HDD PROCESS

Installation of a pipe by HDD is usually accomplished in two stages. The first stage involves directionally drilling a small diameter pilot hole along a designed direc-tional path. The second stage consists of enlarging (reaming) the pilot hole to a diam-eter that will support the pipeline and pulling the pipeline back into the enlarged hole. A HDD drill rig is used to drill and ream the pilot hole and pull the pipeline back through the hole. HDD drill rigs provide torque, thrust, and pullback to the drill string. The drill drive assembly resides on a carriage that travels under hydraulic power along the frame of the drill rig. The thrust mechanism for the carriage can be a cable, chain, screw, or a rack and pinion system. Table 8.1 lists the three gen-eral categories of drilling rigs used in the industry.

Mini Rigs are mounted on a trailer, truck, or a self-propelled track vehicle. These systems are designed for drilling in relatively soft semi-consolidated formations and are used primarily for installation of utility conduits and small diameter pipelines in congested urban areas. They are not suitable for drilling gravel, cobble, or other formations where borehole stability is difficult to maintain.

Medium drilling rigs are used to install larger conduits and pipelines, normally up to 12 inches in diameter, with drill lengths ranging up to 1900 feet. They are par-ticularly suitable for the installation of municipal pipelines, as they are sufficiently

PLASTIC PIPING HANDBOOK 8.2

compact to be used in urban areas, while at the same time have the capacity of installing large diameter products beneath highways, subdivisions, and rivers.

Bores can be installed in unconsolidated to consolidated sediments.

Maxi rigs typically involve a large operation with multiple trailer-mount sup-port equipment and substantial mobilization and demobilization periods. High oper-ating costs make their use somewhat prohibitive in the utility installation market, and they are employed primarily in the pipeline industry. These large units may be used in the installation of large diameter pipes (24-48 inches) and or exception-ally long bores.

In addition to the drilling rig, a variety of support equipment may be required.

Depending on the HDD project, a drilling fluid or mud cleaning and recirculation unit, drill pipe trailer, water truck, and pump and hoses may be required. An exca-vator is needed to dig the entry, exit, and recirculation pits. In urban or environ-mentally sensitive areas a vacuum truck may be required to handle the fluid in the return pits or inadvertent returns.

Bore Installation

The bore is launched from the surface and the pilot bore proceeds downward at an angle until the necessary depth is reached. A small diameter drill string penetrates the ground at a prescribed entry point and the design entry angle, normally between 8-12 degrees. At a prescribed depth or point the drill pipe is bent to follow the proposed drill path and the designed bending radius. Then the path of the bore is gradually brought to the horizontal, followed by another bend before the bore head is steered to the designated exit point where it is brought to the surface. Choosing the proper drill pipe is a key element in the HDD process. The outer diameter and the wall thickness of the drill pipe have limitations that influence the bend radius of the bore. Larger diameter drill pipe cannot bend in short distances and cannot be used on short bores. Smaller drill pipes are more flexible and suited for short bores in the right soil conditions.

During the drilling process the bore path is traced by interpretation of elec-tronic signals sent by a monitoring device, located near the head of the drilling

HORIZONTAL DIRECTIONAL DRILLING 8.3

TABLE 8.1 Typical Characteristics of HDD Rigs

Mini Rigs Midi Rigs Maxi Rigs

Thrust/Pullback ⬍20,000 lbs. 20,000–80,000 lbs. ⬎80,000 lbs.

Maximum Torque ⬍2000 ft.lbs. 2000–20,000 ft.lbs. ⬎20,000 ft. lbs.

Drilling Speed ⬎130 RPM 130–200 RPM ⬍200 RPM

Carriage Speed ⬎100 ft/min. 90–100 ft./min. ⬍90 ft./min.

Carriage Drive Cable or Chain Chain or Rack & Pinion Rack & Pinion

Drill Pipe Length 5-10 ft. 10–30 ft. 30–40 ft.

Drilling Distance ⬍700 ft. 700–2000 ft. ⬎2000 ft.

Power Source ⬍150 HP 150–250 HP ⬎250 HP

string. At any stage along the drilling path the operator receives information regard-ing the position, depth, and orientation of the drillregard-ing tool, allowregard-ing him to navi-gate the drill head to its target. After the pilot string breaks the surface at the exit location, the bit is removed from the drill string and replaced with a back-reamer.

