On 12th November, the operation was rigging up of Client 3rd party (Gasway) gyro sheave, after finishing TBT, derrickman was sent to monkey board to install the sheave on DC finger as per Gasway JSA and previous job performed, at the same time the drill floor crew along with the third-party were preparing rig up of the sheave for installation. Once the sheave rigging was complete, the floorman was instructed by Driller to raise the sheave to monkey board level for installation by DM as agreed in the TBT. The plan was to hang the sheave on DC finger with the sling suspended from the hook. Derrick man was already present on monkey board when the sheave was lifted to the monkey board. DM assumes that the sheave is hanging with the winch line directly. Under the wrong impression DM opens the shackle and this results in the sheave come free and dropped to the rig floor. Weight of the sheave 22.9 kg fell from a height of 25mts.