Un-commanded Lowering of Crane Boom and Hoist Function

The crane involved is a StahlProdukter offshore pedestal-mounted rope luffing crane.

The installation deck crew were conducting routine deck operations with the Crane.

Work was being conducted under routine crane operations permit. The deck crew were in the process of transferring work baskets at the forward end of the vessel.

An empty small work basket (weighing 200kg) was lifted from the main deck bunker station on the starboard side and transferred to a landing location forward of lifeboat 1 on D deck. The basket was safely landed on D Deck and left on the hook with bridles raised whilst the load handlers loaded the basket with scaffolding equipment.

The banksman noted the hoist and boom had lowered – he assumed the crane operator had given some slack to account for any vessel motion. The Banksman turned to view load handlers briefly, and on looking back at the crane, he observed that the boom and hoist had lowered and that the bridles had folded into the basket, and the headache ball was just above the basket.

Banksman informed the crane operator, who stated the joystick had been deactivated and observed that a PLC (Programmable logic controller) failure light had illuminated on the crane cab panel. The boom was raised using the joystick, the load disconnected, and the crane returned to the rest without issue.

Findings:

• PLC failure led to an unsafe condition whereby the crane boom and hoist wires were paying out un-commanded.

• The failsafe brake solenoid valves were energised, causing the brakes to be released.

• A spurious current signal would have to be received by the hoist & and luffing lower proportional control solenoid valves, which results in the crane boom and hoist being ‘driven down’.

• The PLC did not fail safe - A PLC watchdog function was available but had no ‘executive action’ on PLC fault condition.