Dr Ramsay Fraser

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Advances inStructural Design

against

Fires & Explosions

Ramsay Fraser

Tuesday, 18 June 2013



Advances in

Structural Design against

Fires & Explosions

FABIG Session - Piper 25 Conference 2

Examples of current practice

for assessment of structural response to blast & fire

Review some learning from 1988 to 2013

Knowledge transfer

to the next generation of structural engineers



Latest analysis techniques for structural

response to blast load -

Greenfield project design

Wall response for spatial and temporal consideration of blast load



Design of 110m long blast wall on new Semi-Sub

Central Processing Facility (CPF)




CPF Topsides

South Blast Wall

- to protect LQNorth Blast Wall

- to protect large

inventory equipment



Pressure pulse (P,A,T) across the 110m long wall

- spatial and temporal consideration of blast load




NLFEA analysis - external deck - 110m x 25m wall




20 DLB cases - (pressure, area and time)


Displacement contours (m)

View from Southwest

Maximum displacement

was 320mm (12.5in)





Brownfield project assessment

Global topside response




Platform layout – FEED design




Structural model for response to blast




Blast overpressure (internal and external)



LQ response to blast




LQ response to blast + ties at drill deck






Brownfield project assessment

Living Quarters “shield” wall – closely spaced walls



Existing wall


• retrofit design required to protect the North wall of accommodation module.

• minimize damage to the existing structure from a blast load of 0.77bar with 95 ms duration

• limited space; minimum of 21’’ clearance between wall and primary topside truss

• hard connections are only permitted along the perimeter of the existing wall



Existing wall




Fluid structure Interaction (FSI) by

coupled Eulerian-Lagrangian (CEL)

analyses.


The air gap was modeled by Eulerian elements

that surround and extend past the two walls

In an Eulerian analysis nodes are fixed in space,

and material flows through elements that do

not deform.

Years ago tests were performed on onshore

buildings that showed the response of a shield

wall was transmitted to the building if the

sheild wall was too close.





Closely spaced walls - Air Pressure (PSI)




Existing wall – plastic strain


W Air – distributed loadW/O Air – local bearing





Blast load and structural response

Review of learning from 1988 to 2013



Explosion hazard management


Major AccidentHazard

Structural CapacityGas Build Up Explosion Modelling

Module GeometryGas Composition

Vent AreaIgnition location

etc

Parts CountFrequency DataIgnition Probability

Wind RoseAverage Wind Speed

Gas ConcentrationGas Dispersion

eg CSTR models orCFD (Star ccm or Fluent)

Phenomenologicalor CFD

eg SCOPE orCFD (FLACS)

Safety CriticalElement

Structural DrawingsExplosion Profile(pressure,area,duration)

Material Properties

RISK ASSESSMENT ( CONSEQUENCE OR QRA MODEL)

SDOF orNon Linear FEA

SDOF (BIGGS) orFEA (ABAQUS)

EXCEEDENCE CURVE -> SLB, DLB -> max credible wall capacity (1 to 3 bar)

Legal Requirement

Company Values

Stakeholder Information



Single degree of freedom analysis


0

1

2

3

4

5

6

7

0 7 1 3 2 0 2 6 3 3 3 9 4 6 5 3 5 9 6 6 7 2 7 9 8 5 9 2 9 9 1 0 5

1 1 2

1 1 8

1 2 5

1 3 1

1 3 8

1 4 4

1 5 1

1 5 8

1 6 4

1 7 1

1 7 7

1 8 4

1 9 0

1 9 7

2 0 4

2 1 0

2 1 7

2 2 3

2 3 0

2 3 6

2 4 3

2 5 0

Time (ms)

D u c t i l i t y

0

50

100

150

200

250

300

350

400

450

O v e r p r e s s u r e ( m b a r )

Dynamic Response

Elastic/Static Response

Blast Overpressure

μ = 6.0

p_residual = 142.2mbar



1990’s - structural designers lack awareness of

blast –ve pressure




1990’s - structural analysts lack awareness of

overpressure on adjacent roof & floor




Industry mitigation errors in the 1990’s


SCEs (eg fire pump) adjacent to blast wall. SCE then gets

impaired when the wall deforms!

