Investigation On Dynamics Of Pipe During Pullback In Horizontal Directional Drilling Installations

The prediction of pulling force is an important issue in horizontal directional drilling(HDD) research, which provides the basis for the design of crossing project, selection of drill rig,stability evaluation of pipeline during constructing process, and development of drag reduction technology.There is plenty of room for improvement in the accuracy and reliability of the existing methods to predictpulling force since some affecting factors are neglected or too many empirical coefficients are used incalculations. Four components of resistance force are analyzed and physical models are created to calculatethe corresponding component. A theoretical model for predicting pulling force is built and the method ofdetermining initial parameters is discussed. The influence of initial parameters on prediction results, thesensitivity of calculation and the contribution weight of component are studied using the theoretical model.Based on the above issues, drag reduction technology is developed. The main objectives and conlusions ofthis study are as follows:A theoretical model for predicting pulling forces during pulling back phase in horizontal directionaldrilling projects is built. The analytical method develops the method given by Polak et al. in three aspects:(1) the soil is described by Winkler model and it constitutes an elastic support for pipe during pullback,(2)considering the nonlinear rheological properties of slurry, power law model is used in calculations, and (3)the resistance exerted on drill string is considered and pulling force at carriage is calculated. Comparisionof field test data and prediction results shows that the accuracy and reliability of the model is high.The reliability of model improves as the empirical initial parameters decrease.Determining methods for three initial parameters, i.e. the ellipticity of pilot bore, therheological model to describe slurry and corresponding rheological parameters, the frictioncoefficient between pipe and soil, are developed.The state of pipe during pipe soil interacting is similar to a chock and the phenomenonis defined as Chock Effect. The coefficient of Chock Effect is used in calculations of weightand weight friction of pipe, frictions due to directional changes of pilot bore, and resistanceexerted on drill string to consider the Chock Effect. The finite element analysis resultsindicate that the coefficients of Chock Effect increases when external load increases, theover cut ratio decreases, or the ellipticity of pilot bore increases.Initial parameters affect the calculation of pulling force nonlinearly. The sensitivity analysis of initial parameters illustrates that the structure of pilot bore, the over cut ratio, thefriction coefficient between pipe and soil, the density of slurry affect the prediction resultgreatly while the ellipticity of pilot bore, the velocity of pull back, and the discharge of slurryhave less affection. The contribution weight for each component of resistance force changesalong with the installation distance.Drag reduction technology is developed in different stages of HDD installation, such asthe site selection and design of crossing project, straightening process, and pipe sending.Studies show that soil of which subgrade reaction coefficient is large should be given priorityto cross in the design of pilot bore. The cutting point of sensitive area and non sensitive areawhen the over cut ratio affects the pulling force is optimal over cut ratio. The reasonablelength of straightening process is equal to the minimum length not causing great increases inpipe soil interaction force, which is derived from pipe bending effect. The increment ofpulling force due to Capstan Effect at pipe entry point is large and drag reduction technologyof roller is developed to weaken the effect.