Abstract: The present invention has at least one rack chock which is supported pivotally and tiltably at one end by a pin and cross head assembly located at a position between at least one leg rack and the hull of the platform. The other end of the rack chock is connected to the jack foundation with an actuator assembly. The pin and cross head assembly is disposed slidably within a guide for permitting the teeth of the rack chock to engage the leg rack teeth without visual monitoring and manual control. The actuator assembly connected to the other end of the rack chock controls the swinging movement of the rack chock and its engagement and disengagement with the leg rack. A pair of jacks urges upper and lower wedges with tiltable surfaces converging onto the rack chord's upper and lower surfaces, thus obtaining a optimal surface to arrest the relative movement between the rack chock and the leg rack.