New damage stability requirements for ships

When water is running into a ship following an accident, different scenarios can occur. The ship may sink due to flooding of so many compartments that there is not enough buoyancy to keep the vessel afloat. This was the case for RMS TITANIC that sank in 1912, 2.5 hours after hitting an iceberg (Fig. 1). With today’s rescue means, 2.5 hours would be enough time to save most of the people on board.

A much more serious scenario is capsizing due to loss of transverse stability as this can happen within a few minutes. The disasters with HERALD OF FREE ENTERPRISE (Fig. 2) and ESTONIA are examples of this. Due to the short time period needed to capsize and the heeling of the vessel, rescuing is extremely difficult and thus naval architects have to deal thoroughly with damage stability when designing a vessel. The problem can be presented by two main questions: what is the size of the opening to the sea following damage? and which parameters govern the stability of the damaged vessel?

The extent of damage depends on the type of accident: collision, grounding, damage in abnormal wave conditions and damage due to improperly closed watertight openings. Many of these scenarios are very difficult to analyse in detail due to the complicated mechanics in high-energy accidents. Hence, the ship designer relies on regulations from the UN body IMO (International Maritime Organization). These regulations have in general been developed and revised following disasters at sea. The current regulations are to a large extent based on rule damages that specify the nominal extent of damage for a ship to survive without capsizing.  The survival criteria are based on hydrostatic properties of the ship with only marginal considerations of dynamic effects.

Ship-ship collisions are relatively frequent causes of ship losses

Whereas the HARDER project significantly modified the probability distributions for the damage extent, fewer changes were proposed for the probability of survival following damage. The survival probabilities are still based on the hydrostatic stability properties of the ship in the damage configurations. However, research is ongoing in order to define damage conditions that in a real seaway will lead to capsizing within a short period of time.

The IMO Intersectional Working Group on Subdivision and Damage Stability has in February 2005 made revisions to the HARDER proposal for discussion at the plenary session at the MSC80 meeting. It is expected that the results of this research will have a major impact on the design of future ships.

Capsizing of Herald of Free Enterprise  

Because the rules are initiated by specific events in the form of disasters they have developed differently for different types of vessels. A request has recently been made by IMO for a harmonization of the rules to make them into a single set, valid for all types of ships. As a start, damage stability following a collision is considered. The general framework is probabilistic in the sense that all possible damage sizes are weighted with their probability of occurrence and probability of survival. The resulting number, the Attained index A, should then be greater than or equal to a Required index R, taking into account the expected consequences of capsizing (loss of life, environment damage, loss of cargo and ship). Whereas A is calculated for each individual ship, R is defined by IMO in terms of number of people on board and size of vessel.

The formulation of a consistent, harmonized damage-stability regulation has been the subject of an EU-FP5 project, denoted HARDER, and the outcome of the project has been used by IMO for subsequent discussions and modifications. In the HARDER project a simulation tool (Fig. 4) was developed by MEK. The results from the simulations were compared with an updated damage database also collected in the HARDER project and in general good agreement was found.

Simulation procedure developed at DTU Mekanik, for collision risk evaluation