GB1441165A - Motion-compensated crown block system - Google Patents

Abstract

1441165 Maintaining constant tension in cable OCEAN SCIENCE & ENG Inc 9 July 1973 [26 July 1972] 32671/73 Heading B8B [Also in Division E1] A motion-compensated crown block sheave system for use in a drilling rig on a vessel comprises a crown block sheave 25 carried for movement along a straight path by a framework 22, and fluid pressure piston and cylinder means 55 pivotally connected to the sheave and the framework and inclined to the said path. The cylinders are connected to accummulator 69 and the pressure therein is controlled by means sensing movement of the sheave relative to the sea bed; such means may include a line (90) connected to the sheave 25 and to a sea-bed anchor (95) or riser pipe (107), Figs. 9, 10, 10a (not shown). Because of the inclined arrangement of the cylinders the required pressure changes are minimized, since as the sheave moves the inclination changes so that the vertical component of the force exerted changes in the opposite sense to the change in total force due to change in volume of the cylinders. This effect may be augmented by providing further fluid pressure cylinders 128, Fig, 8, which act downwardly when the sheave 25 is at its lowermost position and upwardly when it is raised with respect to the framework. A dashpot 73 is connected to the sheave and the framework; alternatively damping means may be incorporated in the cylinders or be fixed to the framework, Figs. 15, 16 (not shown). Idler sheaves 32 over which cable portions 33, 34 pass after leaving the sheave 25 are connected to the sheave 25 by links 85 and are either connected to the frame work by further links 80 or are slidable laterally in rails 137, Fig. 8, so that the distance between sheaves 25 and 32 is constant and wear on the cable is minimized.