Description of Incident
When recovering a Subsea Xmas Tree, the load bearing capability of the wellhead connector was exceeded once the buoyancy effect of the seawater was removed.
The Wellhead Protection Structure (WHPS) sank to the seabed in an uncontrolled manner from the splash zone to land approximately 2m from the well centre. The WHPS was recovered at a later date.
To reduce the complexity of the recovery operations, a strategy to recover the WHPS, wellhead & tree as a single lift had been engineered. This required the WHPS to remain engaged onto the wellhead during the lift.
However, during the recovery the WHPS disengaged from the wellhead and sank back to the seabed. There was no other infrastructure remaining in the area. After following a management of change process lifting operations were resumed and the wellhead and xmas tree were recovered to the rig.
Later inspection of the WHPS (once recovered by vessel and returned to shore) led to the conclusion that the WHPS-to-wellhead connector had not been activated fully during its installation, prior to the well going on production.
Key Learning:
Equipment data gaps during decom planning are common but can often be bridged by additional engineering and risk assessment. If, however, the data gap cannot be resolved (in this case evidence of full activation of the WHPS-to-wellhead connector) a more conservative recovery plan should be used.
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