How it should be done - towing AHTS - open ocean.
[QUOTE=c.captain;181626]I always say to mates, if in doubt about anything, pull the fucking throttles back to zero if necessary then call meā¦many shit situations can be made much better without speed or thrust.[/QUOTE]
There are very few times when towing astern that pulling the throttles back to zero is a good idea. Frequently (usually?) it could be catastrophic.
[QUOTE=Capt. Phoenix;181667]There are very few times when towing astern that pulling the throttles back to zero is a good idea. Frequently (usually?) it could be catastrophic.[/QUOTE]
It will sure as shit get the engineer away from the TV. . . . .
[QUOTE=anchorman;181660]There can be some limiting, but the shaft generators (and main engines) are normally rated high enough for all consumers, which include the winch & thrusters, under normal operating conditions.
DP or manual, the shaft generator doesnāt care where the command is coming from. The reality is, thrusters are generally in joystick (auto head), and possibly with sway axis, or athwartship - depending on DP terminology. Normally, but at times, full manual control may be required if situation dictates it, but that is more handling the vessel versus power consumption.[/QUOTE]
There are the full amount of power available for straight towing. If you put ALL available power on the propeller you get X BP, if you divert some to the thrusers and winch you get Y BP. The larger the thrusters and the smaller the available total power the larger % is lost to the propellers, thus the BP.
The less pull you have available to maintain tension in the wire the less control you have to maintain relative position, or keeping the wire/chain off the seabed in this case. You can maintain heading but still be push off sideways from you intended track.
My experience is not from the boats, but from the rig/barge side as Towmaster/Rigmover. I have sometimes had to hook up a second boat on the bow of the AHT to get the right trajectory. (Or use a J-hook in the middle of the chain, as was done in the BD case)
In case somebody wonder, I have anchored rigs in up to 2200 m. WD and Drillships rated for 600 ft. in up to 1800 ft., using insert wires.
[QUOTE=ombugge;181682]There are the full amount of power available for straight towing. If you put ALL available power on the propeller you get X BP, if you divert some to the thrusers and winch you get Y BP. The larger the thrusters and the smaller the available total power the larger % is lost to the propellers, thus the BP.
The less pull you have available to maintain tension in the wire the less control you have to maintain relative position, or keeping the wire/chain off the seabed in this case. You can maintain heading but still be push off sideways from you intended track.
My experience is not from the boats, but from the rig/barge side as Towmaster/Rigmover. I have sometimes had to hook up a second boat on the bow of the AHT to get the right trajectory. (Or use a J-hook in the middle of the chain, as was done in the BD case)
In case somebody wonder, I have anchored rigs in up to 2200 m. WD and Drillships rated for 600 ft. in up to 1800 ft., using insert wires.[/QUOTE]
Vessels with shaft generators are always variable pitch wheels, with constant RPM shafts (of course needed for a shaft generator to work). If there is limiting, it will be in terms of pitch reduction, or pitch limiting function on main wheels (as you mentioned). In most cases, under normal conditions you will not see it. On the bigger vessels,32,000HP class, you can have your thrusters near 100%, and picking up +350 tons on the winch while pulling near 300 tons of bollard pull without any issue, heat will be an issue in summer time before anything. Most vessels are not near as capable, but that does not mean you experience any type of bollard pull capability loss every time using thrusters & winches while pulling. The curve is exponential, so it will be in the higher pitch range +80% if you see any pitch limitation, but normally by then, you are in a position where the winch is not needed, bow thrust still required - but nothing in the stern. You are basically on a dead pull and rudder power is pretty high at that point, and since you are high in the exponential curve, 5% means a lot, so 5% extra on one main versus the other can essentially act like a bow thruster to a certain extent.
and Iād assume thatās factored into the equation where what the boat is rated for bollard pull considers a certain load to wheels, thrusters etc? Just curious
[QUOTE=Capt. Phoenix;181667]There are very few times when towing astern that pulling the throttles back to zero is a good idea. Frequently (usually?) it could be catastrophic.[/QUOTE]
I am not talking about with a barge astern but when running out an anchor. If an anchor boat gets all cocked up like the BOURBON DOLPHIN, is it not better to let the whole mess go slack than to keep fighting it with the potential of perilous consequences?
[QUOTE=z-drive;181689]and Iād assume thatās factored into the equation where what the boat is rated for bollard pull considers a certain load to wheels, thrusters etc? Just curious[/QUOTE]
Pitch limiting is not a consideration for bollard pull rating, nor should if be - way too convoluted. You would almost have to put a disclaimer - a short story about certain (theoretical) conditions may result in 10-15% reduction in something you would never likely experience due to flexibility in terms of power management philosophy, and then it will take the rest of the day to explain what that means.