The pilot hole is then back-reamed, enlarging the hole to the desired diame-ter while simultaneously pulling back the line product behind the reamer.

This is typically referred to as a “continuous” borehole. In some situations with small diameter product pipe or conduit, the pipe can be pulled straight into the pilot hole after the drill is completed. However, in most HDD operations the bore-hole has to be reamed to enlarge the bore-hole to accommodate pulling in the product pipe. Generally the borehole is reamed to 1.5 times the outside diameter of the product pipe. The purpose of this is to provide an annular void between the prod-uct pipe and the drillhole for the drilling fluids and spoils and for the bending radius of the product pipe.

Sometimes it is necessary to ream the borehole without pulling back the pipe.

After the drillhole is reamed the product pipe or conduit is pulled back through the reamed hole filled with the drilling fluids. It is best to fabricate the product pipe on the exit side in one section so it can be tested and pulled in one continu-ous pullback. The drill pipe is connected to the product pipe or conduit using a pull-head or swivel. The swivel is used to prevent rotational torque from spinning the product pipe. A reamer is placed between the pullhead and the drill string to keep the drillhole open.

Drilling and Steering

Drilling curved and horizontal boreholes requires specialized drilling equipment.

This equipment is contained in a bottom hole assembly (BHA) that consists of a drilling tool, a bent sub-assembly, and a steering/tracking tool. Pilot hole directional control is achieved by using a non-rotating drill string with an asymmetrical leading edge. The asymmetry of the leading edge results in a steering bias. When a change of direction is required, the drill string is rotated so that the direction of the bias is the same as the desired change of direction. The drill string may also be contin-uously rotated when directional control is not required. Normally, the leading edge will have an angular offset created by a bent sub or bent motor housing. The most common types of down-hole drilling/steering tools used in the HDD industry are compaction tools and down-hole mud motors.

Compaction heads consists of a wedge shaped drilling bit, which is used for cutting and displacing the soil as well as for steering. To bore a straight hole the drill string is rotated and pushed simultaneously. When a correction in direction is required, rotation stops and the drilling head is preferentially oriented in the bore-hole. Then the drill rig pushes the entire drill string forward. As the slant on the face of the wedge is pushed against the soil, the entire assembly is deflected in the desired direction. After the steering correction is completed, rotation is resumed until another correction is needed. Compaction type drilling tools are most often used

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in mini and midi size drill rigs to drill through soft to medium consolidated soils, as well as loose and dense sands. When gravel or hard clay is encountered, compac-tion heads tend to wear rapidly. They are not suitable for drilling in rock formacompac-tions.

When drilling with compaction heads, steering difficulties are often encoun-tered when trying to drill in very soft soils. This is caused when the resistance to the deflector plate is not sufficient to offset the tendency of the drill string to drop vertically under its own weight. To solve this problem use a larger deflector plate.

Steering can be improved by increasing the flexibility at the head of the drill string. A common method is to add a length of smaller diameter more flexible drill rod behind the drill bit.

Mud (down hole) motors are used in ground conditions ranging from hard soil to rock. Mud motors convert hydraulic energy from the drilling mud being pumped from the surface to mechanical energy at the drill bit. This allows for the bit to rotate without drill string rotation. Positive displacement motors are typically used in HDD operations. These motors generate torque and rotation at the drill bit from the flow output of the mud pump. Directional control is obtained by a small bend in the drill string just behind the cutting head. As with the compaction heads, once the correction is made, the complete drill string is rotated to continue boring straight in the new direction. This method costs more than compaction heads and is less common in the utility installation industry.

The advantage of mud motors is that cutting of the formation is done by the mud motor, reducing the drill string rotation requirements, thus making it possi-ble to drill long boreholes to substantial depths. The main disadvantage to mud motors is that they are more expensive in comparison to compaction heads and require hundreds of gallons of drilling fluids per minute.

