Study On Borehole Wall Stability Using Horizontal Directional Drilling Of Trenchless Technology

Horizontal directional drilling,with obvious advantages in safety construction and environmental protection,has become one of the mostly adopted trenchless pipeline installation methods in crossing projects of rivers,lakes,mountains,roads,existing buildings(structures),etc.Recently,there are many Chinese engineering projects implemented by horizontal directional drilling engaging in trenchless laying of underground pipelines,such as power pipelines,communication pipelines,water supply and drainage pipelines,as well as oil and gas pipelines.Borehole wall instability is also commonly seen in horizontal directional drilling construction.Most of prior papers focus on the experience learnt from specific engineering projects and qualitative analysis of construction methods and technical measures.With insufficient research studying concept of connotation and mechanical mechanism in borehole wall stability,the relevant theory lags behind engineering practice.This thesis focuses on the borehole wall stability of horizontal directional drilling by adopting theoretical analysis,numerical simulation and experimental research.Specifically,elastic-plastic mechanics,cavity expansion theory,constitutive relation and strength theory,nonlinear finite element method and other theories are comprehensively used to study the mechanical problems related to the borehole wall stability in horizontal directional drilling.These are combined with our research group and experimental research results for verification.This thesis is structured as follow:1.Various forms and hazards of borehole wall instability in engineering practice are summarized,and a classification method for the borehole wall stability is proposed based on the idea of geotechnical structure.According to unified symbol conventions and parameter definitions,the widely used classical strength criteria(Tresca,Mohr-Coulomb,Huber-Mises,DP series criteria,Matsuoka-Nakai,Mogi-Coulomb,Lade-Duncan,UST,etc.)are re-derivated with unified expression equations for stress invariants.Equivalent area circle yield criteria are also put forward based on unified strength theory.The UMAT is developed based on the return mapping algorithm for verification.2.By optimising the mechanical model and assumptions,the elastic-plastic analytical solutions of borehole wall are deduced in detail.There are eight situations falling into two categories: infinite space and finite space.The new solutions in this thesis take into account boundary conditions,initial stress,non-associated flow law,strength theory effect,dilatancy,large and small strain,elastic strain in plastic zone and so on,which enriches the theory of cavity expansion in geotechnical materials and therefore provides reference for application in related fields.Besides,theoretical formulas of horizontal directional drilling proposed by prior studies are analyzed along with the deficiencies of Queen’s equation and Alberta equation being pointed out.3.Considering the in-situ stress state,the distribution characteristics and expansion law of the plastic zone of the borehole wall under action of drilling fluid are studied in detail.The results of finite element numerical simulation using ABAQUS show that the Young’s modulus and dilatancy angle have little influence on the plastic zone,while the influence of lateral pressure coefficient is significant,and its value determines the distribution shape of the plastic zone.The general equation of approximate plastic condition method is strictly derived and the approximate plastic condition method,Ruppneyt solution and point stress method are also programmed and calculated.The results of these analytical solutions are then compared to accurate post-processing results of finite element numerical simulation.Among these analytical solutions,only the approximate plastic condition method reflected the X-shaped expansion of the plastic zone,and the strength theory effect has been proved to have significant influence on the distribution characteristics of the plastic zone as well4.Taking the horizontal directional drilling Wei River crossing project as the background,the model test was carried out with specific process of modelling test.For example,the design of test box,layered filling,sensor installation and test,high-density resistivity test,drilling and reaming.The results of strain test and the detection results of the high-density resistivity method are then analyzed in detail.Plus,collapse phenomenon in the first and the second reaming is calculated during test process.The maximum plastic failure radius calculated by the approximate plastic condition method is close to that obtained by high-density resistivity method,where it provides a mechanical explanation for the collapse of the borehole wall.5.The whole process of the elastic-plastic mechanical response of the borehole wall under the action of the drilling fluid pressure is analyzed,and the relevant definitions of the characteristic drilling fluid pressure are proposed using corresponding calculation methods.For the isotropic in-situ stress state,the drilling fluid pressure interval that maintains elastic state of the hole wall is conservative,while for the anisotropic initial stress state such interval may not exist.Besides,the influence of strength index on the calculation results of characteristic drilling fluid pressure should not be ignored.6.Based on the failure criterion of connectivity of plastic zone of borehole wall,the definition and calculation method of safety factor for horizontal directional drilling are given.There are different theoretical methods being calculated for comparison and the existing problems are pointed out with reasonable methods outlined.The safety factor has a clear mechanical meaning and provides a perspective for unified evaluation of different theories.The fundamental mechanical challenges of wall stability in trenchless horizontal directional drilling of trenchless technology are investigated in this thesis.The inadequacies about this topic in prior studies are discovered and several new concepts and approaches are introduced based on mathematical derivation and logical analysis.It will deepen the understanding of factors affecting the stability of the borehole wall,which will be providing more comprehensive approaches for predicting and calculating drilling fluid pressure.This thesis also forms a more reasonable stability evaluation methodology and theoretical foundation for technical measures to prevent instability of borehole wall.