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Frequently Asked Questions
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Powerhouse Tug & Barge (PTB) System
Frequently Asked Questions (FAQ):
1. For American flag units, the moment you provide propulsion machinery on the barge, the US Coast Guard manning requirements kick in and you will need have to have crew on the barge - this means SOLAS-level outfitting, lifeboats, and much added cost.
Answer: Please note that other than the electric azimuth drive, which needs virtually no supervision, no other propulsion machineries will be onboard the barge. Traditions, conventions, designs etc. changes with time and the regulations and policies are amended accordingly. I see no reason for any regulatory authority to insist for crews onboard for a system that does not need continuous supervision. The electric motor onboard does not need watch keeping. Even machinery room with IC and other equipment with moving parts are allowed to have unmanned/unattended machinery space. All you need is reliable control, monitoring and safety systems in place with enough redundancy.
Please note that USCG still does not approve LNG tanks below main deck level while rest of the world allows it. USCG rather allows LNG tanks to be at the superstructure close to the crews, which is more of a safety hazard. We need to fight to change these illogical logics rather than agreeing to them.
1A. You will also lose the deeper, B25 load line if you are manned reducing cargo capacity.
Answer: Same logic as per my feedback to item#1. There is no reason for not getting this consideration for an unmanned barge with only an electrical propulsion unit onboard with no IC engine.
2. Given the size of the crew will be based on the gross tonnage of the barge, it will be a crew as large as on a ship - so you end up with no crew size advantage - in fact you will have two crews. This will be at least double what an AT/B that pushes, will have. Your crew will need to be larger than a pushed AT/B crew.
Answer: Same logic as mentioned above. There is no reason to have crews onboard the barge.
3. The soft connection you are showing will make it impossible to get crew over to the barge from the tug while underway. You will need to stop, come around alongside the barge and board crew for routing maintenance and for emergencies. An AT/B that pushes has no such issue.
Answer: I just described a simple possible connection for calm weather and at the end of the presentation has invited companies to come forward to design the articulation system. I have been connected with studying tug & barge systems for over 30years and operating and maintaining them with all kinds of connections for over 20years. All of them are good for operations within certain sea conditions and beyond it they are not run. Same will happen to PTB system. I am confident that a suitable articulation and a gangway for normal operations can be designed without much effort and with lot of confidence.
4. We have now refined AT/B designs to the point where our new "RAPID" class AT/B's use no more power than a ship for the same deadweight and speed. So there is no speed advantage over a conventional pushed AT/B. But with the flexible connection you have, there would be a severe maneuvering penalty with your design, especially in large seas.
Answer: Hope you will agree that to accommodate a near efficient propulsion gear (especially an optimum size propeller) you had to maximize the cubic volume of the tug and shape/shave the stern of the barge and still haven't achieved the most achievable overall flow around that area. In PTB system the ship shape barge with optimum propulsion gear size at no extra premium will obviously have both propulsion and economic advantage over ATB.
In ATB system a bow thruster is virtually a must to add on the barge for maneuvering. In my recent experience I found that an ATB system needed more tug assists than the self-propelled vessels of similar or even larger sizes in the fleet.
As I have mentioned in my paper, at large seas the PTB will switch to towing mode. In normal operation, when the tug is connected at the bow with tow cable at the center and two guy cable from the sides and a 'yet to be designed pivoting system' between the tug and the barge I don't think maneuvering will be an issue. I will definitely need input or cooperation for the connection system design from experts in the field. I strongly believe that for every problem in this world, there is at least one solution, if not more.
5. Plus in towing the barge, the water that the bow sees on the barge is already accelerated and therefore resistance is increased if the tug is too close - if it is too far away you may have yaw stability issues if your barge engines or steering go down.
Answer: Again, in normal mode the tug will be attached at the bow of the barge and will be pushed as an integral part of it. With a good design this will work as an added length and better the angle of entrance of the overall system.
In towing mode in weather and emergencies the tug will be at optimum distance ahead of the barge like traditional towing arrangement. With the tug's own propulsion and remote/wireless operations of the barges propulsion & maneuvering system (preferable a water jet type thruster at the bow which can also be used to assist in propulsion) run by the emergency generator onboard the barge, there will be enough redundancy for propulsion and maneuverability of the PTB system in all operational modes. Multiple azimuth drive on the barge, if installed, will have better redundancy. For electric motor driven azimuth with right-angle drives, options of installing standby motor can be considered.
6. Towing that shape in large seas or even calm ones will be difficult as noted above and yaw stability becomes a big issue. You run the risk of tripping the tug towing close aboard, if the barge steering goes awry and the barge takes a shear to one side or the other.
