Steering Head
Although this tool also varies with the rig and the project, the most common type is the "duckbill" head. Inside it is a cavity (the sonde housing) for the transmitter used for electronic locating. The drilling fluid flows through channels around this cavity to the nozzle, which also cools the electronics hydraulically.
Both the duckbill and the tricone bit are fitted at the very front of the steering head at a fixed angle to the head's axis. It is this angle that allows the head to change direction underground. When the rig stops rotating and only pushes, the head moves in whichever direction the angled bit at the front is facing.
The bits at the tip are chosen from different categories according to the ground: carbide-tipped, cone or toothed bits are some of them. In rock or very hard formations, fluid-driven drilling with a "mud motor" is used — a method that needs more equipment and organisation than ordinary heads. As the name suggests, the bit is driven by the pressurised flow of drilling fluid (the motor). Depending on the capacity of the motor, an ordinary fluid-driven bore needs at least about 0.5 m³ (500 litres) of drilling fluid per minute.
Choosing the steering head for the ground
Choosing the steering head starts with the geotechnical survey. In soft and medium ground such as clay, silt and sand, the duckbill head both advances quickly and gives the best steering response; the blade width and bend angle (usually 1–3°) are chosen according to the stiffness of the soil. In gravelly ground, aggressive carbide-tipped heads are used; in hard and weathered rock, tricone bits driven by a mud motor. The wrong head shows up as an inability to hold the intended direction on the pilot bore, or as premature wear.
The sonde housing inside the head matters as much as steering: if the sonde overheats, its signal degrades and neither the walk-over nor the magnetic system can give a reliable position. The fluid channels on the head are therefore designed to cool the sonde and pass enough flow through the nozzles. Deltek selects the head and bit on every HDD project according to the soil samples, and works with its own mud-motor equipment on crossings that require rock drilling.
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