A Rational Approach to Life-Cycle Stability Management for Passenger Ships

Stability has been a primary focus of the maritime industry and of immense interest to the IMO from the outset. Despite several attempts to resolve stability-related issues, the problem of stability remains one that has yet to be resolved. Reasons for this, range from the complexity of the problem itself to misconceptions in its very nature, particularly concerning compromised conditions of the ship in question. More specifically, whilst stability of ships is an extremely interesting scientific problem and a determining factor in ship design, the impact on the operation of passenger ships, a matter of crucial importance from the dual point of view of safety and economics, has hardly ever received the attention it deserves. Currently, intact stability and damage stability share the same stage from a regulatory perspective and, consequently, they have equal impact on design and operation-related decisions, an example of which is the use of combined intact and damage stability GM limit curves (e.g., IACS Rec 110 Rev1). However, in line with goal-based regulations and standards, design and operational decisions should be risk-informed in which case, matters relating to damage stability are of higher concern, simply by virtue of the fact that damage stability is by far the greater risk contributor. In fact, for passenger ships (>100 m in length), the level of risk associated with intact stability is indiscernible in contrast to that of damage stability. More importantly, in the operational loading conditions of such vessels, damage stability is a more dominant constraint. Hence, such ships are designed based on damage stability considerations alone. However, since life-cycle risk management is still an active research subject, stability management is being addressed somewhat haphazardly by allowing for design GM margins, drawing from experience on the average deterioration of GM over the life cycle of passenger ships. This is typically 1% to 2% per year on average, which leaves roughly 50% of ships on the wrong side of this averaging process and this, in turn, limits operation substantially with severe economic impact. This paper delves in this direction by drawing on current ship design and operational practice and presents an innovation with application to a large cruise ship to demonstrate how life-cycle damage stability management could be tackled in a structured, rational and cost-effective manner.