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Pressure Protection of Inlet – A Case Study
Stavanger, 23.10.2012
Kim Henry Kristiansen| Process Specialist Engineer
Advanced Process Control and Safety, Bergen
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Background info
■ Reservoir pressure reduced
■ Maximize choke valve capacity to increase gas production
■ Limiting factor Overpressure Protection
■ Mal-operation of Riser ESV
■ Pipeline - 12 inch, 15 km long
■ Gas production (Fluid GOR: 8000-9000 Sm3/Sm3)
6 November, 2012Slide 2
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System drawing
6 November, 2012Slide 3
Mal operation
Maximize capacity
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Modelling
■ Model choice:
■ Existing training simulator
■ K-Spice (software from Kongsberg Oil & Gas Technologies)
■ Verification of model (significant part of the job)
■ Piping and equipment
■ Important valves: PSV, choke, PV (Pressure valves)
■ Tuning of pipeline roughness
■ Tuning of pressure drop in flare system
6 November, 2012Slide 4
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Accept critera – Allowable overpressure
■ Design code BS 5500 (all vessels)
■ MAAP Design pressure + 10 %
■ EN13445 can be used for “Exceptional load cases”
■ FEM analysis
■ Can allow for pressures up to test pressure
■ Inlet Separator
■ MAAP = 80 + 8 = 88 barg
■ Test Pressure = 1.5 x Design Pressure = 120 barg
6 November, 2012Slide 5
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Test Pressures
6 November, 2012Slide 6
Test Pressure =
1.5 x Design Pressure =
120 barg
Test Pressure =
1.25 x Design Pressure =
100 barg
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Accept criteria – Allowable overpressure
■ Inlet Separator and Pressure Vessels downstream
■ MAAP chosen due to time constraints in the project
■ To allow for pressures above MAAP FEM analysis on 8 vessels
needed.
■ HP KOD
■ EN13445 already applied
■ Test Pressure: 9.6 barg
■ Piping on outlet nozzle only tested to 9.2 barg.
6 November, 2012Slide 7
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Analysis performed (through Dynamic Simulations)
■ Primary overpressure protection
■ Secondary overpressure protection (PSV capacity)
■ Highest flare load
■ Reaction Forces (Rho*v2) Evaluation of pipe supports
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Analysis - summary
■ Secondary Barrier Test
■ Single Source Scenario (Separator not in operation, outlets closed)■ Cv = 2x108
■ Results very sensitive to PSV characteristic
■ Additional Source Scenario (Separator in operation)■ Cv = 2x108 with limited gas production (background production)
■ Decision to apply administrative control: Operational procedures and
messages on operator screens in CCR
■ NB: If administrative control fails Pressure should not exceed Test
Pressures
■ Max Flare Load■ Cv = 2x90
■ Turns out to be limiting for choke capacity!!!
■ Results very sensitive to control parameters for Pressure Valves (PV)
■ Continue with existing control parameters
■ Control parameters must not be changed without performing new analysis
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Secondary Barrier Test – Single Source Scenario
6 November, 2012Slide 10
Closed
Closed
In-active
• No primary pressure barrier
• No background production
• No Additional flaring
• No compressor trip
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Single source scenario
■ According to existing design documentation Single source
scenario probably limiting for choke valve capacity
Our startingpoint for the analysis
■ Pressure in separator Very sensitive to PSV characteristic
■ No exact characteristic available from manufacturer
■ Pop action at 2-3 % overpressure
■ Full flow capacity at 10 % overpressure
■ Blow down 15% - 25% (Percentage below set point pressure. Valve
closes at this pressure)
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Possible PSV characteristics
6 November, 2012Slide 12
0
10
20
30
40
50
60
70
80
90
100
90 95 100 105 110 115 120
P e r c e n t a g e L i f t [ % ]
Pressure (percentage of set point) [%]
Characteristic A, B og C
Kar. A - Opening
Kar. B - Opening
Kar. C - Opening
Cv=2x90
Cv=2x108
Cv=2x113
Chosen characteristic
(in agreement with customer)
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Chosen PSV characteristic
6 November, 2012Slide 13
0
10
20
30
40
50
60
70
80
90
100
70 75 80 85 90 95 100 105 110 115
P e r c e n t a g e L i f t [ % ]
Pressure (percentage of set point) [%]
PSV Characteristic
Opening
Closing
25 % Blowdown
Conservative for flare load
Pop to 90 % only.
Conservative for pressure
Designed for liquid and vapour.
Typical when in gas service:
- Pop action
- Large blow down percentage
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Secondary Barrier Test – Additional Source Scenario
6 November, 2012Slide 14
Freeze
Trip
compressors
Freeze
In-active
In-active
In-active
Background
Production
• No primary pressure barrier
• Background production
• Compressor trip
• Additional flaring
• No help from control system
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Additional Source Scenario
■ The Additional Source Scenario gives higher pressure build-up than
the Single Source Scenario
■ Reduction of gas production before start-up of pipeline would help
■Solution:
■ Use administrative control to limit gas production before start-up.
■ BUT! Pressure should not exceed test pressure if administrative control
fails.
■ …No reduction of calculated choke Cv compared to the single source
scenario
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Max Flare Load
6 November, 2012Slide 16
active
Trip
compressors
active
active
active
active
Background
Production
• No primary pressure barrier
• Background production
• Compressor trip
• Additional flaring
• Control System working
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Max Flare Load
■ Assumes that administrative control have failed High gas
production before incident
■ Pressure build-up in the HP Flare system is limiting the choke valve
capacity !! Cv = 2x90
■ Evaluation of valve capacity – PV on Inlet Separator
■ 30-40 % more capacity than needed
■ If PV goes fully open in short time – capacity reduction is a good idea
■ With “slow” control parameters the extra 30-40% is not utilized.
■ Conclusion: Control parameters more important than valve capacity
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Max Flare Load
■ Evaluation of control parameters – PV’s
■ Keep existing “slow” control parameters
■ Choosing wrong parameters could give too high pressures in HP Flare
■ Change of PV control parameters should not be done without performing
new analysis.
■ Some examples are shown on the next 3 slides
■ Example 1: Using existing control parameters for all 3 PV’s.
■ Example 2: The Inlet Separator PV has got “fast” parameters.
■ Example 3: The 2 PV’s on the glycol contactors have got “fast” control
parameters.
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Ex. 1: All 3 PV’s have “slow” contr. par. (Existing)
6 November, 2012Slide 19
106
Acceptable
overpressure
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Ex. 2: PV on Inlet Separator has fast contr. par.
6 November, 2012Slide 20
Could help
reducing PV
capacity
106
Unacceptableoverpressure!
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Ex. 3: PV’s on Contactors have “fast” contr. par.
6 November, 2012Slide 21
106
Unacceptableoverpressure!
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Main conclusions / Lessons learned
■ Achievement: Choke capacity increased by 50%
■ Initial assumption was wrong: The limiting scenario was not the
single source scenario.
■ Thorough evaluation of the accept criteria and design basis are of
high importance.
■ Perform sensitivity studies. Surprising dynamic effects are often
revealed.
■ PV Control Parameters and the PSV characteristic are important
factors and should not be underestimated.
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