3 Pressure Protection of Inlet

8/9/2019 3 Pressure Protection of Inlet

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© 2012 Aker Solutions Preferred partner

Preferred partner

Pressure Protection of Inlet – A Case Study

Stavanger, 23.10.2012

Kim Henry Kristiansen| Process Specialist Engineer

Advanced Process Control and Safety, Bergen


2/23


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


Mal operation

Maximize capacity


4/23


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



5/23


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



6/23


Test Pressures


Test Pressure =

1.5 x Design Pressure =

120 barg

Test Pressure =

1.25 x Design Pressure =

100 barg


7/23© 2012 Aker Solutions Preferred partner

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.



8/23© 2012 Aker Solutions Preferred 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



9/23


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



10/23


Secondary Barrier Test – Single Source Scenario


Closed

Closed

In-active

• No primary pressure barrier

• No background production

• No Additional flaring

• No compressor trip


11/23


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)



12/23


Possible PSV characteristics


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


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


14/23


Secondary Barrier Test – Additional Source Scenario


Freeze

Trip

compressors

Freeze

In-active

In-active

In-active

Background

Production


• Background production

• Compressor trip

• Additional flaring

• No help from control system


15/23


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



16/23


Max Flare Load


active

Trip

compressors

active

active

active

active

Background

Production


• Background production

• Compressor trip

• Additional flaring

• Control System working


17/23


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



18/23


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.



19/23


Ex. 1: All 3 PV’s have “slow” contr. par. (Existing)


106

Acceptable

overpressure


20/23


Ex. 2: PV on Inlet Separator has fast contr. par.


Could help

reducing PV

capacity

106

Unacceptableoverpressure!


21/23


Ex. 3: PV’s on Contactors have “fast” contr. par.


106

Unacceptableoverpressure!


22/23


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.



23/23


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