Snubbing units (sometimes known as hydraulic workover units [HWU]) are used to perform well intervention on live or dead oil and gas wells. They are used to drive pipe into a well bore and overcome well pressure that is preventing conventional drill rig operations. Snubbing units consist of hydraulic machinery to provide force on the drill string, as well as hydraulic jacks, slips, and a high basket serving as the control post.
Due to the size of most snubbing units, snubbing operations are costly to set up, and are typically used only for the most demanding interventions, or those requiring the well to be killed.
A typical snubbing unit. Note the unit's height relative to surrounding vehicles.
Image credit: RigZone