# High-Expansion Foam: A Risk Control Option to Increase Passenger Ship Safety during Flooding
## 1 Introduction  
Survivability of passenger ships after an accident is a parameter that needs to be assessed during the ship design process to evaluate the potential risk of loss of life associated with the accident itself [1, 2, 3]. This has become (and will always be) even more relevant due to the incremental growth of passengers in new cruise ship designs [4]. According to the statistics of casualties involving cargo and passenger ships in the last 10 years [4], the following ranking can be provided: 27% loss of control, 26% collisions, 15% contacts, 13% groundings/standing, 12% damage, 5% fire, 1% flooding, and 1% capsize. The majority of the casualties are related to collisions and groundings/contacts, which are damage types that intrinsically implicate flooding and the potential sinking or capsizingof the vessel [5]. To this end, regulations impose restricting requirements for the damage stability of passenger ships [6; 7]. The normative request of incrementally higher standards for damage stability impacts the internal layout of a new passenger ship design [8]. The standard answer of naval architects concerning the request of increasing survivability after an accident corresponds to an increased subdivision of internal spaces [9; 10; 11]. However, reacting to more restrictive safety requirements through a fractal fragmentation of the internal ship volume also has a physical limit for new designs [12], decreasing the ship earning potential (more weight and less usable space) without further increasing its safety. Moreover, a substantial reconfiguration of internal spaces may be technically impossible or too expensive for the refitting of existing ships for fulfilling new safety requirements [13]. An efficient and cost-effective way to boost the safety of passenger ships should aim at overtaking the actual plateau of the safety level reached by modern designs, looking more into residual risk mitigation rather than total risk prevention. Mitigation of the residual risk implies the adoption of technological innovation where solutions are likely to be found in the shorter-term and, more than likely, be cost-effective. On the other hand, for the specific cases of damage stability, residual risk mitigation requires a thorough understanding of the loss modalities of a ship due to flooding, thus the availability of appropriate methods, tools, and techniques capable of meaningfully addressing the physical phenomena involved in the flooding process [14; 15]. In this sense, the application of risk mitigation systems based on detailed analyses of the flooding process designed jointly with the application of materials granting sufficient water tightness and mechanical properties could drastically increase the safety level of a passenger ship after a flooding accident [16; 17]. An effective result of this radical change in the approach to flooding risk management for passenger ships is provided by the AREST (Adaptive REconfigurable Safety Technology) systems, related to a University of Strathclyde Patent (patent no. PCT/GB 2017/050681), combining accurate flooding vulnerability analysis with the use of high-expansion foams to restore or limit the loss of buoyancy due to flooding. Such a system is composed of passive solutions, active solutions, or a combination of both of them. Traditionally, mitigation measures address only one of the following options for risk reduction: limiting flooding occurrence, increasing the residual stability or limiting/preventing flooding progression. Ship reinforcement [18] is a design measure that aims at reducing the potential dimension of a breach, thus limiting the flooding occurrence. Permeability reduction aims at increasing vessel residual stability by using different strategies, such as counter ballasting, application of foams, systems employing C02-based fire systems to activate air-bags inside damaged compartments, etc. [19]. The fragmentation of flooding progression is achieved by increasing the number of watertight doors [20] or changing the internal layout of the ship [21].