Advanced structural fire design of offshore structuresT. HolmasSINTEF Civil and Environmental Engineering, NorwayJ. AmdahlNorwegian Institute of Science and Technology (NTNU), Dept. of Marine Structures, NorwayABSTRACT: Resistance against extreme loading has to be documented for offshore structures. Fire is acontinuos threat as large amounts of oil and gas are passing through the installations. In a conventional designprocedure, the ‘cold’ structure is optimized for the different mechanical loads. The responsibility of thestructure is then transferred to the safety department, which normally recommend use of passive fireprotection as the only way to avoid structural failure during fire. Including fire in the design process willresult in more fire resistant structures which means reduced use of passive fire protection and thereby reducedinitial and maintenance costs. Better understanding of the structural behavior under fire exposure opens alsofor extended use of light alloy structures. Design against accidental fires should be included in the designprocess conducted by the structural engineers in close cooperation with the safety department.1 INTRODUCTION expansion will vary from one structural component to another. The thermal expansion results in member axial forces and bending moments. Thin walledAdequate resistance against extreme loads has to be components are the most vulnerable, and if the endsdocumented for offshore structures. Accidental fires are restrained member buckling take place even forare a continuos threat as large amounts of oil and gas moderate temperature increase, (∆T=100°C). As theare passing through the installations. The yield stress and E-modulus reduce, and after a givenrequirements to the structures are based on general time the whole structure may collapse.safety studies, and are often expressed as minimumtime to failure of the most important structures In a conventional design procedure the ‘cold’exposed to on or more fire scenarios. structure is designed and optimized by the structural engineer for the different loads. The “responsibility”Structures exposed to fire will be heated up, and the for the structural performance is transferred to theheating rate is dependent on the intensity of the fire, safety department. The major measure to avoidthe surface/mass ratio of structural components, the structural failure is by using passive fire protection,surface properties of the actual material and finally i.e. thermal insulation slowing down the heating ofthe presence of passive fire protection covering the the structural components. The requirements to thesurface. protection is expressed as maximum allowable temperatures after a given time (for instance 400°CThe main effects of the heating are thermal after 1 hour). The necessary thickness of the passiveexpansion, reduced elastic modulus and yield stress fire protection is taken from tables depending on theand creep. Thermal expansion takes place from the surface to mass ratio (HP/A ratio), the fire exposure,very beginning, and due to the fact that the different maximum accepted structural temperature and thecomponents will be heated differently (different fire duration. Normally, this work is carried out byexposure and surface/mass ratio), the thermal safety engineers who seldom have their background
from structural engineering and thus may have The simply supported axially free beam shown inproblems to suggest alternative structural design. Figure 1 carries the midspan force by bending, andOne consequence of using the temperature criterion in connection with linear design, no axial forces arefor determining the passive fire protection is that the introducedsmallest components get the largest thickness andvice verse.The fire scenarios used in design are often limited tostandardized uniform ‘fires’ (e.g. 200 kW/m2) whichare assumed to expose all structural members “all Figure 2. Fixed beam with middle hinge.way around” from the very beginning to the veryend of the fire. Such assumptions are often overlyconservative, yielding excessively high costs for fire The hinged beam described in Figure 2 which isprotection. In some cases it may exclude the use of f axially fixed in