Experimental And Numerical Study On Reverse Circulation Reaming In Horizontal Directional Drilling

As the critical lifelines to mitigate unbalanced distribution of natural resources and to keep cites functioning well,underground utilities play an important role in guaranteeing a healthy national economic development and continous city propersity.Because of its advantages such as short construction period and better economic benefits,Horizontal Directional Drilling?HDD?is widely used in the underground utilizes constructions and is becoming an indispensible technology for the installation of pipelines and conduits,especially in rivers,lakes or other areas where open-cut construction is not allowed.With a substantial increase in pipe diameter and crossing distance,HDD is advancing toward large diameter,long distance and complicated geological conditions.As the pipe diameter incrases,a larger borehole is required and the drilling fluid velocity within the borehole is consequently decreased,resulting in the formation of cutting beds at the borehole bottom.To improve the cuttings removal efficiency in large-diameter HDD reaming,the reverse-circulation reamer was innovatively designed based upon the ejecting theory.Centered on the proposed reverse-circulation reamer,the following work has been done:?1?The overhanging auxiliary drill pipes were assumed as a cantilever beam,based on which the mathematic correlation between the length of overhanging auxiliary drill pipes and borehole diameter as well as drill pipe diameter was deduced.With jet-pump's performance equations,the variations in reverse-circulation reamer's head and flow ratio were analyzed and then the entry point was chosen as the base for the reamer's design.Moreover,the critical velocity and head loss within the auxiliary drill pipe were determined using the available slurry transportion theories,based upon which the reamer's structural parameters were calculated with jet-pump design theory.?2?The impacts on the reverse-circulation reamer's noncavitating and cavitating performances of primary flow Q₁,suctiond pressure P₂and rotary speed n were investigated and the results indicated:the proposed reverse-circulation reamer was structurally feasible and its non-dimensional q-h curve could be approximated as a straight line with an intercept of 0.44 and a slope of 0.26;neither Q₁ nor P₂ makes an evident difference to the reamer's non-cavitating performance,but the growth in Q₁ or reduction in P₂ would undermine the reamer's anti-cavitation capability;reamer's rotation does exert influence on its non-cavitating or cavitating performance.?3?The impacts on the reverse-circulation reamer's cuttings transportation efficiency?_p of particle size d_P,drilling fluid viscosity?_F and rotary speed n were experimentaly studied and results indicated:an increase in d_P would lead a reduction in?_p,while using bentonite slurry would result in an increase in?_p?compared with using water as drilling fluid?;?_p increased initially and then decreased with the increase in n,and a critical rotary speed n_c at which the highest?_p was achieved was idenifid;n_cincreased with the increase in d_P and it reduced to zero as bentonite slurry was used.?4?Numerical simulation was conducted through FLUENT to investigate the characteristics of the reverse-circulation reamer's internal flow and also to optimize its structural parameters such as nozzle-to-throat space l_nt,throat length l_t and diffuser angle?_d.The numerical results indicated:within the primary flow issuing from the nozzle in the form of a high velocity jet existed a flow core zone and a contraction of the flow core zone was obsvered as l_nt increased;the velocity difference between the mixing layer and flow core zone got reduced as the flow advanced toward the throat exit;the optimal l_nt,l_t and?_d were 1.0?1.25d_n,?3.83+2.88q?d_t and 5°respectively.?5?FLUENT-DPM model was used to investigate the impacts on the reverse-circulation reamer's cutting transportation efficiency?_p of the number N_S and diameter d_S of suction inlets and also to investigate the impacts on the reamer's performace of particle size d_P and concentration S_V.The numerical results indicated:the optimal d_Swas 2?3d_P and the optimal N_S in rock strata was 3?4;the reamer's performance was reduced due to the addition of particles and the increase in d_P or S_Vwould resulte in a greater reduction in the reamer's performance.The originality and innovation of this paper was listed as follows:?1?The application of reverse circulation technology during the reaming process was studied resulting in the development of a novel reverse-circulation reamer.?2?Laboratory experiment was conducted to investigate the effects of various parameters on the reamer's performance and cuttings removal ability.?3?CFD was used to investigate the characteristics of the reamer's internal flow field and the impacts of the suction inlet's layout on its cuttings removal ability.