SCEs (eg Riser ESD ) supported off the floor of the wellbay.

A wellbay blast can then shear off the riser and the ESDV!

Blast Wall strengthening (and stiffening) was performed by

carbon fibre bonded to column flanges. This made the wall

respond elastically, which increased the end reactions, which

made the welded connection the weak link and REDUCED

the failure capacity of the wall!



Lessons Learned...


Run a simple check first before advanced analysis

Run sensitivities first before advanced analysis

Look for cliff edge effects

Apply Proportionality and Transparency in design/ assessment

In probabilistic analysis - ignition location can change the blast

wall loads significantly and gas build up can change the gas

volume significantly...

So beware of jumps in the exceedance curve and remember

engineering judgement and feasible blast wall capacities



Heat transfer

Methodology



Structural response to fire load


FAHTS (Fire And Heat Transfer Simulations) or ABAQUS

Calculate steel temperature for duration of fire

USFOS (Ultimate Strength For Offshore Structures) or ABAQUS

Calculate response of structure as a function of steel temperature

CFD (eg KAMELEON) or code values of heat flux

;heat flux radiated from fire and

temperature of air surrounding steel (for conduction)

Define fire scenarios

fire type, location, geometry and intensity.

Create FE model geometry

Convert from CATIA V5 or PDMS via .dgn format




response to fire load -

Greenfield project design

Topside fire analysis & PFP optimization



PFP scheme




Plastic utilisation – no PFP







response to fire load -Brownfield project assessment

Hull (Deck) fire analysis



Fire analysis of Production jack-up deck




Process deck above hull




Fire load


Cavity 1

Cavity 2

Cavity 3

fire load applied for 2 hours



Thermal analysis – temperature contours





1

2

3

4

5

6

7

Process deck – temperature time history




Main hull deck – temperature time history

1

2

3

4

5




Bottom hull deck – temperature time history

2

1

3

4

5




Plastic strain in Process & Main decks



Fire analysis of entire deck




Temperature – time


0

100

200

300

400

500

600

700

800

900

0 1000 2000 3000 4000 5000 6000 7000 8000

N o

d a l T e m p e r a t u r e ( C )

Time (seconds)

Temperature (Node 1)





0

100

200

300

400

500

600

700

800

0 1000 2000 3000 4000 5000 6000 7000 8000

N o

d a l T e m p e r a t u r e ( ° C )

Time (seconds)






Assumed CurveAssumed Curve

Main Hull Deck Tank Top and Hull Bottom



Temperature at 40mins (no deck PFP)




Plastic strain & collapse at 40mins (no deck PFP)




Temperature at 1hr (underside of main deck PFP)






Summary & Conclusions




Summary & Conclusions


30 years is long enough for

a communication gap to develop from

one engineering generation to the next.

“as time passed during the

period of development, the bases of the

design methods were forgotten and so were

their limits of validity. Following a period of

successful construction, a designer, perhaps

a little complacent, simply extended the

design method once too often”




Today we have better structural analysis tools, however...

Today’s structural analysts are less familiar with the fundamentals

of structural response to blast and fire load.

Today’s structural designers don’t have access to the lessons learned.



technical development of staff at speedco-founded distance learning MSc at Aberdeen University


MSc in Offshore Structures

Year 1 - Certificate Stage

• Conceptual Design of Topside Modules

• Fatigue and Fracture Mechanics

• Structural Dynamics

• Design of Connections

•Year 2 - Diploma Stage

• Blast and Fire Engineering

• Brownfield Engineering

• Petrochemical Structural Engineering

• Finite Element Methods

Year 3 - MSc Stage• Conceptual Design of Jackets and Subsea

Structures

• Design of Jacket Attachments

• Design of Stiffened Plates

• Re-Assessment of Existing Structures by

Structural Reliability Analysis



Lessons learned & knowledge management




Advances inStructural Design

against

Fires & [email protected]

[email protected]

Tuesday, 18 June 2013

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Dr Ramsay Fraser

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