Horizontal Directional Drilling Article

Different types of drilling heads/ tools are used in the pilot-hole process. Selection of drilling head is dependent on the type of soil strata. Soils with cobble stones or rock having voids or incomplete layers of rock are not considered suitable for drilling. The purpose of the geo-tech investigation is not only to determine the feasibility of HDD crossing, but also to establish the most efficient way to accomplish it. On the geo-tech information governs the determination of best crossing route along with selection of drilling tools and execution methodology.

Following shall be investigated during the geo-tech survey:

• Soil identification to locate rock, rock inclusions, gravelly soils, loose deposits, discontinuities and hardpan
• Soil strength and stability characteristics
• Groundwater level

(Supplementary geo-technical data may be obtained from existing records, e.g. recent nearby bridge constructions, other pipeline/ cable crossings in the area.)

Geo-tech investigation is performed by analyzing the soil sample extracted from bore-holes drilled along the pipeline route called as bore log data. For long crossings, bore logs are typically taken at 200 m intervals. For short crossings i.e. crossings which are less than 300 m length, as few as three bore log may be sufficient. 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. A thumb rule is to take borings at least 10m to either side of bore path. Although these are good general rules, the number, depth and location of boreholes is best determined by the geo-tech engineer.

Geo-technical data for River Crossings: River crossings require additional information such as a study to identify river bed, river bed depth, stability (lateral as well as scour), and river width. Typically, pipes are installed to a depth of at least 6m below the expected future river bottom, considering scour. Soil borings for geotechnical investigation are generally conducted to 12m below river bottom.

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. Small radius of curvature induces bending stresses and increases the pullback load due to the capstan effect. 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. Higher tensile stresses reduce the pipe’s collapse resistance. Curvature requirements are dependent on site 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.

The designed drilling profile consists of a series of straight lines and curves. The straight lines are referred as tangents. The straight sections are those in which the drilling hole curvature is ideally zero. This implies that any pipe section can be considered as straight section if the curvature of that section is less than that necessary to make the pipe deviate beyond the walls of the hole, which is roughly 1.5 times larger in diameter than the pipe itself.

The length of straight section AB, CD and EF can be reduced to zero by entering the proper combination of horizontal length of the crossing, exit height, entry height along with suitable radius of curvature.

Guidance note: Drilling path described above is suitable for most of the pipeline crossing performed by HDD methodology. It is advised that the user plots the HDD profile on the surveyed AutoCAD drawing before inputting the drilling profile parameters. User shall re-assure the suitability by checking that calculated value for AB, BC, CD, DE and EF is in sync with the plotted HDD profile length.

The steering tool is placed within the Bore Hole Assembly (BHA). Generally, the BHA is made up of non-magnetic drill collars. The “lead collar” of the BHA is placed on the alignment of the particular crossing. After the alignment, the steering probe is energized with electrical current (wire-line steering) and a bearing for the drill path is established and logged into the surface computer. The drilling rig is set precisely on line with a transit. The non-magnetic “lead collar” (with steering probe) and the directional deviation tool are started exactly at the designated entry point. In most cases, one Non-Magnetic Drill Collar (NMDC) is used behind the BHA. A 10 m non-magnetic collar shall serve as a buffer between the steering probe (in the “lead collar”) and the steel drill pipe. Drill pipe is often highly magnetized due to the continual making up and breaking out the tool joint connections and can affect the tool parameters.

Once the drilling bit exits out (punch out) of the pilot hole, the lead pieces/ drill pipes are unscrewed. The hole opener/ reamer is then attached to the leading pipe to start reaming operation. The reaming operation consists of using an appropriate tool to open the pilot hole to a slightly larger diameter than the carrier pipeline. The percentage oversize depends on many variables including soil types, soil stability, depth, drilling mud, borehole hydrostatic pressure, etc. Normal over-sizing may be from 1.4 to 1.5 times the diameter of the carrier pipe. While the over-sizing is necessary for insertion, it means that the inserted pipe will have to sustain vertical earth pressures without significant side support from the surrounding soil.

The pipe shall be strung and welded, on the rollers, in the same line as the drilled hole from entry side to exit side. The welds of the pipe may be subject to visual inspection and/ or non-destructive testing (NDT). After welding of the total pipe string, in a single segment length, it is pre-hydrostatically tested at a pressure of 1.25 times × design pressure of the pipeline. After successful completion of pre-hydrostatic testing, test header is removed and pull head is welded on the rig side of the pipe string. The near to hole section of the pipe string is lifted with the help of adequate lifting equipments to make a necessary over bend.

The pullback operation involves pulling the entire pipeline string in one segment (usually) back through the drilling mud along the reamed-hole pathway. The pulling equipment is attached to the leading end of the drill pipes string, and the prepared pipe string is fed gently into the bored hole. Proper pipe handling, cradling, bending minimization need to be followed. Axial tension force readings, constant insertion velocity, mud flow circulation/exit rates, and footage length installed should be recorded. The pullback speed ranges usually between 1 to 2 feet per minute.