Around 01:04 am,20.July 2019, Rig 111 was laying down 4” drill pipe. One of the singles was put into the power catwalk groove. Then, the driller coach lowered the travelling block until the DP lay in the groove of the trough, and open the elevator and continued lowering down the elevator. But the DP was still hung on the elevator with the residual contacting half inside elevator, which caused the DP box end continue to be lowered and the pin end of DP raised. When lowering the elevator, the friction force suddenly released, and the pin end of the DP rested on the back board of power catwalk and slipped down through the gap between the power catwalk skate cap and top of back board and dropped to the pipe rack area.