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Rigging failure – clump weight dropped to seabed

What happened?

A clump weight used for anchoring a weather buoy was being deployed over the sea when the long link chain attached to the vessel crane hook failed resulting in the clump weight dropping to the seabed.

The 2.6t clump weight was the anchor component of a mooring arrangement for the weather buoy.  The load was rigged using a supplied mooring arrangement, which followed the manufacturer’s drawing in the weather buoy manual. Instead of a 450kg weight it was decided to use the 2.6 tonne clump weight already available and which had been shipped to the location for deployment.  A service specialist prepared a deployment procedure which was discussed with those involved in the task.  Shortly after the load was submerged in the sea the chain link attached to the quick-release mechanism snapped, and the load dropped to the seabed in an uncontrolled manner.

The findings revealed:

  • The rigging failed because a larger clump weight (2.6t) was selected than the design allowed for (450kg). This decision was based on previous experience where the standard weight (450kg) did not adequately secure the weather buoy;
  • The design and installation procedure were not revised to accommodate the heavier clump weight;
  • The chain selected was not suitable for lifting.  Declaration of conformity by the chain manufacturer confirmed that the working load limit was “2.5 Tonne Not for lifting”.


  • All planned deviations from procedures should be subject to a formal Management of Change (MoC) process, the risk of the change adequately assessed and the change authorised by a competent person before the work proceeds;
  • Reinforce to the riggers and personnel involved in lifting operations:
    • The need for thorough lift planning, and,
    • That they make a thorough check that lift rigging is certified, fit for purpose and appropriate for that specific lift.

Members may wish to refer to 

Safety Event

Published: 6 December 2020
Download: IMCA SF 33/20

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