Single point failure resulting in a dropped object

What happened

A small part weighing 0.3kg fell 20 meters from an OMC-160 wind speed and direction sensor installed on a vessel’s main mast. Two wind speed and direction sensors were installed on the vessel’s main mast; they consisted of a wind speed sensor and a wind direction sensor. The wind direction sensor was of the wind vane type unit and had a counterweight (length 6 cm/diameter 3 cm; 300 grams) fixed to it. This counterweight was only secured to the wind vane by a single grub screw.  The grub screw came loose causing the counterweight to fall off. There were no injuries, nor any damage to the vessel or equipment.

What went wrong

• The counterweight was secured with only one screw, and this single point failure was not identified during the previous DROPS surveys;
• The OMC-series wind sensors are designed for minimal maintenance and as such were not part of the vessel’s planned maintenance system;
• The original grub screw worked loose because of vessel vibration.

Preventative action

• The crew secured the counterweight by drilling one more hole on the opposite side of the existing one, and thus used two new screws to secure it. In addition, Loctite was applied to secure these screws;
• After checking for similar situations on the main mast, the same modification was done to the counterweight on the second direction sensor;
• Check if there is anything similar in your work area that could fall from height following a single point failure, or, is not included in the vessel planned maintenance system – check all instruments on masts;
• Check fixings at height very thoroughly. Particularly worthwhile and important when access is difficult.

Members may wish to refer to

• Dropped object: Tank dog [grub screw holding the anchor pin in place had worked loose]
• High potential dropped object near-miss: antenna fell to deck [grub screws came loose]
• Dropped object near-miss: Antenna parts worked loose and fell to deck