Baijan SAVALAN Abu Dhabi - Presentation Slides - Risk based flare location study

1. Baijan Savalan – Senior Safety Engineer
TECHNIP, ABU DHABI
Risk Based Flare Location
Study - A Case Study
2015 HSED E-Seminar

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Table of contents
1. Safety Moment
2. Introduction To Topic
3. Case Study
4. Conclusion
5. Way Forward

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Safety Moment - Tyre Safety and Suitability

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 Safe flare location is conventionally decided based on thermal
radiation and dispersion modelling of worst case flaring scenarios
consequence. However due to constraints in meeting large
separation distance/ stack height , risk based flare location study
was used to define safe location. This study was performed for an
offshore EPC Project.
Introduction

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Case Study – Offshore Complex
Facility
Flare Stacks

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Separator/s
Well
Fluid
Flare KO
2X100%
VRU
2 X 100%
GDU
2X100%
GSU
1X100%
Pressure
Control
Valve
GLC –
1st &
2nd
Stage
GLC
3rd &
4th
stage
Sweet gas
to Fuel Gas
Conditioning
and then to
Consumers
Compressed
Gas to GLC
suction
Off-
Gas
GDU
Off-Gas
GSU/
Acid
Gas
To Gas Lift
Wells & Gas
Injection
Compressor
Vent/Flare Header
QOV
Assist Gas –
Normally
Fuel Gas
Other Vent
Streams, BDVs,
PSVs
Simplified Scheme – Flare/Vent Gas Recovery
Zero flaring concept

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Flaring Scenarios - Assessed
Flaring Scenarios
 Emergency Flaring – Limiting for Radiation – No dispersion concern
 Start-up case – No radiation concern- No dispersion concern
 Operating upset (Vapor Recovery Unit Failure Case) – less flow (1% of
emergency/design flow) and very high H2S content 29% - Limiting scenario
for flare location.
 Flare design is a single tip (HP tip)
 No normal flaring
Specific Case - Flare flameout toxic dispersion is limiting
case for flare location

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The Concern…
Worst Case - Toxic Dispersion - PHAST
GTP RP SP UTP AP
CD
MD
WD
AWD

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Possible Mitigations
 Relocate the flare stack – 750m from nearest process platform –
Not feasible at EPC..!
 Elevate the Flare to reduce impact on nearest platform
 Stack is already 70m MSL
 Further increase means
 Cost & Schedule impact
 Installation/ constructability issues
 Maintenance/ Tip handling issues
Conventional approach resulted in infeasible solution for
flare location

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The Solution..
 It was decided to apply risk based approach to determine flare
location
 Perform the flame-out dispersion using CFD for worst case
 Perform FMEA Workshop & fault-tree analysis to arrive at
likelihood of event
 Compare the risk using client risk matrix – using likelihood and
consequence

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Fault Tree Inputs
Flare System Design
 Redundant flares (duty & standby)
 Redundant pilots (6 pilots per flare tip)
 Continuously lit pilots (three)
 Redundant ignition systems – High energy +
continuous sparking
 Redundant pilot gas supplies
 SIL 2 PLC for Flare Package
 Redundant thermocouples per pilot (2+2 spares)
 Redundant electrical power supply to ignition
systems
 2 electrical feeders
 EDG supply
 Independent UPS supply to each feeder

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Results – Fault Tree Analysis

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CFD Dispersion Results
Results indicated H2S exposure to around 24-35 ppm at 2 m/s and <5 ppm at 5 m/s.

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Conclusion
 Overall likelihood of event 3.6x10-5 /yr
 Low consequence of flameout event.
 Based on clients risk matrix - low risk

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Way Forward..
 For facility handling sour gas i.e. high H2S content – potentially
large separation distance may be required to satisfy toxic
exposure criteria during flame-out conditions.
 Use of risk based approach may be advisable to predict feasible
separation distances for flare.
 For such system, in terms of design
 Recommended to have redundancy in pilots and ignition
system
 Reliable pilot gas system
 Pilot gas back-up system.

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