During piggyback drilling operations, a driller inadvertently operated the sample winch control lever instead of the power swivel fast rotation control lever as intended. 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 11kg) separated from the wire and fell approximately 8m onto the drill floor below. There were no injuries and no equipment was damaged.
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 450kg. 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 actions were taken?
- 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.