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Wednesday, 21 December 2016

Conditions for risk based ship survivability approach: a study on the analysis of fire risk


Published article:Liwång, H. (2016). Conditions for risk based ship survivability approach: a study on the analysis of fire risk. Naval Engineers Journal, 128(3), 31-45.
Some comments from the article
The purpose for introducing a risk-based approach is to identify risks in the intended operation of the ship and use this information to guide the concept development and ship design using a risk-based ship design approach. The uncertainty in the design decision making is generally high when novel concepts are developed. Therefore, a rational ship design support process is necessary to avoid erroneous assumptions that affect design choices. Risk analysis is a knowledge model that may reduce this uncertainty. Based on this need, the proposed analysis models are in this paper used to identify critical aspects and gaps in the analysis process, specifically for naval ships.
Risk controls must be analyzed with respect to susceptibility, vulnerability and recoverability; the total effect of these aspects must be understood to evaluate survivability. The fire risk cannot be analyzed without a general analysis of a ship’s susceptibility and vulnerability with respect to relevant threats, which indicates that the analysis depends on relevant multiple operational scenarios.
Physical descriptions of fire depend on the ship specifications, and for the same operational scenario, the ship design concept will vary due to differences in the ship tactics, susceptibility and vulnerability. The example in this paper demonstrates that the ignition frequency for weapon-ignited fires depends on the location of the compartment; this is not the case for accidental fires. Different design concepts will also require different passive and active fire protection depending on the differences in the design and how it is manned. For example, if an FRP concept is considered, the combustible nature of FRPs can contribute to the fire in extreme fires; in other cases, with higher expected frequencies, the thermal insulation of FRP will yield a smaller fire zone and contribute to survivability.
The critical systems must be identified to analyze the ship kill levels. These critical components and systems depend on the ship design and assumed tasks after a hit (often described as the ship survivability levels). Typical critical systems include the propulsion system and power supply.
The importance of firefighting on naval ships is highlighted in the study which show that firefighting is the most important aspect for reducing the probability of catastrophic consequences from complicated ignition cases because the built-in protection is insufficient for stopping the fire escalation. The reaction times and effectiveness with respect to firefighting onboard naval vessels are difficult to compare with other firefighting conditions thanks to extensive training, a high level of readiness, high number of crew members relative to the ship size and good firefighting equipment availability.
Certain problems have been raised for risk-based approaches, especially for defining the scenario, such as limited research and perceptions. However, these problems are consistent among most analysis approaches, but heavy use of complicated tools may hide these aspects and make validation more complicated. Further, uncertainties are particularly challenging, especially for analyzing antagonistic threats. On the other hand, a probabilistic approach offers a framework that is consistent from theory to the first principle tools, which has been found to improve the decision-making process when selecting among candidate survivability design principles.

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