# Fire Safety Risk Analysis of Conventional Submarines
## 4 Fire Safety Risk Considerations--Technology Review
### Fire Risk--Hydrogen Fuel Cell  
#### 4.4.1 Failure Modes  
The PEMFC, reformer and exhaust treatment systems consist of many electrical and mechanical components, most of which do not pose a significant fire safety risk. The key components of their AIP system with respect to fire risk are the burner and reforming reactor in the reformer system, LOX and fuel storage tanks, hydrogen purification vessels and associated fluid distribution systems. A review of the Hydrogen Tools Lessons Learned database which contains over 300 incident reports for failures of hydrogen and fuel cell technologies shows that the most common probable cause was Equipment Failure, happening 26% of the time [60]. This far outweighs the next most common cause of Human Error (10%) and appears to be the main risk factor. Equipment failure and human error are the main causes of failure. Other 64% can be taken from [60].  
Of the recorded incidents citing equipment failure as the probable cause, over 50% were due to piping, valves, flanges and or tubing. Fluid leaks often occur at the interfaces, interconnection points or in joints and discontinuous sections of distribution lines. It is at these connection points that seals, flanges, gaskets, fasteners and other components can fail and cause a fluid leakage [61]. Pipe ruptures, split tubes and similar failures have occurred but are less common. It should be noted that only one equipment failure event was listed as being caused by the fuel cell itself, with a short circuit causing melting of a tube filled with hydrogen which caused a small fire [60].  
As with the hydrogen fuel cell, LOX system failures relating to loss of fluid containment are the most critical with respect to fire risk. Although not flammable itself, oxygen supports and accelerates combustion. An oxygen leak will lead to a localised higher than normal concentration of oxygen which can significantly reduce the ignition temperature and energy required to ignite materials [62].  
Failure modes of steam reformers are largely dependent on design and configuration, however those concerning fire risk also relate to loss of fluid containment. The furnace can operate at pressures of up to 100 bar, the reactor can operate in excess of 50 bar depending on design and temperatures in excess of 800 \({}^{\circ}\)C depending on the feedstock. Failure modes are similar to distribution systems and other comparable systems like heat exchangers, ranging from flange, gasket and valve failures to corrosion and fatigue related cracking and fracture [61].  
The primary failure modes which could lead to a significant fire incident relate to containment of the feedstock, hydrogen gas and LOX. Common failures of a hydrogen fuel cell AIP system are summarised in Table 4. It should be noted that for the purposes of this research and any subsequent failure mode and risk assessments, it is assumed that control systems and related components have been designed to an appropriate Safety Integrity Level such that the probability of critical failure caused by the control system has been reduced to negligible levels.