A rig worker was in the process of cutting drilling line. The 500-ton blocks had been hung on the off-driller side of the derrick with a 7/8″ wire rope loop secured with four U-bolt clamps. As the worker pulled drilling line off the drum, the U-bolt clamps on the wire rope look slipped. The blocks fell about 3-1/2″ causing the worker to be thrown toward the drum. He struck the equipment in the drum area and received multiple serious injuries.