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The Articulated Tug/Barge

More and more, owners are moving toward pushing barges at sea — a transition made possible by the application of the Articulated Tug/Barge (ATB). The ATB has provided the tug and barge industry with a vessel that can effectively compete with ships in two areas: speed and weather reliability. The ATB is designed to shine in both areas, and it does so without sacrificing the economic benefits of tug and barge construction and operation. The modern ATB tug is a fully functional ocean tug, capable of towing on a hawser if required. The ATB barge is designed so that it can work with other tugs and be towed. The difference is 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. Delays of two to three days are not unheard of.

A firm at the forefront of ATB design has been Ocean Tug & Barge Engineering (OT&BE). Centra] to OT&BE's ATB concept is the use of the proprietary Intercon connection system, of which OT&BE's principal was a co-patentor. In continuous operation for more than nine years now, this system has reportedly operated without need of a single disconnection at sea due to weather. The six vessels now working with the system have operated in seas that match conditions simulated during the extensive model testing program Intercon undertook prior to construction of the first system. Robert P. Hill, president of OT&BE, filed this report on the progress of the ATB.

The ATB design is not a compromise between pushing and towing, but is touted as the most efficient design possible in pushing the barge. ATB speeds are now at more than 12 knots service speed, and with the new generation currently being designed, speeds of more than 13 knots are possible. Here are some possible benefits of the ATB, particularly with an Intercon ATB.

Reduced bow damage to barges because the tug crew can feel excess pounding.

Reduced fender replacement (virtually none in 7 years) and tow wire replacement costs.

As much as 25 percent fuel savings vs. towing.

Reduced engineering and construction costs vs. a tanker of equal deadweight.

More control over barge and less dock damage, emergency maneuvers without fear of breaking wires or tripping the tug. Higher average speeds, light and loaded.

Ship-reliable ETAs. Ability to sail through a wider range of weather conditions. Reduced port time/elimination of transfer time from pushing to towing gear and back.

Availability of a large, high horsepower ocean tug for emergency and salvage work, and a means to deploy a spill boom in an accident.

All functions controlled from pilothouse.

Presently, OT&BE is providing the contract design for a 33,000- dwt ATB for Penn Maritime of Stamford, Conn., including the design of the barge and the contract design of the tug based on an adaptation of OT&BE's new Voyager class ATB tug. Penn built the first totally new Intercon ATBs in 1994-5.

OT&BE is also participating in the Crowley Maritime ATB project by providing subcontracted ATB consultation for the barge design undertaken by Elliott Bay Design Group (Seattle). In addition, OT&BE is providing the design for the 12,000-bhp tugs that will push the barges, with a second adaptation of the Voyager class tug, along with conversion engineering for a group of existing tugs. This unit is also an Intercon ATB.

OT&BE client Maritrans Inc. (Philadelphia) was the first Intercon ATB customer and has a total of four units in service. The first, the Intrepid I Ocean 250, went into service in 1986. The latest, the Liberty I Maritrans 300, has a new type of Intercon system specially designed for use in lightering service. It was delivered in October.

OT&BE engineered both the barge and tug modifications to this unit, and is working with two shipyards, providing conceptual design services for Maritrans. River/Ocean Systems has begun design of a 10,000-lt. dwt ATB bulk carrier using a contra-rotating Aquamaster Z-Drive propulsion system. The first is to be built in 1996, with major design responsibilities for the ATB-related engineering and barge hull design given to OT&BE. OT&BE is working with both the owner and Leevac Shipyards on the project. The Voyager Class The Voyager class tug took into account the comments and ideas of shipyards, owners, crews and regulatory bodies. It is designed to be built economically, and to provide the horsepower and fuel capacity to allow for both higher speed and longer range. Equipped with — and primarily designed to — an Intercon connection, the tug is adaptable to all other systems, with only minor overall changes. With a length (between perpendiculars) of 150 ft. (45.7 m), a beam of 44 ft. (13.4 m) and a depth of 24 ft. (7.3 m), the tug is designed with full towing capability and a single tall pilothouse. It can be produced with a number of propulsion packages. Current vessel contracts include one group featuring a pair of EMD 20-710G engines, for a total of 10,000 bhp specified. Another group calls for a pair of Caterpillar 3616 engines, producing 12,000 bhp. A third group of vessels will drop down to 8,400 bhp with a pair of EMD 16-710G engines. The tug design has the size and tankage to accommodate a heavy fuel installation. The hull is designed to provide minimal resistance in the stern notch of the barge, with lines developed to provide smooth flow from the barge to the tug hull. Tankage is designed to trim the tug properly in the presence of the ATB connection equipment, and the stability of the design exceeds current standards. The tug is also designed for ease of maintenance. The tugs will be classed by ABS and will meet the requirements for U.S. Coast Guard designation as Dual Mode tug/barge units.

Accommodation is provided for up to 10, expandable if required. The tugs can meet SOLAS requirements based on an International Tonnage certificate.

OT&BE has also developed the Atlantic class ATB tug, which is 120 ft. (36.5 m) long, with a 36-ft. (10.9-m) beam and 20-ft. (6.1-m) hull depth. This design is arranged for up to 5,600 bhp.

The Intercon Connection The operation of the Intercon connection system is straightforward. It 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 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 multiangle 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 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 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 lowtorque, 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 highspeed 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 percent 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 also offers 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. This is of major importance in the product trades.

It allows for two-porting, eliminates ballasting of either vessel, and means the crew has 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 three degrees relative to each other, with full system operability.

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, 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.

 
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