Duty/not duty, which is the question?

This DP event occurred on an FPSO. The FPSO is fitted with one bow tunnel thruster and two stern azimuth thrusters; however, at the time of the event the starboard aft thruster was out of operation due to an ongoing upgrade project and the forward thruster had been stopped and isolated due to diving operations occurring at the time.

Diving was being undertaken from a light diving craft (LDC). The FPSO was lying with the wind to its side in order to shelter the LDC during the diving operations. The FPSO turret was locked in line with the diving isolation matrix.

The engineer on duty was completing planned maintenance and had a routine job to grease the port azimuth thruster steering gear pump, 2.

The engineer went down to the local controls of the port azimuth thruster, the steering pump that was due to be greased was running and set up as the duty pump, so the engineer switched the control over to place pump 1 as lead steering pump. As he switched the duty pump over pump 2 stopped and pump 1 started, as per the logic; however, the pressure dropped after stopping the running steering pump and the low-pressure alarm did not clear within the time limit set in the logic. The system shut down running pump 1 as a protective measure, leaving the port azimuth thruster with no steering. Pump 2 did not auto-start.

With no steering control of the port azimuth thruster, and no other thrusters operational, the heading of the FPU could not be maintained.

The Central Control Room (CCR) directly notified the LDC diving supervisor that they had lost power on one thruster and the diving operations were aborted. The divers were able to safely return to the surface without incident, whilst the LDC coxswain monitored the FPSO heading.

Figure – Setup prior to loss

Investigation of the event concluded that:

• The quick response of the CCR to notify the LDC meant for a swift recovery of the divers to the craft with no incident; however, at the time of the event the CCR were not actually aware of whether the divers were in the water or onboard the LDC.
• The CCR did not relay the entire severity of the incident. No alarm was raised onboard the vessel.
• Due to the routine nature of the planned maintenance, insufficient risk assessment had been conducted.
• There was no consideration for if a failure were to occur and how this would affect the heading control.
• A thorough risk assessment of the diving operations had not been considered, including emergency preparedness.