MARITIME SAFETY

36 www.motorship.com February 2012

WITH OWNERS ordering ever larger cruiseships to achieve economies of scale, thereare an increasing number of vessels under

construction and on the drawing board whose designfeatures deviate from some of the more prescriptiveaspects of Chapter II-2 of SOLAS. As a result theyrequire a safety assessment of their alternative designarrangements in order to gain approval.

Under SOLAS II-2, for designs that deviate from aparticular statutory requirement, IMO sets guidelinesfor agreeing the alternative design arrangements andthese are in two basic parts. The first involves theanalysis of the design in qualitative terms. The secondstage involves quantitative calculations to prove thedesign performance. Fire screening falls within thefirst procedure.

To meet the needs of naval architects and shipowners working on large cruise ship designs, Glasgow-based Safety at Sea has developed an improved firerisk screening methodology, as an evolution of anearlier model developed in 2007. This latest screeningtool can estimate and assess the risk contribution ofevery space which may be considered a potentialplace of fire origin onboard a passenger vessel.

The risk contribution of these spaces is added up toestimate the risk contribution of specific areas of thevessel, or fire zones, in relation to the risk contribution ofthe vessel as a whole. According to Luis Guarin, director,safety engineering: “The method is particularly usefulwhen undertaking preliminary assessments ofalternative designs, as it facilitates the task of makinginformed choices about casualty scenarios forquantitative and fire engineering analysis.”

When dealing with large fire zones, the results ofthe screening process can be used to identify specificlocations where there are the highest levels of risk, inorder to support the definition of particular casualtyand design scenarios for the qualitative analysisphase. In addition, the results of the fire screening canbe used to identify areas onboard which may requirespecial attention in relation to fire safety, even if theyare not subject to alternative design and arrangements.

Taking a large cruise ship design as an example,

one of the most common ‘triggers’, as far as alternativedesign arrangements are concerned, is the existenceof fire zones larger than 48m in length or 1600m2 indeck area. A hypothetical example is shown in Figure

1, illustrating a representative cruise vessel with twomain vertical zones (MVZs) exceeding 48m in length.

In this particular case, out of the possible 31 firezones deviating from SOLAS requirements, the designteam has to identify a number of representative firescenarios for subsequent quantitative analysis.

In its earlier fire screening model, Safety at Sea useda ‘first-principles’ approach, based on simplified firedynamic calculations. Practical difficulties associatedwith that approach, and the observation that relevantfire protection arrangements influencing all of the variousprobabilities have a degree of correlation with thespace category (CAT) defined in SOLAS Regulation II-2/9,led to the development of a more practical approach.

The new approach draws on available casualty dataas a function of the CAT of each space associated with aparticular incident/accident.

According to Guarin: ”Rather than relying on expertjudgment and opinions alone, we have developed amethod of screening the complete layout of the vesselbased empirically on statistical data. This allows for asystematic evaluation of all the fire zones to assesswhich are most likely to experience fire and whichareas are most vulnerable to fire. It also allows the

Taking a systematicapproach toscreening fire risksUK-based company Safety at Sea Ltd describes its new fire riskscreening methodology, designed to meet the requirements oflarge ships, and larger cruise vessels in particular.

Figure 2: Correlation between fire zones deviating from SOLAS requirements and firezones attaining the highest levels of risk to people from fire

Figure 1: Typical layout of the decks and main fire bulkheads for a representative cruise vessel with 7 MVZsand 15 decks.The fire zones deviating from the maximum size allowed by SOLAS are highlighted

_The Motorship_Feb_2012_1-884455342.e$S_The Motorship 30/01/2012 10:27 Page 36

designers to screen a vessel for all risks associatedwith fire, so they can focus on those areas where therisks are highest.”

The input data used in the screening consists ofinformation for each space onboard a vessel which couldbe considered a possible fire origin.

As part of the screening process, Safety at Seaassigns various attributes to each space beingevaluated. The space use definitionis selected from alist of categories for which a historical frequency offire outbreak exists. This data, taken into account inconjunction with the actual size of the space,determines the likely frequency of fire outbreak.

A ranking of fire zones in order of risk can then beundertaken, on the basis of frequency of fire outbreak,frequency of fire escalation and/or risk to human life.Figures 2 and 3 show a summary of outcomes fromthe screening process.

Certain generic trends have been identified bySafety at Sea as a result of using the screening tool.For a typical cruise vessel layout, the company saysthe following spaces are the most significant in termsof frequency of fire outbreak:● large public areas (stairs, lobbies, corridors);● engine room / machinery spaces;● galleys;● passenger cabins;

● laundry / dry cleaning facilities; and● garbage treatment / incinerator room.As far as the frequency of fire escalation is concerned,the most significant areas are the engine room /machinery spaces, external deck areas, galleys, largepublic areas and cabin balconies, while in terms of firerisk, the most significant locations are the engineroom / machinery spaces, luggage handling areas,galley, Internal promenades and lifeboat deck areas.

The casualty data used to develop the newscreening approach was taken from cruise vesselsonly, and included more than 600 fire incidents,collected over a period of around four years. Guarinsays: “Using the new fire screening method, we areable to harness casualty and fire incident data and usestatistical tools to quantify the different profiles that

are involved. The process is faster and can evaluatelarge areas quickly. Moreover if the design is changed,calculations can be redone relatively easily.”

To date the Safety at Sea fire screening process hasbeen used during the design of Oasis of the Seas, thelargest cruise ship afloat.

The fire risk screening framework developed bySafety at Sea can be used not just at the early designdevelopment stages of a newbuilding, but also forexisting ships.

The screening process, if applied to a number ofvessels, can be used to benchmark the fire risk acrossa fleet. In this way design elements and trends thatlead to changes in the risk profile can be identifiedand fed back into the design process.

The same principles can be applied to cargo ships;Safety at Sea has completed a fire risk screening studyfor a leading bulk carrier operator. Guarin says:”Wehad to enhance the screening process to accom-modate the fact that the availability of historical datais very limited. This was done by a more explicit linkbetween safety objectives, functional requirementsand specific SOLAS requirements and performancecriteria. In this way we used the fire screening process,to assess the risk associated with old ships and helpthe operator decide if upgrades to meet current rulesretrospectively are necessary.”

MARITIME SAFETY

February 2012 www.motorship.com 37

Figure 3: Fire risk to human life, breakdown per MVZ for a hypothetical ship

_The Motorship_Feb_2012_1-884455342.e$S_The Motorship 30/01/2012 10:27 Page 37