Selection of Coupler Model

1) Selection of Type    Couplers of ARTICOUPLE- and TRIOFIX-series comprise several types having different features and the type must be chosen to comply with the character of the service intended. The descriptions in the pages "TYPES" and "PUSHING AT SEA" may be useful in forming a judgement upon the type meeting the client's demand best. It is practically possible to find a coupler type adaptable to any type of demand expected.    In simple transports between sheltered harbours, it is recommended to use pure mechanical tooth-engagement types ---ARTICOUPLE-K or TRIOFIX-TK or -TR --- which are of simple construction and practically have no wearing parts.    Couplers with F --- friction components --- ARTICOUPLE-F and -FR and TRIOFIX-TRF ---are recommended in the following cases :

--- When it is desired to keep connection even during loading and/or unloading in ports in spite of gradual change of draught of the barge, particularly when electric power for cargo-handling must be supplied from the pusherboat --- such as bulk cement tanker barge.    --- When the draught of the barge changes during navigation, such as the case of dump-barge or dredge barge.    --- When connection must take place under the influence of waves.

It is to be kept in mind, however, that the rubber-linings contained in couplers with friction components will need replacement, troublesome and costly, at several years' intervals. It may even be necessary to consider whether facilities for renewal of rubber-lining will be available without particular difficulty. Another problem is that the couplers with friction components have a certain limit in realizable size and, when the pusherboat and barge are too big, couplers of such types are often hard to manufacture.    The choice between ARTICOUPLE and TRIOFIX may be to the owner's preference, and the merits and demerits of these two series are as described in the pages "TYPES". When higher speed is desired, TRIOFIX is generally better, or, in many cases, it is the only way to reach the goal, but with some additional costs. However, it should be noted that TRIOFIX gives nothing more than a chance to utilize good hull forms for attaining a good speed, and mere adoption of TRIOFIX cannot improve the speed if the hull forms are not improved accordingly. 2) Coupler Load Analysis    For the final selection of the coupler model, the components of load acting on the coupler are estimated for the sea state expected in the intended routes.    Theoretical analysis is carried out for every inquiry, in general rule, by solving numerically on a computer a series of simultaneous equations of motions by applying the Strip Method for the combination of the pusherboat and the barge concerned.    For the coupler load analysis, the following data on the characters of the pusherboat and the barge are needed :

--- Dimensions, hull forms and displacement ;    --- Position of coupler ;    --- Positions of centre of gravity, centre of buoyancy and other hydrostatic data ;    --- Running speed ;    --- Etc., etc.

Among these, except cases of retrofitting, the hull forms are not settled yet at the stage of inquiry in most cases. The manufacturer has dozens of hull form models in stock and will select among them such models as may be most similar to the intended vessels. Even when the engine power and deadweight are decided only and the dimensions of hulls are not decided yet, the manufacturer's expert naval architects will choose proper dimensions of the vessels concerned for the client. In this connection, the clients are kindly requested to utilize the Tables in the article INQUIRIES contained herein.    The coupler load components are sensitive functions of wave period (length) and angle of encounter, and they are considered proportional to the wave height when the wave period and angle of encounter are same. What is needed for selection of model is to find out the maximum amplitudes of load components which may be probable in the severest sea state expected. The analysis is arranged to obtain the amplitudes of longitudinal, transverse and vertical components of load and the bending load on the side coupler pin as the resultant of longitudinal and vertical components calculated taking into account their phases, in a wide range of wave period and in the full range of angle of encounter from right ahead to right astern, to find out their maxima. Elements of motion of hulls, such as pitching angle, are obtained at the same time. The model is selected to cover all the maximum amplitudes of load components and the bending load. When hull forms have been finally settled, re-analysis of coupler load is carried out for confirmation.    The data obtained by the theoretical analysis generally agree fairly well with the results of the tank experiments, and the differences between the theoretical and experimental data are taken into account in the form of safety factor involved in the design of the coupler.    The couplers of Taisei Engineering Consultants, Inc. have been approved by the following classification societies :

American Bureau of Shipping,      Bureau Veritas,      Det Norske Veritas,      Indian Register of Shipping,      Korean Register of Shipping,      Lloyd's Register of Shipping,      Nippon Kaiji Kyokai,      Registro Italiano Navale,      Russian Maritime Register of Shipping.

