EMERGENCY VESSELS BLOWDOWN SIMULATIONS

AGENDA

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INTRODUCTION

TYPICAL FIRST STUDIES IN TOTAL UPSTREAM

INTERNAL RULES BASED ON API

METHODOLOGY

STUDY RESULTS

DEVELOPMENT WORK - COLLABORATION BETWEEN SIMSCI & TOTAL

PROPOSED ENHANCEMENTS TO BE DONE

CONCLUSION

INTRODUCTION

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Why to depressurize ? Loss of control

(fire, power failure, leak…)

Pressurized hydrocarbons

Hazard for installation and people (explosion, oil spill …)

Solution: Emergency depressurization

Pressure decrease leads to temperature drop because of the Joule-Thomson effect and of the liquid vaporization

Main concerns • Wall vessel temperature

Vessel material to be chosen to resist low temperatures • Maximum flowrate to be flared

Hydrocarbons sent to the flare

Reliable predictive simulation tools

needed

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TYPICAL DEVELOPMENT STUDIES IN TOTAL UPSTREAM

Short time periods + Fluctuating data Calculations done internally with process simulators

Longer periods + detailed studies External tools

Depressurization calculations are done at early stage of the studies: The flare size or safety zones have a significant impact on the facilities layout or on a platform size. It can be of major importance in the choice of a concept.

RADIATIONS Calculations are done through flaresim (softbits) software

It models thermal radiation and noise footprints generated by flare systems and predicts the temperature of exposed surfaces

Project Pre-project Conceptual

studies Screening

A few weeks 3 months 6 months

DISPERSION Calculations are done through PHAST (DNV) software

It models atmospherics dispersion to determine the safety zones.

INTERNAL RULES DERIVED FROM API 521

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Initial conditions

- Initial pressure =system design pressure

- Initial temperature = MinOT (or ambiant T if lower)

- Initial liquid level in the drum, the calculations shall be conducted for both LAL and LAH. The worst case shall be retained.

Final conditions

- P down to 8 bara or 50% of design pressure (whichever is the most stringent) in 15 minutes. For cold cases, P down to atmospheric pressure to get the right minimum temperature.

METHODOLOGY

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Comparisons to the results of reference software

LNGDYN and BLOWDOWN

Very few experimental data

4 commercial software

Pro/II Hy..s Dynsim Un…m

Two Reference tools considered in Total: Blowdown (from Imperial College of London) and LNGDYN (From Technip) not available for Total initial development studies (have to be subcontracted to ICL

or Technip, which is not compatible with the initial studies planning)

Why do we use pro/II for dynamic simulations? • PRO/II used for performing all the process simulations in conceptual and pre-projects studies → more comfortable to use the same tool. • Dynsim too onerous at these early stages of a project • More efficient to use pro/II features such as controller to adjust the orifice size

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Objectives of the study

• Decide if PRO/II can be used in our studies by comparison of the results to those of the reference tools • Determine margins to be applied on simulation results during conceptual and project studies • Focuss on vessel depressurization (no multi-units were checked and no pipes) with no water phase

OBJECTIVES AND RESULTS OF THE STUDY

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Total and SimSci worked together to address the weaknesses of pro/II depressurization tool. This work led to significant enhancements into pro/II software

1. Definition of the required improvement by Total

2. Prioritization from Total

3. Reviewing of the detailed specifications issued by SimSci

4. Tests of issued versions

Pro

ject

p

rogr

ess

Conclusions of the validation study

• This study exhibited relatively good predictions for pro/II for flowrate and temperature

Recommended margins: 10% on flowrate predictions and 10°C on Minimum temperature

• Drawbacks: Only one wall temperature, poor flexibility on vessels geometry (head types, wall thickness…) , no orifice models

RESULTS AND WORK AHEAD

DEVELOPMENT WORK - COLLABORATION BETWEEN SIMSCI & TOTAL

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USABILITY

Report temperature downstream the relief valve

Add Depressuring Time vs. Pressure Summary to Depressuring report

Indicate whether flow through valve is critical or subcritical

Re-order product streams

Create pseudo-streams corresponding to instantaneous vent rates at specific times

Supply wall and liquid temperature profiles

Documentation on depressurization unit

VESSEL CONFIGURATION

New Valve Mode – Orifice: It will automatically detect if the flow is sonic or subsonic and use the appropriate equations.

Different head type (hemispherical, flat…)

Valve Policy & user control of flow rate and depressuring time

Rigorous Dynamic Wetted Area calculation

Support Density and thickness in Vessel Geometry

Require final pressure in depressuring unit to be exactly the specified PFINAL

FURTHER ENHANCEMENTS PROPOSED TO BE DONE

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Two main concerns for Total

• Solid CO2 predictions: The aim is to predict accurately amounts and conditions of formation of Solid CO2 during cold Blowdown for gas with a high CO2 content

• Two wall temperatures: Twliq for the liquid side and Twgas for gas side

• We do not expect a multi equipment depressurization (Dynsim to be used in such cases)

Twgas

Twliq

CONCLUSIONS

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Pro/II Software is able to perform vessel depressurizations at early stages of upstream development projects

Comparison were done with reference tools in Total

Pro/II leads to satisfying results compared to these reference tools (appropriate margins have been defined)

Software development project permitted to successfully improve Pro/II depressurization tools

Further enhancements are currently studied to have a more powerful tool