Load sharing imbalance causes loss of position – DP incident

Comments:

The power plant was configured to work in closed bus mode. No 2 port, No 2 centre and No 2 starboard generator were online. No 1 port and No 1 centre generator were on standby. No 1 starboard generator was on long term isolation due to turbocharger overhaul.

The vessel’s power management system (PMS) developed a fault in an I/O module for No 2 centre generator. This fault caused the generator circuit breaker status to freeze in the ‘closed’ position. The alarm indicated a generator fault causing the engineer to disconnect the faulty generator. The breaker opened on the generator and due to the fault on the I/O module the status of the breaker was still showing connected and a load sharing imbalance was noticed by the PMS which attempted to correct it. As the disconnected generator could not take any load, the imbalance persisted and the PMS drove the bus frequency down to the point where it triggered the frequency based thrust limitation function on all thrusters leading to loss of position and an emergency disconnect.

Considerations from the above event:

• It is recommended that the failure modes and effects analysis (FMEA) should consider the PMS interface so that this type of failure mode is identified.
• It would appear the PMS was not properly tested for operations in closed bus tie operations.
• The fact that frequency issues occurred suggests advance generator protection was either not present or not functioning correctly.
• The vessel’s DP operations manual and well specific operating guidelines (WSOG) must be questioned.
• The value of the current factor (3.9kts) should be questioned.
• It should be noted that when thrusters are operated via the IJS they are still under the control of the PMS.