A top drive well drilling installation comprising a drilling tower, vertical rails supported by the drilling tower, a trolley guided along said one or more vertical rails, and a hoisting device for moving the trolley up and down. The installation further comprises a tubular stem which is to be connected to the upper end of a drill string for rotation therewith about an axis of the drill string, and a top drive unit including a motor adapted to rotate the tubular stem and thereby the connected drill string to drill a well, wherein the top drive unit is supported by the trolley. Furthermore an elevator is provided which is adapted to—in an operative position—suspend the drill string, e.g. during tripping, and an elevator support assembly is provided which is adapted to absorb the load of the suspended drill string and adapted to move the elevator between an operative position on the drill string axis and a retracted position. According to the present invention, the elevator support assembly is embodied as a direct drill string load bearing connection between the elevator and the trolley, independent from the top drive unit.
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