Why do larger passenger ships often use a two-compartment damage standard?

In naval architecture, the two-compartment damage standard is a safety measure commonly applied to larger passenger ships to protect against the risk of a collision occurring at the boundary between two adjacent watertight compartments.1

While a one-compartment standard requires that a ship remain afloat if any single compartment is flooded, the two-compartment standard provides a higher level of redundancy by ensuring the vessel can survive the flooding of two adjacent compartments. This design strategy is intended to mitigate the consequences of hull breaches, as a collision often impacts the area where bulkheads meet the hull, potentially compromising the integrity of two neighboring spaces simultaneously.1

It is important to note that while subdivision is vital for safety, it is impractical to design a passenger ship to survive all possible flooding scenarios, as excessive subdivision would severely limit the interior space required for passengers and operations. Naval architects use floodable-length curves to determine the optimal placement of watertight bulkheads to balance safety with functional design.12