Depressuration Report

Depressurisation Report

NO-HLD-10-AET2-001021

Rev. Status Date Revision memo Issued by Checked by Approved by

00 IFR 25.05.12 ISSUED FOR REVIEW GK/SLT M NOS

This document has been generated by an Electronic Document Management System. When printed it is considered as a for information only copy. The controlled copy is the screen version and it is the holder's responsibility that he/she holds the latest valid version.

Document number NO-HLD-10-AET2-001021DEPRESSURISATION REPORT

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Document Type : REP System / Subsystem : NA Discipline : PRO Rev Date : 25.05.12 Contractor document number : 4100H11.002 Page 2 of 62

This document is the property of COMPANY. It must not be stored, reproduced or disclosed to others without written authorisation from the Company. DEPRESSURISATION REPORT-REV00.DOC

TABLE OF CONTENT

1. Summary............................................................................................................................................... 32. Introduction ........................................................................................................................................... 33. Abbreviations ........................................................................................................................................ 44. Depressurisation Criteria....................................................................................................................... 5

4.1. Process Sectionalisation........................................................................................................... 54.2. Calculation Design Basis .......................................................................................................... 54.3. Initial condition .......................................................................................................................... 6

4.3.1. Calculation results fire case ............................................................................................... 64.3.2. Equipment and piping integrity during fire............................................................................. 7

5. Depressurisation methodology.............................................................................................................. 75.1. General ..................................................................................................................................... 7

5.1.1. Pipe....................................................................................................................................... 75.1.2. Equipment............................................................................................................................. 7

5.2. Fire Case .................................................................................................................................. 75.3. Cold case minimum design temperature ............................................................................... 85.4. Manual Depressurisation .......................................................................................................... 9

6. Depressurisation and Cold temperature result...................................................................................... 96.1. HP Depressurisation................................................................................................................. 9

6.1.1. HP3-20-RO0010 1st Stage Separator .................................................................................. 96.1.2. HP4-20-RO0024 2nd Stage Separator ............................................................................... 106.1.3. HP6- 35-RO1025 Gas Export Compressor 1...................................................................... 116.1.4. HP20- 35-RO1225 Gas Export Compressor 2.................................................................... 126.1.5. HP8-31-RO0015 1st Stage Recompression ....................................................................... 136.1.6. HP9-31-RO0036 2nd Stage Recompression...................................................................... 146.1.7. HP10-31-RO0056 3rd Stage Recompression..................................................................... 156.1.8. HP11-31-RO0074 4th Stage Recompression..................................................................... 15

6.2. LP Derpressurisation .............................................................................................................. 166.2.1. LP1-20-RO0042 3rd Stage Separator ................................................................................ 16

6.3. Maintenance Depressurisation ............................................................................................... 176.3.1. 10-RO-1222 Brent gas well (Typical) To HP....................................................................... 18

7. References.......................................................................................................................................... 19APPENDIX A - DEPRESSURISATION LOAD SCHEDULE....................................................................... 20

Depressurisation Load Schedule ............................................................................................................ 20Manual Depressurisation Load Schedule ............................................................................................... 21Summary of Low temperature calculations............................................................................................. 22

APPENDIX B - NEW*S Documentation ..................................................................................................... 23


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1. SUMMARY

The purpose of the depressurisation system is to reduce the pressure in hydrocarbon systems. Depressurisation can be planned operations, for example before maintenance, or it can be at an emergency in case of fire, to ensure that the system will not be subject to stresses above design at extreme temperatures. In addition, the depressurisation shall reduce the hydrocarbon inventory in order to minimize the amount of hydrocarbons fed to a fire in case of a rupture.

The main segments modelled in the 3D-model have been simulated in NEW*S. For the Gas Export and the 4th stage Recompression segment further investigation is needed to keep the minimum temperature above -46C. The blowdown time need to be prolonged or it may be limited by initiating depressurisation at a higher temperature than the minimum ambient of - 10C. The blowdown rate currently exceeds 10.15 MSm/d, calculations need to be rerun with longer depressurisation time until 10.15 MSm/d is not exceeded.

