Case Study – Maintenance – Not a Good Idea During DP Ops

1 Overview of Event

An Offshore Supply Vessel (OSV) was conducting an ROV survey in open water, the bustie was closed creating a single power grid, as per Task Appropriate Mode (TAM) configuration.

The OSV was in Auto DP using 2 Position Reference Systems (PRS’s) selected into DP Control all with automatic weighting, the PRS’s selected were:

• x2 DGNSS

The power plant was set up as below with 2 generators (G2 & G3) connected.

During routine maintenance on Tx1 the 480Vac bustie was closed, and the two feeder breakers for Tx1 were opened to allow Tx1 to be isolated.

G2 & G3 were connected to the 690Vac switchboard.

Upon completion of the maintenance it was noted that the 690Vac feeder to Tx1 would not close.  Troubleshooting determined that a Port 690Vac switchboard ‘REF’ protection relay was faulty and needed to be exchanged.

The vessel continued conducting DP operations.

It was decided to exchange the ‘REF’ relay.  After remounting the replacement relay and in the process of terminating the new relay, an unknown live wire inadvertently contacted a surface and grounded.  This grounding caused the remaining 690Vac circuit breakers on the Port bus to trip and open, since they were connected via “daisy chain” configuration by design for protection purposes.  This then caused the bus tie to open as per design in order to protect the Starboard power bus.  Port Azi and BT1 failed as a result.

The bus tie opened upon sensing low voltage on the Port bus.  No loss of Position was experienced. 

2 What can be concluded?

• That the vessel was configured in such a way that a single failure would exceed the WCFDI.
• The event occurred as a result of the decision to replace a critical switchboard component while remaining on DP.

3 Additional Comments

• This was recorded as a Human Factor triggered event.
• The power system was configured as per a TAM presumably with the knowledge and understanding of all stakeholders that the vessel could lose position.  The TAM may have been agreed, but it’s unlikely that the increased risk caused by conducting maintenance would have been.
• It was not clear if an ASOG was in place and if it was being followed.  While operating in TAM will allow for a lesser fault tolerance, the system integrity has been further compounded by  vessel maintenance activities.
• It is not clear whether the closed bus configuration had been fully analysed in the FMEA.  If so then the analysis appears to have missed the failure mode associated with common power supply.  If not considered within the FMEA then the decision to operate closed bus had additional risk of unknown failures.
• The event highlights the risk of undertaking maintenance / repair of components critical to the integrity of the DP system (in this case the main switchboard).  It also highlights the importance of management of change procedures.
• This case study highlights the dangers of working on a live switchboard and also highlights the risks of not ensuring appropriate isolation of electrical components.  This event could easily have resulted in personal injury.

4 Guidance that would be relevant

The following IMCA Guidance would be relevant to this case study:

• IMCA M220 – Guidance on operational planning