which is introduced into the torque track tt as a result of the rotation of top drive drilling units is resisted by the strong back frame arrangement which transfers most of the torsional loads and forces into substructure 24 rather than mast 22 one possible configuration and assembly for a torque track tt is disclosed in further detail in u s patent application ser no 217 689 filed mar 24 1994 the present invention provides improvements to the subject matter of u s pat no 4 865 135 referring to fig3 a system dd according to the present invention has a derrick 11 shown schematically by dotted lines the derrick 11 supports a block arrangement 13 which moves up and down the derrick the block arrangement supports a swivel 15 which is connected to a mud hose 17 the mud hose 17 will be connected to a source of drilling fluid top drive units 19 a and 19 b are also supported by the blocks below the swivel 15 in the embodiment shown each top drive unit contains an electrical motor within a housing 18 which is supplied with electrical power from the drilling rig the housings 18 also contain a drive mechanism connected to the electrical motor for rotating a drive stem 21 the drive stem 21 is adapted to be connected to the upper end of the string of drill pipe.