A New Approach for the Water - On - Deck - Problem of RoRo - Passenger Ships

Since the ESTONIA accident in 1994, the so called water on deck problem for RoRo-Passenger Ships has been subject to many investigations. Being the central part of the Stockholm Agreement (MSC Circ.1891 and EU directive), the water on deck problem was included in the damage stability calculations in addition to SOLAS 74/90 II-1/8. Although some of the assumptions are not physical sound, it is obvious that the safety level of RoRo-Passenger Ships has significantly been improved by including the water on deck problem in the safety regime. Unfortunately, the SOLAS 2009 does not explicitly address this problem, and there have been indications that the present safety level of the SOLAS 2009 seems not to cover the Stockholm Agreement for most of the smaller RoRo-Passenger Ships/ Ferries. However, when accidents of ships are analysed where water on the vehicle deck plays the dominating role, one finds that in most cases the problem is more related to intact stability. This is due to the fact that the involved ships were not damaged below the waterline, and this does especially hold for all problems related to firefighting on the vehicle deck. Therefore we tried to formulate the water on deck problem as an intact stability criterion. In a first step, the stability limiting amount of water on deck needs to be determined. Then, in a second step, righting levers for the intact condition including this amount of water on deck can be computed, and some defined intact stability criteria can be applied. When determining the amount of water on deck which shall be used as design value, it is useful to analyse the relevant accidents. As a matter of fact, the ships accumulated water on deck due to various reasons, and the crew continued their operation until the situation became irreversible. They were not aware that they had run into a dangerous situation. This led to the idea to use the alteration of the roll period with water on deck as a suitable design criterion) and as an indicator for dangerous situation which easily can be measured by the crew). Consequently, we performed numerical roll decay tests with several RoRo-Passenger ships, where we varied the amount of water on deck. As an interesting result, we found that when increasing the amount of water on deck, the roll period first increases slightly and then changes drastically with a steep gradient. As a good rule of thumb we found that when the roll period doubles, a significant amount of water has accumulated on deck, but the ship still has a significant remaining stability margin against capsizing. Thus we used this approach to come to a reasonable design value for the minimum amount of water to be considered on deck. We also found a significant influence from centre casings on the amount of water on deck, which has to be considered. The proposed stability criteria have to be complied with for the intact condition including a dedicated amount of water on deck. These loading conditions were defined in such a way that all ships which are fully compliant to Stockholm Agreement do also fulfil our new approach, which is quite robust.