KFX Paper ComputationalSystem

8/12/2019 KFX Paper ComputationalSystem

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Computational Industry Technologies AS

Kameleon FireEx

An Advanced Computational System for Calculation of Fire Structure Interaction

Abstract

During the last 15-20 years new technology has been developed and introduced to the industry,

allowing a rigid analyzing of the interaction between fire and structure and thus improved fire design.

The most important factor for achieving cost reducing design and safer systems for personnel is

detailed knowledge of the development and behaviour of real fires in combination with advanced

structural response simulations. Such advanced methods are tracing the accidental events from the

actual hydrocarbon leak, ignition and combustion, heating of structural components up to material

softening/weakening.

The Kameleon FireEx system comprises the CFD simulator Kameleon FireEx (KFX) for detailed fire

simulation, automatically linked to the FEM code Fahts/Usfos for detailed non linear structural

response analysis.

This article focuses on the methodology being used by ComputIT for fire analysis and fire design, a

methodology that has been applied for a large number of industrial analyses with good results.

Simulation results are presented as examples, and achievements in projects are given.

Introduction

Fire is a continuous threat for most offshore oil and gas installations. It is required that the structureresists fires with a given probability long enough to evacuate the platform. The platform integrity is

also crucial with respect to the economic consequences of fire accidents.

In the past, design of structures for fires is mainly based on prescriptive standards for the size of fire

heat loads. Typically, a conventional fire design procedure might be: 1) Find a standard heat flux, for

example 200 kW/m2, 2) Let the entire structure be exposed to this heating, 3) If the steel temperature

exceeds 400°C, insulate the steel surface with passive fire protection (PFP).

The total surface of the structural components of a platform, representing the potential exposed to fire,

is typically 10- 50.000 m2 (for comparison the area of a football ground is approximately 7.000 m2). If

all surfaces should be protected, the total weight of the PFP would be 50 - 100 ton with correspondingcosts of magnitude 5-20 mill €.

A question that may be raised is: Do we need to protect all surfaces? The experience of ComputIT is

that for many installations the answer is no.

This article focuses on the methodology ComputIT is using for fire analysis and fire design, a

methodology that has been applied for a large number of industrial analyses with good results.

Article in Fabig Technical Newsletter, Issue # 41, January 2005. The Steel Construction Institute, UK




The KFX Integrated fire analysis

During the last 15-20 years new technology has been developed and introduced to the industry,

allowing for a much more rigid analyzing of the interaction between fire and structure and thus

improved fire design.

It has become quite common to use Finite Element Method (FEM) which is a rigid and accurate

method for calculation of the structural response to fires. However, in combination with this it is also

quite common to use very simplified methods for calculating the fires loads, which might be

insufficient in combination with rigid FEM analysis.

Typically a pool fire is taken as a heat load of 150 kW/m2 and a jet fire as 250 kW/m2. This is a strong

simplification of real fire behaviour, and the consequences will often be very conservative design with

respect to structural protection and insufficient design with respect to personnel safety, evacuation etc.

A fire will in general vary by time, both in intensity and location, and all surfaces will not be heated by

the same load during a fire, nor will one specific surface be heated by the same load during the wholefire. Often might also 400°C be accepted if the integrity is maintained due to redistribution of forces.

The most important factor for achieving cost reducing design and safer systems for personnel is

detailed knowledge of the development and behaviour of real fires in combination with advanced

structural response simulations. Such advanced methods are tracing the accidental events from the

actual hydrocarbon leak, ignition and combustion, heating of structural components up to material

softening/weakening.

The ComputIT methodology is in principle given by:

• The fire simulations (transient and 3D) including temperature calculation in solid materials, are

carried out by the CFD fire simulator Kameleon FireEx (KFX). The KFX simulations accountfor the actual release of hydrocarbons, which in many cases are limited due to process

segmentation and shut down systems, and is in detail calculating the heat loads, radiation,

smoke production, smoke distribution etc in real process environments.

• Detailed structural analyses are carried out by the Finite Element (FEM) codes Fahts and Usfos

to check the structure’s load bearing capacity. The temperatures are intermediate results only.

• Identify weak parts and reinforce/insulate these components.

• The coupling between the fire simulations and the structural simulations are done

automatically as a part of the KFX system

• The results of the fire and the structural response simulations may be visualized by video

technique for a realistic impression of the fire and its consequences.

