Know A Ship - RoRo Ships- Part Two

RECAP:

Last time we spoke about RoRo Vessels in the first article. A reading of the article gives some idea about the history and utility of the these vessels.This article will focus on the ship from a designer's perspective.

THE DESIGN:

The main requirements against which a design is to be developed will generally include certain statements.

• Passenger Market for which it is intended to cater- deluxe,middle market or the popular "Package Tours".
• Passenger Numbers which will determine accommodation and cabin types.The number of vehicles and the mixture of their types should be incorporated into their design.
• Normal service speeds and maximum service speeds in different legs of cruises.
• Sea areas of interest and times of the year.
• Maximum Duration of cruises between restoring and refueling at ports.
• Details of loading and unloading ramps at each of the ports the ship is intended to use.Particulars of the ramp,distance to center of the ramp from side which ship will be berthed against.

The dimensions are largely determined by the vehicle deck(s) layout.Large vehicles heavily influence the design.The ramp is divided into lanes,usually two or more.The length determines the positioning of engine casing and access stairs.In less maneuverable vehicles,the bow to stern door straight run is preferred. This keeps the weight differences between lanes minimized and the hoist-able car decks can be easily arranged in the position.

With  the ship's length known, the beam of the ship can be determined by calculating the multiples of vehicle widths and access space.Now, deck capacities are estimated using data on how cars and heavy vehicles should be arranged.The tween deck height favors heavy vehicles and the ship sides are suited for cars.When further optimization is required,the cars are run down ramps to lower or up to higher decks as required

Bow Ramp Mechanism on a Ro-Ro Ship

Layout of a Ro-Ro Vessel

.

The tween deck space is suited for larger vehicle cargo

Now the berthing requirements are examined and modified if required to suit the ship design.

Ship weight calculations are carried out and suitable block co-efficients  and drafts are determined.

These days owing to some previous tragedies.(Read Estonia and See Herald of Free Enterprise), certain Ro-Ro exclusive safety regulations are now factored too.

• Increase free-board to car deck to check heavy inflow of water into this deck.
• Providing heads that ensure that water drains away through drains/scuppers.
• Sometimes,inflatable air bags at ship sides are also considered. 

With the depth to car deck fixed the design can now be developed as required considering the machinery weight,power estimates with decision on type of machinery with diesel,diesel-electric and gas turbine electric being main contenders. 

Spacing and number of water-tight bulkheads are decided upon. Positioning of public rooms are done and accommodation of crew and officers are allocated respectively based on the frame spacing.Preliminary stability assessment is done using established methods. Now the shipbuilding process proceeds to the next stage i.e. ship calculations.

Lift-on/Lift-off (LO/LO) Ships:

LoLo vessels (acronym for Lift on - Lift Off) are vessels that can transport a range of different products as a result of their flexible cargo space,high container capacity and on board crane facility. Lift-on/Lift-off (lo/lo) cargo is generally containerized cargo that must be lifted on and off vessels and other vehicles using handling equipment.

A LoLo operation is when cargo is loaded and discharged over the top of the vessel using cranes or derricks. LoLo vessels can load and unload cargo at Roll On-Roll Off (Ro-Ro) ports, Load On-Load Off (Lo-Lo) ports and at unserviced jetties, using its own cranes.

 At-sea demonstration of their Large Vessel Interface
 Lift-on/Lift-off (LVI Lo/Lo) crane system.

Cargo vessels can be equipped with a variety of cargo handling systems the most commonly encountered systems being classified as either lo-lo or ro-ro (the vessels may sometimes be referred to as lo-lo or ro-ro vessels even though it is possible to have both loading capabilities on the same vessel). 

Each vessel type and cargo handling system has applications that may be more appropriate for certain specific commodities and cargo. In addition, each has it's own advantages and disadvantages in terms of cost and service characteristics. Matching the technology to available port infrastructure, commodity markets, and labor constraints will have a strong influence on the economics of a particular operation. LSD 

Article By: Sudripto Khasnabis

Author's Note: This article is the second one about Roll-on/Roll-off  Vessels.The next article will talk about vessels with standardized cargo dimensions-The Container Ships.The section shall proceed regularly with similar articles about other ship types with a common aim of making the reader aware of the general principles on which their design and construction is based and their utility and characteristics. The videos and the figure do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com