What happened:
The operation was pulling the BOP and the drill crew were disconnecting a riser pup joint which meant removing the riser bolts completely.
While removing one of the bolts a roughneck dropped it onto the riser spider. Two half moon covers had been fitted between the riser joint and riser spider but there was a gap between them and the bolt fell through the gap and landed on the mezzanine deck below.
Weight of bolt was 20.7 kg and the distance of fall was 7.8 metres.