A number of incidents have been reported where loads have been dropped during subsea operations, with the following causes being identified:
- Failure of mechanical latches;
- Modified ‘off-the-shelf’ rigging;
- Lack of understanding of the dynamics of subsea loads.
Below is a brief summary of a number of incidents.
1 – Dropped umbilical handling basket
An umbilical handling basket was dropped during recovery from the stern A-frame of a monohull construction vessel. Subsequent investigations showed that the lifting bracket had become detached. Another basket was similarly damaged. Previously the baskets had been deployed from a semi-submersible construction vessel where no incidents had occurred. The cause of failure is attributed to an inadequate appreciation of the low terminal velocity of the basket due to its high in-water drag. This resulted in snatch loads whilst lifting in a relatively high sea state from the stern of the monohull vessel. No guidance on operational sea state for types of deployment vessel was documented in the procedures. Remedial measures included strengthening the lifting bracket on the baskets and the specification of limiting sea states from future deployment vessels.
2 – Dropped clump weight
A 1 tonne displacement buoy connected to a 1.25 tonne clump weight became detached from a lift wire and fell 20 metres to the seabed. Subsequent investigation with an ROV showed that the 6.5 te shackle used to connect the load had become undone, with the bolt, securing nut and locking R clip missing. The load landed 1 metre away from a flowline jumper, which could have been damaged. The cause was attributed to an R clip becoming displaced from the shackle bolt, enabling the nut to back off and release the bolt. It is thought that vessel motions transmitted to the lifting wire plus high drag low weight of the combined load enabled the shackle R clip to contact the load and become displaced. Subsequent movement then rotated the bolt to the point where the load became detached. The contractor involved advised that all lifting operations should be conducted within sea state limits identified in the risk assessment for the specific vessel/load/rag combination. It also recommended that conventional R clips should not be used to secure shackles as they can become dislodged by the load and that split pins bent fully back or some equally secure alternative was preferred.
3 – Dropped clamp and buoy
A 225 kg clamp plus buoy was being transferred from a crane hook to a subsea winch hook in a moonpool of a semi-submersible construction vessel when the load was lost. The incident was attributed to the use of a single master link in the rigging. Two links should have been used, one for each lifting device. The load transfer procedure was modified for future operations of this kind.
4 – Modified latch failure
An incident occurred when an ROV was inadvertently released from a TMS that had a modified latch mechanism. The ROV and TMS were on deck and in the process of being latched into the A-frame when the ROV fell approximately 0.5 m onto the A-frame deck. No one was hurt and damage was minimal. A full investigation is underway but initial conclusions are that some of the latch rollers could have been seized in the open position. The contractor involved has instructed a check of latching mechanisms.