Hydro Papers

Fire safe aluminium structures

Abstract

In this white paper, you will get an overview of regula-tions applicable to aluminium structures that need to be fire protected as well as the procedures and criteria according to which needs to be tested.

You will also learn that the material behavior of alumin-ium is well known and as a result, structures in alumin-ium can be designed and effectively protected against fire. A wide variety of proven and certified materials are available. Also with some additional fire protection, aluminium can achieve considerable weight savings compared with steel.

Hydro manufactures aluminium plate, sheet and cus-tomized extrusions based products. One of its markets is Marine & Offshore. Hydro can also assist in the design of fire exposed aluminium structures.

The regulations for the fire protection of aluminium structures including their impact on the comparison with similar steel structures.

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Fire safe aluminium structures

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Introduction ...................................................................................................................................................................................................... 4Regulations and certification ..................................................................................................................................................................... 4 Fire testing ....................................................................................................................................................................................................... 5Fire-rating criteria .......................................................................................................................................................................................... 6 Typical benefits of aluminium .................................................................................................................................................................... 6 Material properties of aluminium vs. steel ............................................................................................................................................ 7 Consequences ................................................................................................................................................................................................ 8 Protective materials ....................................................................................................................................................................................... 8 Keeping the weight down with fire protected aluminium ................................................................................................................. 9 Conclusion ........................................................................................................................................................................................................ 11Literature ........................................................................................................................................................................................................... 11

Contents

Regulations and certification

Global conventions and regulatory bodies exist to ensure that vessels and offshore facilities comply with the safety standards in construction, equipment and operation. Any aluminium ship or offshore structure must comply with the following requirements:

1. International Convention for the Safety of Life at Sea (SOLAS) is an international maritime treaty between signatory flag states, governed by the International Maritime Organization (IMO). One of the committees in IMO is the Maritime Safety Committee (MSC). The SOLAS Convention is generally regarded as the most important of all international treaties concerning the safety of merchant ships

2. The flag state has the authority and responsibility to enforce regulations over vessels registered under its flag.

3. The NORSOK standards are developed by the Norwegian petroleum industry to ensure adequate safety and cost effectiveness. If possible, they can replace oil company specifications and serve as references.

4. The 2010 FTP Code (Fire Test Procedures) provides international requirements for laboratory testing, type- approval and fire test procedures for products referenced under SOLAS chapter II-2 (which includes regulations on fire protection, fire detection and fire extinction).

5. Class agencies (IACS) take the above rules and standards, and, combined with their proprietary know-how, they formulate an own set of rules. These enable them to audit, witness, approve and certify ships, offshore structures, products and designs.

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Aluminium is commonly used in marine applications and gaining ground in offshore structures, due to the material properties of the metal. It offers weight savings, and savings in installation and maintenance costs.

However, when it comes to the ability to handle fire exposure, there are many misperceptions about aluminium. These pose a risk to increased advantage taking of the benefits of this material. By providing a solid but basic overview, this white paper aims to overcome these misperceptions.

As the health and safety of crew and passengers are of utmost importance in the offshore and marine industries, it is crucial that the necessary design rules are followed. Fire accidents that occur at sea could face long response time before rescue teams are able to offer assistance.

Taking the leap from a linear to a circular economy

Today, our economy remains too linear and dependent on the extraction, trade and the conversion of raw materials into goods, and finally, the disposition of the product as waste or emissions. While a linear economy harvests, produces, uses and disposes materials (what is often referred to as cradle-to-grave), a circular economy strives to reduce resource use, losses and waste to a minimum, keeping resources in circulation as long as possible and ultimately recovering and regenerating materials and products. Thus, circular economy is much more than recycling and use of renewable resources. It means having better designed products which can easily be dismantled and effectively recycled, and then put into use again and again (cradle-to-cradle).

Introduction

Nowadays, design as well as verification are done by means of computer software. Final verification, however, often is done by laboratory testing. The 2010 FTP Code describes how these tests must be performed. The code can be divided into two categories:

• Reaction to fire: As much as possible preventing a fire from developing from small to bigger. Potentially aluminium-related sections are parts 1 (non-combusti- bility), 5 (surface flammability / flame spread) and 10 (test for fire-restricting materials for high-speed craft). However, as aluminium is non-combustible it cannot add fuel to the fire. Therefore, negative results for the use of aluminium are not expected based on this category.

