Flex rig

From Wikimarcellus

A flex rig is a specialized drilling rig with rails at the bottom to allow it to slide approximately 80 feet in one direction and 10 feet to the left or right without dismantling it. These rigs are designed to drill multiple wells on a pad without having to tear the rig apart and moving its pieces with a crane. Tearing a rig apart is an expensive operation, and using a flex rig's ability to move about a pad is a more economical option for drilling multiple wells in close proximity.