Ventilation rate with two different CFD-codes

Jan Terje Birkeland

FLACS User Group meeting, Bergen, May 3, 2017

Ventilation rate with two different CFD-codes

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• Requirements to ventilation in areas where gas leakages may occur

• Recommendations from software providers

• Comparison of ventilation simulations with KFX and FLACS

Ventilation rate with two different CFD-codes

WHO WON?

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Not a competition…

Not compared to measured ventilation rates

Not a master’s thesis

Main deliveries from an engineering perspective

4

QRA Technical

safety

CFD:

Explosion Gas dispersion Ventilation Fire

Working environment

/ Environmental aspects

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Why compare?

• Illuminate an –initially- relatively simple and normal issue in offshore development projects where CFD tools are used

• There is often limited opportunity (time/resources) to evaluate all aspects of the modelling, however quality check and grid sensitivity is performed

• Guidelines from software provider are therefore contributing to assuring quality

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Requirements to ventilation in areas where gas leakages may occur, NCS

• Facility regulations §14: – Ventilation in indoor- and outdoor areas shall cover the need for air

exchange and provide acceptable air quality.

• Guideline to Facility regulations §14: – To fulfill the requirements for ventilation as mentioned in the first

subsection, the standards NS-EN ISO 15138, NORSOK H-003 og S-001, chapter 16.4 should be used.

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Requirements of natural ventilation in areas with danger of gas leakage, NCS

• NORSOK S-001: – Natural ventilation in hazardous areas shall be as good as possible and

shall as a minimum provide an average ventilation rate of 12 AC/h for 95 % of the time. The ventilation rate shall be provided throughout the area to avoid stagnant zones.

– Potential stagnant zones shall be evaluated and precautions taken where considered necessary.

• NORSOK H-001 and NS-EN ISO 15138: approximately same wording

𝐴𝐶𝐻 =𝑄

𝑉 × 3600

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Recommendations for ventilation simulations

• Simulation volume

• Grid

• Boundary conditions

This presentation does not include all of the recommendations found (e.g. avoiding long, narrow cells in area of interest)- only the most relevant for the comparison are presented.

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Recommendations- simulation volume

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Geometry area

Sufficient size - must be evaluated by CFD analyst, although some guidelines exists. • FLACS: Minimum 2-3 x the geometry length in all directions.

Run sensitivity in case of doubt

• KFX: Create enough room downstream obstructions to avoid recirculation on the outlet boundary

Simulation volume

Recommendations - grid

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Uniform grid ( ca. 1 m)

Grid stretched towards the boundaries (applies to all directions)

• FLACS: Uniform grid at the geometry. 1.5 m cells are OK – outside of the geometry, the grid is stretched towards the boundaries • KFX: Use dense grid at high velocity gradients

Recommended boundary conditions FLACS:

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Inflow +X: WIND

Outflow -X: NOZZLE

Parallell -Y: WIND

Parallell +Y: WIND

Ground/Sea -Z: NOZZLE or EULER with solid surface

Parallell +Z: WIND

Parallel: WIND: Forces the wind to flow in parallel, acts approximately as a symmetry condition.

WIND

NOZZLE

WIND

NOZZLE

Ensure that the chosen boundary conditions are the ones that fit best for the issue.

Simulation volume:

Recommended boundary conditions KFX:

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Inflow +X: WIND

Outflow -X: WIND

Parallell: WIND: Lets the wind flow out if the direction is out (Free boundary)

Ground/Sea: SOLID

WIND

WIND

WIND

WIND

Simulation volume:

Parallell -Y: WIND

Parallell +Y: WIND

Parallell +Z: WIND

Basis for comparison of FLACS and KFX

• Same 3D model:

FPSO modelled in FLACS,

imported to KFX

• Same grid,

same

simulation volume

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Starting point for comparison of FLACS and KFX

• Same scenarios: 7 m/s wind from two directions

– Some more prevention of flow in module with wind direction 3 than wind direction 1

– Some more wind pushed up above the blockage, towards +Z boundary, with wind direction 3 than wind direction 1

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Starting point for comparison of FLACS and KFX

• Same wind profile at the inlet boundary

• Sea surface

• Incompressible flow in KFX, compressible in FLACS- will normally not affect ventilation simulations