• Fire resistance: Preventing fire from spreading from one compartment to another (compartmentalization). Aluminium-related sections are parts 3 (‘A’, ‘B’ and ‘F’ class divisions) and 11 (fire-resisting divisions of high-speed craft). Two crucial concepts when talking about fire resistance are “integrity” (no holes, also big) and “insulation” (keep it cool). These will be explained further below.

Apart from the requirements which need to be verified by testing, FTP also specifies testing equipment, types of tests, measurement, testing criteria, and heating curves. Because it is highly relevant for aluminium, we will discuss only heating curves.

There are three major heating curves for fire testing. In order of descending severity, they are:

• Jet fire (J-class): Produced when gaseous or liquid fuel, released as a jet from a pressurized pipe or container, is ignited. Flame erosion plays an important role. Tempera-tures instantly increasing to a stable level of 1100°C after 5 minutes.

• Hydrocarbon pool fire (H-class): Involving liquid fuels, such as oil, in an open pool. Temperatures quickly build-ing up close to 1100°C after 15-20 minutes staying stable thereafter.

• Cellulosic fire (A-class): Involving solid cellulosic fuels (e.g., wood). Most fires inside rooms fall within this category. Temperatures gradually building up to just over 900°C after 60 minutes.

Fire testing

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standard fire curves

1200

1000

800

600

400

200

0

°C

0 10 20 30 40 50 60 minutes

Jet fireHydrocarbon fireCellulosic fire

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Typical benefits of aluminium

In case of a fire accident, the health and safety of crew and passengers are of utmost importance. Next to design rules, also verification of the design by testing is an important aspect to secure the operational safety of structures.

Collapsing structures or high temperatures can create very unsafe environments for human beings. 2 major aspects of a structure's behavior during fire should therefore be tested: structural integrity and insulating capabilities.

Structural integrity, also at elevated temperatures, is the abil-ity of a structure to withstand its intended loading including its own weight, without failing due to fracture, deformation, or fatigue. This includes preventing flames and gases from

passing through gaps in the structure caused by too much deformation.

Insulating capability is a material´s ability to reduce or prevent the transmission of heat.

Structures are rated depending on their capability to provide structural integrity and insulation for a certain time interval. For marine and offshore structures and installations, fire resistance due to cellulosic fire is specified according to IMO Res. A.754 (18). A common fire rating for aluminium struc-tures is the A-class. An H-class rating may also be required e.g. specified according to MSC.307(88). Based on these IMO resolutions, the performance criteria for these classes are:

Fire-rating criteria

Aluminium is a light weight material and usually offers 30 – 50% weight reduction compared to common carbon steels. Aluminium’s weight saving potential is a result of its high strength-to-weight ratio.

It offers design flexibility due to its malleability and the possi-bility to integrate many different kinds of functionalities into extruded solutions.

Aluminium is corrosion resistant with virtually no main- tenance needed.

It does not loose mechanical properties in arctic climate the way most steels do.

Last but not least, it can be recycled over and over again without any loss of quality.

class integrity insulation"A-60" 60 min 60 min"A-30" 60 min 30 min"A-15" 60 min 15 min"A-0" 60 min 0 min"H-120" 120 min 120 min"H-60" 120 min 60 min"H-0" 120 min 0 min

During an integrity test of the insulation, the unexposed side of the door will not rise more than 140°C (284°F) above the original temperature, nor will the temperature at any point, including any joint, rise more than 180°C above the original temperature, within the time specified.

For structural integrity, the following requirements shall also be satisfied for the test duration:• Flaming: There shall be no flaming on the unexposed

face• Cotton wool pad: There shall be no warm gases that

can ignite a standardized cotton wool pad• Gap gauges: By use of a gap meter, control that no

gap more than 6 mm wide and 150 mm long, or single opening with diameter more than 25 mm exist

If the core temperature of aluminium approaches 200°C, the strength is still 80-to-90 percent of what it was at room temperature. However, beyond this point, the strength starts to decrease considerably. Hence, in the case of divisions of aluminium, the average temperature of the structural core shall not rise more than 200°C above its initial temperature at any time during the minimum test duration relevant to the classification.

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Material properties of aluminium vs. steel

Comparing aluminium’s material properties with those of steel, relevant in fire situations, shows the following:

• Aluminium is non-combustible. It does not burn, unless in form of tiny flakes or powder. This is the same as for steel. E.g. it is easy to set fire to a piece of steel wool.