Equipment and piping integrity during fire will be part of the detail engineering work.

2. INTRODUCTION

The main objective of this report is to describe the design of the Hild depressurisation system with respect to:

Design rates and capacities to the flare systems.

Sizes and sizing basis for major blowdown devices.

Sizes of depressurisation lines in the flare system.

Uncertainties and areas for further work.

The design is in accordance with all applicable guidelines, standards and company requirements. Where this is not the case, Company will be notified for review and approval of the deviation.

Reference is made to relevant standards and philosophies listed in section 10 and to the following design documents:

NO-HLD-10-AET2-001106 Safety Analysis Flow Diagram, Oil Separation

NO-HLD-10-AET2-001107 Safety Analysis Flow Diagram, Oil Export

NO-HLD-10-AET2-001108 Safety Analysis Flow Diagram, Gas Recompression

NO-HLD-10-AET2-001109 Safety Analysis Flow Diagram, Gas Treatment And Export

NO-HLD-10-AET2-001110 Safety Analysis Flow Diagram, Flare, Closed & Open Drain

NO-HLD-10-AET2-001112 Safety Analysis Flow Diagram, Fuel Gas System

NO-HLD-10-AET2-001126 Utility Flow Diagram, Flare System


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3. ABBREVIATIONS BDV Blowdown valve

ESDV Emergency Shutdown Valve

F Fire

FPSO Floating Production Storage Offloading (Vessel)

HP High Pressure

ISO International Standard Organisation

KO Drum Knock Out Drum

LAH Level Alarm High

LAL Level Alarm Low

LSHH Level Switch High High

LP Low Pressure

LSLL Level Switch High High

NEW*S Fluid properties and process simulation program from Bubblepoint AS

PAH Pressure Alarm High

PDMS Plant Design Management System from Aveva.

PFP Passive Fire Protection

PSHH Pressure Switch High High

PSLL Pressure Switch Low Low

PSS Process Shutdown System

PSV Pressure Safety Valve (Relief valve)

P&ID Piping and Instrument Diagram

RO Restriction Orifice

SD Safety Shut Down

SOP Settle-out pressure

UFD Utility Flow diagram


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4. DEPRESSURISATION CRITERIA

The ISO 23251 /3/ suggests a depressurisation down to 6.9 barg or 50% of design pressure, which ever is lower, within 15 minutes. According to NORSOK S-001 /4/ the use of passive fire protection shall be minimized by rapid depressurisation.

Simultaneous depressurisation is the preferred method for evacuation of hydrocarbons. It is considered as the safest depressurisation method due to less complex configuration than for staged depressurisation.

Low temperature depressurisation calculations needs to be performed in order to validate the selected minimum design temperature.

4.1. Process Sectionalisation

The process is divided into depressurisation sections by sectionalisation valves (SD or ESD valves). Reference is made to GS EP SAF 261 /5/, chapter 5.1.3 defining the criteria for whether a BDV is required. Ref. Table 4-1.

Table 4-1 Criteria for BDV. BDV required

That cannot be isolated No That can be isolated, but cannot be exposed to fire.

NoPipingorVessel

That can be isolated and can be exposed to fire:

Flammable gas Liquefied HC

Liquid HC Two-phase Toxic gases

P > 7 barg and PVgas > 100 bar.m3

Mgas or Mliq > 2 tonnes of C4 and more volatile

No P > 7 barg and PVgas > 100 bar.m3

As required for protection of personnel

The depressurisation facilities consisting of an actuated block valve (BDV), orifice plate, pipe expander and a manual isolation valve (locked open, full bore) in that order, all in upstream system pipe spec.

