Crew on a DP vessel were performing maintenance on the main power distribution bus circuit breakers; maintenance which was several years overdue. Additionally, the maintenance was conducted during a critical Outer Continental Shelf activity. In support of the circuit breaker maintenance, the vessel was transitioning from a ‘closed bus’ operation to an ‘open bus’ configuration with 50% of the vessel’s thrusters operating on each bus.
After opening the bus tie, a generator protection circuit failed to function properly, and this combined with a design flaw in a power transformer protection circuit causing half of the vessel’s thrusters to stop.
The vessel’s engineer attempted to restore power to these thrusters by closing the bus tie without synchronising two live buses (crash sync). Design features and operational procedures to prevent such an action and consequences were not sufficiently in place. The design deficiency allowed a power transient to cause a total loss of thrust and therefore loss of position.
Repair and maintenance Electrical system on DP vessel
1. The vessel did not have a defined Critical Activity Mode of Operation (CAMO). Ensure a vessel has a defined CAMO and is operating in its CAMO during critical OCS activities.
2. Ensure the DP operations manual and SMS both appropriately address DP equipment inspection, repair and maintenance requirements. A vessel should not perform maintenance that may cause a loss of position during a critical OCS activity.
3. An equipment failure, an operational error and multiple failure modes not identified in the vessel’s Failure Modes and Effects Analysis (FMEA) combined to produce the loss of position.
4. Ensure a structured competence assurance programme is applied to all key DP personnel. At a minimum, DP personnel should be required to demonstrate proficiency in understanding the redundancy concept and emergency procedures in intervening for failed redundancy. Intervention proficiency in restoring power and thrust should be demonstrated during drills and annual trials.