A supply boat was to off load Oil Base Mud (OBM). To enable access to the OBM hose take-off point on the starboard rail of the vessel several pieces of cargo had to offloaded. Initially the vessel’s Bosun and two vessel Able Seamen (AB) were on the boat deck. A rack was transferred from the starboard side of the metal plated deck area to the rig and the Bosun departed the deck. Then a framed wire line unit was prepared for transfer. Tag lines were attached to the wire line unit, the lifting gear checked and the two AB’s started to walk away from the load to wait for the crane hook to be lowered. As one of the ABs turned his back to the unit, a large wave or swell impacted on the port stern quarter and caused the vessel to roll to starboard causing the wire line unit to suddenly move across the deck. One AB felt the unit begin to move and shouted a warning but the other AB was struck on the back and catapulted across the metal plated deck where he was pinned. The vessel then rolled to port and the wire line unit slid back across the deck. The injured AB fell to the deck unconscious and the other AB rushed across to the injured AB to help but realized he was very badly injured and since he did not have radio communication he went for help. The injured died from his injuries.