Safety assesment of small container feeders

In this paper the intact stability of small container vessels is investigated, both in calm water and in a seaway.
The research was triggered by the capsizing of the container feeder ‘Dongedijk’ near Port Said in August 2000. It is shown that a combination of over-loading, a ‘flat’ part in the righting lever curve and a large trim to the stern can cause an accumulation of water on deck due to heel when changing course, which results in an extreme heeling angle and finally capsizing of the ship.
The stability sensitivity for cargo weight and position together with the occurrence of the above-mentioned static undefined regions within the stability curve is further being looked at. A static study is performed for two vessel designs: one with a minimal freeboard and one with a higher freeboard but with the same hull shape and deadweight capacity.
As it was suspected that a low-freeboard vessel with similar stability characteristics could be susceptible to instability in a seaway, non-linear numerical simulations were performed in order to determine the probability of capsizing for various wave conditions. For the calculation of these probabilities the program FREDYN was used. In order to validate the FREDYN calculations for this ship type and to collect more data about the absolute risk values, model tests have been performed. The results of the model tests and the calculations in a seaway are compared and discussed.
The findings of this study contribute to the safety at sea in two ways.
- Information about the stability sensitivity of a ship is of importance for crews when operating this kind of ships as well as for the designers when optimising a design.
- The results from the simulations in seaways can contribute to the possible revising of current stability criteria values on the short term and provide information for a new set of rules on the long term.