Thruster loads on ASOG

Good day everyone

I would like to bring one issue for your attention which I had recently to deal with. Let’s imagine typical power and propulsion configuration for DP2 vessel as below:

4 x DG, 4 independent BUSes for each A,B,C,D)
2x Stern Azimuths
3x Bow Tunnel Thrusters

We have the following setting during DP operations:

Now, my doubt is that someone has established on ASOG 45% of load as the limit for Azimuths to turn the status to Yellow which is reasonable as one Azimuth would take 90% of load in case if other one fails, that should still allow to bring the vessel to the safe location.

The question is why the relevant limits on three tunnels are set to only 30% of load on ASOG? I still try to figure out the right explanation to this and I was even in a bit of arguing with other SDPO’s (this is exactly third vessel where I see this). Some of those questioned were not able to give me any answer that would make a sense for me, someone said that 90%:2 is 45% for Azimuths then 90%:3 is 30% for Tunnels which is an answer I can’t accept.

What I insist is that as per CAMO/ASOG we have to consider always worst single failure which could be a fail of any tunnel too. In other words I would consider the limit on tunnels of 60%, not only 30% if we have this case analogical to stern propulsion.

Your comments will be much appreciated.

Now I’m no FMEA auditor, but it seems to me that the ASOG was written for an event in which the vessel would be down to one bow thruster, and one azimuth drive. So if any two bow thrusters dropped out, the remaining one would have to increase thrust by three times to hold position. So a max load of 90% divided by 3 yields 30%. Probably written by some desk jockey.

Hope that all makes sense.

Looking at that diagram I notice that BT 1 is on the port bus, while BT2 and 3 are both on the stbd bus. I haven’t worked on a DP2 vessel with 3 bow thrusters, so would I be correct in saying that design is taking into account that BT 1 offers more yaw (heading) force?

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Which makes sense…single point failure of the stbd bus would give the load to the port side with the port azi and BT 1.

Admittedly, it has been 3 yrs since I was doing DP work, but IIRC, we are on the right track.

Thrust curves are exponential. 45% on each azi thruster does not necessarily equal 90% on a single unit. It should be less. I see where 30% on the tunnels could make sense. Your capability plots will show that you are the least capable with forces on the beam, requiring more spinning reserve as well as being more susceptible to cavitation. ASOGs are risk based, maybe not totally aligned with the consequence analysis model.

Thank you for your prompt answers.

Well, what I see is ASOGs are rather related to consequence analysis, IMCA M220 says “worst single failure”. What I didn’t mention before is that our BUSes can be separated also to 4 individual as there are breakers between yellow and red, also between green and blue. So, worst case scenario would be a loss of STBD side but that wouldn’t happen after single failure, there should be ie. either malfunction of BUS breaker between DG3/DG4 in case of short circuit or both of those engines trip.

Hence, I still say that 45% of stern load and 60% of forward is a maximum what can provide redundancy for station keeping.

If WCF is one bow due to one bus/DG then its 60% IMHO
The ASOG/TAGOS should also have a number for when you are down to 2 bow thrusters.