Firefighting (FiFi) Tank Outlet Blockage
- Safety Flash
- Published on 23 October 2020
- Generated on 14 February 2026
- IMCA SF 30/20
- 2 minute read
What happened?
A member reports a blockage of the outlet of a dive chamber fire fighting water tank, caused by the failure of a floating plug.
On this particular DSV the saturation diving chamber complex has six firefighting water tanks. Each tank has a water outlet at the bottom with a floating plug that blocks the outlet if all the water drains from the tank. See line drawing of the bottom of one of the FiFi tanks.
The floating plug assembly consists of a buoyant float and a nylon conical endcap plug with recessed O-ring, which sinks in water. The floating plug assembly is highlighted red in the line drawing. The float is connected to the endcap plug with a threaded rod through the centre, as shown on Figure 3. The floating plug is retained in the tank by a guide tube.
What were the causes?
The endcap plug unscrewed itself over time and eventually detached from the float. Because the endcap plug is heavier than water it sank to the bottom of the guide tube and blocked the tank outlet.
That would result in no water being supplied from the tank, and therefore it could have life-threatening repercussions in the event of an emergency. In normal circumstances it would only be detected during routine 6 monthly maintenance. (IMCA emphasis).
Actions
The floating plug assemblies were removed from all 6 x FiFi tanks on the vessel and the endcap plugs were permanently bonded to the floats to prevent this failure from happening again.
Each DSV should review the water storage tanks in their system for any similar failure mechanisms. This failure was identified in a dive system FiFi tank, it could equally occur in other water tanks in any vessel.
IMCA Safety Flashes summarise key safety matters and incidents, allowing lessons to be more easily learnt for the benefit of the entire offshore industry.
The effectiveness of the IMCA Safety Flash system depends on the industry sharing information and so avoiding repeat incidents. Incidents are classified according to IOGP's Life Saving Rules.
All information is anonymised or sanitised, as appropriate, and warnings for graphic content included where possible.
IMCA makes every effort to ensure both the accuracy and reliability of the information shared, but is not be liable for any guidance and/or recommendation and/or statement herein contained.
The information contained in this document does not fulfil or replace any individual's or Member's legal, regulatory or other duties or obligations in respect of their operations. Individuals and Members remain solely responsible for the safe, lawful and proper conduct of their operations.
Share your safety incidents with IMCA online. Sign-up to receive Safety Flashes straight to your email.