Loss of subsurface buoys

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. 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.

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.