One of the most effective ways of reducing downtime on projects using compact and medium-sized horizontal directional drilling (HDD) units is to properly maintain drill-string components—drill pipe, connectors, transmitter housings, drill bits and backreamers. Because of the nature of the HDD process, many preventive-maintenance procedures for the drill string can be performed on the jobsite.
It's important to maintain a drill's engine, hydraulics, drill-fluid systems and other components. But drill string presents special maintenance challenges. The components are exposed to tremendous stress, abrasive soils, and impact with rocks and other obstructions. Failure of any component while making a pilot hole or during product pullback is costly and time-consuming, and it can shut down a project.
Drill pipe is the link between the drill unit and the downhole cutting tool—a bit making the pilot hole, or a backreamer enlarging the hole. Drill pipe transfers thrust, torque and pullback. Drilling fluid flows through the pipe to cool downhole transmitters, lubricate cutting components and product, and wash away cuttings.
Although it's seldom done, manufacturer manuals recommend pre-conditioning new drill pipe before putting it into the ground. Start by thoroughly lubricating the entire surface of the threads and shoulders of the pipe with a copper-based tool joint compound. Join the pipe and tighten the joint. Break the joint and repeat the process twice.
In regular operation, threads and shoulders of male joints must be lubricated as each length is added to the drill string. Machines with automatic pipe-handling systems typically have an auto-lubricator that should be checked—reservoir topped off and proper function of the system confirmed—every 10 hours of operation.
Even a small amount of dirt on the pipe threads can prevent proper connection and cause the drill string to break. That's why end caps are supplied to protect both the male and female threads when pipe is not in use. Caps should be used any time pipe is transported in dusty conditions, or stored for an extended period.
With repeated use, the stresses of drilling weaken drill pipe. Because the first drill pipe is underground longest, it endures more shock loading and wear than any other pipe segment used in a bore. Share the wear throughout the drill string by moving the first pipe out of the first position at the end of each job. For machines with automatic pipe-handling systems, this will require a bit of forethought. At the start of pullback, leave one space empty in the active column of the drill-pipe magazine. When the first pipe comes out of the ground, it can be placed manually in the open spot further back in the pipe magazine.
Proper maintenance calls for careful inspection of each pipe segment for wear, cracks, bends and damaged threads. Threads are easily damaged by misalignment during make up, so pipe should be inspected and cleaned after each job. Scars in the pipe shaft may indicate damage, and bent pipe is a sure sign of trouble.
One of the most common causes of bending is oversteering—trying to change direction in a smaller space than the pipe's rated bend radius allows. Pipe can be bent if the drill frame is set up at a steep angle and the operator tries to level the bore too quickly (too little setback). Hitting large rocks or other buried objects can also bend pipe. Pipe with bends in the mid-portion may be repairable, but bends near an end may break from drilling stress and should be replaced.
A cracked or damaged transmitter housing cannot properly protect the transmitter, which provides critical information about location and bit-face orientation needed to guide the path of the bore. Hardware that secures the access cover on the transmitter housing—depending on the brand and model, it's a door with bolts, slide component, or roll pins—should be inspected often and replaced as necessary.
Dirt and debris in the housing can interfere with indexing, which affects readings, so clean its interior before starting a pilot bore. Check the isolators that cushion the transmitter from shock for cracks or tears, and replace when necessary.
Remove batteries from the downhole transmitter as soon as an installation is completed. Leaving batteries in place when a transmitter is not being used can cause corrosion and weaken the springs. Before each job, inspect and clean the contacts and springs—flattened springs can interrupt power. Use fresh batteries on each new job. To help maintain uninterrupted power, some manufacturers recommend using dielectric grease on the battery contacts. Before putting the transmitter into the ground, verify that the system is sending and receiving signals.
Drill bits and backreamers are the high-wear components of an HDD drill string. Generally, the more difficult the soil conditions, the faster bits and backreamers wear. Selecting the correct cutting tool for soil conditions is perhaps the most important factor affecting their performance and life. Using the wrong bit also can put unnecessary stress on the drill rig.
Never attempt to make an installation with a worn or damaged bit or backreamer. Some types have replaceable teeth; and broken, worn, or missing teeth should be replaced immediately. Renewing the hardfacing on wear surfaces also can extend cutting-tool life. Clean all fluid openings to permit free flow of drilling fluids, and check back flow valves. It can be a smart investment to routinely replace the inexpensive ball and spring check valves used in some housings every 100 hours or so.
A drill head or backreamer is attached to the first pipe—some with built-in swivel, others with separate connectors called subs or crossover subs. The pipe or duct being pulled into the bore is attached to the drill string behind the backreamer, requiring an additional connector. Inspect threaded ends of downhole tools and connectors and keep threads clean. Lubricate threads each time a connection is made. Worn components should be immediately replaced with new.
Up on the drill, the pipe-handling system and the sub saver directly affect drill-string makeup. Keep pipe-handler components clean. Lubricate sliding parts, and adjust the system according to the manufacturer's recommendations to ensure that each drill rod is positioned so threads are not damaged when adding lengths of pipe.
Vise alignment is critical so that the drill rod is centered in the vice for connection. When pipe joints aren't lining up, typically it's because the rear of the drill unit is raised or lowered slightly. Gripper pads in vices are wear items that require periodic replacement, although some can be flipped over to expose a second surface. Vice jaws should be regularly inspected and cleaned.
The sub saver is a replaceable attachment that protects the drill frame's spindle from wear. The sub saver is attached to the spindle and drill pipe is connected to the sub saver so that wear from repeated pipe make-up and breakout occurs on the sub saver, not the spindle. Keep components clean and lubricated. Typically, the sub saver wears out before a machine's supply of drill pipe and must be replaced to avoid damage to pipe.