Was involved in a project where the casing shear rams inadvertently activated, shearing the drillpipe while drilling. Now it is public on a government database I can share what was reported below in the EOWR. Seems the root cause was the EHBS / Autoshear system.
"Drilling the 12 ¼” section resumed from 3948.0 - 3961.0 mMDRT, at 3961.0 mMDRT the driller experienced a sudden loss in pump pressure from 3,600 - 110 psi and decrease in string weight from 560 - 300 klb indicating the drill string had parted.. The drill string was then pulled to surface and from drill pipe tally and visual inspection of the sheared drill pipe it was confirmed that the drill string had been sheared by the casing shear rams. Initial investigation indicated that the EHBS system had malfunctioned. As the exact cause was unknown, it was decided to bleed off the subsea accumulators before running any assembly through the BOP. This would render the EHBS system inactive but still permit BOP control from surface within permissible operating times.
The EHBS system was investigated in attempt to determine the root of the failure and the BOP/LMRP was function tested and serviced.
The software was analysed line by line and it was proven that the software could not fire the EHBS without external input from surface. On surface the EHBS sequence can be fired from the EDS sequence or the EHBS test mode. The retrieved data indicates that the EDS sequence was not activated. Therefore the only possibility is that the EHBS test mode key was turned hence activating the system. If the key is only turned 35 degrees and flicked quickly it will return to the normal position but still fire the EHBS sequence. This action will also not register on the event logger, hence there will be no record of the event.
Maintenance activities performed on the BOP during this time included the ram blocks being removed and re-dressed, chart testing of the hydraulic operators, a new flex joint and LMRP connector was installed, the mandrel on the lower annular upper housing was replaced and the blue and yellow MUX cables were also replaced.
The decision was made to continue drilling with ROV intervention required to complete the EHBS sequence.