Hoist apparatus with dual mast structure and compound power transmission

Abstract

A hoist rig for lifting and lowering a pipestring is disclosed. The rig includes first and second vertical mast structures, a traveling block for engaging and transporting the pipestring, and a compound power transmission system for raising and lowering the traveling block and pipestring. The compound power transmission system includes traveling sheaves mounted on the extensible portion of the linear actuators, crown sheaves mounted on the upper ends of the mast structures, and first and second cable systems having end portions secured to the traveling block and having intermediate portions disposed in cooperative reeved engagement with the traveling sheaves and crown sheaves of each mast structure for displacing the traveling block in response to extension or retraction of the linear actuators. In a preferred embodiment, at least one additional linear actuator is secured to each mast structure, the piston element of each actuator being secured to the upper end of the mast structures, and the extendable housing elements being mechanically coupled to one another for a concurrent movement along the mast structures. In this preferred arrangement, at least one additional array of crown sheaves and traveling sheaves is provided and at least one additional independent cable system is cooperatively engaged with the additional array of sheaves to displace the traveling block in response to operation of the linear actuators.