| The erosion of clay with traveling jet has wide engineering usage.In offshore oil & gas industry it was used in the sub sea pipeline burying.The sub sea pipeline was an important part of the oil & gas exploration.It could convey the oil or gas quickly continuous,so more and more pipelines are laid on the sea bed.For the purpose of protecting the pipeline from damage of wave impact and fishing,it should be burred under the sea bed 1-3m.Most of those works are done by jet trencher,for its reliability.Because the mechanism of jet-soil interaction is very complex,and the circumstance of the trenching is turbid,the jet trench depth pridection is a difficulty in the jet trencher design.So the research of jet trenching has great practical importance of offshore oil & gas industry.In this paper the interaction of jet and stiff clay was studied by experimental method.Key factors of jet trenching were investigated including shear strength of clay,jet pressure,nozzle diameter and the translational speed of nozzle.We made a group of nozzles,including 9mm,12 mm and 16 mm.The clay was mixtures of nature clay and cement,the shear strength was controlled by the variable cement content.The jet pressure was controlled the pump system,ranged from 2bar to 6bar.Two kinds of clay were made which shear strength was 60 kPa and 90 kPa.A single axis gantry mechanism controlled the translational speed of the nozzle.4 kinds of translational velocity were tested,including 7 cm/s,14 cm/s,21 cm/s,and 28 cm/s.The interface of jet-soil was found to be a logarithmic like curve,which was associated to the key parameters.The curve was longer and had less curvature when the diameter of nozzle,the jet velocity or erodibility coefficient of clay increased.It was shorter and had greater curvature when the threshold stress of soil or the velocity of nozzle traveling increased.On the jet-soil interface,there was a sine relationship between the erosion rate and the velocity of the nozzle traveling.At the exit of nozzle the erosion speed was equal the velocity of nozzle traveling and at the ultimate depth of trench the erosion speed was 0.The transverse profile was found to be a water drop shape.When the nozzle moved fast,the trench was shallow and wide,being water-drop shaped.When the nozzle moved slowly,the trench was narrow and deep.The efficiency of jet trenching was found be higher when the jet worked with low pressure and large diameter.So in practical engineering the jet trencher should tend to be designed with low pressure and large diameter.When the vane shear strength of soil,jet pressure and translational speed of nozzle were kept the same,the trench depth and diameter of nozzle accorded with the geometric similarity.Thus the trench depth of large nozzle could be got by test small one.According to this theory a new concept jet trencher-non contact jet control flow jet trencher was desgined.The parameter of the trencher was confirmed,and the trencher succeeded in practical engineering.The boundary layer shear stress was the main reason that caused jet trenching.The scouring theory of cohesive soil was more reasonable to illuminate the process of traveling jet trenching,which is much different from the theory based on the bearing capacity in soil mechanics.The essence of traveling jet trenching could be considered as jet erosion.Thus the key parameters were velocity of jet(jet pressure),diameter of nozzle,translational speed of nozzle,threshold stress,and erodibility coefficient of clay.We found that the trench depth increased with the velocity of jet,the nozzle's diameter and erodibility coefficient of clay,inversely with the nozzle translational velocity and clay's threshold stress.According the erosion theory,a new method was introduced to predict the traveling jet trenching,and the results was shown in a figure.When four of the five key parameters were confirmed,the rest one could be got according to the figure. |
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