R o l l - O n / R o l l - O f f And Container Carriers

Doros A. Argyriadis, Goran Nilsson and Harry Petsis*

A N e w Concept In M a r i n e Transport Is Seeded Only In Times Of Economic U p h e a v a l And Bears Fruit Only If It Is A d a p t a b l e To Economics Of A C h a n g i n g Pattern The most c h a l l e n g i n g and unique problem the naval architect faces when developing the design of a new vessel is that of combining the best technical know-how with the best economic analyses, then adding a bit of magic forecasting and talent.

From the technical know-how of the past dozen years or so, many specialized vessels have evolved. If one includes in a comparison the tankers and bulkers and their many combinations, but excludes such unique vessels as passenger and cruise ships, one can perhaps distinguish the following basic types of cargo ships: 1. LASH, SEABEE or barge vessels of different types.

2. Container and pallet ships which move cargo to and from the ship "over the rail." 3. Ferries, train carriers, auto carriers and roll-on roll-offs which transport or allow movable and wheeled vehicles to carry cargo to and from them.

4. B r e a k b u l k cargo carriers which may carry any cargo, including containers and wheeled vehicles, by utilizing self-contained suitable lifting equipment and moving all cargo over the rail.

5. B u l k e r s , tankers, etc. designed to carry uniform or nearly uniform goods in quantities and in bulk.

The first three types fall into the category of what the authors would call specialized types of ships. Historically, the oldest type of these specialized carriers must be the ro ro vessel.

The container vessel, on the other hand, was slowly emerging in the maritime arena in the 1950s.

The 1960s saw its emergence in the more competitive international trades.

Somewhat later, but in an almost parallel development, LASHtype vessels were developed for special routes, carrying non-selfpropelled barges.

Meanwhile, the ro/ro vessel was slowly emerging. Ferries in some parts of the world began carrying more than passenger vehicles and started looking at heavier cargo and trucking as a paying proposition. Figure 1 shows an artist's impression of a modern deepsea ro ro, clearly indicating cargo access and transfer equipment onboard.

A ro ro vessel is not the answer to all problems and in many cases the adaptability of the ro ro to a specific area or service may not be clear-cut and may be weighed one way or another by many parameters and special situations.

Design Parameters It may be worthwhile to consider some design philosophies and parameters in the selection of a ro ro and container carrier.

Basically, "a horizontally wide open" vessel is desired. No transversal obstructions in the form of bulkheads can really be tolerated in the main cargo spaces, and pillars or other deck supports must be kept to a minimum. In this respect, it becomes obvious that, below the freeboard deck, a double-skin construction must be utilized for strength as well as for damaged stability reasons.

A stern ramp is a necessity. It must suit the requirements of the trade as much as possible, without l i m i t i n g the utilization of the ramp or vessel too much. In larger ships, a ramp that can be operated in at least two directions is the minimum acceptable. A slewing ramp, which allows operation in three directions is the most desirable but may be prohibitive in cost, Figure 2.

Containers must be carried in all decks. The standard unit, known as the TEU (20-foot equiv- alent) is the basis of all studies, but 40-foot boxes must not be disregarded.

The movement of containers over the stern ramps of ro/ro/container vessels is done in several forms. TEUs are moved in an athwartships attitude by large forklift trucks or they are placed, several together, on low "bodies" in systems such as MAFI and LUF. Forty-footers are moved on truck bogies, MAFI or LUF units, or with shore cranes over the rail on accessible decks. The use of specialized bogies is cumbersome and expensive, requires special tractors that are not always available, and severely limits inclination of ramps within and without the vessel. It is therefore felt that TEUs can move by forklift trucks and 40-footers should be loaded and discharged by shore cranes.

Whenever a 40-footer is utilized in quantity, it usually means sufficient facilities ashore are available.

The odd 40-footer moving in and out of undeveloped ports can still be handled by a truck bogie and stowed inside the vessel by a forklift with a top spreader.

Some of the specific design parameters for a ro/ro/container carrier are: Stern ramps and internal ramps: The gradient should be no more than 1 to 7 and with approach slopes on both ends of about 1 to 9. A width of at least 7 meters will allow athwartships carriage of TEUs and double-lane truck traffic. A quarter stern ramp will have to be wider at the entry to the vessel to allow unimpeded movement of trucking without backing and filling for turns. Axle loads of about 65 tons for forklifts and 45 tons for trailer trucks must be allowed. Total loads per 12.19-meter-long unit is about 80 tons. The tire print pressures are 12 kilograms/square centimeter.

The maximum permissible load on the quay (fully laden ramp) is limited to 2 tons/square meter.

Heights must allow clear passage of the highest vehicle expected, keeping in mind movement envelopes of tractors and bodies. The tidal ranges for stern ramps is from plus 4.0 to minus 2.5 meters.

Stern ramps of a minimum twoway utilization, preferably threeway, are recommended. Internal ramps should preferably be of the movable type to conserve cargo space, except perhaps in very large vessels.