both ends, is assumed to carry theex light weight material such as aluminum alloys. same force through axial forces only (pure membrane). In this simple example, the change inBy contrast to such simplistic considerations, geometry due to stress induced strain is disregarded.advanced computational fluid mechanics (CFD)codes recently developed allows accurate simulation The two static systems will have a very differentof the combustion process from the ignition to behavior during fire.termination. It is possible to trace differences inexposure of the various structural components For the simply supported beam, the elongation of thegiving a far better description of the real fire. beam due to thermal expansion will not change the system, but the material degradation due toSimilarly, advanced non-linear structural analysis increased material temperature will reduce thetools are available. By simulating the mechanical bending capacity.response of structures exposed to fire it is possible todocument and evaluate the consequences of a fire in For the hinged beam the thermal elongation of thea much more physically correct manner. It is beam introduces lateral deflections as indicated bypossible to trace failure of components, force the dotted lines. This deflection changes the loadredistribution and global failure during the fire. With carrying system in a positive manner: Increasedthis knowledge it is possible for the structural deflection results in increased capacity.engineer to suggest a more optimum design of thestructure with respect to the effects of fire. In To demonstrate these effects, the following simpleparticular, alternatives to passive fire protection can example is given:be evaluated. Assume a steel pipe with diameter 1.0m and thickness 50 mm spanning 10 m. The steel with2 FUNDAMENTAL BEAM BEHAVIOR initial yield stress 350 MPa is assumed to be degraded according to Eurocode 3, /4/.To describe the behavior of complex structuresexposed to fire, it is necessary to understand the For the simply supported beam the ultimate midspanfundamental behavior of the structural members. force Pu is taken as:2.1 Simplified beam behavior Pu = 4 σy Wp / L (1) where σy : Current yield stress Wp : Plastic sectional modulus L : Span lengthFigure 1. Simply supported beam.
For the hinged beam following ultimate midspan increase of the midspan deformation, and theforce is taken: capacity drops. In Figure 3 the “ultimate” capacity is Pu = 2 σy A sin( ϕ ) (2) shown as function of temperature for the twowhere simplified cases. The current maximum of the two σy : Current yield stress cases are plotted in Figure 4. A : Cross section area ϕ : Angle = ArcTan ( 2δ/L ) 12 δ : Midspan deflection Ultimate Load [ MN ] 10 8The midspan deflection caused by thermal 6expansion only is expressed as follows: 4 2 δ2 = [ L/2 ( 1 + αT ) ] 2 – ( L/2 ) 2 0 = L2/4 ( 2 αT + α2 T2 ) (3) 0 200 400 600 800 1000 Temperature [ C ]where α : Thermal expansion coefficient T : Temperature increase of beam Figure 4. Ultimate capacity of the simplified beam.For the simply supported beam the capacity as given This simple “linear” approach shows that membraneby Equation (1) is governed by the degradation of effects represents a substantial reserve which shouldthe yield stress. The equation is used for varying be utilized in connection with extreme loadtemperature of the beam which means changed yield situations where requirement to deflections are lessstress. The initial capacity of 6.3 MN is kept than under normal, service conditions.unchanged up to 400° C, and then the capacity dropsaccording to the material degradation curve. 2.2 Advanced beam behavior Ultimate Load [ MN ] 12 10 In the above example, pure bending and pure Pure Bending 8 membrane effects have been demonstrated under fire 6 Pure conditions. In a real case both membrane and 4 2 Membrane bending effects take place at the same time limited 0 by current plastic interaction of the cross section. 