples,jet distance being 15 d₀ to 20 d₀,the erosion volume produced by the caitating jet can be increased by 73.8 times,which shows the erosion advantage of the cavitation jet to the hard soil.Combined with the simulation and test results,recommendations of combined arrangements of the nozzle assembly for application to field trenching machine have been set up.This paper provides reference and guidance for cavitating jet application in seabed trenching engineering.