Horizontal Directional Drilling Process
Horizontal Directional Drilling (HDD) has been used since the 1970's. This type of drilling is a marriage of directional drilling of oil wells and conventional road boring. This method is used for crossing rivers, highways, airport runways, shore approaches, pipeline corridors, and areas congested with buildings. Crossings have been installed for the use of pipeline carrying oil, natural gas, petrochemicals, water, sewage, ducts for fiber optics and electric cables, and many other products. Directional drilling has come a long way since the 1970's where it was mainly used in the United States Gulf Coast in alluvial soils, now there have been crossings through granite, sand, cobble, glacial till, and many other soil types.
HDD has the least environmental impact out of any other method of pipeline transportation. The HDD technology offers maximum depth of cover under the obstacle being crossed. This offers maximum protection and minimizing maintenance costs. On river crossing which hold boating traffic, with HDD the traffic is not interrupted. This is because the work is done from a safe distance away from the bank. In most cases the HDD process is the least expensive of other methods and is known for having a predictable and short construction schedule.
Horizontal Directional Drilling Background
Horizontal Directional Drilling Stages
Stage 1: Pilot Hole
The pilot hole is the beginning of the HDD process. The pilot is first drilled to help guide the following hole opening operations. The drill rig is set up at a prescribed entry angle above the ground, usually between 8 and 20 degrees. The Pilot Hole is achieved either by excavation by jetting or by a down hole monitoring. Depending on the conditions of the soil the pilot is drilled along a pre-determined alignment in which the path is selected by traditional methods. The drill then continues under and across the given obstacle. Drilling fluid is pumped through the drill pipe to the drill head at which time is jetted through or pumped through a drill motor. The end of the Drill Pipe is to core the pilot hole. The drill fluid is then recycled and re-injected into the drill stem. The pilot process can take several days, depending on the conditions of the soil and may require changing of the drill stem or drill head. This path is set up by the design profile which is made up of straight tangents and a long radius arc. The directional control is brought about by a small bend in the drill string just behind the cutting head. The pilot drill string is not rotated except to orient the bend. This drill path is monitored in the drill cabin by the navigator and the driller.
This path is monitored by a combination of activities. First before the drill rig was set into its center line, a navigator will come to the project site and take a survey of the land the drill will cross. They will survey the center line and a distance to each side of the center. The distances to the side will be the track for the wire line that will be spread the length of the project. The second part of this operation comes from the steering tool on the pilot drill string. This steering tool is a part of the pilot drill sting. It is a measured distance behind the cutting head and is powered by a wire line strung inside the drill pipe. This wire line allows for the electromagnetic readings. The steering tools electronic package detect the relation of the drill string to the earth's magnetic field and its inclination. This information is the relayed to the drill cabin. The information relayed from the steering tool includes the magnetic azimuth (right/left) and the inclination (up/down).
This monitoring helps the navigator and driller know how accurate the drill sting is to the designed profile. The ideal bore begins with a straight, tangent section inclined at the angle of entry to the ground. This is needed to gain maximum depth and cover from the given obstacle and also to aid in steering reaction. The drill string is then steered into an upward sweeping curve transitioning into a horizontal section before turning upward toward the exit point. The angle at exit point is usually between 5 and 10 degrees. Once the pilot hole is complete the cutting head and steering tool are disconnected from the drill string and the drill string is set up for the hole opening/reaming stage.
Stage 2: Hole Opening
Once the pilot hole has been completed the hole needs to be enlarged to a diameter suitable to have a smooth pull back. This is accomplished by opening the holes diameter in successive stages of size. There are two types of reaming, forward reaming and back reaming. Forward reaming is when a reaming bit is attached on the rig side and pushed through with the aid of an excavator pulling the pipe side tail string. Back reaming is when the reaming bit is attached on pipe side and the pulled back with pipe being added from pipe side.
The number of reaming passes depends mainly on the size of the product pipeline. Aside from that the number of passes and type of reaming bit depend on soil conditions. In loose or soft soils the number of reaming passes will typically be less than that of hard soils or rock. This is due to torque limitations, cleaning plant capacity, and pump capacity. In the end the final bore diameter must be larger than the product pipeline diameter to reduce frictional pull back loads and also facilitate the flow of the drilling fluid around the product. General rules of thumb for reamed diameter is: for a product line less than 8 inches the reamed diameter will be 4 inches larger than the product pipeline, for a product pipeline between 8 and 24 inches the reamed diameter is the product pipeline diameter times 1.5, and for product pipeline greater than 24 inches the reamed hole diameter will be 12 inches greater than the product pipeline. This extra space is called annular space. Reamed diameter size will vary depending on soil conditions and the foreman preference from previous experience.
Stage 3: Hole Cleaning
After the hole has been opened to a desired diameter the hole needs to be cleaned from and excess cuttings this is called a "mud pass." This is done by sending one of our custom built ball reamers down into the hole. The ball reamers diameter is a couple inches smaller than the hole diameter. With this ball reamer the pipe is pressurized and clean mud is sent down hole causing lighter cuttings to be sent out of the hole, the heavier cuttings will be pushed or pulled out depending on which direction the reamer is coming from (rig/pipe side). The number of mud passes depends on bore diameter and soil conditions.
Stage 4: Pipeline Pull Back
Once the drilled hole is enlarged and cleaned, the product pipeline can be pulled back through it. Before the product pipeline is pulled through it is tested and placed on rollers. After the testing has been completed and the pipe is certified to be pulled through a pulled head is attached to the pipeline. This pull head has a swivel on it and connected to that is a swivel. The swivel prevents any translational rotation from the reamer's rotation into the pipeline string. This allows for a smooth pull through the drilled hole. The product pipeline is then positioned and supported to enter the bore. The drill rig rotates the drill pipe string and send drilling mud for lubrication in the hole and pulls the product pipeline back simultaneously. The pullback continues until the product pipeline is visible on rig side.
At the end of the HDD demobilization and clean up take place.