A member has reported a recent incident which occurred during diving operations. A dive team was engaged in removing rock dump from a subsea pipe, in order to position a hyperbaric welding chamber. A combination of manual rock removal and air lifting operations were being utilised to clear the area.
Approximately six hours into the bell run, there was a break in air lifting operations, to allow the divers to manually remove exposed rocks. The air lift was left in a vertical position approximately four metres from the divers. The divers intended to resume air lifting, so the air line was not vented. When the rock had been cleared, the decision was made to lay down the air lift to disconnect the air hose and crane, as the bell run was approaching completion. The crane began lowering the air lift, at which time the diving supervisor requested that the air line be vented. The air lift was lowered as the air line was venting.
When the air lift was at approximately 30 degrees to the seabed, the divers asked whether the crane was still coming down. It was confirmed that it was. The air lift landed in a controlled manner. As the air line was vented, a headache ball dropped from approximately 12-13 metres to the seabed – one metre from the working divers.
The company involved has identified the following contributing factors:
- The main contributing factor in this incident was the air line not having been vented when the air lift was laid down. The buoyancy in the air line resulted in an uplift effect, indicating to the diving supervisor and divers that the load was still on the crane. Consequently the crane wire was continually slackened, resulting in a catenary in the wire. When the air line was vented at the surface, the buoyancy in the 4″ hose was removed, resulting in the uncontrolled descent of the headache ball and excess crane wire;
- The diving supervisor had asked for the airline to be vented, but no confirmation that this had been achieved was obtained;
- The implications of the buoyancy in the airline were not fully understood;
- The company’s air lifting procedure clearly stated that the 4″ hose was to be bled prior to the air lift being laid down. Despite this instruction, the operation proceeded without confirmation that this step had been completed.
The company involved has noted that this incident demonstrates the importance of all employees engaged in a task having a full understanding of the associated hazards and of the implications of not strictly following safety procedures.
In addition, the company has set out the following guidance for its personnel on aspects to be considered when planning, risk assessing and carrying out air lifting operations, which others may wish to consider:
- Air lift operating procedures to include a step which requires confirmation from surface and divers that venting is complete prior to lay-down of the air lift;
- Air lift job hazard analysis should clearly highlight the hazard of buoyancy in the air line;
- Personnel conducting air lift operations should review this incident to increase awareness of the associated hazards.
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