Tracking

In HDD applications tracking is the ability to locate the position, depth, and orien-tation of the drilling head during the drilling process. The ability to accurately track the drill is essential to the completion of a successful bore. The drill path is tracked by taking periodic readings of the inclination and azimuth of the leading edge of the drill string. Readings are recorded with a probe that is inserted in the drill col-lar as close as possible to the drill bit. The three most common type of tracking tools are:

1. Electronic beacon systems (walkover)

2. Combination magnetometer-accelerometer systems 3. Inertial navigation systems

A “walkover system” consists of a transmitter, receiver, and a remote moni-tor. A battery-powered transmitter is located in the bottom hole assembly near the front of the drill string and emits a continuous magnetic signal. The receiver is a portable, hand held unit, which measures the strength of the signal sent by the

HORIZONTAL DIRECTIONAL DRILLING 8.5

transmitter. This information is used to determine the drill heads position, depth, and orientation. The remote monitor is a display unit installed at the drilling rig in front of the operator. It receives and displays the information provided by the receiver. This information is used to navigate the drilling head below the surface.

The data is recorded to provide the as-built profile of the bore path.

When access to a location directly above the borehole alignment is not possible, or when the depth of the bore exceeds 100 feet, other types of navigation systems should be used. Two systems commonly employed are the magnetometer-accelerom-eter system and the inertial navigation system. The magnetommagnetometer-accelerom-eter-accelerommagnetometer-accelerom-eter system uses three magnetometers to measure the position (azimuth) of the tool in the earth’s magnetic field and three accelerometers to measure the position (incli-nation) of the tool in the earth’s gravitational field. The steering tool sends infor-mation via a wire line to a computer at the surface where the azimuth, inclination, and tool face orientation are calculated. As far as operating depth and distance from the drilling rig, this steering tool does not impose any limitation on the rig’s oper-ating range. Disadvantages of this system include susceptibility to magnetic infer-ences from buried metal objects and power lines. Some magnetic-accelerometer systems use a secondary survey system to account for local magnetic influences on the downhole probe. The secondary survey system induces a known magnetic field at the ground surface through a copper wire surface grid. A computer pro-gram connected to both the surface magnetic field and the steering tool compares the magnetic field measured by the steering tool and the theoretical magnetic field induced by the system, and compensate for local magnetic interference.

The inertial navigation system uses a system of three gyroscopes and three accelerometers to measure the azimuth and the inclination of the steering tool, respectively. The gyroscopes are aligned to true north at the ground surface before the survey is made. Any deviation from true north during the survey is detected by the gyroscopes and relayed to the surface where the azimuth, inclination, and drilling tool orientation are calculated by a computer. Because of the cost and sen-sitivity of these systems they are used mainly for calibration purposes.

Drilling Fluids

Drilling fluids are commonly called drilling mud or slurry. Drilling mud is mixed on the surface and pumped down the drill string. The mud comes out at the drill bit and is either left in the annulus of the borehole or circulated back to the surface.

Drilling mud is a mixture of water, premium bentonite, and if needed, small amounts of polymer. Bentonite is a non-hazardous material.

Drilling fluids have many uses or functions. The main purposes of HDD drill-ing fluids are:

• To establish and maintain the borehole integrity

• To transport drill cuttings to the surface by suspending and carrying them in the fluid stream that flows in the annulus between the wellbore and the drill rod.

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• To clean the build-up on the drill bits or reamer cutters by directing high-velocity fluid streams at the cutters. This also cools the bits and electronic equipment

• To reduce the friction between the drill string and the borehole wall aided by the lubricating properties of the drilling fluid

• To stabilize the borehole, especially in unconsolidated soils, by building a low permeability filter or mud cake lining and exerting a positive hydrostatic pressure against the borehole wall preventing collapse as well as preventing formation fluids from flowing into the borehole or drilling fluids from exiting the borehole into the formation (lost of circulation)

• To provide hydraulic power to downhole mud motors if used

A drilling fluid is composed of a carrier fluid and solids (clay or polymer).

The carrier fluid carries the solids down the borehole where they block off the pore spaces on the borehole wall. The blockage is referred to as a filter or mud cake. The ideal mud cake will form quickly during construction of the wellbore and prevent intrusion of drilling fluid into the formation. At times additives such as detergents are added to the drilling fluids to counteract some of the formation characteristics such as swelling and stickiness.

Drilling fluids that are not properly contained on the surface can cause prob-lems. A drilling plan should include the procedures for handling the drilling fluids as they return to the surface. Pre-dug pits and trenches or a vacuum truck should be a part of the bore planning. In addition, a drilling fluid disposal plan is a require-ment for the HDD project. After all the federal, state, and local regulations are met, spreading the used bentonite slurry on pastures and fields, or pipeline rights of way with the land owner’s permission can benefit the contractor and the land owner.