0 200 400 600 800 1000 With increasing temperatures, the yield surface shrinks. In the following, the example described in Temperature [ C ] the previous section is used without the middle hinge inserted.Figure 3. Bending capacity versus membrane as functionof temperature. In Figure 5 the plot of midspan bending moment versus axial force at temperature 250°C shows thatFor the axially restrained beam the capacity as given after pure bending, compression forces areby Equation (2) is governed by the degradation of introduced due to thermal expansion.the yield stress as well as the angle ϕ. With an initialdeflection equal to zero, the capacity curve of thehinged beam starts in zero when the stress inducedstrain is disregarded, (sin (0)=0). Increasingtemperature results in increased deformation whichmeans improved membrane load carrying, seeequations (2) and (3). When the temperature passes400°C, however, the material degradation is strongerthan the improved geometric stiffness due to further
1,5 forces will be introduced due to thermal expansion, N / Np and the beam will carry the load by bending 1 throughout the whole heating process. Figure 7 0,5 describes the force state for the simply supported 0 axially free beam at 600°C. -1,5 -1 -0,5 0 0,5 1 1,5 -0,5 M/ Mp -1 1,5 N / Np -1,5 1 0,5Figure 5. Yield surface and force path at 250° C. 0 -1,5 -1 -0,5 0 0,5 1 1,5 -0,5Due to second order effects, the midspan bendingmoment increases with the increasing compression -1 M/ Mpforce. At same point the member buckles, and the -1,5axial force is being relieved. At this point theresponse is governed by the plastic interactionbetween axial force and bending moment. Later the Figure 7. Yield surface and force path at 600° C.response travels through the initial condition with No axial fixation.pure bending, see Figure 6. However, by furtherincrease of the deflection the axial force will turn Similar to the simplified case described in 2.1 thefrom compression to tension. ultimate capacity of the beam as a function of the temperature with different boundary conditions areThe yield surface is reduced continuously. New calculated utilizing the non-linear computer codeequilibrium conditions must be established always USFOS /1/. The beams are first heated up to thelimited by the current size of the yield surface. The actual temperature and then the mechanical load ismechanical load which is kept constant during the applied. Each calculation terminates when theheating, is carried more and more by membrane midspan deflection exceeds 0.75 m in this example.effects. The force state at 600°C is shown in Figure Stress induced strain as well as thermal expansion6. effects are included. The example demonstrates the much higher capacity of an axially fixed beam 1,5 compared to one with free ends. This documents the N / Np 1 importance of designing the joints for the ultimate 0,5 member forces including accidental loading rather 0 than optimizing the connections for the actual forces -1,5 -1 -0,5 0 0,5 1 1,5 in “cold” condition. -0,5 -1 M/ Mp 20 Ultimate Load [ MN ] -1,5 15 Fixed EndsFigure 6. Yield surface and force path at 600° C 10 Free Ends Both ends translation fixed 5 0The beam now carries the lateral force by pure 0 200 400 600 800 1000membrane action. Temperature [ C ]Without fully fixation in both ends, the beam will Figure 8. Ultimate beam capacity versus temperature.have a very different behavior. No compression
2.3 Column behavior cause column buckling before the mechanical load is applied.Columns have a very different behavior under fireexposure than beams. No inherent reserves are The ultimate load is given relative to the initialavailable due to large deformation – rather the “cold” situation.opposite is the case as any disturbance from straightconfiguration will reduce the ultimate capacity. In the “free” case the buckling load is little influenced by temperatures up to 400°C as theSeveral factors influence the ultimate capacity of the ultimate load is primarily governed by the materialcolumn: yield stress. A slight decrease caused by the E-• Reduction of yield stress modulus degradation is observed for this particular• Reduction of stiffness (E-modulus) column.