Pressure Protection of Inlet - A case study Kim ... - Aker Solutions
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Pressure Protection of Inlet - A case study Kim ... - Aker Solutions

Preferred partnerPressure Protection of Inlet – A Case StudyStavanger, 23.10.2012Kim Henry Kristiansen| Process Specialist EngineerAdvanced Process Control and Safety, Bergen© 2012 Aker Solutions Preferred partner

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 Sm 3 /Sm 3 )© 2012 Aker Solutions Slide 2 6 November, 2012Preferred partner

System drawingMal operationMaximize capacity© 2012 Aker Solutions Slide 3 6 November, 2012Preferred partner

Modelling■ Model choice:■■Existing training simulatorK-Spice (software from Kongsberg Oil & Gas Technologies)■ Verification of model (significant part of the job)■■■■Piping and equipmentImportant valves: PSV, choke, PV (Pressure valves)Tuning of pipeline roughnessTuning of pressure drop in flare system© 2012 Aker Solutions Slide 4 6 November, 2012Preferred partner

Accept critera – Allowable overpressure■ Design code BS 5500 (all vessels)■ MAAP Design pressure + 10 %■ EN13445 can be used for “Exceptional load cases”■■FEM analysisCan allow for pressures up to test pressure■ Inlet Separator■■MAAP = 80 + 8 = 88 bargTest Pressure = 1.5 x Design Pressure = 120 barg© 2012 Aker Solutions Slide 5 6 November, 2012Preferred partner

Test PressuresTest Pressure =1.5 x Design Pressure =120 bargTest Pressure =1.25 x Design Pressure =100 barg© 2012 Aker Solutions Slide 6 6 November, 2012Preferred partner

Accept criteria – Allowable overpressure■ Inlet Separator and Pressure Vessels downstream■■MAAP chosen due to time constraints in the projectTo allow for pressures above MAAP FEM analysis on 8 vesselsneeded.■ HP KOD■■■EN13445 already appliedTest Pressure: 9.6 bargPiping on outlet nozzle only tested to 9.2 barg.© 2012 Aker Solutions Slide 7 6 November, 2012Preferred partner

Analysis performed (through Dynamic Simulations)■ Primary overpressure protection■ Secondary overpressure protection (PSV capacity)■ Highest flare load■ Reaction Forces (Rho*v2) Evaluation of pipe supports© 2012 Aker Solutions Slide 8 6 November, 2012Preferred partner

Analysis - summary■ Secondary Barrier Test■■Single Source Scenario (Separator not in operation, outlets closed)■ Cv = 2x108■ Results very sensitive to PSV characteristicAdditional Source Scenario (Separator in operation)■ Cv = 2x108 with limited gas production (background production)■ Decision to apply administrative control: Operational procedures andmessages on operator screens in CCR■ NB: If administrative control fails Pressure should not exceed TestPressures■ 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© 2012 Aker Solutions Slide 9 6 November, 2012Preferred partner

Secondary Barrier Test – Single Source Scenario• No primary pressure barrier• No background production• No Additional flaring• No compressor tripClosedIn-activeClosed© 2012 Aker Solutions Slide 10 6 November, 2012Preferred partner

Single source scenario■ According to existing design documentation Single sourcescenario 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 % overpressureFull flow capacity at 10 % overpressureBlow down 15% - 25% (Percentage below set point pressure. Valvecloses at this pressure)© 2012 Aker Solutions Slide 11 6 November, 2012Preferred partner

Percentage Lift [%]Possible PSV characteristics100Characteristic A, B og C90807060Cv=2x113Chosen characteristic(in agreement with customer)Cv=2x905040Cv=2x108Kar. A - OpeningKar. B - OpeningKar. C - Opening302010090 95 100 105 110 115 120Pressure (percentage of set point) [%]© 2012 Aker Solutions Slide 12 6 November, 2012Preferred partner

Percentage Lift [%]Chosen PSV characteristicPSV Characteristic1009080Designed for liquid and vapour.Typical when in gas service:- Pop action- Large blow down percentage7060Pop to 90 % only.Conservative for pressure5040OpeningClosing30201025 % BlowdownConservative for flare load070 75 80 85 90 95 100 105 110 115Pressure (percentage of set point) [%]© 2012 Aker Solutions Slide 13 6 November, 2012Preferred partner

Secondary Barrier Test – Additional Source Scenario• No primary pressure barrier• Background production• Compressor trip• Additional flaring• No help from control systemFreezeIn-activeIn-activeTripcompressorsBackgroundProductionFreezeIn-active© 2012 Aker Solutions Slide 14 6 November, 2012Preferred partner

Additional Source Scenario■ The Additional Source Scenario gives higher pressure build-up thanthe 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 controlfails.■ …No reduction of calculated choke Cv compared to the single sourcescenario © 2012 Aker Solutions Slide 15 6 November, 2012Preferred partner

Max Flare Load• No primary pressure barrier• Background production• Compressor trip• Additional flaring• Control System workingactiveactiveactiveTripcompressorsBackgroundProductionactiveactive© 2012 Aker Solutions Slide 16 6 November, 2012Preferred partner

Max Flare Load■ Assumes that administrative control have failed High gasproduction before incident■ Pressure build-up in the HP Flare system is limiting the choke valvecapacity !! Cv = 2x90■ Evaluation of valve capacity – PV on Inlet Separator■■■■30-40 % more capacity than neededIf PV goes fully open in short time – capacity reduction is a good ideaWith “slow” control parameters the extra 30-40% is not utilized.Conclusion: Control parameters more important than valve capacity© 2012 Aker Solutions Slide 17 6 November, 2012Preferred partner

Max Flare Load■ Evaluation of control parameters – PV’s■■■Keep existing “slow” control parametersChoosing wrong parameters could give too high pressures in HP FlareChange of PV control parameters should not be done without performingnew 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” controlparameters.© 2012 Aker Solutions Slide 18 6 November, 2012Preferred partner

Ex. 1: All 3 PV’s have “slow” contr. par. (Existing)10 6Acceptableoverpressure© 2012 Aker Solutions Slide 19 6 November, 2012Preferred partner

Ex. 2: PV on Inlet Separator has fast contr. par.Could helpreducing PVcapacity10 6Unacceptableoverpressure!© 2012 Aker Solutions Slide 20 6 November, 2012Preferred partner

Ex. 3: PV’s on Contactors have “fast” contr. par.10 6Unacceptableoverpressure!© 2012 Aker Solutions Slide 21 6 November, 2012Preferred partner

Main conclusions / Lessons learned■ Achievement: Choke capacity increased by 50%■ Initial assumption was wrong: The limiting scenario was not thesingle source scenario.■ Thorough evaluation of the accept criteria and design basis are ofhigh importance.■ Perform sensitivity studies. Surprising dynamic effects are oftenrevealed.■ PV Control Parameters and the PSV characteristic are importantfactors and should not be underestimated.© 2012 Aker Solutions Slide 22 6 November, 2012Preferred partner

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