4.2. Calculation Design Basis The depressurisation calculations will be performed in the NEW*S software Version 3.30, 2011 (From Bubblepoint AS). The software has been validated by Imperial College on the Skarv BP FPSO project and gives satisfactorily results compared with the BLOWDOWN software (Imperial College). Further it should be noted that the methodology utilised in the NEW*S software is acknowledged by the API 521 committee (Reference is made to API 521 /3/ chapter 5.15.2.3. In this section there is made a reference to a more rigorous method. Reference [141] effectively describes the thermodynamic background for the NEW*S software presented at the GPA conference in 1993). Reference is made to Appendix B


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4.3. Initial condition

No credit is taken for fire-fighting equipment. The flash is assumed to be isentropic and compensated by heat from the wall heated by the external fire load. Initial condition and design criteria for calculating depressurisation loads for sections exposed to fire are as follows:

Temperature, ref. Table 4-2.

Pressure, ref. Table 4-2.

Vessel liquid level, ref. Table 4-2.

Heat input is 100 kW/m2

Backpressure is 1 bara

Combined piping and equipment volumes, wetted areas and metal mass to be used.

For Emergency depressurisation AET will follow the Hild Field Development Basis of Design /6/ section 15.2.15 quoting that the platform shall be designed in accordance with GS EP SAF 261 /5/. AET suggest using PSHH or settle-out pressure based on PSHH at suction and discharge side of compressor and normal operating temperature. Ref GS EP SAF 261 section 5.2.2 quoting "The initial pressure to be considered shall be the maximum operating pressure, which will normally correspond to the PSHH."

This will derogate from GS EP EPC 103 /7/, section 13.2 quoting "The initial pressure will be the system design pressure/safety relief valve set pressure, except for the compressor systems for which the settle out pressure will be considered."

Table 4-2 Initial conditions, depressurisation Case Emergency depressurisation Planned depressurisation Low temperature Purpose Maximum total

depressurisation flowrate to flareSizing of depressurisation orifice and depressurisation piping

Total depressurisation flowrate expected upon planned shutdown

Minimum design temperature case

Fire? Yes No No Initialtemperature

Normal operating Normal operating Minimum ambient

Initial pressure PAHH / Settle out pressure Normal operating pressure / Settle out pressure

Based on constant volume flash from PSV set pressure to minimum ambient temperature

Initial liquid level LAHH/LL to be checked Normal operating LAHH/LL to be checked

4.3.1. Calculation results fire case Reference is made to Depressurisation Load Schedule attached in Appendix A, which gives a total blowdown rate of 10.15 MSm3/d to the HP Flare and approx 0.16 MSm3/d to the LP Flare.


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According to GS-EP-SAF 261 /5/ it is a good principal to fire insulate liquid containing vessel and piping to reduce boiling and thereby avoid longer depressurisation time. This principle is foreseen to result in fire insulation on all piping, pressure vessels, filters, electrical heaters and shell and tube heat exchangers containing liquid.

Passive fire protection is foreseen required on the 2nd Stage Separator and the 3rd Stage Separator. The fluids in the 2nd Stage Separator and 3rd Stage Separator contain significant quantities of water. As the segments are depressurised, the vapour phase will not be saturated with water, which subsequently will result in a significant mass flux of water to the vapour phase as the temperature increases and the pressure drops. These vessels are recommended to be fire insulated to prevent a steam explosion.

4.3.2. Equipment and piping integrity during fire HOLD - will be part of detail engineering work.

5. DEPRESSURISATION METHODOLOGY

5.1. General

Sensitivity checks on depressurisation rate have been performed on different years for the 1st Stage Separator section, worst case is HILD-Gas_Alone-2018-07 will be used for Fire Case

NEW*S will be used for calculations outlined in section 5.2.

NEW*S will be used for low temperature calculations outlined in section 5.3.

5.1.1. Pipe The PDMS 3D model will be used to determine pipe work volumes within each segment (limited by sectionalisation valves). The inner diameter is determined using Piping And Valve Material Specification ref /1/. In addition, 15% of the calculated pipe work volume is added to include a margin in this early stage of the project. Volumes will be rechecked as part of detail engineering work.

5.1.2. Equipment The equipment volume is based on the Master Equipment List /8/. The end head of all vessels are assumed to be elliptical. An additional 10% of the total calculated volume is added to include a margin.

