Loss of subsurface buoys
- Safety Flash
- Published on 14 August 2007
- Generated on 24 July 2026
- IMCA SF 07/07
- 2 minute read
A Member has reported an incident in which a number of subsurface buoys – used as part of the anchoring system for a semi-submersible accommodation unit – broke loose and drifted away.
What happened?
Due to operational restrictions on certain fields it had been necessary for certain operations to add fibre inserts, wire inserts and/or subsurface buoys to keep anchorlines over pipelines and the concrete base structure of fixed platforms.
For two such operations the field layout was such that these buoys were underwater when the submersible accommodation unit was stood off from the platform, whereas when the submersible accommodation unit was alongside, the rigging of the anchor chains meant that some of these buoys came close to the surface.
Mooring of submersible accommodation units needs to ensure sufficient clearance to pipelines both in alongside, gangway-connected mode and in stand-off position when the rig is winched away from the fixed platform.
Since the submersible accommodation unit was in the alongside position most of the time, these buoys were subject to wave loads throughout the operation. As a result, on both these operations some of the buoys close to the surface broke loose and drifted away.
Actions
The company developed an alternate technique using ‘chain through’ type buoys, which improved the operation of buoys close to the surface.
The mooring chain passes through the centre of the buoy and is secured on each side.
This removes the need for a pennant,which is often a weak link, and also places the buoy much deeper and clear of surface wave action.
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.