Local Section Paper: GL/GR, 10/29/1999

INTRODUCTION

The marine industry is witness to an increase in the popularity of Articulated Tug/Barge (ATB) combinations.

A tug pushing a barge has less total resistance than a tug towing a barge because the combined speed-length ratio, V J..f L, is smaller for the combination than the sum of the two individual vessels.

As operators strive to spend more time pushing, the devices to allow the vessel to push in a seaway become more complex. A history ofU.S. patents is shown in the references. This paper attempts to describe various methods of connecting the tug to the barge in the push mode. It also describes the loads in a typical pin connection system.

The author's company has been involved in the Detail Design of five ( 5) recent A TB units. In each case, a Finite Element Analysis (FEA) is used to verify the stresses in a complex, redundant structure.

This paper attempts to show the limits imposed upon the design as understood by the author.

PUSHING MODE AND DUAL MODE ITB

U. S. Coast Guard Regulations, Ref. (1), consider an Integrated Tug/Barge (ITB) to be classed as a "Pushing Mode" or "Dual Mode" ITB.

In a "Pushing Mode" ITB, the tug is not expected to separate from the barge except at controlled drafts. The two (2) vessels are essentially treated as a single vessel for tonnage measurement, manning and inspection.

In a "Dual Mode" ITB, the tug and barge are treated as separate vessels for inspection purposes. The tug must meet the towline stability criteria, Ref. (2), and be equipped to tow the barge on a hawser. The two (2) vessels must be able to safely disconnect in a reasonable time.

When the two (2) vessels are connected with some freedom of motion, especially pitch, the combination can be referred to as an Articulated Tug/Barge (ATB).

Vessels that are rigidly connected by mechanical means shall carry the same navigation lights as a ship, per Rule 24 (b) of Ref. (3). Interpretative Rule 82.3 ofRef. (3) does not include vessels connected by lines, hawsers, wires, or chains as mechanical means. These vessels need to follow Rule 24 (f).

The reader is encouraged to study Ref. ( 1) carefully to determine the type of A TB applicable to a given design.

1

TYPES OF INTERFACE CONNECTIONS

Rope Connection

The most simple type of connection between the tug and barge( s) is to tie the vessels together with soft line or wire rope. This can be used for protected waters and in a limited seaway. This type of connection is common on the inland rivers. The tug's bow is lashed to the barge. There can be a shallow notch in the barge or no notch.

Lines are led from the stem of the tug to the aft comers of the barge. The port and starboard lines can be led around pulleys and connected to the tug's towing winch to tighten up on the lines. When making a tum, the outside wire stretches and the inside wire tends to slacken, often falling into the water.

Mechanical Systems

The inability of the rope connection to perform in all but calm water led to the design of tug/barge connection systems.

It is interesting to read the myriad of devices claimed to improve on the simple rope connection method. References ( 4) through (35) describe all manner of connection systems dating from 1858 to 1997. Of these, the ones put into practice and still in use are:

The Sea-Link System was developed on the U.S. West Coast by L. Glosten, Ref. (7).

The Artubar System evolved from the early work of E. H. Fletcher, Ref. (13).

A popular system in use today was developed by Robert Bludworth, Refs. (26) and (28).

The Articouple System was developed by T. Yamaguchi, Refs. (25), (27), (36) and (3 7). It uses a hydraulic cylinder to extend and retract a transverse ram from the tug's sides into a vertical rack in the sides of the barge notch.

The Intercon System, Ref. (33), is nearly the same as the Articouple System; the major difference being Intercon uses a power screw and geared electric motor in lieu of a hydraulic cylinder to move the ram.

The Bark River System was developed by the author for Clyde Van Enkevort under the name All Purpose Marine Products. It is essentially the same as the Articouple, using a hydraulic cylinder to extend and retract the ram.

The older patents have expired after 19 years, leaving all but the latest patents open to the imagination of the designer.

2

CONCLUSION (Cont'd)

The lowest load on the sides of the connector notch are with a head having no angle or slope to the notch sides. The loads normal to the surface of the sides are the actual longitudinal loads imposed by the seaway and no additional transverse loads are imposed on the ram. The seaway transverse loads are taken by the end of the ram pushing directly on the center structure of the connector notch. The problem with the functional ability to readily connect with a close fitting pin and the rattle caused by the clearances, as previously discussed, poses an undesirable choice.

If the sides of the connector notch are tapered to provide a positive contact between the ram teeth and the barge notch sides, and no contact is made on the end of the ram for transmitting transverse loads, we have the transverse and longitudinal seaway loads resolved into normal and transverse loads as shown herein.

Ifthe angle ofthe sides is small, the normal loads on the teeth and barge rack structure are high, but the transverse loads on the ram are small. For larger angles, the opposite is true. The nprmalloads on the teeth and rack structure are lower, but the transverse load on the ram is higher.

The selection of the angle is influenced by the following:

1. Bearing pressure between the fixed ball and movable head (teeth) of the ram as a result of normal loads on the connector sides.

2. Ability to transmit some of the transverse seaway loads between the end of the ram and a compressible pad on the longitudinal side of the connector notch, thus reducing the normal loads on the connector sides.

3. Size and cost of the actuating and holding mechanism of the ram, whether by hydraulic cylinder or power screw.

4. Cost of the structure to support the forces normal to sides of the barge connector rack.

Items 1 and 3 above are examined by exercising sound engineering principles. Item 2 depends upon the materials available and the construction tolerances to insure a reasonable transfer ofloads. Item 4 usually results in a complex, redundant structure that is best checked by a Finite Element Analysis (FEA). Modeling techniques of distributing the loads from the teeth connector ram to the sides of the barge connector structure must be carefully addressed.

As with most designs, a compromise or trade-off must be made. This paper offers an insight as to the loads to be expected in the design of an Articouple type connector.

20