• Uneven exposure and associated uneven thermal expansion over the cross section causing column The “fixed” case the ultimate capacity is influenced curvature by the temperature already from slightly above• Thermal expansion which may lead to column 100°C. This is mainly caused by thermal expansion buckling depending on the column boundary forcing the column into a bent configuration. This conditions out-of –straightness reduces the ultimate capacityFigure 9 describes the ultimate load as function of significantly when the column is subsequentlytemperature of a steel column. The temperature is loaded in compression.uniform over the cross section (no gradients), andfollowing column data is used: The two curves represent the limits for “real life”Outside diameter: Do = 355 mm column behavior; in practice the capacity curve willThickness: T = 25 mm be somewhere between the two extremesLength: L = 6mThe material properties are assumed to be degradedaccording to Eurocode 3, /4/. 3 BEHAVIOR OF PLANE FRAMESThe column is first heated up to the actualtemperature, then the axial compression force is The next case to be studied is a portal frame bracedapplied until the column buckles. The ultimate with an X-trusswork, Figure 10 and a K-trusswork(peak) force level is recorded for each case. Figure 11. The frame is loaded with a horizontal force, which is primarily carried by tension and compression in the braces. The X-truss represents an Ultimate Load [ Relative ] 1,2 indeterminate structure in the sense that once the 1 compression brace fails, it load can be shed to the 0,8 Free tension braces. The K-truss is a determinate 0,6 Fix structure. Equilibrium requires that the compression 0,4 brace carries the same force as the tension brace. 0,2 Hence, failure of the compression brace signifies 0 global failure of the frame. 0 200 400 600 800 1000 Temperature [ C ] The frames are first subjected to uniform heating followed by application of the mechanical load. TheFigure 9. Ultimate load of free and fixed column ultimate strength is recorded for all cases and normalised against the capacity at normalIn the “free” case, the column is free to expand temperature.axially. In the “fixed” case the column upper node isfree to move inwards, but is fixed in the outwarddirection. This means that thermal expansion may
Ultimate Load [ relative ] 1,2 1 0,8 K_Truss 0,6 X_Truss 0,4 0,2 0 0 200 400 600 800 1000 Temperature [ C ] Figure 12. X-Truss versus K-truss behavior during fire.Figure 10. Horizontal loaded X-Truss exposed to fire. 4 OFFSHORE STRUCTURE EXPOSED TO FIRE 4.1 Background The steel frame bridge composed of tubular members shown in Figure 13 connects two offshore oil platforms in the Northern Sea. The bridge has two main functions: Supporting hydrocarbon pipelines and human traffic. InFigure 11. Horizontal loaded K-Truss exposed to fire. connection with fire the bridge is a part of the escape routes which must remain intact for a given time. InDuring heating very small thermal strains are addition it is of great importance that the pipelinesinduced in the K-brace analogous to the axially free do not break and then cause escalation of the fire.case of Section 2.3. Failure of the compression braceis again predominantly governed by the reduction inthe yield stress and elastic modulus, but a furtherreduction in the buckling strength is also attributedto a frame induced lateral deformation.Conversely, due to the static indeterminacysignificant thermal strains are induced in the X-truss-work, closer to the axially fixed case in Section Figure 13. Bridge connecting two offshore oil platforms2.3 The compression brace fails early in the heating supporting hydrocarbon pipelinesprocess, but this does not of, course, signify ultimatestrength. Global failure occurs when the tensionbrace reaches yield. From the safety studies the actual heat flux (or gas temperature) associated with the assumed “fire-on-Figure 12 displays the normalised capacities for the sea” scenario is found. The bridge is not located inframes. It is seen that both curves lies within the the center of the fire and is assumed to mainly bedomain spanned by the axially free and axially fixed exposed to smoke gases, (~40 kW/m2) It is requiredcases plotted in Figure 9. The X-truss suffers the that the bridge should withstand the fire withlargest relative reduction of ultimate capacity. The substantial margin at least for 1 hour.absolute strength is nevertheless larger.