• Both use k-ε turbulence model

• There are some differences we cannot affect: – Calculation of porosity

– Sub-grid modelling

– Modelling of boundary conditions

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Comparison FLACS and KFX

Initial result: needed a larger simulation volume in FLACS than in KFX in order to avoid recirculation on the outlet boundary:

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Y boundary : 11 x geometry lengths away

X boundary : 4 x geometry lengths away

Uniform grid (1 m) around

FPSO

Stretched grid towards the boundaries

- Z boundary: sea surface +Z boundary: 10 x geometry lengths away

KFX basecase

FLACS basecase

As KFX, but downstream X: 7x and Y: 17x geometry lengths away

Comparison FLACS and KFX

Wind direction

Volume flow [m3/s] FLACS

Volume flow [m3/s] KFX

% difference KFX / FLACS

1 842 892 +6%

3 269 217 -19%

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Basecase: WIND boundary conditions

Comparison FLACS og KFX - basecase, wind direction 3

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FLACS

KFX

• Varied horizontal extent of simulation volume

• Varied height of simulation volume

• Varied between boundary conditions WIND and NOZZLE for +Z boundary in FLACS

– WIND acts approximately as a symmetry boundary

– NOZZLE lets the airflow out

• Only boundary condition WIND for +Z in KFX

– Is set as «free boundaries» if the flow is outwards

• Tested boundary conditions SYMMETRY & EULER in FLACS

Additional sensitivities FLACS and KFX

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Used simulation volumes, horizontally

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Medium: 4x / 11x geometry

Small: 2x / 5x geometry

Large: 7x / 17x geometry in downstream direction (FLACS only) XL: 10x / 23x geometry in downstream direction (FLACS only)

Used simulation volumes, vertically

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+130 → 200 (approx 3 x above geometry height)

+680 → 750 (approx 10 x above geometry height)

Geometry, h=70 m

Approx. height above geometry for +Z boundary:

Ground/Sea

Initial scenarios compared

Cases Simul. volume +Z boundary FLACS +Z boundary KFX + Z boundary

1 Medium 750 - WIND/Free

3 Medium 200 - WIND/Free

11 Large 750 WIND -

13 Large 200 WIND -

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• Basecase FLACS is case 11

• Basecase KFX is case 1

Basecase according to recommendations

KFX sensitivities simulation volume

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Basecase according to recommendations

Variation is within ± 5 %

Lower +Z boundary→ some decrease in volume flow

Less simulation volume → some increase in volume flow

FLACS sensitivities Sim. Vol. and BC

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Basecase according to recommendations

Variation is larger than for KFX, but simulation volume is too small (recirculation on outflow boundaries)

Further scenarios FLACS

Cases Simul. volume +Z boundary FLACS +Z boundary

21 XL 750 WIND

22/12/2 XL/Large/Medium 750 NOZZLE

13 Large 200 WIND

14/4 Large/Medium 200 NOZZLE

15 Large 1500 WIND

16 Large 1500 NOZZLE

17 Large 750 SYMMETRY

18 Large 750 EULER

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Result of further sensitivities

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Varies within about ± 10%, and the trends are corresponding :

• NOZZLE lets more airflow out from the simulation volume the closer the border and the less the simulation volume is

• WIND and too low +Z boundary pushes more airflow in

• SYMMETRY on +Z boundary gives similar results as with WIND, EULER a bit lower

• XL sim vol and 1500 +Z boundary had little impact

Basecase according to guidelines

Noted about results & time and resources vs. accuracy

• FLACS requires longer distance to downstream borders than KFX in order to keep the results from being affected - at least for this FPSO example.

• The results become somewhat different, probably not because of following the guidelines for FLACS and KFX, but inherent code differences.

• For modules with inadequate ventilation, these differences can mean acceptable or unacceptable ventilation when 12 ACH is considered directly.

• BUT - More important - focus on how design may improve instead of running a lot of simulations to get within the requirements (there will always be some uncertainty with the simulations).

• Choice of simulation volume and grid resolution affects the accuracy of the results and time/resource need for the job

• Choose that which is adequate / good enough for the purpose

• We, the users, should keep in mind that guidelines are not strict rules, and think for ourselves

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