Answer: As mentioned above towing is only an emergency mode and will be no worse than towing a ship. In case of PTB and the above mentioned options there is less risk than any other system in use.
Please note that in PTB system the barge will be designed like a ship for optimum propulsive efficiency or an existing ship will be retrofitted as a barge and will have as good maneuverability and sea keeping characteristics as of a ship. A boxed shaped barge which may take shear to one side and needs multiple skegs for stabilizing it, is not suggested in PTB system.
7. I'd be leery of a cheaper articulation system. The forces that unit will see at sea are enormous. We have model tested AT/B's where the tug leads, instead of being in the stern. This is the basis of some of our container carrier AT/B designs - but the testing clearly shows that in large seas, the tug is driven into the waves and it is not safe, as it moves and has greater relative motion - far more than the barge, especially in short period seas.
Answer: You are absolutely right. But as mentioned earlier in my proposal, at adverse sea condition the operation will be switched to towing mode and there will be no concern of the articulation system. It is of no doubt and argument that a smaller tug will need stronger articulation system to push a heavy and robust barge in front than a heavy barge pushing a lighter and smaller tug in front. I welcome any suggestion on design of this articulation system. Again this will be used only in normal operating condition. In bad weather the tug is in towing mode significantly forward of the barge.
8. Attached close to the tug as you seem to be, if anything happens to the propulsion of the tug and to the connection, the tug runs the risk of being run down by the barge.
Answer: Very genuine concern. There may be following scenarios in both the barge and the tug:
a. Failure of barges propulsion system in normal mode when it is pushing the tug in front:
b. Failure of tugs propulsion system: Tug's propulsion system is for emergency and independent use only and may also be used as bow thruster for the PTB configuration, eliminating the need for bow thruster on the barge. In case of its failure the crews onboard can try to fix it. The tug may have more than one azimuth for redundancy, if failure is of great concern. As long as one of the propulsion system is working, either of the barge or the tug, there should not be any risk for any of the equipment.
Please note that in case of an ATB system, if the tugs propulsion fails there is more risks involved, as there is no redundancy. In ATB system if the connection fails along with the propulsion system on the tug they will separate from each other and will go totally out of control.
9. Your note about AT/B tugs being "soft and unstable" is not universally correct. The tugs we design and others design are extremely seaworthy out of the notch. Our boats are model tested to Force 12.
Answer: I have experience with a number of tugs that have been retrofitted for ATB system raising its pilothouse and became very tender out of the notch. In a recent case, possibly in Great Lakes, one of the ATB tug taken out of the notch was so tender that it needed tug assist to be brought to dry dock for service. You know way better than me what it takes to design a tug boat with high pilot house. Additionally it restricts the tug going under low level bridges, restricting their operations. Please note that I come from mostly operational background and practically witnessed the issues with towing and ATB configurations. In most cases the actual operators does not get much chance to talk to the designers and these operational issues get buried or taken as normal and beyond control.
10. We already have diesel/electric AT/B designs published, where the barge feeds power to the tug, and vice-versa but no propulsion itself is on the barge. Conveying large amounts of power between two articulated vessels of any kind is difficult - and breaking that connection in an emergency is fraught with problems, particularly if the barge then has no control.
Answer: The technology and practice is already there and we should not be at all worried about it. Additionally, what I am also suggesting is to incorporate in the quick disconnect power transmission system a device that will shut off the power supply as soon as there is a break in any of the phases.
11. There are very few large tugs "existing" that can be refit for this service without also needing large investments in the tug itself to update it, deal with steel and machinery, issues etc. The ones left in the US right now are few and in very bad shape. We track them.
Answer: While this proposal is perfect for new designs, for existing tugs it is suitable for the following retrofits:
Needing engine replacement due to environmental compliance:
For ships with the above circumstances the propulsion system can be retrofitted with PTB concept, even if the immediate intention is not to put in PTB configuration. This will basically be a diesel-electric conversion with the option of its power supply to be installed onboard the ship or on a powerhouse tug.
12. You are claiming it will be safer than existing articulated systems. We alone have over 50 AT/B's we designed out working now and there have been zero problems with the articulation systems or the units themselves. It is hard to claim better than that.
Answer: I think all the above explanations will justify my claim. The tug being at the stern of the barge is inevitably a navigational issue. Early day ships were mostly been designed with pilothouse in the forecastle with the deck crews been called "Front-end crews" with engineers being at the stern of the ship called as "Aft-end" crews for navigational and operational convenience. Later the cargo vessels had stern superstructure with pilothouse but no high cargo on the deck. Containerships with bad line of sight needs significant pilot and tug assists in confined areas. Proposed PTB configuration will have significant positive contribution on the marine transportation operations and economics.