RO-RO Ship Safety
The roll-on/roll-off ship is one of the most successful types operating today. Its flexibility, ability to integrate with other transport systems and speed of operation have made it extremely popular on many shipping routes. The roll-on/roll-off ship is defined in the November 1995 amendments to Chapter II-1 of the International Convention for the Safety of Life at Sea (SOLAS), 1974 as being "a passenger ship with ro-ro cargo spaces or special category spaces..." One of the ro-ro ship's most important roles is as a passenger/car ferry, particularly on shortsea routes.
But despite its commercial success, the ro-ro concept has always had its critics. There have been disturbing accidents involving different types of ro-ro ship, the worst being the sudden and catastrophic capsizing of the passenger/car ferry Herald of Free Enterprise in March 1987 and the even more tragic loss of the Estonia in September 1994.
Although ro-ros have proved commercially very successful, some concern has been expressed about ro-ro ships from the safety point of view virtually ever since the first ro-ro ships were introduced. The whole design concept is different from that of traditional ships because of the introduction of a number of elements which make ro-ro ships unique.
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. With ro-ro ships the installation of unpierced transverse bulkheads is a major obstacle, at least on the upper "through" decks: the whole idea of the ro-ro ship depends upon being able to drive cargo on to the ship at one end and off again at the other. The installation of fixed transverse bulkheads would prevent this. Although ro-ros are all fitted with the watertight collision subdivision, and engine-room bulkheads below the freeboard deck prescribed by SOLAS, the huge vehicle decks make it possible for water to enter very rapidly and fire can also spread very quickly for the same reason.
The cargo access doors at the stern and bow of the ship represent a potential weak spot, as do the side doors with which some ro-ro ships are equipped. Over the years such doors can become damaged or twisted, especially when the door also serves as a ramp.
The movement of cargo on the vehicle deck can affect the intact stability of the ship, causing it to list. The sudden inrush of water following damage to the hull or failure of watertight doors can be even more serious (and rapid). The fact that ro-ro ships generally have a very large superstructure compared with other types means that they can be more affected by wind and bad weather.
Cargo access doors fitted on cargo-only ro-ros are often very close to the waterline. This means that a defective trim or a sudden list, caused, for example, by the movement of cargo, can bring the access threshold below the waterline, resulting in a sudden inrush of water (if the door is open) which will in turn result in the list increasing and a possible capsizing of the ship.
A list can cause cargo to break loose if it is not correctly stowed and secured. The problem is made worse because the crew of the ship cannot normally see how the cargo is stowed inside or on the trailer in which it is transported. A heavy load which breaks loose can cause other units to follow suit. The result can be an increased list, the spillage of dangerous substances and, in extreme cases, damage to the hull and ship's structure.
The high sides of many modern ro-ros, including passenger ships, pose problems regarding lifesaving appliances: the higher a lifeboat, for example, is stowed the more difficult it can be to launch, especially if the ship is listing badly.
These factors indicate that ro-ros are highly sophisticated ships which require very careful handling. This makes them exceptionally vulnerable to human error.
Because of the publicity surrounding accidents involving passenger ro-ro ships such as the Herald of Free Enterprise, Scandinavian Star and Estonia it is sometimes assumed that this type of ship is much more dangerous than others. This is not borne out by statistics. The World Casualty Statistics for 1994 published by Lloyd's Register of Shipping show that passenger/ro-ro cargo loss rate per thousand ships was 2.3 - the same as the average figure for all ships.
However, when one considers loss of life at sea the picture changes. Between 1989 and 1994, the Lloyd's Register figures show that 4,583 lives were lost in accidents at sea. Of these 1,544 were lost in accidents involving passenger/ro-ro cargo ships - exactly one third, even though ro-ro ships make up only a small fraction of world merchant marine tonnage. This would seem to indicate that although passenger ro-ro ships are involved in an average number of accidents the consequences of those accidents are usually far worse.
An important study concerning the safety of ro-ro ships (including cargo ships) was submitted to IMO in 1983 by Norway. The study was compiled by the classification society det Norske Veritas and covered the years 1965-1982. Of 341 casualties during the period, 217 were defined as serious and 36 resulted in the total loss of the ship. The study showed that the most common causes of serious casualties were collisions (24%); machinery damage (17%): grounding (17%); shift of cargo and operational (16%); fire and explosion (14%). The figures changed significantly when total losses were studied. Here the most common cause was shift of cargo and operational faults (43%); collision (25%) and fire and explosion (18%). The dNV study showed that total losses as a result of a collision were much higher for ro-ros than for other ships (with only a 9% occurrence). Both collisions and uncontrolled shifts of cargo more frequently led to serious consequences with ro-ros.
The paper noted that more than 70% of all ro-ro total losses due to collision resulted in loss of lives while 60% of ships reported to have capsized or sunk following a collision did so in less than ten minutes. Nearly all of the total losses involved ships of less than 110 metres in length. A further important point made by the dNV study was that the ro-ro ships most frequently exposed to serious casualties and total losses were the pure ro-ro and freight-only types. Pure ro-ros had a high percentage share of all casualties and especially of total losses. Passenger ferries, on the other hand, had a fairly high percentage share of all categories but the serious casualty/total loss frequency was relatively low.
The study also showed that the total loss rate for ro-ros was significantly lower than the average for the world fleet (under 0.25% over the 13-year period compared with about 0.55% for the world fleet).
A quick look at some of the best-known accidents involving ro-ro ships also indicates some of the major problem areas. Several of them involved water getting on to the vehicle deck through the cargo doors, either as a result of a mistake or an accident.
The first ro-ro ship to be lost at sea was the Princess Victoria, a rail ferry which sank on a voyage to Belfast in 1953 when heavy seas stove in the stern door: 133 lives were lost. At least 264 people died in 1966 when the Greek ferry Heraklion sank in heavy seas on a voyage to Piraeus. Although not a ro-ro, the ship did have a large car deck without subdivisional bulkheads. This deck flooded when the loading hatch was smashed by a vehicle which had broken loose. The cargo ro-ro Hero was lost in 1977, partly as a result of water entering through a leaking stern door. In September 1994 the passenger ro-ro Estonia was lost with more than 900 lives when the bow door was torn off by heavy seas. The car deck flooded and the ship capsized within a few minutes.
These accidents happened in heavy seas, but other ro-ros have been lost through water entering doors in port or sheltered waters. They include the Straitsman, which sank when the stern door was opened as the ship approached land, with the crew unaware that the door sill was below the waterline: and the Seaspeed Dora, which capsized in 1977 when a movement of cargo caused the ship to list sufficiently for water to enter through an open bunkering door. In the case of the Herald of Free Enterprise, water entered through the bow door which had been left open.
Ro-ro ships which have sunk rapidly as a result of a collision have included the Jolly Azzurro (1978), Collo (1980), Tollan (1980), Sloman Ranger (1980), Ems (1981), European Gateway (1983) and Mont Louis (1984). Among ships which have been lost following a shift of cargo are the Espresso Sardegna (1973), Zenobia (1980) and Mekhanik Tarasov (1982, in very bad weather).
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