This document describes a case study where a risk-based approach was used to determine the location of a flare stack for an offshore oil and gas facility. Conventional analysis found that meeting separation distances for toxic gas dispersion in a flameout scenario was infeasible. A fault tree analysis and CFD modeling were used to determine the low likelihood of a flameout event. This, along with the low consequence, resulted in the risk being determined as low according to the client's risk matrix. This allowed a feasible flare stack location to be identified using the risk-based approach.

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

2. 2015 HSED E-Seminar
2
Table of contents
1. Safety Moment
2. Introduction To Topic
3. Case Study
4. Conclusion
5. Way Forward

3. 2015 HSED E-Seminar
3
Safety Moment - Tyre Safety and Suitability

4. 2015 HSED E-Seminar
4
 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

5. 2015 HSED E-Seminar
5
Case Study – Offshore Complex
Facility
Flare Stacks

6. 2015 HSED E-Seminar
6
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

7. 2015 HSED E-Seminar
7
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

8. 2015 HSED E-Seminar
8
The Concern…
Worst Case - Toxic Dispersion - PHAST
GTP RP SP UTP AP
CD
MD
WD
AWD

9. 2015 HSED E-Seminar
9
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

10. 2015 HSED E-Seminar
10
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

11. 2015 HSED E-Seminar
11
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

12. 2015 HSED E-Seminar
12
Results – Fault Tree Analysis

13. 2015 HSED E-Seminar
13
CFD Dispersion Results
Results indicated H2S exposure to around 24-35 ppm at 2 m/s and <5 ppm at 5 m/s.

14. 2015 HSED E-Seminar
14
Conclusion
 Overall likelihood of event 3.6x10-5 /yr
 Low consequence of flameout event.
 Based on clients risk matrix - low risk

15. 2015 HSED E-Seminar
15
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.

16. www.technip.com
Thank you