Volumes will be updated when supplier information is available.

5.2. Fire Case The purpose of the fire case is to calculate the orifice area and to design the upstream and downstream pipe dimensions of the blowdown line.

Start conditions:


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Normal operating temperature

PSHH or settle-out pressure based on PSHH at suction and discharge side of compressor.

High high liquid level in separators

High high liquid level in scrubbers

Credit for insulation only if PFP, i.e equipment and pipes provided with insulation for other purpose are regarded as uninsulated.

The heat input is 100 kW/m (Global fire, ref /4/)

Add all calculated blowdown rates (Simultaneous opening of the BDV for all fire zones) and find the total blowdown rate,

If the blowdown rate exceeds 10.15 MSm/d, calculations to be rerun with longer depressurisation time until 10.15 MSm/d is not exceeded.

As a first estimation, the pressure drop requirement is to reach 6.9 barg within 14 minutes. A time delay of 1 minute is assumed in order to allow sectionalisation valves to close and blowdown valve to open. A total depressurisation time will then be 15 min, ref GS EP SAF 261 /6/. This default depressurisation time, is for vessel wall thickness of 25 mm; for thinner walls, the depressurisation time shall be reduced i.e. 3 minutes for each 5 mm. For thicker walls, the depressurisation time cannot be longer than 15 minutes unless a specific study is validated by Total.

The depressurisation time of the compressor segments and the TEG Contactor segment needs to be revised when supplier information with respect to maximum pressure drop per time [bar/min] has been clarified. Currently it is assumed that these segments will withstand the gradients resulting from an API blowdown.

5.3. Cold case minimum design temperature The purpose of the Cold Case is to determine the minimum operating temperature of the blowdown segment and the minimum operating temperature downstream the orifice.

The low temperature calculation is performed on the main vessel in a process segment, ignoring all piping. This is regarded to be a conservative approach as the piping holds much more heat capacity in the steel relative to heat capacity of the process fluid, and would contribute to increase the minimum design temperature if included in the calculations. The low temperature calculations performed in NEW*S give separate minimum temperatures in both the fluid and the pipe/vessel wall for both the liquid and vapour phase. The lowest steel temperature reported is applied as the basis for the minimum design temperature and material selection for a segment. However, it is evaluated that the depressurisation nozzles on the separators and the connected depressurisation lines will locally reach a significantly lower temperature due to the good heat transfer caused by a high flow rate throughout the depressurisation. Therefore, the minimum gas temperature is applied as the basis for material selection and minimum design temperature for these parts of the segments whenever it affects the material selection.

Low temperature calculations will be performed for high and low pressure segments.

Reference is made to Depressurisation Load Schedule attached in Appendix A for cold temperature results.

Low temperature calculations are performed with the orifice size calculated in fire case and initial conditions are given below

Start conditions:

The initial pressure is based on constant volume flash from settle out PSHH or PSV set pressure and normal operating temperature to an ambient temperature of -10 C, as stated by Company /6/.


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Pipe work metal and volume excluded.

Use LSLL in vessels. LSHH to be checked as well.

Use the orifice size calculated in Fire Case

5.4. Manual Depressurisation Equipment that may be isolated and maintained during operation is equipped with manual depressurisation line to Flare. These depressurisation lines are equipped with a restriction orifice that shall be sized to keep temperatures and flow rates within the design limitations of the equipment.

Reference is made to the Depressurisation Load Schedule attached in Appendix A, a separate sheet gives the overview of maintenance blowdown with rates and corresponding cold temperatures.

6. DEPRESSURISATION AND COLD TEMPERATURE RESULT The input data collected from the spreadsheet Hild Equipment volumes.xls is included in a Unit Defined Command Procedure file (UDC-file) for calculation of pressure profile (time dependent pressure) during depressurisation. An UDC-file is used by NEW*S to read input data to defined units in NEW*S and is actually a collection of most of the commands needed to run a depressurisation simulation. In the UDC-file, all lengths are in meter and all times are in minute.

