Conductor pipe

The Conductor Pipe is a large diameter pipe that is set into the ground to provide the initial stable structural foundation for a borehole or oil well.
The Conductor pipe is the first string of casing and is the largest diameter casing to be installed in a well. 
It can also be referred to as a "drive pipe" because it is often "driven" into the ground with a pile driver.
Conductor pipe is typically set, on petroleum wells, before any drilling operations are performed. It is usually set with special pile-driving or rigs, though the drilling rig is sometimes used to save time and money. Conductor pipe’s purpose is to protect shallow sands from being contaminated by drilling fluids, and to help prevent washouts (which occur near the surface due to loose and unconsolidated top soils, gravel beds, etc.).
In the offshore drilling industry, the conductor pipe is set in the seabed, and is a key structural foundation for the subsea wellhead. When drilling an offshore well, a marine riser is connected to the well, and this is subjected to large environmental forces such as current and waves which are transferred to the conductor. The forces can be large and also cyclical which can potentially cause fatigue damage to the conductor, especially in harsh environmental conditions. A Conductor tensioner unit (CTU) can be used in shallow water on jackup rig to withstand such damaging forces.
Conductors are sometimes driven into position in very soft formations with large hammers. Sometimes, they are jetted into position and at other times, a separate cheaper rig (even truck mounted) may be used to drill the hole for the conductor. Offshore, two conductors may be installed in a well. The first conductor might be driven into position and not reach a depth sufficient for installation of a BOP. Usually secondary conductors are cemented to surface.