North American Society for Trenchless Technology Conference (2014)
Significant friction forces can develop between the inner surface of the borehole and the pipe wall throughout the pull back (Baumert et al., 2002) stage of HDD installations. A custom testing apparatus was designed and fabricated capable of mimicking the deposition of the filter cake during an installation, while measuring the friction force between the pipe and the surface of the “borehole”. Tests were performed under controlled laboratory conditions for a range of geological materials, with and without the presence of a filter cake. Parameters in the experimental design include the type of geological material (sand, silty sand, clay, silty clay, and pea gravel), magnitude of the normal over burden force (56 lb, 70 lb, and 80 lb) acting on the pipe’s crown, drilling fluid composition (bentonite based fluids for sand and silty sand, polymer based fluids for clay and silty clay, and micro-fiber additives plus bentonite for pea gravel), pulling rate, and the contact surface area between the pipe and the filter cake. Tests performed with and without the presence of a filter cake quantified the benefit associated with the introduction of properly designed drilling fluid formulations in terms of reduced friction coefficients, and subsequently the required pull force. The outcomes of this study are expected to improve the understanding of the relative contribution of various parameters to friction forces developed at the pipe-borehole interface, as well as assist consulting engineers and other industry professionals in estimating the worst and best case scenarios, in terms of pull-load values, for specific installations based on anticipated geological conditions.