The simulation gives, besides pressure in the segment with time, blowdown rates with time, time to reach backpressure from flare and remaining volume and mass of fluid in the segment with time.

Based on experience from previous project the Coefficient of Discharge used for all restriction orifice plates with critical flow have been CD= 0.83932.

All the UDC and results obtained with the process simulation tool NEW*S, will be found in the folder \depressurisation simulations\

The folder name will indicate the corresponding RO.

6.1. HP Depressurisation

6.1.1. HP3-20-RO0010 1st Stage Separator 20-RO0010 is installed on the 1st Stage Separator, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001210.

The RO is sized based on volume from the depressurisation simulations\Hild Equipment volumes.xls.

As input for the calculation stream 20010 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 85 Total vessel volume, m3 63.62 Initial flow rate, kg/h 60103.2 14 minute to reach 7.9 baraCalculated orifice diameter, mm 38.74


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References are made to NEW*S simulations for orifice calculation see: depressurisation simulations\HP3-20-RO0010

All relevant data are included in NEW*S UDC HP3-20-RO0010.udc.txt and simulation result is printed in HP3-20-RO0010.doc

Low temperature calculation is performed in NEW*S on the 1st Stage Separator, 10-VZ2001, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 62.5

Calculated minimum temperatures: Vapor (nozzle temperature),C -51.80Liquid, C -22.53Minimum metal temperatures: Vessel (vapour space), C -22.29Vessel (liquid space), C -21.32Minimum downstream temperature in the flare system at t= 2.45 is -71.22

Reference is made to NEW*S simulations stored in: depressurisation simulations\Cold temperatures\ HP3_cl-1sep

All relevant data are included in NEW*S UDC HP3_cl-LL.udc.txt and result is printed in HP3_cl-LL.doc

The minimum design temperature of -46C in the segment and a cold sleeve in the depressurisation nozzle on the Separator with minimum design of -60C is sufficient.

6.1.2. HP4-20-RO0024 2nd Stage Separator 20-RO0024 is installed on the gas outlet of the 2nd Stage Separator, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001211.


As input for the calculation stream 20020 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 32 Total vessel volume, m3 184.53 Initial flow rate, kg/h 46236 14 minute to reach 7.9 baraCalculated orifice diameter, mm 56.05




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Low temperature calculation is performed in NEW*S on the 2nd Stage Separator, 10-VZ2002, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 21.45

Calculated minimum temperatures: Vapor (nozzle temperature),C -44,92Liquid, C -14.17Minimum metal temperatures: Vessel (vapour space), C -16.21Vessel (liquid space), C -13.92Minimum downstream temperature in the flare system at t= 6.84 is -49.32

Reference is made to NEW*S simulations stored in: depressurisation simulations\Cold temperatures\ HP4_cl- 2sep

All relevant data are included in NEW*S UDC HP4_cl.udc.txt and result is printed in HP4_cl.doc

The minimum design temperature of -46C in the segment and a cold sleeve in the depressurisation nozzle on the Separator with minimum design of -60C is sufficient.

6.1.3. HP6- 35-RO1025 Gas Export Compressor 1 35-RO1025 is installed on the Gas Export Compressor 1 outlet, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001231.


As input for the calculation stream 35003A from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 117 Total vessel volume, m3 33.87 Initial flow rate, kg/h 61165,2 14 minute to reach 7.9 baraCalculated orifice diameter, mm 29.09



Low temperature calculation is performed in NEW*S on the Gas Export Compressor Suction Scrubber 1, 10-VZ3501, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 92.9


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Reference is made to NEW*S simulations stored in: depressurisation simulations\Cold temperatures\ HP6-ExeA

The results need further investigation. To keep the minimum design temperature of -46C in the segment, the blowdown time need to be prolonged or it may be limited by initiating depressurisation at a higher temperature than the minimum ambient of -10 C.

6.1.4. HP20- 35-RO1225 Gas Export Compressor 2 35-RO1225 is installed on the Gas Export Compressor 2 outlet, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001233.