Stern doors: Stern doors must have a clear height of at least the maximum expected for any vehicle plus the envelope of the body when passing through the maximum gradient of the ramp. Stern door width should be somewhat more than the width of the ramp and they should, of course, be watertight.

As both stern ramps and stern doors are, in many cases, the only shore access equipment onboard oceangoing ro/ros, it is imperative that alternative means of opening or closing them be provided.

Elevators: E l e v a t o r s should have minimum lifting capacities of 80 tons in large ro/ros and no less than 40 tons in any ro ro except the smallest ones. They should not reduce clear heights at any deck and, if flaps are used for their loading approaches, vehicle movement envelopes and slope limitations should be observed.

Elevator lengths should be about 18 meters. If the vessel is expected to trade heavily in the European Economic Community area and is to often carry rigid trucks with trailers, then their length should be 18.5 meters to accommodate this type of vehicle.

Widths should be at least sufficient to accommodate the width of a forklift truck (3.20 meters).

They should be capable of loading from one side as well as from front or back to allow placing of containers on them by forklift trucks. Axle weights should be equivalent to the maximum axle weight of the decks. Speeds should be about 7 meters per minute for normal loads, with minimum 4 meters per minute for maximum all-up capacity. If an elevator is the only means of transporting cargo to and from a deck, alternative means of operation must be considered.

Car decks: Car decks should be stowable to the overhead and lockable with a minimum amount of lost space. Heights under stowed car decks should be sufficient for stowing two 8-foot 6-inch TEUs with forklifts, and under lowered car decks enough to allow the unimpeded passage of a 4.11-meter-high truck. Space between car deck and overhead, when lowered, should be minimum of 1.7 meters and possible 1.85 meters to allow small truck stowage.

Tie-down points should be spaced half a meter apart. Deflection of car decks, when loaded, must be considered. Finally, cardecks should be in suitable panel sizes and port and starboard sections.

Each section and panel should be independently lowerable or hoistable.

Decks: Decks obviously must be designed to withstand the maximum load (axle and unit) expected to travel over them. All decks should be capable of supporting a forklift truck carrying a fully laden TEU (high cubage), and deck heights must be for two high-cubage containers, plus stacking cones and quick-locking devices between containers, plus lifting clearance for a forklift truck with side or front spreaders lifting a TEU on top of another, both of the 8-foot 6-inch type.

Decks should also be equipped at regular intervals with tie-down devices for trucks and wheeled cargoes (minimum four per side of a 12.19-meter-long body) placed in line between lanes. Lanes are usually assumed to be 2.9-meters wide, allowing a clearance of 200 millimeters per side per truck.

F r o n t and back clearances of 12.19-meter bodies or truck unit (15 to 18-meter total length) is usually 100 millimeters each end.

Hatch covers: Hatch covers are another cargo access equipment of an oceangoing ro, ro ship. The uppermost deck must be fitted with hatch covers that are watertight and flush with the deck.

Pontoon covers are the most logical answer as they can be lifted with shore cranes that would lift containers, but hydraulic cleating would be necessary to minimize releasing and stowing time. Sizes of covers must be such as to allow unhindered vertical loading of containers 40-feet long and a multiple (plus clearance) of 8-feet in width. Strength of the covers must be the same as the rest of the deck.

There are, of course, many other parameters and design philosophies to be considered and each study will have an individual character of its own, molded around the trade route considered, the size of the vessel contemplated, and the type and amount of cargo which the original studies indicated to be available.

Cargo transfer equipment: Having arrived in very general terms at the "optimum" size of vessel required for the route and having established the basic factors, parameters and design philosophies, the naval architect must now decide on the number and type of ro ro equipment and other cargo access equipment that is to be incorporated in the design. At this point, it may be worthwhile to call in for consultation the experts of the trade and listen attentively to the advice they can give.

Maneuverability: The oddity of the lines of a ro/ro/containership creates unusual results in turning circle, crash stop, zigzag, and maneuvering.

Good maneuvering demands the incorporation of a bow thruster(s).

Some designers also favor the inclusion of a stern thruster. The authors believe that machinery rooms of these vessels are crowded enough without such additions, which, if controllable-pitch propellers are used as main propulsion, would have a doubtful degree of utilization. The authors feel that the inclusion of controllable- pitch propellers is almost a must.

Shallow-water maneuvering and speed characteristics are expected to be affected by stern and run aft flatness, but no tests have been conducted, and operational experience is insufficient to establish the degree of afterbody shape influence.

Rudder areas must be increased over conventional cargo vessel areas to compensate for these lower than standard maneuvering characteristics. A t o t a l rudder area of 1.65 percent of immersed underwater lateral area at scantling draft should be sufficient.

Finally, it is true that a vessel which is over-complicated relative to the technical environment in which she is expected to work represents a poor investment. The designers' constant aim should be to obtain the least expensive ship in the long run and the one that would bring the best secondhand price. This means maximizing simplicity and reliability, compatible with fitness for the purpose intended.

Maritime Reporter Magazine, page 12,  Jun 1980

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First published in 1881 Maritime Reporter is the world's largest audited circulation publication serving the global maritime industry.