4.2 Conventional fire design simulate the individual member temperature histories. The design finite element model of theIn a conventional design procedure, the structural bridge is automatically transferred to surface shelldepartment optimizes the steel based on the elements by the code in order to capture the thermalmechanical loads only. The structural response due effects over the cross section.to member heating is disregarded except forchecking of necessary clearances in connection with Simulations of the unprotected structure result inthermal expansion. member temperatures up to approx. 650°C. At this temperature the steel has “lost” more than 50% ofThe structural engineers presuppose that the member the initial strength. The temperature history for eachtemperatures do not exceed a given temperature, individual structural member are transferred400°C is a widely used temperature threshold. At automatically to the mechanical response modulethis temperature the steel maintain most of the initial USFOS. Both FAHTS and USFOS have been(cold) properties. verified against large scale testing of a fire exposed 3D tubular frame /3/.The “responsibility” of the structure is thentransferred to the safety department. In connection with the USFOS simulations the following analysis procedure is used:The safety engineers will normally use passive fireprotection (PFP) to protect the structure. The • Apply deadweight (loadfactor=1.0)thermal insulation will slow down the heating of the • Apply member heating (results from FAHTS)structural members avoiding the members to reach f • Increase deadweight up to system collapseex 400°C within 1 hour. Correct thickness of theselected PFP product is found by utilizing the Figure 14 shows the collapse mode of the originalsurface to mass ratio (HP/A ratio) which means in design configuration. The compression members inpractice that the smaller members (easiest to heat the upper girders at midspan buckle with a loadup) get the largest thickness of PFP and vice verse. factor of the permanent loads equal to 1. This is an unacceptably small margin.In this particular example approx. 1500 m2 surfacewas to be protected. With a minimum thicknessapplied of a typical spray-on product this results inapprox. 8 ton passive fire protection.4.3 Advanced Fire DesignApplication of passive fire protection on exteriorsurfaces which are exposed to rough Northern Seaclimate represents a maintenance challenge.Covering the whole surface with f ex a spray-onproduct makes it hard to inspect welds etc for fatigue Figure 14. Collapse mode due to fire. Original designcracks. Corrosion on the steel surface may cause thePFP to loosen and fall off. The first modification is to increase the wall thickness of the upper girder tubular members. TheAvoiding use of passive fire protection is then of idea is to prevent member buckling.great interest. New simulations are carried out with the modifiedAssuming the heat fluxes from the safety studies to structure. However, now it is observed that memberbe design requirements, the computer code FAHTS buckling takes place at the supports, see Figure 15.(Fire And Heat Transfer Simulations) /2/ is used to System failure corresponds to a load factor =1.05.
The increase of the steel weight caused by the increased wall thickness of some compression members represents 1% of the total permanent load. The saved weight by using no passive fire protection represent the same mass which means that the total weight of the bridge is unchanged. The fabrication and maintenance costs are, however, substantially lower.Figure 15. Collapse mode due to fire and with 5%overload. First modification.Increasing the wall thickness of the most exposeddiagonal members gives a substantial increase of thecollapse load. In Figure 16 the midspan deflection Figure 17. Final Design. Situation after 1 hour fire andversus load factor is presented for the three cases. 60% overload.Linear behavior is observed for all 3 cases up to aload factor of 1.0 causing a midspan deflection ofapprox. 0.12 m. The fire is then applied causing the 5 CONLUSIONSdeformation to increase to approx. 0.5m mainly dueto reduced E-modus. Further increase of the The importance of including the accidental fire inmechanical load results in a very early system the design process is documented.collapse for the two first cases. For the third case(final design), the system does not collapse before a Advanced computer codes which have been verifiedload factor of 1.6 is reached. A rather ‘linear’ path is against large 3D tests simulating structural behaviorobserved up to the peak, and the slope is approx. 1/3 during fire might be an efficient and reliable tool forof the initial slope which corresponds well with the the structural engineer.fact that the E-modulus is reduced to approx. 1/3. Increased knowledge about structural behavior 1,80 under fire opens for more fire resistant structures, 1,60 1,40 reduced fabrication and maintenance costs and 1,20 extended use of light metal structures. Load factor 1,00 Original Design First Modification 0,80 Final Design 0,60 0,40 6 REFERENCES 0,20 0,00 0,00 0,20 0,40 0,60 0,80 1,00 /1/ USFOS. Ultimate Strength of Frame Midspan Deformation [ m ] Offshore Structures. User’s Manual, SINTEF Report STF71 F88039.Figure 16. Midspan deflection versus load factor for /2/ FAHTS Fire And Heat Transfer Simulationsthree design alternatives. User’s Manual, SINTEF Report 1995. /3/ Laboratory Test of a 3D Steel FrameThe bridge situation at the peak load level of 1.6 is Exposed to Fire. SINTEF Report, 1995.presented in Figure 17 with unscaled displacements. /4/ Eurocode 3: Design of Steel Structures Part 1.2: Structural Fire Design