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DNV Software

Offshore Consequence ModellingHow you can use DNV software to easily model offshore scenarios

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Contents

BackgroundOnshore v. OffshoreOffshore Consequence Modelling

ExamplesSmoke ModellingTurbine Exhaust Modelling

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Offshore versus Onshore

Onshore considerations

Relatively open

Off-site Impacts

Immediate impacts to the public and/or

workers

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Offshore Risk Considerations

Enclosed/confined spaceImpingementEscalationConfined explosionsFlaresGas turbine exhaust

EscapeImpairment of Temporary Refuge (TR)Impairment of escape, evacuation and rescue (EER) facilities

Subsea Releases

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Escalation

Flares

Impingement

Smoke

Subsea releases

Gas turbine exhaust

What special consequences may be modelled?

Offshore Consequence Modelling

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Offshore Consequence Modelling

What information do we need?

What are the physical constraints of an offshore platform?

How will this affect my consequence modelling?

What assumptions do I need to make?

Which results are most useful?

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Software Tools

PHASTConsequence modelling toolIncludes models for all aspects of a release from discharge and dispersion to end fire, explosion and toxic effects

NEPTUNEAdvanced risk analysis toolIncludes models for consequence modelling, leak frequency generation, event tree modelling and offshore specific requirements (ship collision, EER, etc.)

Model SpreadsheetsExpert consequence modelling toolIndividual spreadsheets for each consequence model Offers more modelling flexibility than is available through the standard software

DNV Software

Practical Examples

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Smoke Dispersion

Smoke generation is caused byFires from initiating releaseLarge fires due to escalation

Smoke dispersion can lead toImpairment of EER facilitiesImpairment of TR

We want to look at distance to concentration results, specifically:Does the cloud reach the TR?How long is it over the TR?Will it ingress into the TR?

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Smoke Dispersion Assumptions

Combustion products are primarily a mixture of nitrogen, steam, carbon dioxide and carbon monoxide.

High proportion of nitrogen in airNitrogen tends to dominate in hot combustion products⇒ Nitrogen properties representative of mixture

For hydrocarbon fuels the stoichiometric ration is 15 kg air/kg fuel

Well ventilated areasSufficient air for complete combustion⇒ Smoke production rate = 16 x fuel burning rate

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Smoke Dispersion Assumptions

Real smoke plume tends to be longer than predicted dispersions of nitrogen gas

⇒ Artificially extend the plume using calm weather conditions, eg F stability

High temperature However if too hot, combustion products would tend to rise clear and give false impression that TR is not impaired⇒ Temperature ≤ 600 ºC

Concentration of interest 1e4 ppm = 1%Indicative of edge of smoke plume that would lead to impairment of visibility

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Smoke Dispersion Example

The Unified Dispersion Model (UDM) can be used to model smoke dispersion using the previous assumptions.

The UDM is part of PHAST/SAFETI, NEPTUNE and Model Spreadsheets.

For this example we will be using PHAST.

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Smoke Dispersion in PHAST

Smoke generation from a pool fire

User Defined Source Model

Material = NitrogenDirection = Vertical

NB: Pool fire results will include burn rate from which release rate can be calculated.

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Smoke Dispersion in PHAST

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Turbine Exhaust DispersionArea of concern for the helideck

Offshore plant and equipment can cause localised environmental anomalies near the helideck and on available flight pathsCan compromise helicopter safety (lift)Must be taken into account during installation design and modifications

We want to look at temperature profiles results, specifically:

Does the cloud interfere with the helideck?Does the cloud interfere with any flight paths?

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Turbine Exhaust Dispersion Assumptions

Exhaust is primarily a mixture of nitrogen, oxygen, carbon dioxide and steam.

Nitrogen composes approximately 75% of the mixture⇒ Nitrogen properties representative of mixture

Real exhaust plume tends to be longer than predicted dispersions of nitrogen gas

⇒ Artificially extend the plume using calm weather conditions, eg F stability

High temperature, approximately 500 ºC (optional)

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Turbine Exhaust Dispersion Example

The Unified Dispersion Model (UDM) can be used to model turbine exhaust dispersion using the previous assumptions.

The UDM is part of PHAST/SAFETI, NEPTUNE and Model Spreadsheets.

For this example we will be using PHAST.

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Turbine Exhaust Dispersion in PHAST

User Defined Source Model

Material = NitrogenDirection = Horizontal

NB: Release rate and temperature are part of turbine design specifications.

NB: Discharge velocity can be calculated based on release rate and exhaust exit area.

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Turbine Exhaust Dispersion in PHASTFind detailed concentration and temperature information in the Detailed Dispersion report.

Can extrapolate the concentration equivalent for temperatures of interest

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Concentration equivalentsExtrapolated by plotting a graph and generating trend line in Excel®

Temperature profile graphsCan plot the concentration equivalents to get a view of the temperature profiles

Turbine Exhaust Dispersion in PHAST

Temperature Concentration(º C) (ppm)

50 64,000100 148,000200 317,000300 486,000

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Turbine Exhaust Dispersion in PHAST

50º C100º C200º C300º C

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Summary

Offshore consequence modelling presents very different challenges due to

Physical constraints of the platformEnclosed/confined spaceEscalation impactsEscape impacts

Additional upstream aspectsRisers, wellheadsSubsea releases

However, these challenges can be met through the correct application of your existing software.

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2005 DNV Software