Horizontal Directional Drilling Article

HDD Crossing
Horizontal directional drilling (HDD) is a steerable trenchless method of installing underground pipelines in a shallow arc along a prescribed bore path by using a surface-launched drilling rig, with minimal impact on the surrounding area. Directional boring is used when trenching or excavating is not practical. The tools and techniques used in the HDD process are an outgrowth of the oil well drilling industry.

The components of a horizontal drilling rig used for pipeline construction are similar to those of an oil well drilling rig with the major exception being that a horizontal drilling rig is equipped with an inclined ramp as opposed to a vertical mast. HDD pilot hole operations are not unlike those involved in drilling a directional oil well. Drill pipe and downhole tools are generally interchangeable and drilling fluid is used throughout the operation to transport drilled spoil, reduce friction, stabilize the drilled hole, etc. Because of these similarities, the process is generally referred to as drilling as opposed to boring.

The horizontal directional drilling process represents a significant improvement over traditional trenching & backfill methods for installing pipelines beneath obstructions, such as rivers or shorelines, which warrant specialized construction attention.

Installation of a pipeline string by HDD is generally accomplished in following six stages:

HDD crossing of pipeline primarily consists of drilling a small diameter pilot hole (≈ 6 inch to 8 inch) along the drilling path and then enlarging/ reaming the pilot hole upto a diameter which can facilitate the pipe string pull-back (generally 1.5 times pipe diameter). While boring/ reaming operation is being performed, pipe string preparation i.e. welding of line pipes, NDT of weld joints, field joint coating, pre-hydrostatic testing of the prepared pipe string etc. is done opposite to rig side of the crossing simultaneously. After the successful completion of hydrostatic testing, pipe string is pulled back into the enlarged hole.

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 curves are typically sag bend and over bend. The curved sections are considered short enough to assume one constant radius for the entire sweep of that section.

The HDD design calculation is for steel pipes in a banana-shaped drilling profile or drilling path. The banana-shaped drilling profile means the drilling path will start with:

  • An inclined straight section (AB); then it will encounter
  • A curvature (BC) after which it will have
  • A horizontal straight section (CD). Towards the exit side this horizontal straight section will again encounter
  • A curvature (DE) and then end with
  • An inclined straight section (EF)
HDD Drill Profile
Fig. Typical horizontal directional drilling profile

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.

Pilot hole drilling typically is considered the most challenging and time consuming step. As each piece of drill pipe is advanced, the next drill pipe is fitted with a wire inside. This wire is attached to the corresponding wire of the drill pipe previously drilled. This internal wire is the vehicle used for the signal to be sent from the steering probe located in the Bottom Hole Assembly (BHA) to the surface computer. This process is repeated until the bit is advanced along the predetermined path and comes out at predetermined exit location as per the designed drilling path.

Pilot hole pic
Fig. Pilot hole cross-section

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.

HDD reaming/ boring pic
Fig. Reaming/ boring cross-section

Good grade of bentonite is continuously pumped through the reamers to flush the cuttings and stabilize the hole. Similar procedure is repeated for all stages of reaming.

Swab Pass: While pulling the reamer back to the shore if the Driller or the Superintendent feels that the hole is not conditioned or if there is a collapse of the hole, additional swab passes are made with the same size of the reamer. High yield bentonite with quick jelling characteristics is used to preserve the integrity of the borehole during the swab pass.

Drilling Mud: Usually a “drilling mud” such as fluid bentonite clay is injected into the bore during cutting and reaming to stabilize the hole and remove soil cuttings. Drilling mud can be made from clay or polymers. The primary clay for drilling mud is sodium montmorillonite (bentonite). Properly ground and refined bentonite is added to fresh water to produce a “mud.” The mud reduces drilling torque, and gives stability and support to the bored hole. The fluid must have sufficient gel strength to keep cuttings suspended for transport, to form a filter cake on the borehole wall that contains the water within the drilling fluid, and to provide lubrication between the pipe and the borehole on pullback. Drilling fluids are designed to match the soil and cutter. They are monitored throughout the process to make sure the bore stays open, pumps are not overworked, and fluid circulation throughout the borehole is maintained. Loss of circulation could cause a locking up and possibly overstressing of the pipe during pullback. Drilling muds are thixotropic and thus thicken when left undisturbed after pullback. However, unless cementitious agents are added, the thickened mud is no stiffer than very soft clay. Drilling mud provides little to no soil side-support for the pipe.

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.
HDD pipe pull-back pic
Fig. Pipe pull-back cross-section