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Failure of 64mm polyester rope in subsea mooring operations

What happened

A 64mm polyester rope parted during the lowering of subsea equipment to depth. The failure occurred between the Mooring Line Deployment Winch (MLDW) and the pennant, due to overload caused by lack of synchronism between the crane and the MLDW winch. The operation was immediately stopped. No-one was injured, but damage was caused: there was a break in the balustrade of the cable lowering ramp and damage to the window of the MLDW control cabin.

MLDW winch
Parted rope
Place where the MLDW operator stood

What went wrong

Although the MLDW operator three times raised his concern to the supervisor to pay out extra line to reduce the tension, this was not recognised as a STOP WORK or other signal to stop and re-assess the situation.  The supervisor was concentrating on moving the vessel into position as there were simultaneous operations (SIMOPS) taking place. A clear “STOP” was not used.

Our members’ investigation found:

  • Tension on the rope increased leading to it parting;
  • Company procedures were not followed;
  • Cranes were lowered without giving sufficient slack on the rope;
  • There was miscommunication and a lack of situational awareness during the operation;
  • No-one stopped the job – STOP WORK authority was not used.


  • Indicator installed to allow winch operator to see rope tension;
  • Better practice developed on co-ordination of simultaneous movement of crane and winch;
  • Reiteration of and retraining in, existing procedures;
  • Encourage all in their right and responsibility to use the STOP WORK Authority and how to exercise it.

Members may wish to refer to:

Safety Event

Published: 9 December 2021
Download: IMCA SF 34/21

Relevant life-saving rules:
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