The following analysis procedure for structure integrity is used:

• Apply mechanical loads (operational loads and wind) to 100 %

• Apply temperature history

• Increase mechanical loads by 10 %, (at least)

An alternative procedure is to apply step 1 and 2 in once, i.e. to check the stability of the structure at

different stages during the fire. Based on the temperature situation, parts of the structure are assigned

the degraded material properties. If the temperatures exceed f ex 1000°C, the parts are completely

removed from the finite element model (become non structural). For the cases with high temperature,

the alternative procedure is preferred because it is more robust numerically for elements with “zero”stiffness and capacity.





The structure is accepted if:

• Global stability is preserved at all stages of the temperature history

• Component strain level limited to 15%, (ref NORSOK )

The additional weight is applied to get an idea of the residual strength at the end of the fire history, butdoes not represent any requirement as such. In general, member yielding and buckling causing load

shedding is allowed.

Kameleon FireEx

Kameleon FireEx (KFX) is a field model for gas dispersion and fire simulation, developed at

ComputIT/NTNU/SINTEF. The KFX development is a continuous activity at ComputIT, at present

supported by Total, Statoil, the ENI-group, ConocoPhillips and Hydro.

Kameleon FireEx is capable of calculating heavy and light gas dispersion as well as hydrocarbon

liquid pool fires, spray fires, and gas jet fires, in enclosures and in open air. In addition fire suppressionusing monitors, deluge, sprinkler, and water mist systems can be included in fire simulations and

interact with the flow and energy field in the gas phase. KFX includes CAD import capabilities, and is

widely used internationally for safety analysis in the oil and gas industry.

Advanced rendering technique gives, by creation of realistic pictures and animations of fire

developments, a unique opportunity to interpret the results of the transient calculations and to increase

the understanding of the complicated flame physics that is very important in fire safety design.

Kameleon FireEx is based on a Cartesian finite volume technique to solve the averaged basic transport

equations from fluid dynamics. The SIMPLEC algorithm [6] is used for calculating the pressure terms

in the momentum equations based on the pressure correction which must be imposed in order to obtainmass conservation. The turbulent transport effects are modeled as gradient diffusion with turbulent

viscosity calculated from the k-ε model of turbulence. The averaged reaction rates for the species mass

fractions are modeled according to the Eddy Dissipation Concept (EDC) for combustion by Bjørn F.

Magnussen [1]. The thermodynamic properties are calculated by the CHEMKIN [4] package and

tabulated. The source terms for soot are modeled according to the Eddy Dissipation Soot model by

Magnussen [1]. It includes a transport equation for the soot nucleus that is the starting point for the

soot particle growth. The soot mass fraction is used to find the correct radiative absorption coefficient

of the control volumes. The absorption coefficient is used in the Discrete Transfer Model by

Lockwood and Shah [2] to calculate realistic radiative transfer in flame and between the flame and its

surroundings.

Fire mitigation by water systems is treated by a two phase spray model. A Lagrangian description is

selected for the behavior of the droplets [13], [14]. The cloud of droplets is represented by a number of

discrete numerical droplets each representing a group/class of real droplets [14]. The number of

physical droplets represented by each numerical droplet, i.e. the total number of numerical droplets to

be used, depends on the computational capacity available.

When calculating the effect of for instance deluge on fire evolution it is assumed a certain particle size

or particle size distribution. Based on the prescribed or assumed release rate the number of droplet

parcels are calculated and then released in the prescribed direction from the nozzles.





FEM tools Fahts/Usfos

An important feature of the KFX system is the link to the finite element code Usfos. The response

program consists of two parts; one for temperature response, Fahts, and one part for mechanical

response, Usfos.

The KFX results give the heat flux at the individual structural component surfaces. The structural

model is automatically transferred to surface shell elements in order to receive the correct heat flux and

to capture thermal gradients over the cross section, caused by uneven fire exposure and/or partly

protected members. The heat exposure, (radiation heat flux and convective heat flux), is then varying

from point to point on the structure depending on the actual point’s coordinates and surface orientation

(f ex if the surface is facing against or away from the fire, etc.).

