Uncontrolled ascent – loss of lift bag to surface

A member has reported the loss of a lift bag to surface.

During a surface-supplied diving operation from a dynamically positioned diving support vessel (DP DSV) in approximately 24 metres of seawater, a 10 tonne parachute lift bag had been attached to a pipeline as an installation aid in accordance with the member’s procedures as part of a sleeper rectification program. At the previous site a successful/uneventful installation had been accomplished only a few hours earlier using 2 x 5 tonne and 2 x 10 tonne lift bags.

The bag had been partially inflated in order to obtain a vertical position and volume sufficient to allow an inspection prior to any additional inflation. At this point the diver observed and reported a shift of the bag upwards relative to the rigging strops secured to the pipe. Shortly thereafter, the bag slipped away from its rigging and ascended to surface.

• The diver reported the loss of control immediately and the bridge was notified;
• The main rigging and inverter line remained attached to the pipeline;
• It was determined that the shackle joining the top end ‘cradle ring strap’ had mistakenly been removed 4 hours earlier when the bag had been retrieved to deck from the previous deployment.

The member reported that no injuries had been sustained and the lift bag had been damaged, but the diver could have potentially experienced an embolism or have been fatally injured if he had been dragged to the surface.

The member noted the following root causes:

1. A lack of awareness on the part of the rigger who had de-rigged the bag;
2. A detailed pre-deployment inspection regime had not been in place;
3. The cradle-ring-strap and single pin shackle arrangement at the top of the bag had ‘routine’ly been used as a common securing point for both the main rigging and the inverter line which represented a single point failure;
4. The single pin shackle had been easily removed by hand as it did not require a tool, nor did it have any distinguishing markings (e.g. paint) to indicate that it formed an integral component part of the cradle-ring-strap configuration.

Rectification taken and recommendations made by the member company:

1. Activities were ceased and an investigation carried out which included two onshore managers;
2. The model of bag used was quarantined until further notice;
3. A pre-deployment model specific check list was produced for a similar model lift bag which utilised a continuous loop cradle sling;
4. A ‘stand down’ was held and personnel involved in all phases of lift bag usage, including the handling and deployment, were familiarised with the safety critical elements and specifics of the equipment;
5. Black Box footage of the event was to be developed into a training aid and lessons learnt material;
6. Communicate to vendors the issue of an inherent single point failure;
7. Communicate to the vendors the need for manufacturer and model specific check lists and this was to be included as a future requirement in the member’s purchase procedures. The member noted that the manufacturer supplied checklist with the bag was a generic check list found in industry guidance documents;
8. Review and update the company work instruction for the use of lift bags and include lessons from this incident and any relevant improvements forthcoming by the vendors.

Members are reminded of IMCA guidance – Guidance on open parachute type underwater air lift bags – and of recent IMCA safety flashes 02/08 and 15/08.