[QUOTE=anchorman;181688]Vessels with shaft generators are always variable pitch wheels, with constant RPM shafts ā¦[/QUOTE]
Not always. Variable speed SGs in various sizes have been available and widely used for the past 5 or 6 years at least. Both ABB and RR have well proven systems.
[QUOTE=z-drive;181689]and Iād assume thatās factored into the equation where what the boat is rated for bollard pull considers a certain load to wheels, thrusters etc? Just curious[/QUOTE]
When you do Bollard Pull test it is at 100% pitch, with all available power onto the wire, something that will not be repeated until the next test, or in dire emergency.
If, for some reasons, like beam wind or current during the test, thruster power or rudder angle may be needed to maintain heading/position during the straight pull, some lost BP is inevitable.
I agree that if you have 32000 Bhp available the loss of pulling power due to thruster use is not a big problem, but when you have a relatively large hull and small power (10-15000 kW) stealing up to 2500 - 3000 kW to power the thrusters means a lot.
I also agree that it is not often you would need ALL thrusters at full power and at the same time max. power on the shafts.
FYI: The Bourbon Dolphin was 75.2 m. LOA and had 4 x 3000 kW Main engines (16000 Bhp) Total thrusters; 2946 kW, or nearly 25% of available power at 100% MCR: http://www.shipsandoil.co.uk/ulstein-verft/ulstein-a102
[QUOTE=Steamer;181736]Not always. Variable speed SGs in various sizes have been available and widely used for the past 5 or 6 years at least. Both ABB and RR have well proven systems.[/QUOTE]
Very interesting. I have never been exposed to that particular application on AHTS vessels. There is a variable mode - engine RPM/pitch match, but that is not while power generation (I am pretty confident in that, but I will need to check this one to be sure - never used the variable feature since it was impractical for day-to-day operations)
That is true. I have worked on several SLM generators (none of them reliable) either as main engine take offs or shaft take offs. Fixed blade propellers.
I believe Anchorman and Wxbound is talking about two VERY different worlds.
Anchorman is talking about something like this: http://www.dof.no/en-GB/DOF-Fleet/AHTS/Skandi-IguaƧu.aspx
While Wxbound may be more into something like this: http://supplyboats.leefelterman.com/index.php/69-osv-217.html
[QUOTE=anchorman;181753]Very interesting. I have never been exposed to that particular application on AHTS vessels. There is a variable mode - engine RPM/pitch match, but that is not while power generation (I am pretty confident in that, but I will need to check this one to be sure - never used the variable feature since it was impractical for day-to-day operations)[/QUOTE]
On Chouestās 280s (OSVs) you couldnāt run the shaft gens in combinator mode, you had to be in fixed RPM mode.
[QUOTE=Capt. Phoenix;181768]On Chouestās 280s (OSVs) you couldnāt run the shaft gens in combinator mode, you had to be in fixed RPM mode.[/QUOTE]
You have to have variable frequency converter to operate Shaft Generators with variable RPM: http://www.km.kongsberg.com/ks/web/nokbg0240.nsf/AllWeb/AC15755EC2442854C125788F0031E4F6?OpenDocument
Automated RPM/Pitch control has been around since the 1980s,(also known as āeconometreā) where you can dial in the required speed.
The system would set the most fuel efficient balance between Pitch/RPM while maintaining that speed through the water in variable conditions. (Before GPS)
It is 10 years today since the Bourbon Dolphin incident which killed 8 people.
Donāt know if this report on the incident has been posted here before: http://maritimeaccident.org/library2/the-case-of-the-toppling-tug/
At least it changed the way things are done on Norwegian AHTS almost immediately, but it is estimate that it will take until 2020 or more before it becomes IMO rules.
Even the design of AHTSs have changed, as seen in this article in the local paper: http://www.smp.no/pluss/2017/04/10/Bourbon-ulukka-har-endra-skipsdesignen-14582183.ece?cx_front_click=baseline_test&cx_front_click_place=17&cx_front_click_articles=1
May they rest in peace: http://www.smp.no/nyheter/soere/2017/04/11/ā-Forliset-pregar-framleis-offshoremiljĆøet-14583685.ece
Thanks for reviving this thread. Chevron was at fault for using the smallest AHTS on location to do the heavy work.