ES417211A1 - Motion compensated crown block system - Google Patents

Abstract

A motion compensated crown block system in which a crown block sheave means is movable along a vertical path defined by a framework usually forming part of a drilling rig which is supported on a vessel. The crown block sheave assembly, which carries the hook load, is principally supported by inclined fluid pressure cylinder and piston means having one of their ends pivotally connected to the sheave means and their other ends pivotally connected to the framework in lateral spaced relation to the generally vertical pathway of the sheave means. Changing inclination of the cylinder and piston means during relative movement of the crown block sheave assembly and vessel provides variation in the piston reaction vertical force component which is caused to be nearly proportional to the change in fluid pressure in the cylinder and piston means. Load variation of less than 5 percent of the hook load is achievable over the full path of travel of the sheave means and is in the order of 2 percent over a major portion of said path in which the crown block sheave assembly travels. Means for sensing motion of the crown block sheave means relative to the sea bed are also provided so that the crown block sheave means will be virtually stabilized to maintain unchanged relation with respect to the sea bed during heaving of the vessel as caused by wind and wave action. Means are also provided for reducing to a minimum travel of the sheave cables over idle sheaves during travel of the crown block sheave means in its vertical path.