Case Study – Closed Bus Ruined the Day

1. Overview

A DP 2 Cable laying vessel was working closed bus within a wind farm, two out of the four generators were connected to the bus.  Each connected generator was loaded to approximately 55% kW load, as stated in the vessels ASOG/TAGOS.

2. What happened?

An alarm was activated on the vessel management system indicating to the operator that there was a problem with the power balance on the bus along with bus frequency alarms.  The operator in the ECR observed that one generator was at full load while the other generator was at negative load, shortly after this the thrusters started to phase back, causing the vessel to drift off position.

The normal operating frequency of the bus is 50Hz.  An observant control room operator would have observed that the frequency on the bus was now reducing to 43Hz, however as the thrusters phased back the frequency would rise and the thrusters would attempt to ramp back up, as they did the frequency would fall again, this cycling effect was causing multiple continuous alarms.  The control room operator took the decision to e-stop the highly loaded generator, the vessel immediately blacked out.

The vessel lost heading and position resulting in damage being sustained to the cable, vessel and a monopile when  the vessel made contact with one of the monopiles located in the field.

3. What can be concluded?

The following can be concluded:

• The vessel did not have the necessary protection that would automatically open the bustie, isolating the fault to a single redundant group.  Which would have equaled to the WCFDI.
• The vessel did not have the necessary protection functionality to disconnect the faulted generator, this would have prevented a blackout.
• Operating closed bus within the wind field with fixed structures was clearly not considered a Critical Activity Mode situation and as a result having a closed bus had significant consequences.  The vessels FMEA should have guidance as to the limitations of operating in closed bus, the DP operations manual along with the ASOG should give guidance of operations that can be conducted with closed bus configuration.
• The operator should have manually opened the bustie – In case of uncertainty, opening the butie would have averted the blackout by isolating the problem to a single redundant group.  If a vessel is operating closed bus it should have the adequate protection in place to isolate a fault and stop it propagating to other redundant groups, it is preferable to have inbuilt automatic functions to perform such actions.

• Why did the operator removed the wrong connected generator? – The operator has decided to remove a generator, they had two options, the question is which generator they should remove at this time.  At the time, the root cause of the issue was unknown.  The operator was presented with an excessive amount of data and background noise.  Upon observing one engine operating at 100% load, they assumed it was the culprit and stopped the engine.  If the operator had observed what was occurring with the bus’s frequency, they would have recognised that a low frequency was being experienced, this can only occur in one way.  Frequency is a function of the rotational speed of the alternator, and the engine governor is responsible for regulating engine speed. In this case, one generator was displaying a negative load on a low frequency bus, indicating that it was the faulty generator because the frequency was being physically lowered – low frequency low load ‘must go’. However as stated above it is preferable to have inbuilt automatic functions to perform such actions.

4. Relevant guidance

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

• IMCA M 117 – The Training and experience of key DP personnel
• IMCA M 220 – Guidance on operational planning
• IMCA M 252 – Guidance on position reference systems and sensors for DP Operations