Definition - What does Upper String mean?
Upper String is the upper portion of a drill string. A Drill String is a string or a column of a drill pipe present on a rig which is used to transmit torque and drilling fluids to a drill bit. Upper string works as a hanger device to hang different tools. A drill string is hollow inside hence it allows drilling fluids to be pumped down and circulated back up to the annulus. It is a combination of the bottom hole assembly, drill pipe and other tools that are used to make drill bits turn at the bottom of the borehole.
Petropedia explains Upper String
A complete drill string assembly and all its parts, i.e., upper string, lower string, etc, is supported by a top drive that allows it to advance to the downhole . The top drive also rotates the drill string assembly on the surface. It is used to maximize the hanging length and strength by using a lighter drill pipe on the bottom for reducing its weight and a stronger drill pipe on the top where the bending stress is higher. The Drill String is run using a dual elevator system that uses two heavy lift elevators for supporting a pipe-tool joint instead of the kind of tooth that dies in slips.