• Steel melts at about 1500°C and aluminium at about 650°C.

• Increased temperatures affect the strength of both aluminium and steel. Eighty percent of its Rp0.2 remains at 190°C for aluminium (6082), whereas for steel this is 475°C.

• Aluminium’s thermal conductivity is about four times higher than steel.

• Aluminium’s specific heat capacity is about double that of steel.

• Aluminium’s reflectivity index is up to 19 times that of steel. This is also the case for weathered aluminium.

property valuealu valuesteel

Moduleofelasticity(Mpa) 70000 21000Tensilestrength,ultimate(Mpa) 310 (6082T6) 420 (S355)Tensilestrength,yield(Mpa) 290 (6082T6) 350 (S355)Meltingpoint(°C) 660 1425-1540Specificheatcapacity(0-100°C)(J/g.°C) 0.9 0.47Thermalconductivity(0-100°C)(W/m-K) 170 44-52Linearexpansion(0-100°C)(x10-6/°C) 23.5 12Electricalresistivityat20°C(x10-6Ω-cm) 3.32 17Density(g/cm3) 2.7 7.8Reflectance/Reflectivity(%) 55-90 40-60Emissivityε(heatradiation) 0.09 (sheet) 0.8 (weathered)

Picture 1 - Burning steel wool

Table 1 - material properties aluminium and steel

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Consequences

Even though at elevated temperatures aluminium loses its strength earlier than steel, some of the properties of aluminium can partly compensate for this and provide even an advantage during fire, compared with steel.

• Heat transfer and reflectivity are two of the properties that help offset aluminum’s earlier loss of strength at elevated temperatures. They result in less heat being absorbed, and, when absorbed, the heat is transferred away from the heat source much quicker.

• The emissivity of aluminium is 7-to-9 times lower than steel. This means less heating-up of the direct surroundings.

An example is the extensive testing that has been done on aluminium helidecks, where it has been observed that, on many occasions, the aluminium structure showed no damage. It is not uncommon to see that the steel structures failed first in these tests, such as the structures used to keep the fuel

on the deck or to simulate the weight of a helicopter. This difference between aluminium and steel can be explained by aluminium’s excellent reflectivity and heat transfer properties. Heat is not absorbed as easily – it is reflected much more – and it is channeled away much quicker from the heat source, dividing it over the rest of the non-exposed structure. This results in fewer hotspots, lower temperatures and smaller differences in temperature between different parts of the structure, which can reduce potential damage.

This shows that an aluminium structure has a thermal effi-ciency factor which is much better than steel, meaning that its ability to handle heat and fire is better than steel. In any given fire situation, it is too simplistic to assume that the aluminium structure would melt first. The heat input necessary to make a structure of the size of an aluminium helideck unusable, would mean that a like-sized helideck in steel would be rendered similarly unusable.

Protective materials

There are two categories of fire protection systems: active and passive. Active fire protection is a term that can describe any systems or products put in place to detect or combat a fire, such as smoke detection, extinguishers and sprinkler systems. These systems will always need some form of trigger to activate. Passive fire protection requires no activation. Passive fire protection refers to materials built into structures that form a fire-resistant barrier to either slow or prevent the spread of fire without need for external stimuli.

Active systems lie outside the scope of this paper. We will, however, list some passive product categories that are suitable for use in combination with aluminium or steel. Relatively heavy materials that would eliminate the weight savings provided by aluminium, will not be included. Material selection depends on the design requirements. We see the following product categories:

• Blanket material: Flexible blanket made of mineral fibers. The two main materials are stone and ceramic fibers. They provide protection for temperatures between 800°C (stone) and 1200°C (ceramic). They are light and are flexible in terms of shape.

• Intumescent paint systems: Sprayed-on and inert at low temperatures. These provide good insulation due to a complex chemical reaction at temperatures typically of about 200-to-250°C, when they swell and form an expanded layer of low conductivity char.

• Cladding-like calcium silicate boards.

Picture 2 - Fire protection blanket material

Keeping the weight down with fire protected aluminium

To better understand the benefits of aluminium even with the weight impact of fire protection, we have compared 1m or 1m2 of steel and aluminium in three typical cases: I-beam, deck and bulkhead. For every case, the comparison is based on equal bending stiffness of the steel and aluminium structure. This assumes, that enough space is available to increase the height of the aluminium structure.

