The straight sections are those in which the drilling hole curvature is ideally zero. The straight lines are referred as tangents. The designed drilling profile consists of a series of straight lines and curves. Curvature requirements are dependent on the site’s geometry (crossing length, required depth to provide safe cover, staging site location, etc.), but the degree of curvature is limited by the bending radius of the drill rod and the minimum elastic bending radius of the pipe. Higher tensile stresses reduce the pipe’s collapse resistance. The capstan effect is the increase in frictional drag while pulling a pipe around a curve due to a component of the pulling force acting normal to the curvature. Small radius of curvature induces bending stresses and increases the pullback load due to the capstan effect. One of the key considerations in the design of the drill-path is creating as large a radius of curvature as possible within the limits of the right-of-way. Although these are good general rules, the number, depth, and location of boreholes are best determined by the engineer. A thumb rule is to take borings at least 10m to either side of bore path. The borings should be near the drill-path to give accurate soil data, but sufficiently far from the borehole to avoid pressurized mud from following natural ground fissures and rupturing to the ground surface through the soil-test bore hole. For short crossings (which are less than 300 m length), as few as three bore logs may be sufficient. For big crossings, bore logs are typically taken at 200 m intervals. Soil strata investigation is performed by analyzing the soil sample extracted from bore-holes drilled along the pipeline route - called bore log data. Soil identification is done to locate rock, rock inclusions, gravelly soils, loose deposits, hard morum, boulders, brittle rock, black laterite, pebble stones, mountain blend, fast stak and bluff stones, etc. Soil Samplings Investigations during pedological survey The geo-tech information governs the determination of best crossing route, along with selection of drilling tools and execution methodology. If the strata becomes very hard (>90 MPA) then one has to use the rock cutting tools accordingly - like Mud Motor, Air Hammer, ARMOR etc. The purpose of soil testing is not only to determine the feasibility of HDD crossing, but also to establish the most efficient way to accomplish it. Selection of drill bit depends on the type of soil strata.
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