Robert P. Hill – Ocean Tug & Barge Engineering Corp.
I have never been one to be able to simply leave things alone. As a kid, I used to spend a lot of time by the docks in my home of Troy, NY, watching the tugs and barges that plied the then-busy New York State Barge Canal. Unlike living at the coast where you would see a barge leave port on a hawser, I constantly saw them pushed by tugs fit into notches at the barge stern. So for a boy of 12, pushing barges seemed the only logical way to move them. That logic would remain with me over the years and through my career as a naval architect. I would grow up to learn of coastal towing and in fact I would often go to sea doing just that. But what started as a "given" for me, became an obsession. Everything I'd learned over the years taught me that pushing was simply a better way to do things, yet in coastal and ocean service, pushing was severely limited in application. I decided at a very early stage of my career to try and fix that.
Now in 2016, Ocean Tug & Barge Engineering has become the leader in AT/B or "Articulated Tug/Barge" design. As co-inventor of what would become the proprietary "INTERCON" connection system, and with over 35 years of experience engineering systems and vessels for the tug/barge industry – 34 of that working with AT/B’s - I've watched the tug and barge industry begin a slow transformation. More and more, owners are turning away from hawsers and heavy towing gear, and moving toward pushing barges at sea - a transition made possible by the application of the AT/B. Whether the connection system be Intercon, Marine Specialties, Artubar or others, the AT/B has provided the tug and barge industry with a vessel that can effectively compete with ships in two areas where ships were long held to be better performers. These are the areas of speed and weather reliability.
The AT/B shines in both of these areas, and it does so without sacrificing the economic benefits of tug and barge construction and operation. Unlike previous "rigid" tug/barge systems (which were never really "tugs" and "barges"), the modern AT/B tug is a fully functional ocean tug, capable of towing on a hawser if ever required. The AT/B barge is designed such that it can work with other tugs if required and be towed. The difference comes in the connection between the tug and barge.
Towed barges are often weatherbound when sea states or weather along their trade route are projected to be risky to operation. This has been particularly true along the U.S. East Coast, where tank barges often gathered at the approach of bad weather, in protected locations to await more favorable conditions. Delays in sailing of 2-3 days are not unheard of. Lower cargo rates and the days of bloated inventories were once enough to allow shippers to overlook the delays. But in the modern world of reduced inventories in terminals and the need for scheduled deliveries, weather delays with towed barges are an increasing problem. Coupled with that is the increased exposure to financial penalties created under OPA90. A parted hawser in bad weather could in some cases be the virtual end of a company.
It is a given, that in any barge, pushing it will result in at least some increase in speed as opposed to towing it. That difference can run to as much as 33% in an even moderately well designed hawser unit. With the advent of the AT/B, we design not for a compromise between pushing and towing, but for the most efficient design possible in pushing the barge. AT/B speeds are now up above 12 knots service speed and with the new generation we are engaged in designing, speeds over 13 knots are now possible. Yet, we are still building a real tug and barge. The reduced construction costs make this concept ideal for a number of services and can play a big part in making American operators and shipyards competitive in the world marketplace, not only the protected Jones Act market. Also, an AT/B requires no larger a crew than for an equal towed tug/barge unit.
The following are all possible benefits of AT/B operation:
- Greatly reduced bow damage to barges because the tug crew can feel excess pounding they cannot feel at the end of a tow wire
- Greatly reduced fender replacement (virtually none in 7 years) and tow wire replacement costs.
- Lowering of back injury claims from not having to handle backing wires and towing gear.
- More control over barge and less dock damage, emergency maneuvers without fear of breaking wires or tripping the tug.
- Higher average speeds both light and loaded.
- Ship-reliable ETA's, providing improved scheduling capability.
- Greater crew comfort and reduced hazards from handling tow wire.
- As much as 25% fuel saving vs. towing.
- Ability to sail through a wider range of weather conditions.
- Ability to enter ports with severe bar situations.
- Reduced port time/elimination of transfer time from pushing to towing gear and back.
- Improved fuel economy from reliable autopilot function - less rudder carried to compensate for loose backing wires or fighting a tow line in seas.
- Availability of a large, high horsepower ocean tug for emergency and salvage work, and a means to deploy a spill boom in an accident.
- No crew needed on deck to connect or disconnect - all functions controlled from pilothouse.
- Reduced construction costs vs. a tanker of equal deadweight.
- Reduced engineering costs vs. a tanker of the same deadweight.
- Annual delivered tonnages competitive with a tanker due to the higher speed of an AT/B vs. towed barge.