Comparison I-beam

Steel A-60 Aluminium A-60 Weight advantage ALU

excl. fire protection incl. fire protection

60% 54%

• 1m beam, 50kg

• Blanket: 64kg/m3, thickness=45mm, 2.88kg/m2

• Adds 2.6kg

• 1m beam, 20kg

• Blanket: 70kg/m3, thickness=50mm, 3.5kg/m2

• Adds 4.3kg

Even with the somewhat heavier protection blanket, the weight saving for the aluminium I-beam is still a very impressive 54%.

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Photos: courtesy of Morgan PLC

• 1*1m2 deck, 42kg

• Blanket: 48kg/m3, 2.4kg/m2, thickness=50mm• FireMaster® Marine Blanket Plus• Single-sided protection

• Adds 2.9kg

• 1*1m2 deck, 20kg

• Blanket: 70kg/m3, 3.5kg/m2, thickness=50mm• FireMaster® Marine Blanket Plus• Single-sided protection

• Adds 4.2kg

Comparison deck

Steel A-60 Aluminium A-60 Weight advantage ALUexcl. fire protection incl. fire protection

52% 46%

Decks are usually not fire protected on the outside. Hence, an aluminium deck is protected single sided, similar to steel. It can be seen, that even with the 45% additional weight of fire protection material, still an impressive 46% of overall weight saving remains.

• 1*1m2 bulkhead, 42kg• Same geometry as above deck• Semi-rigid slab: 130kg/m3, thickness=2*30mm incl. stiffener• Searox SL 640, Rockwool• Single-sided protection

• Adds 10.1kg

• 1*1m2 bulkhead, 20kg• Same geometry as above deck• Semi-rigid slab: 130kg/m3, thickness=2*30mm incl. stiffener• Searox SL 640, Rockwool• Double-sided protection

• Adds 17.9kg

Comparison bulkhead

Steel A-60 Aluminium A-60 Weight advantage ALUexcl. fire protection incl. fire protection

52% 27%

In the most unfavorable comparison, an aluminium bulkhead needs to be protected double sided (steel only single sided), to guarantee both the structural integrity as well as the insulation requirement. This is only the case if the fire can occur on both sides of the bulkhead. If the fire can only occur on 1 side, the comparison will be similar to a deck. It should be noted, that

usually only a share of the total aluminium (vessel) structure needs to be protected double sided.

It can be seen, that even with the 80% additional weight of fire protection material, still a highly interesting 27% of overall weight saving remains.

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Literature

[1] SOLAS Convention[2] International Code for the Application of Fire Test Procedures (2010 FTP Code)[3] Efectis Nederland B.V.: Aluminium and Fire Safety in the Maritime and Offshore industry, march 16th 2017[4] THERMOPEDIA™.[5] TALAT Lectures 2502: Material Aspects of Fire Design fig. 2502.01.05[6] SteelConstruction.info: Fire damage assessment of hot rolled structural steelwork[7] Matweb.com[8] The Center for Occupational Research and Development: MODULE 6-5, MIRRORS AND ETALONS (fig. 7),1987.[9] Fire test Bayards helideck - See comments @ 0.50min.[10] Fire test Aluminium Offshore helideck – See Lloyds Register comments @ 5.35min.[11] Fire test Bayards helideck – witnessed by UK Civil Aviation Authority (CAA). See comments @ 2.40min.[12] Commentary on SOLAS Ch II, Reg 18, helicopter facilities, by Aluminium Offshore.

The material behavior of aluminium is well known. As a result, structures in aluminium can be designed and effective-ly protected against fire – and a wide variety of proven and certified materials are available. Also with some additional fire protection, aluminium can achieve considerable weight savings compared with steel. The different examples above show weight savings of 27-to-54 percent of the overall weight.

Although there are some instances where aluminium needs more protection than steel, the additional cost for (locally) protecting aluminium must be viewed in the bigger picture of the complete vessel or offshore facility. This picture will likely show major weight- and maintenance savings. Therefore, the additional costs for fire protection should be compared to the monetary savings of the reduced weight and maintenance cost for the complete structure.

Conclusion

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Photo cover: courtesy of Morgan PLC

© Hydro/05-2020

Hydro is a fully integrated aluminium company with 35,000 employees in 40 countries. Rooted in more than a century of experience in renewable energy, technology and innovation, Hydro is engaged in the entire aluminium value chain, from bauxite, alumina and energy to primary aluminium, rolled and extruded products and recycling.

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