Different surfaces receive individual heat flux, and for instance for column elements only the outer

side will experience the fire. Internally, radiation between the inner surfaces will transfer heat from the

most exposed side to colder parts. Some keywords about the temperature module (Fahts) :

• Model file compatibility with Sesam, Sacs, Staad and more

• Fully integrated with the Kameleon FireEx database

• Efficient non linear time domain solver

• Unprotected and insulated members

• Fully or partly insulated cross sections

• Internal radiation inside hollow members

• Automatic re-meshing of beams to shells (surface)

• Shielding

• Temperature dependent material properties

Usfos includes non-linear geometry effects, material yielding and thermal effects, (expansion, yield

stress and E-modulus degradation) and calculates instability of individual components as well as

system collapse. Some keywords about the mechanical response module Usfos:

• Model file compatibility with Sesam, Sacs, Staad and more

• Non Linear material and geometrical effects

• Detects Local buckling and Global instability

• Beams and shells

• Static and Dynamic analysis

• Efficient solver

• General Accidental Limit State simulation

Case example of integrated fire analysis

A case where a floater is exposed to severe accidental fire scenarios is briefly discussed below as an

example of the integrated fire structure analysis. The scope has been evaluation of mechanical

consequences of accidental fires that could heat the columns which are unprotected. It is required that

the structural integrity is maintained for a certain time during an accidental fire. This study is carried

out with use of realistic fire simulations and realistic heat loads, realistic temperature distribution in

structures, and realistic load bearing.

The study and its results as such are not discussed in this article, only a few results are shown asillustration.





Due to an initial risk analysis the leakage is modeled as a horizontal gas jet directed normal to one of

the platform legs. In cases where there is leakage of both oil and gas, the oil leakage is assumed to

settle on the sea as an oil pool which evaporates and burns around the column. For some cases the

effect of water cooling of the column has also been included in the structural analysis.

A number of different fire scenarios are defined in accordance with the risk analysis, and below are

shown two examples of the fire simulations.

Figure 1: KFX simulation of oil and gas fire close to a platform column

The structural system is a 3D frame structure (deck) supported on 4 columns. The columns are

stiffened with both ring stiffeners and vertical stiffeners. Inside the column, another stiffened cylinder

with 5 m smaller diameter exists. Simulations are carried out for non-protected and partly protected

(PFP columns and stools), also in combination with water cooling of some of the equipment. Below

are shown response examples for the worst case and with all protection means activated.

Parts of the outer skin of the column will be heated rapidly due to the intense heating, but it will take

time before the inner structures will respond similarly. If a rescue vessel is able to start the cooling of

the outer skin of the column after 30 min using fire monitors, the outer skin will cool rapidly and thematerial “recovers”, and also the heating of the inner structures stops and gradually reverses due to

loss of energy towards the cold surface.





Figure 2 below describes the temperature field (outer and inner structures) after 30 and 60 minutes for

the most severe fire case corresponding to fire situation number two in Figure 1. The colour images

show the extent of the cooling zone.

Figure 2 Temperature field after 30 and 60 min. Cooling after 30 min.

The structural integrity is checked for the different platform conditions, from the completely

unprotected situation assuming no cooling from a rescue vessel to several modified systems. A

substantial number of numerical simulations have been performed for various fire scenarios and

different mitigating actions.

With no protection the column will fail for the worst situations. Figure 3 below shows the mechanical

response for the most severe case when water cooling and PFP in a limited area is utilized. To the left,

the situation after ½ hour just before the cooling takes place, is presented, and after 1 hour (image to

the right). The plots clearly state the effect from the cooling, where the contacting plus “recovery” of

the steel reduce the mechanical utilisation of the southern part of the column.





Figure 3 Plastic Utilization field after 30 and 60 min.

Some examples of the results from this analysis are :

• The fire simulations of the unprotected structure in this particular study show temperatures up

to ~1300° C on the outer skin and ~1000°C on the inner (shielded) structures.

• The simulations have shown that the floater could collapse due to the heating from fire on sea

accidents within ½ to 1 hour.

• The most critical detail seems to be the load transfer from the deck to the entire column cross

section, the four stools. The stools and parts of the ring beam could be heated to relatively

high temperatures, and are likely to fail.• If the load transfer structures (stools/ring beam) were intact during fire i.e. cooled or passive

fire protected, and with cooling of a sector of the column, the floater is likely to survive all

fire scenarios.

