Dual draw works heavy hoisting apparatus

摘要

A dual draw works hoisting apparatus. A single derrick structure is provided for two drilling rigs. The crown block platform receives the crown blocks for both drilling operations such that both crown blocks are slidable along the crown block platform. The hoisting lines are reaved in a conventional manner, except for the dead lines. The dead lines from the two crown blocks are connected together by means of a sheave assembly which is locked in position during normal independent operation of the crown blocks and their respective travelling blocks. To provide increased lifting capacity for lifting the drilling riser, the crown blocks are moved closer together on the platform such that the travelling blocks may be attached to a spreader bar to be positioned above a drill slot and attached to the drilling riser. The sheave assembly connecting the two deadlines is unlocked to allow relative movement between the two deadlines during simultaneous use of the draw works for both travelling blocks. This allows the dead line loads to be equalized even though the separate draw works do not rotate at precisely the same speed.