Unexpected Descent and Ascent of Mattress Lifting Frame

Description of Incident

A subsea load close to working divers started moving with no intervention from the crane operator. The incident occurred during movement of concrete mattresses using a mattress handling frame and the Main Crane. The frame was held with the vessel crane set-up in Active Heave Compensation (AHC) mode, at approximately 3m above seabed. Divers were in the process of reconnecting the hooks to the mattress handling frame ahead of recovery to deck when the frame was seen to descend to 1m depth above seabed.

An “all stop”, followed by a “come up on thecrane” command was issued and within 3 seconds of the command the frame was observed to ascend at anunexpected rate back to its original position approx. 3m above seabed without crane operator input on the joystick. Both Diver 1 and Diver 2 were in close proximity to the frame just before it ascended. There were no injuries nor damage to equipment or subsea assets.

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