The paper presents a critical review of recent developments regarding the pi and si factors, adopted in Chapter II-1 on subdivision and stability of ships in the harmonized Convention on Safety of Life at Sea (SOLAS) convention. The two factors adopted from an European Union (EU)-funded research project HARDER are questioned. The paper strongly suggests that International Maritime Organization restores in SOLAS the previous sound and simple formulations for the factor pi from SOLAS 2006, and adopts the factor si, embedded in the accumulation of water on deck.
Many factors affecting the final consequences of ship hull damage are random in nature and their influence is different for different ships. For this reason, the probability of collision survival is taken as a genuine measure of ship safety in the damaged condition (i.e., a measure of merit of ship's subdivision).
A relatively rigorous procedure for dealing with this concept was first introduced by Wendel in 1960 (Wendel 1960) who initiated this novel approach to the evaluation of subdivision. The idea appeared to catch on and was later developed by Comstock and Robertson (1961), Volkov (1963), Wendel (1968), Aleksandrov (1970), and the International Maritime Organization (IMO) (1974), which resulted in establishing and adopting the equivalent regulations for passenger ships. IMO (2009) continued this work resulting in harmonized regulations for all types of ships, contained in the 1974 sOLAs convention, in Chapter II- in parts B to B–4. In the convention, the probability of collision survival is referred to as the attained subdivision index A.
Generally, three measures of merit can be postulated for judging the effectiveness of ship's subdivision:
overall (global) index of subdivision, reflecting the average degree of subdivision for the whole ship that denotes a mean probability of survival for the whole ship in case of accidental flooding, and
local indices of subdivision of two kinds (not in use in the regulations as yet), reflecting the degree of subdivision for individual parts of the ship. They denote mean probabilities of survival either for the cases of flooding in which a given (wing, if any) compartment is involved (alone or together with adjacent compartments), or in which a given transverse bulkhead is involved. For this reason, we talk about one-compartment indices of subdivision and minor damage indices. The formers express the so-called one-compartment standard on the ground of the probabilistic concept, while the latter, a two-compartment standard, so regarded by the practitioners.