Drilling rigs are machines that are used to drill a wellbore at oil and gas reserves for the extraction of hydrocarbons. In onshore drilling, drilling rigs include virtually all components required for the drilling process. While in offshore drilling, drilling rigs include the same components as onshore drilling rigs, but not those of the drilling platforms or vessels themselves. The major components of the drilling rig include a mast or derrick, mud pumps, mud tanks, rotary tables, drillstring, draw works, power generation equipment, and other auxiliary equipment. Oil and gas upstream companies hire drilling rig contractors to drill oil and gas wells.