The M/V Cougar Ace seen listing on its port side in July 2006.

Equipped with built-in ramps and long tiers of decks, Roll On Roll Off (RoRo) ships are essentially floating parking garages that allow vehicles to be driven aboard the ship at the loading port and driven off at another. The use of the vehicles’ own wheels makes loading RoRos much faster and more effective than traditional ships which require massive cranes to unload.

RoRo’s are certainly efficient but, the question remains, are they safe? The truth is that the most tragic RoRo incidents – like the 1987 Herald Of Free Enterprise disaster which claimed 193 lives, the 1994 M/V Estonia tragedy which claimed 852 lives, and yesterday’s sinking of the Baltic Ace – are only exclamation points on less tragic losses like that of the M/V Cougar Ace and M/V Tricolor.

History

Modern RoRo ships can trace their origins back 100 years to the early days of the steam locomotives. Back then, ships were designed to take trains across rivers which were too wide for bridges: the ships were equipped with rails, and the trains simply rolled straight on to the ship, which sailed across the river to another rail berth where the train would roll off again. An example is the Firth of Forth ferry in Scotland, which began operations in 1851.

It was not until the Second World War however, that the idea of applying the RoRo principle of road transport became practical – and was used in constructing the tank landing craft used at D-Day and in other battles. The principle was applied to merchant ships in the late 1940s and early 1950s. It proved to be extremely popular, especially on short-sea ferry routes.

For the shipper, the RoRo ship offered a number of advantages over traditional ships, notably speed. As the name of the system implies, cars and trucks can drive straight on to a RoRo ship at one port and off at the port on the other side of the ship within a few minutes of the ship docking.

But why are they so dangerous?

mv-cougar-ace-infographic

Flooding diagram from the M/v Cougar Ace

On conventional ships, the hull is divided into a number of separate holds by means of transverse bulkheads, many of which may be watertight. In the event of the hull being holed, the bulkheads will limit or delay the inrush of water, resulting in the ship sinking slowly enough for the evacuation of those on board or even preventing the ship from sinking at all. In addition, these transverse bulkheads and compartments limit something called the “Free Surface Effect” which is a huge contributor to a loss of vessel stability.

Having wide open spaces on a ship with only an inch of water on the deck can create a serious problem with regard to vessel stability.  RoRos are particularly subject to this dilemma due to their large open cargo areas and coming up with the right solution to mitigate this issue is one of the top challenges a naval architect might face when designing such a vessel.

Another problem is freeboard, the height of a ship’s side between the waterline and the deck. Cargo access doors fitted on cargo-only (passenger RoRos are now subject to stricter regulations) RoRos are often very close to the waterline. These access doors (often located at the stern and bow of a RoRo) can become damaged or twisted during a collision, especially when the door also serves as a ramp.

Another problem is that the movement of cargo on the vehicle deck can affect the stability of the ship, causing it to list. The sudden inrush of water following a collision can cause vehicles to break loose from their lashings and pile up on the low side of a capsized ship. Even a moderate list can cause cargo to break loose if it is not correctly stowed and secured.

Perilous Escape

All of the above factors can shorten the amount of time that the crew has to escape a RoRo before she sinks but, once the alarm is given, abandoning a RoRo that is listing can be dangerous. In 2006 the crew of the RoRo vessel Cougar Ace was able to escape the ship after she experienced ballast problems in Alaska but, because of the 60-degree list, a member of the salvage team slipped and fell to his death on the vessel’s upper deck.

Even if the crew can maneuver themselves across a heavily listing deck, the high sides of many modern RoRo’s pose another problem: the higher a lifeboat is stowed above the waterline the more difficult it can be to launch, especially if the ship is listing badly.

These factors indicate that RoRo’s are highly sophisticated ships which require very careful handling. This makes them exceptionally vulnerable to human error.

In Defense of RoRo’s

While many experts agree that RoRo’s are dangerous, in 2004, more than 1.3 billion passengers, 188 million cars, 856,000 buses and 28.7 million trailers were carried on 5.9 million crossings globally and non-passenger carrying RoRo’s, like the Baltic Ace, have a similarly impressive safety record in recent years.

As of this moment little is certain about the cause of the tragic Baltic Ace incident, but one thing is certain: the industry’s best minds will continue to focus on improving regulations to make ocean transport safer.

Information sources: IMO.org

gCaptain’s FULL Coverage: Baltic Ace Sinking

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