Ro-Ro is a special type of cargo ship which carries vehicles or wheeled cargo. It is different from other cargo ships in construction, stability, arrangements and operation. Let’s see all these factors in details:
Ships in which the cargo is wheeled or loaded/unloaded on board in a vehicle or platform which is equipped with wheels.
What is Roll-On/Roll-Off ?
“Roll-On/Roll-Off” is the process of loading and unloading of wheeled vehicles.
First time, it is used in 1861 in naval ship But in merchant ship RO-RO Concept came in 1940’s. It has high capacity cargo and of high value. Liner Trading is preferred.
Types of ro-ro ship
- Ferries – (Carriage of passenger with vehicles, earlier)
- Freight – (Carriage of Wheeled cargo)
- Ro-Pax – (Transport of Passengers + Vehicles + Wheeled cargo)
- Con-Ro – (Transport of Container + Wheeled cargo)
- RoRo-LoLo – (Both arrangement)
- Pure Car Carrier – (Transport of cars)
- Pure Car Truck Carrier – (Transport of Cars + Trucks)
Ro-ro ship construction
There are 2 different structural concepts regarding car carriers (RoRo ship).
Hinged deck design (flexible design)
For the hinged deck design, an elastic hinge arrangement between the vertical web and the deck girders increases the ability of the ordinary side web frames to sustain transverse racking deformations of the upper hull. ln consequence, the side webs are then normally more slender than for a conventional design. However, in order to carry the racking moment, the main transverse racking constraining members must be strengthened. However, the total capacity of the racking constraining structures must be the same for a conventional (rigid deck) design as it is for a hinged deck design.
Rigid conventional deck design
A conventional Car Carrier design means that the vertical side webs are in line with the deck transverses. This means that transverse forces on the decks will cause the deck transverses to bend. As a result, when compared to the hinged deck design, the frame section (vertical side and transverse deck girder) is rigid when subjected to transverse forces. A considerable fraction of the racking moment created above the bulkhead deck (freeboard deck) is then mainly to be carried by the frame section itself.
Ro-ro ship Critical parts or region:
- Wheel loaded deck
- Deflection in way of ramps
- Transverse racking
- Racking constraining structure
- Pillar structure
- Fixed ramps
- Ventilation ducts
- Transverse bulkheads
- Bottom girders
- Floors etc.
Ro-ro ship ventilation
- Ventilation systems for RoRo cargo spaces works on the principle of dilution ventilation
- In dilution ventilation, supply air flow to the area is sufficient for the exhaust gases to mix thoroughly with the air and dilute the pollutants
- t controls pollutants by ventilating the entire space
Dilution ventilation is classified into two types.
1. Exhaust air ventilation
In exhaust air ventilation, fans remove air from a RoRo cargo space, and this is then replaced by outdoor air entering through open ramps, doors and other openings. Exhaust air ventilation is employed when sub-atmospheric pressure is required in the RoRo cargo space. The Sub-atmospheric pressure which prevents pollution from the spreading to nearby areas or adjacent spaces.
2. Supply Air ventilation
It operates Just opposite of exhaust air ventilation. Fans deliver outdoor air into the RoRo cargo space and the air is then exhausted through ramps and other openings.
Supply air ventilation usually creates slight pressurisation of the RoRo cargo space. If only supply air ventilation is used, pollutants may mix with the supply air, be pushed up the internal ramps, and contaminate other decks.
Ro-ro ship arrangement
Access way: (Ramps & Doors)
Ramp: (a sloping surface joining two different levels, generally at the entrance and floors)
- (External Ramp & Internal Ramp)
- Stern Ramp and Bow Ramp are external ramp, because it is used externally for loading or unloading.
- Stern Ramp – Fitted at stern
1. Straight stern ramp: The straight ramp, for the tidal variations under consideration, should be about 20 m long by 7 m wide and be able to handle two 80-t vehicles moving on her at the same time with a maximum axle load of 45 t.
2. Angled ramp: The same ship, equipped with an angled ramp with the characteristics of the straight ramp, would require one of 36-m length which would weigh about 130 t.
3. Slewing ramp: Flexibility in loading from both sides as well as stern-to is achieved only with a slewing ramp. This is a common requirement nowadays, and the slewing ramp would have the same overall dimensions as the angled ramp for our model ship.
4. Jumbo ramp: This type of ramp has been recently introduced. It has a clear width at ship’s entry of approximately 25.5 m, a clear width (minimum) of approximately 12 m, and a length of 50 m, weighing only 425 t.
Bow Ramp – Fitted at bow
It is not used now-a-days due to various reasons regarding stability
Internal Ramp: Use internally i.e. It is used to arrange vehicles in various locations inside the ships.
- Slope of the internal ramps is normally between 8° to 6°
- Widths between 7 and 12 meters
- Anti-skid surfaces
Type : Fixed Ramp and Hinged Ramp
- Number of decks increases with the size of ship
- For ships up to 15000 DWT, 2 decks are most common
- Bigger ships may have 3 permanent decks
- Movable car decks are installed in between the permanent decks
- Movable car decks are of light weight construction that allows the stowage of cars
- There are 2 types of movable decks: Hoistable deck & Liftable deck
- Hoistable deck has integrated lifting device but Liftable deck has scissors type lifting process
Internal door and External door
(Watertight door designed for the subdivision of the cargo spaces with a minimum interference)
- Securing should be done as per IMO criteria
- Decks shall be provided with securing points with: longitudinal spacing < 2.5 m and transverse spacing within 2.8 m to 3 m.
- The minimum strength required to without permanent deformation should be 20 Kilo Newton (KN ).
- It is made of chain or any other device made of steel or another material of equivalent strength.
- Strength should be of 120 KN.
- The Lashing which should only be attached to the secure points.
- Angle between the lashing and horizontal and vertical planes should be 30° to 60°.
Arrangement for Loading or Unloading of Vessels:
- Loading or Unloading is done horizontally
- It is done by various cargo handling equipment like Ramp, Doors, Lifts, Movable deck etc.
- Cargo (vehicle) is arranged properly by securing and lashing process
- Vehicle is loaded by a Ramp, it is generally attached with ship
- Earlier it is done by crane (Lo-Lo process)
- Vehicles are arranged at different platforms (decks) with maintain stability of vessel.