• For the case with protected stools/ring beam, the system is likely to survive all fire cases if

cooling from a rescue vessel starts within 30 minutes.

Some Projects and Achievements

ComputIT has carried out a lot of studies of this kind during the last 10 years, and below some main

achievements in projects are given.

Year Installation Achievement/result

1995 Platform Minimum PFP thickness, little or no coat back

1998 Floater 15.000 m2 outer surfaces recommended without PFP

1999 Topside module PFP at only 15% of heat exposed surface (400/2.500 m2)

2002 Barge module No coat back, Access to all welds of interest (gap accepted)

2002 Floater All outer surfaces recommended without PFP (ca 10.000 m2)

1998 Whole deck area recommended unprotected, (exposed to jet-fire)

2002 Utility shaft No PFP on all steel structure surfaces of Utility shaft of concrete platform

1996 Topside module Little and simple protection of topside module. No coat back

2001 Flare boom Unprotected Flare boom, at process level, (aluminum coating)

2004 Floater Effect of water cooling from rescue vessel. Extended response time





Conclusions

• The interaction between fire and structure can easily and realistic be analyzed in detail by the

Kameleon FireEx system

• It is possible to reduce costs significantly and still maintain safety by use of detailed simulationtools

• The methodology has been used in a large number of industrial studies with good results

• Results from detailed analysis have shown to differ significantly from results based on simpler

methods

Further Information

For further details, please contact:

Trond Evanger

ComputIT

PO Box 1260 Pirsenteret

N-7462 Trondheim

Norway

Tel: +47 73 89 59 00

Fax: +47 73 89 59 01

E-mail: [email protected]

Tore Holmås

AkerKværner Offshore Partner AS

Postboks 1, Sandsli

N-5861 Bergen

Norway

Tel: +47 55 22 41 08

Fax +47 55 22 30 10

E-mail: [email protected]





References

1 Magnussen, B.F., and Hjertager, B.H., Olsen, J.G. and Bhaduri, D. The Seventeenth Symposium

(International) on Combustion, The Combustion Institute, p. 1383, (1979)

2 Magnussen B.F., "The Eddy Dissipation Concept", XI Task Leaders Meeting, Energy Conservation

in Combustion IEA (1989).

3 Magnussen, B.F. "Modeling of Reaction Processes in Turbulent Flames with Special Emphasis on

soot formation and combustion", Particulate Carbon Formation During Combustion, Plenum

Publishing Corporation, 1981.

4 Magnussen, B.F. "Heat transfer in Gas Turbine Combustors. A Discussion of Combustion, Heat and

Mass Transfer in Gas Turbine Combustors", Conference Proceedings no. 390, AGARD - Advisory

Group for Aerospace Research & Development, 1985.

5 Launder, B.E. and Spalding D.B Computer Methods in Applied Mechanics and Engineering, No. 3,

pp 269-289, 1972.

6 Tesner, P. A., Snegiriova, T. D., and Knorre, V. G. Combustion and Flame, 17, 253 (1971).

7 Patankar, S.V. and Spalding, D.B. Int. J. Heat Mass Transfer, Vol. 15, pp 1787-1806, 1972.

8 Shah, N.G. "The Computation of Radiation Heat Transfer", Ph.D. Thesis, University of London,

Faculty of Engineering, 1979.

9 Lakså, B. and Vembe, B.E.: "KAMELEON II. A general purpose program system for simulation of

fluid flow, heat and mass transfer". Sintef report STF15 F91048, 1991

10 Vembe, B. E. et.al: “Kameleon FireEx A simulator for gas dispersion and fires”.

11 1998 IGRC

12 Ultimate strength testing of Structural Members with Passive Fire Protection.

SINTEF report STF22 F99840, Trondheim 1999.

13 Ultimate strength testing of Structural Members with Attachments.

SINTEF report STF22 F00836, Trondheim 2000.

14 USFOS: Ultimate Strength Analysis. User’s and Theory manuals.

SINTEF reports. Trondheim 1993 - 2001

15 FAHTS: Fire And Heat Transfer Simulations.

Theory and User’s Manual. SINTEF report, Trondheim 1994.

16 Fire and Flaring Impacts – Temperature and Mechanical Response Analysis of the Kristin Platform,

ComputIT report: R0109, Trondheim 2001.


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KFX Paper ComputationalSystem