| The construction of oil and gas long-distance pipeline is the inevitable demand of sustainable economic development and national energy security of China.Because of the high construction efficiency,low cost and little damage to the environment,horizontal directional drilling(HDD)has been widely applied to the construction of oil and gas long-distance pipeline.However,engineering applications show that irregular hole forms becuses the HDD reamer sinks.And then it causes hole instability,finally the high efficiency,economy and environmental friendliness of the technology are decreased.So far,there was rarely any reference to the principle of reamer sinking and the mechanism of irregular hole formation,especially for the HDD hole stability with irregular hole,,whereas they are the key to safety and high efficiency,low cost HDD construction.Therefore,the reamer sinking regularity and hole wall stability of horizontal directional drill were studied in the article.The contents and achievements are as follows:(1)The reason of HDD reamer sinking was studied.According to HDD reaming process,stress state of the reamer was analyzed.And based on theorem of motion of center of mass and theorem of moment of momentum about center of mass,an analytical model was set up to push differential equations of reamer motion.It can be concluded that heavy reamer and low bearing capacity of formation are the key causes of reamer sinking.(2)A dynamics simulation model of interaction system of reamer with rock-soil was established to study the reamer sinking principle.The simulation results show:a)there exists a limited sinking value of reamer,which establishes a data support for irregular hole formation mechanism;b)With the increase of the weight of the reamer,the sinking of the reamer increases near-linearly,however when the difference between the gravity of the reamer and the buoyancy of the mud is greater than 0.847 times gravity,the sinking value of the reamer reaches the maximum;c)the sinking value of reamer is closely related to the formation,and the stronger the bearing capacity of the formation is,the smaller the sinking value of the reamer is.(3)The mathematical model of HDD hole outline is established to predict the shape of HDD hole;The formation mechanism of gourd-shaped hole was explored.The results indicate taht:a)reamer cuts the hole-wall on the lower formation like a crescent with the gravity of reamer,and after multilevel reaming,gourd-shaped hole is formed due to the superposition of crescent-shaped holes;b)a quantification relationship between the height of gourd-shaped hole and the maximum sinking value of reamer was modeled.And the relationship was validated with the practical hole height,because the relative errors were below 10%.Thus the hole formation mechanism has some engineering value.(4)The study of gourd-shaped hole wall stability was carried out.a)A three-dimensional finite element simulation model of gourd-shaped hole stress field was set up,and then combined with strength theory of rock and soil,wall stability of circular hole and gourd hole was compared;b)The influence of various factors on the hole wall stability of the gourd-shaped hole was detennined by orthogonal test.The analysis results show that:.The primary and secondary order of influence factors on the stability of the wall of the gourd shaped hole is hole shape,mud pressure,drilling depth and density of rock and soil.Moreover the effect of hore shape on the stability of the wall of the gourd shaped hole is significant.And Wall stability of the hole with samller reaming difference is worse.(5)Based on above results,some measures aiming at reducing the degree of gourd-shaped hole were put forward to prevent the HDD hole collapsing,such as single-multistage reaming construction method,reduce the quality of reamer and improving geological condition by grouting. |
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