As input for the calculation stream 35003B from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 117 Total vessel volume, m3 29.91 Initial flow rate, kg/h 53874 14 minute to reach 7.9 baraCalculated orifice diameter, mm 27.3



Low temperature calculation is performed in NEW*S on the Gas Export Compressor Suction Scrubber 2, 10-VZ3502, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 92.9

Calculated minimum temperatures: Vapor (nozzle temperature),C -52.70Liquid, C -33.35Minimum metal temperatures:


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Vessel (vapour space), C -28.0Vessel (liquid space), C -31.92Minimum downstream temperature in the flare system at t= 1.65 is -88.58

Reference is made to NEW*S simulations stored in: depressurisation simulations\Cold temperatures\ HP7-ExeB


6.1.5. HP8-31-RO0015 1st Stage Recompression 31-RO0015 is installed on the 1st Stage Recompression outlet, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001221.


As for thinner walls, the depressurisation time shall be reduced i.e. 3 minutes for each 5 mm. The segment has a wall thickness of 8mm, so as pr governing document /6/ the blowdown time has to be reduced to less than 5 minutes to reach 50% of design pressure. As input for the calculation stream 31008 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 8.1 Total vessel volume, m3 12.73 Initial flow rate, kg/h 4420,2 4.1 minute to reach 5 baraCalculated orifice diameter, mm 30.8


All relevant data are included in NEW*S UDC HP8-31-RO0015.udc.txt and simulation result is printed in HP8-31-RO0015 to 2 bara.doc

Low temperature calculation is performed in NEW*S on the 1st stage Suction Scrubber, 10-VZ3101, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 5.65



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Reference is made to NEW*S simulations stored in: depressurisation simulations\Cold temperatures\ HP8-1r1

The minimum design temperature of -46C in the segment is sufficient.

6.1.6. HP9-31-RO0036 2nd Stage Recompression 31-RO0036 is installed on the 2nd Stage Recompression outlet, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001224.


As for thinner walls, the depressurisation time shall be reduced i.e. 3 minutes for each 5 mm. The segment has a wall thickness of 8mm, so as pr governing document /6/ the blowdown time has to be reduced to less than 5 minutes to reach 50% of design pressure. As input for the calculation stream 31018 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 10 Total vessel volume, m3 7.03 Initial flow rate, kg/h 4280 4.2 minute to reach 5 baraCalculated orifice diameter, mm 27.3



Low temperature calculation is performed in NEW*S on the 1st stage Suction Scrubber, 10-VZ3101, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 6.97





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6.1.7. HP10-31-RO0056 3rd Stage Recompression 31-RO0056 is installed on the 3rd Stage Recompression outlet, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001226.


As for thinner walls, the depressurisation time shall be reduced i.e. 3 minutes for each 5 mm. The segment has a wall thickness of 8mm, so as pr governing document /6/ the blowdown time has to be reduced to less than 5 minutes to reach 6.9 barg. As input for the calculation stream 31028 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 23 Total vessel volume, m3 4.19 Initial flow rate, kg/h 4894 4.6 minute to reach 7.9 baraCalculated orifice diameter, mm 19.2

References are made to NEW*S simulations for orifice calculation see: depressurisation simulations\ HP10-31-RO0056


Low temperature calculation is performed in NEW*S on the 3rd stage Suction Scrubber, 10-VZ3103, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 15.9




6.1.8. HP11-31-RO0074 4th Stage Recompression 31-RO0074 is installed on the 4th Stage Recompression outlet, and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001228.



