High potential near miss: Dropped object during piggyback drilling operations

  • Safety Flash
  • Published on 16 July 2019
  • Generated on 20 June 2025
  • IMCA SF 17/19
  • 2 minute read

During piggyback drilling operations, a driller inadvertently operated the sample winch control lever instead of the power swivel fast rotation control lever as intended.

What happened?

Activating the sample winch caused a tool to rise from its storage position, resulting in it snagging on a safety hoop of the adjacent vertical fixed access ladder.  

The tool (weighing 11 kg) separated from the wire and fell approximately 8 m onto the drill floor below. There were no injuries and no equipment was damaged.

During piggyback drilling operations, a driller inadvertently operated the sample winch control lever instead of the power swivel fast rotation control lever as intended

What were the causes?

  • The operating driller moved the wrong lever unintentionally.

Investigation noted the following:

  • The tool (an ‘overshot’) was connected to the sample winch wire via a weak link shear pin designed to separate at 450 kg. The sample winch has a working load limit of 1.2 tonne.

  • The experienced driller had not operated this particular type of drill rig control console for several years; however, he had received a short period of familiarisation training during the previous shift under the guidance of the back-deck supervisor.

  • The control console ergonomics and inspection and maintenance requirements had not been properly considered:

    • the possibility of inadvertent operation of the wrong lever had not been identified when the piggyback drilling equipment was installed some months before the incident

    • there is no evidence that this had been taken into account (in 1995) when the piggyback drill was designed.

  • There had been ineffective checking and assessment of the suitability or fitness for purpose of this equipment.

What lessons were learned?

  • Undertake a design review and complete a design risk assessment on all similar drilling rigs:

    • to include the operational and human interface with the drill rig and associated equipment

    • planned preventative maintenance check sheets should be updated to reflect the findings of this assessment.

  • Control consoles should be clearly labelled to identify all controls and their function.

  • Develop a thorough familiarisation assessment process for the operation of this kind of equipment.

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