Single Screw Causes $2.2M in Damage

A recent bulk carrier accident was traced back to a single screw.

On July 5, 2020, at about 2:50 in the morning, the 736-foot-long Atlantic Huron self-unloading bulk carrier struck a pier in the Soo Locks in Michigan. Luckily, no one on the 25-person crew was injured, but the accident caused $2.2 million in damage and triggered a National Transportation Safety Board investigation. The accident was traced back to a single screw. 

According to the NTSB, as the Atlantic Huron approached the locks, the crew tried to slow the vessel down, but a problem with the controllable pitch propeller (CPP) system caused the ship to increase speed instead of slowing down, hitting the pier at 6.8 knots - about eight mph. 

The vessel, which weighs 36,920 metric tonnes, was moored. The Atlantic Huron suffered more than $1.6 million in damage, and the pier sustained about $573,000. 

The NTSB investigation found that the crash was caused by a maintenance error and the incorrect installation of a single set screw. There is a reason they tell you to follow the manufacturer's requirements, no matter how simple they seem. 

Blades in a controllable pitch propeller aren't fixed in position but rather fastened to the hub so they can rotate and change pitch. The blade pitch determines the vessel's speed and direction.

NTSB investigators traced the problem to a small set screw installed in the piece of the controllable pitch propeller that controls pitch. 

According to maintenance records, the set screw was last removed and reinstalled more than four years before the accident. So, it's not really on the current crew. 

Throughout the investigation, technicians found no evidence that manufacturer-required thread-locking fluid had been applied. As a result, the set screw backed out, beginning a series of mechanical failures that resulted in the ship moving ahead and increasing speed when it was supposed to be doing the opposite.

The investigation also found that the crew was improperly trained on responding to a loss of propeller pitch control. The NTSB suggested that better training and documentation could've helped the crew react more quickly to the situation. 

The Atlantic Huron reached 7.1 knots before the captain cut the engine and dropped the anchors. Despite their efforts, they still caused quite the collision.  

The ship was built back in 1984 and is owned by CSL Group, operated by V Ships Canada.

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