The AT/B is also construction-friendly, with tug and barge able to be built in specialized yards at lower costs. For both commercial and military sealift applications, the AT/B is quicker to produce, is less complicated than a ship, and can be more quickly moved from the design to construction stage. For developing shipbuilding industries, the AT/B will require less sophisticated facilities and allows market entry at a lower cost. For American yards, the world-leaders in tug and barge construction, the deep-sea-capable AT/B is the perfect low-cost competition to foreign tanker designs.
Central to OT&BE's AT/B concept is the use of the INTERCON connection system. In continuous operation now for over 15 years, this system has operated without need of a single disconnection at sea due to weather. The vessels now working have operated in seas that match the extensive model test program Intercon undertook prior to construction of the first system and during that seven year period the vessels have been weather-bound only in extreme weather situations and all of the loaded trips have been done connected - a remarkable record for a prototype unit.
The operation of the system is straightforward. The system is a single degree of freedom connection that like other systems, establishes a transverse, fixed axis between the tug and barge, around which the vessels are allowed free relative rotation, or pitch. All other movements such as yaw, roll, and heave are restrained. Thus the tug heaves and rolls with the gentler motion of the barge, and unlike systems that allow motions in more than one axis, the system forces are predictable. In the system, the port and starboard sides of the notch wall are fitted with a vertical channel member with the open side facing the barge centerline. Notches, or teeth are incorporated on the fore and aft sides of the channel to eliminate vertical travel. The channel sides are tapered to provide a wider opening to ease connection, and the side taper is flat enough to minimize resultant thrust from higher bow to stern loads imparted on the barge by the tug. The notches lie on the taper of the sides and are of equal angle, peaked to balance forces and to minimize multi-angle planes of contact when engaged by the tug's connecting helmet. The vertical extent of these connection ladders is determined by the relative draft range desired for operation, and they can be supplied in a skeg module, prefabricated at Intercon for installation as a unit into the barge.
The machined steel connecting heads of the tug, which are inserted into the channels port and starboard to make the connection, are configured to match the channel tooth pattern. Each head is mounted on a spherical support to allow auto-alignment to the channel while retaining the greatest load carrying capacity in a minimal space. The connecting head of the tug is thus allowed limited movement in all directions except rotation about the horizontal centerline of the connecting points. Horizontal rotation is transmitted to the ram which supports the head by guide locks on the top and bottom of the head. The ram is a heavy fabricated steel cylinder supported on the O.D. by a bronze bushing. The bushings are mounted in the I.D. of a "Load Box" structure which is a stress-relieved steel fabrication designed to transfer all structural loading, shaped to suit the tug and pre-fabricated and outfitted at Intercon with full lighting, access, wiring, piping etc. for insertion into the tug as a module.
To extend or retract the head, the rams are moved along the horizontal axis by a large, male threaded shaft turning in the female boss on the inside of the ram. The threaded shaft is operated by electric motors through gearing mounted to the housing. Two drive motors are provided - one for low-torque high speed operation, the second for high-torque low speed operation. The high speed motor is direct coupled to the gearing. The low speed motor is connected to the gearbox via an air clutch, to protect it from overspeed in high-speed operation. Emergency drives are also connected to the gearboxes, for use in the event of an electrical power failure.
Control of the system is accomplished locally at the units, and remotely from the pilothouse. Under normal operation, the system is 100% pilothouse controlled, and local control at the units is for maintenance or emergency operation. All parts of the unit are accessible for maintenance and other than the structural fabrications, an effort has been made to provide for mechanical parts which can be purchased off the shelf
Side to side motion of the tug in the notch is completely eliminated, with no need for added fendering or expandable bladders between the tug and the barge notch wall to restrain sliding of the tug. The system offers as well, the ability to connect at any draft of either tug or barge, reducing costly port delays incurred while trying to ballast two vessels to match each other and without the need for maintenance-intensive "movable" connection points on the barge. This of major importance in the product trades. It allows for two-porting, eliminates ballasting of either vessel, and means the crew has absolutely no concern with regard to the relative locations of the tug and barge connection points. Engagement can also be accomplished with the tug and barge heeled up to 3 degrees relative to each other, with full system operability. Compared to systems operated by extension of hydraulic rams, the system does not rely on check valves or other hydraulic devices to hold the rams out.
Connection to the barge requires virtually no crew intervention outside the wheelhouse. The same is true for disconnection. The tug's shape is not altered to fit the system, and so the tug can handle ordinary, non-system barges as well. There are no ungainly projections from the hull either structural or system-related which can damage the vessels during connection/withdrawal or cause the tug to be unsuitable for other work. For an assessment of other connection systems, and where their application can be of benefit, please read our corporate brochure, and inside it, the paper describing the various systems available.
This white paper is written by the Robert P. Hill, N.A., President of Ocean Tug & Barge Engineering Corp. The paper is the sole opinion and expression of the author, and was not edited by any outside source.