I am currently working on a assignment and would like some outside hints help and pointers some of you may have regarding this scenario?
The questions goes as follows:
"As a marine engineer working for a ship owner you have been called to investigate into the reasons for one of the company’s vessels suffering an electrical failure. The bulk carrier was maneuvering into the port of Antwerp when it lost power as the switchboard breaker for No 3 generator tripped out suddenly and then the other running generator (No 2) was also disconnected due to an overload.
Power was restored quickly but the Anchor had to be used in an emergency to bring the vessel to a halt. You will need to travel to meet the ship from your office in central London but Captain has already reported that the ship does not appear to have suffered any damage to its structure. The Chief Engineer has however reported that the generator running at the time has been checked, runs OK but will not now connect to the main electrical switchboard, leaving the ship short of electrical power for the next part of their journey.
Upon your arrival you uncover further issues that lead to the root cause of the electrical failure, when you discover in the log book that the switchboard “preferential tripping” system had not been tested for the past three months. Discussion with the engineers has also identified that the essential tripping did not operate quickly enough, in reducing the electrical load on the single remaining generator. As a consequence, this engine stopped due to its overloaded condition causing the power failure. Answering the assignment Your answer should be in TWO parts with the first part giving a critical analysis about your preparation for and proposed sequence of actions to be taken during the investigation of this incident.
The second part should consist of a professional style report complied for your senior managers commenting upon the : • Root cause or causes of the failure of the generator and why the power generation system did not work, in the way that it was designed to • Likely components, that will need replacing, on the switchboard • The extent of the disruption to the operation of the vessel while the repairs are completed • Sequencing of the work required
• H&S issues to be followed and who will supervise the work"
Thank you in advance for your assistance…
While I enjoy a good troubleshooting problem as much as the next guy, I’d hazard a guess that the point of the assignment is to get YOU to use what you’ve learned to critically think it through.
If you want some assistance perhaps you could start by posting what you think the answer is first.
Sorry if it my post came through in the wrong way. I should have probably asked targeted questions.
I am guessing there was probably fault with the preferential tripping system which did not reduce load to Gen2 when Gen 3 tripped out. What i am unsure of is what are possible reasons for Gen3 to trip out so suddenly? Except for crew overloading system by starting machinery without having enough power to run them? or is this just part of a larger troubleshooting that would need to be tested onboard?
And how long would breaker replacement/repair and preferential tripping system testing and repair usually take? I am not well versed in electrical repairs so figuring out the disruption to the vessels schedule is stumping me a fair bit.
Taking the second part first, breaker replacement can be fairly quick and straightforward depending on the type of breaker system. Many Marine switchboards use breakers that can be racked out, that is, from the front of the breaker with special crank tools it is physically disconnected from the panel and can be slid out on tracks like a drawer from a cabinet. Assuming there’s a spare main breaker onboard (and there really should be, the tankers I worked on stored it at the end of the console), it’s a quick swap. Arguably the longer part of that evolution is planning the procedural safety aspect.
Alternately I’ve worked on some poorly designed switchboards that had bolted in breakers. In that case you’d need to add significant time to LOTO the entire board segment and properly deenergize so you can open rear panels and physically unbolt the bad breaker. Again, H&S planning will take time.
It sounds pretty obvious that there is a problem with the preferential tripping system, known by the crew and unreported. When thinking of how to test that system, consider how it works. If you have an overload situation you have too much current. The preferential trip mechanism needs to sense that current, right? How? And how could you simulate high current without actually overloading the system again?
That would depend a lot on if the current sensing system uses a current transformer or shunt. For AC I would guess the former and if you can figure out the ratios and voltages you can probably rig up a fake signal to have the current be whatever you want. Shunts too can be gamed like this, but I have only seem current transformers on AC loads.
Maybe this doesn’t scale to big ships, but we used to rent load banks on trailers to connect to generators to test them.
As for time required to replace a main breaker, it does depend on several factors, primary of which is the age of the vessel and availability of spares. Many ships 10+ years old may not have a spare breaker on board, and chances of locating an exact replacement may not come easy. Like any mechanical product, obsolescence is a factor. Sure, Mitsubishi, Schneider, Terasaki, or any other manufacturer may still be in business, but a drop-in replacement could not be available. That means modification of the switchboard and subsequent unknown time required. It could be 6 hours, could be 6 days. It depends on location of the ship (Antwerp has qualified trades available on short notice. Other ports could vary considerably). Some breakers can be racked out, others are stab in connection, and yes, fixed base breakers are still around.
Preferential trip testing can be foregone on many ships. Many Chief Engineers are hesitant to test it due to probable issues in bringing machinery back online. Like truly overspeeding an engine, there is an inherent risk. That risk increases with vessel age, and also with lack of testing. Machinery not exercised has a higher rate of failure when called on to do its function. How long to test preferential trips? Depends on the ship. A standard bulk carrier should take no more than 2 hours at most, depending on how thoroughly the class surveyor wants to check. The preferential trips are tested by the yard and again at sea trials, but some ships will never check them again as part of a PMS routine. Presuming your case study vessel did have this routine in place, it could be verified by checking past PM’s in the maintenance software, not only the logbook. Alarm history could also confirm operation/testing.
What could cause the generator to come offline? Again, it depends on the age of vessel and what level of automation is installed. Some automation will not allow an incoming consumer to start without adequate power reserve. I have sailed on ships vintage year 2000 which had this ability. I have been on ships year 2015 that did not. Likewise, I have also seen the Chief Officer start a ballast pump without calling the engine room (which had only one generator online, insufficient for this extra load). He quickly realized his mistake and stopped the pump, but the time between start and stop was so short, it caused the generator engine to shut down on overspeed because the governor was in the process of compensating for the extra load. On a different take, I have seen preferential trips activated due to unknown reasons - no fault could be identified, it simply malfunctioned and issue could not be repeated. Nobody in investigations likes to hear this, but some mysteries do exist.
Portable load banks definitely scale for testing large ship generators, generally used for commissioning. But in this case if you want to simulate load safely just for testing the preferential trip I was thinking diagnostic current injection on the sensing circuit. Increase and hold to see whether it’s a current sensing issue or a time delay issue. Like you said though you could start further down the line with inducing a voltage signal depending on the type of trip mechanism.
if you don’t have a spare breaker aboard you might consider not leaving the pier !!! #3 prob tripped out on too much current so the breaker itself would be my first suspect.
the overload best done by faking it, actually running that power is a hazard best avoided but yes, you sure do want to know the shut downs work.
My guess is you are doing the Diploma in Marine Engineering through Lloyds Maritime Academy? Send me a private message and I can email you what I have been writing.