Hull Design

As a general rule, the coupler main bodies are installed above the freeboard deck.    Large- and medium-sized ARTICOUPLE-pushers are designed to have long forecastle to contain the coupler system inside, and the coupler main bodies are fixed to forecastle structure by welding. Smaller ARTICOUPLE-pushers are often built as flat-deck ships and the coupler main bodies mounted on foundations are installed on the main deck forward. TRIOFIX-pushers are designed mostly as complete-superstructure vessels so that the stern end may have a sufficient height above water, but, very small ones may be designed as flat-deck boats.    In ordinary arrangement with a notch at the stern of barge to receive the bow of pusher, the occurrence of heavy eddies at the bottom part of nozzle is the main reason for the low speed of the pusher-barge train.Ê In order to reduce eddies, the clearance between the hulls of barge and pusher must be reduced and, for this purpose, the pusher will have a full bow, and a fine stern to smoothen the flow running astern. In ordinary cases, the barge has a deeper draught than the pusherboat and this difference of draught plays an important role in generation of eddies. Such a "step of draught" can be reduced through cutting up the stern of barge.    In ARTICOUPLE-connection, a certain clearance is needed to permit free pitching of pusher relative to the barge and eddy-generation in this clearance is inevitable.    In TRIOFIX-connection without relative motions, such a wide clearance is not needed and eddy-generation can be reduced to the minimum if the hull forms relationship is well designed. For attaining satisfactory results, however, the total design must be built up from the stage of determination of principal dimensions and, with successful design, it may be possible to reduce the difference of speed, as compared with ordinary cargo ships, even to less than half a knot. The manufacturer has some know-how on hull forms design, proven by tank tests, to utilize the hull forms relationship for suppressing eddy generation, and is ready to contribute to realization of high-speed TRIOFIX-trains through preparing designs or furnishing advices, if so requested.    The stern of the ARTICOUPLE-pusher subjected to heavy pitching when swung by the pitching of barge in waves should be shaped wide and flat so that the stern end part may follow the up-and-down motions of water surface without jumping up or plunging down as if the bottom were stuck to the water surface. Then, the pitching of pusherboat and the disturbance of flow into propellers will be smaller. The "above-water reserve buoyancy" at the stern is important for ARTICOUPLE-pusher.    On the contrary, a wide and flat stern of a TRIOFIX-pusher will increase the coupler load remarkably and, accordingly, it is recommended that the TRIOFIX-pusher will have a fairly fine stern.    Different from tugs (towing boats), the size of rudders of the pusherboat should correspond to the total size of the pusher-barge combination, and this often becomes an issue when retrofitting a tugboat. Use of propeller-nozzles is preferable in most cases in improving the propulsive efficiency. Further, nozzles are considered to decrease the vertical motions of stern (pitching) in waves, and also to have an effect to rectify the flow into propellers under pitching and prevent drop of speed to some extent. Steerable nozzles are considered to assure a sufficient manoeuvrability in steering.

Multi-Barge Trains

Most of push-barge trains of river service are formed as rope-connected multi-barge trains. In wavy sea, however, the loads on the connecting means between two heavy barges are so large that construction of multi-barge train is not easy. Particularly, pushing of tow or more barges connected side by side is impracticable and, at the moment, the practicable limit is to push three barges connected in line by articulate couplers. Similarly, pushing of a dumb barge or two dumb barges connected in line by a self-propelled barge is practicable.    The articulate connection is the only applicable method of connection in multi-barge trains    Multi-barge trains are convenient, for example, when a same cargo, such as oil, is transported from a loading terminal to two or three unloading terminals. In Argentine, a project of push-barge train, pushing a dumb barge of 3500 DWT by a powered barge of 3150 DWT, was realized for the transport of oil in rivers and estuary many years ago. The coupler was ARTICOUPLE-K.    In forming a multi-barge train, it is strongly recommended to design the unit barge short and wide to reduce the coupler load.