New design equation for maximum allowable mud pressure in sand during horizontal Directional drilling

Abstract

Shear failure (blow-out) is known to control failure and mud loss in sand during Horizontal Directional Drilling (HDD). A series of experiments investigating the influence of burial depth on the stability of horizontal boreholes have been reported in a previous publication. Numerical analysis is employed in this paper to establish a new criterion for horizontal borehole stability in sand based on successful comparisons between experimental and numerical data. A new design equation for calculating the maximum mud pressure is developed based on a parametric study considering different friction angles, lateral earth pressure coefficients at rest, and ratios of the borehole springline depth H below the ground surface to the borehole diameter D (H/D). After comparisons to the experiments, the dimensions considered in the numerical models are expanded to eliminate overestimation of the maximum allowable mud pressure at deep covers (so the new design equation overcomes a significant limitation with a previously unpublished design equation). Finally, the new equation for maximum allowable mud pressure is evaluated using three case studies, with results also compared to those from the Delft Equation, the current state of practice that is widely used for calculating allowable mud pressures in industry. During development of the case studies, a new ‘dangerous’ construction condition is defined where expected mud pressures are close to or exceed the mud pressure calculated from the new design equation.