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Pressure Protection of Inlet – Inlet – A A Case Study Stavanger, 23.10.2012 Kim Henry Kristiansen| Process Specialist Engineer E ngineer Advanced Process Control Control and Safety, Bergen
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)
System drawing Mal operation Maximize capacity
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
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
Test Pressures Test Pressure = 1.25 x Design Pressure = 100 barg Test Pressure = 1.5 x Design Pressure = 120 barg
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.
Analysis performed (through Dynamic Simulations) ■ Primary overpressure protection
■ Secondary overpressure protection (PSV capacity)
■ Highest flare load
■ Reaction Forces (Rho*v2) Evaluation of pipe supports
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
Secondary Barrier Test – Single Source Scenario • • • •
No primary pressure barrier No background production No Additional flaring No compressor trip
Closed
In-active
Closed
Single source scenario ■ According to existing design documentation Single source scenario probably limiting for choke valve capacity Our starting
point 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)
Possible PSV characteristics Characteristic A, B og C 100
Chosen characteristic (in agreement with customer)
90
Cv=2x113
80 70 ] % [ 60 t f i L e g 50 a t n e c r 40 e P
Cv=2x90 Kar. A - Opening
Cv=2x108
Kar. B - Opening Kar. C - Opening
30 20 10 0 90
95
100
105
110
Pressure (percentage of set point) [%]
115
120
Chosen PSV characteristic PSV Characteristic 100 90 80
Designed for liquid and vapour. Typical when in gas service: - Pop action - Large blow down percentage
Pop to 90 % only. Conservative for pressure
70 ] % [ 60 t f i L e g 50 a t n e c r 40 e P
Opening Closing
30
25 % Blowdown Conservative for flare load
20 10 0 70
75
80
85
90
95
100
Pressure (percentage of set point) [%]
105
110
115
Secondary Barrier Test – Additional Source Scenario • • • • •
No primary pressure barrier Background production Compressor trip Additional flaring No help from control system
In-active
Freeze
In-active
Trip compressors
Background Production
In-active Freeze
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
Max Flare Load • • • • •
No primary pressure barrier Background production Compressor trip Additional flaring Control System working
active
active
active
Trip compressors
Background Production
active active
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
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.
Ex. 1: All 3 PV’s have “slow” contr. par. (Existing)
106
Acceptable overpressure
Ex. 2: PV on Inlet Separator has fast contr. par.
Could help reducing PV capacity
106
Unacceptable overpressure!
Ex. 3: PV’s on Contactors have “fast” contr. par.
106
Unacceptable overpressure!
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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