Abstract
Drilling with simultaneous casings of pneumatic DTH (down-the-hole) hammers has been extensively used in various complex formations because it can rapidly, safely and economically drill holes. The maximum depth and the skin friction associated with air DTH hammer drilling with a casing are very important parameters for project design and construction. By analyzing the conversion of impact energy and strain energy based on the energy method during the drilling process with a casing, we focused on the final stage of casing drilling and proposed a theoretical calculation method for the maximum depth and skin friction. At the same time, combined with the relevant technical specifications, the value of skin friction is improved and supplemented by analogy and induction. Finally, the theory was analyzed and validated through relevant construction examples with high accuracy, and then, the strain of the casing string under the action of a double impactor was analyzed with the concept of a neutral point. This distinguishes it from the stress wave method for calculation; its advantage is that both the strain energy and impact energy are incremental parameters at a certain time, without considering the influence of other forces during the construction process, which can greatly reduce the computational difficulty. The important calculation parameters include the casing wall thickness, casing outer diameter, and impactor power, which are exponentially related to the maximum depth and have a relatively small impact on the calculation results. The way to significantly increase the depth of drilling with a casing is to install multiple impactors on the casing string to change the force mode of the pipe string.