The driller was using the top drive system to pick up a 50-foot slick pup joint of marine riser from the raised catwalk. When the riser reached an angle of about 30 degrees, two riser inserts fell about 15 to 20 feet to the drill floor. The pup joint was lowered back to the deck. No one was injured when the riser inserts fell. The 23-pound inserts should be held in place by circlips. When the riser joints are made up, the riser bolts screw into the inserts, connecting the joints together. The riser joints had recently had an annual MPI inspection by a third party inspector. The inserts and circlips were removed for the inspection. The inserts were reinstalled: the riser protector was then put in place. The riser was not inspected prior to picking it up from the deck to the catwalk as the riser is taken directly from the riser rack to the catwalk using the gantry crane. Upon inspection, it was found that the other four inserts had no circlips installed to hold the inserts in place.