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As for thinner walls, the depressurisation time shall be reduced i.e. 3 minutes for each 5 mm. The segment has a wall thickness of 18mm, so as pr governing document /6/ the blowdown time has to be reduced to less than 10 minutes to reach 6.9 barg. As input for the calculation stream 31038 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 51 Total vessel volume, m3 29.06 Initial flow rate, kg/h 41609 9.4 minute to reach 7.9 baraCalculated orifice diameter, mm 36.9

References are made to NEW*S simulations for orifice calculation see: depressurisation simulations\ HP11-31-RO0074

All relevant data are included in NEW*S UDC HP11-31-RO0074.udc.txt and simulation result is printed in HP11-31-RO0074 doc

Low temperature calculation is performed in NEW*S on the 4th stage Suction Scrubber, 10-VZ3104, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 38.3




6.2. LP Derpressurisation

6.2.1. LP1-20-RO0042 3rd Stage Separator 20-RO0042 is installed on the 3rd Stage Separator, and routed to the LP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001213.


The minimum temperature of -46C in the downstream LP flare system is the restriction when designing the flow orifice for the 3rd Stage Separator. Calculations performed in NEW*S indicate a maximum rate of 10491 kg/h to be within the design criteria of -46C for the segment.


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As input for the calculation stream 20030 from HILD-Gas_Alone-2018-07 have been used. Else, Operating pressure, bara 4 Total vessel volume, m3 191.68 Initial flow rate, kg/h 10491 Less than 14 minute to reach 1.5 baraCalculated orifice diameter, mm 67.02

References are made to NEW*S simulations for orifice calculation see: depressurisation simulations\LP1-20-RO0042

All relevant data are included in NEW*S UDC LP1-20-RO0042.udc.txt and simulation result is printed in LP1-20-RO0042.doc

Low temperature calculation is performed in NEW*S on the 3rd Stage Separator, 10-20VZ2003, with an orifice size giving the same pressure profile as the one calculated in the fire case. Initial conditions and result are: Operating temperature, C -10.00000 Operating pressure, bara 6.13


Reference is made to NEW*S simulations stored in: depressurisation simulations\Cold temperatures\ LP1_cl-3sep

All relevant data are included in NEW*S UDC LP1_cl.udc.txt and result is printed in LP1_cl.doc

6.3. Maintenance Depressurisation All calculations of maintenance blowdown valves are stored in the attached folder:

depressurisation simulation\Maintenance manual valve

The folder name will indicate the corresponding FO.

For low temperature calculation, reference is made to:

depressurisation simulations\Cold temperatures\Maintenance manual valve


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6.3.1. 10-RO-1222 Brent gas well (Typical) To HP 10-RO-1222 is installed on the Brent gas well and routed to the HP Flare header. Reference is made to P&ID NO-HLD-10-AET2-001200.

The Mach number in the downstream flare system is the restriction when designing the flow orifice on the typical Brent gas well. Calculations performed in Flare net indicate a maximum rate of 1900 kg/h. Input stream used for the calculation is 20010, PSHH=94 bara and normal operating temperature is 66.48C.


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7. REFERENCES

1. NO-HLD-10-AET2-100005, Piping And Valve Material Specification 2. Scandpower, Guidelines for the Protection of Pressurised Systems Exposed to Fire Rev 2,

31.03.043. BS EN ISO 23251:2007 4. Norsok S-001- Edition 4, February 2008. 5. GS EP SAF 261, Emergency Shut-Down and Emergency De-pressurisation (ESD & EDP). Rev.02 6. NODOC01-#928856-V8, HILD field Development Basis of Design, Functionality Description &

Operating Requirements - Rev 7. 7. GS EP ECP 103, Process Sizing Criteria. Rev.05 8. NO-HLD-00-AET2-000502 Master Equipment List 9. Determination of Temperatures and Flare Rates During Depressurization and Fire. (Sverre Over,

Ellen Stange and Per Salater, Presented at 72nd annual GPA Convention March 15-17, 1993, San Antonio, Texas

10. Size Depressurisation and relief Devices for Pressurised Segments Exposed to fire. (Salater, Overaa, Kjensjord, CEP (AIChe) sept. 2002)

11. NO-HLD-00-GEO-955616, #955616 -Metocean Specification For The Hild Field 12. GS EP SAF 262, Pressure protection relief and hydrocarbon disposal systems. Rev.02 13. NORSOK P-001, Process Design, Ed.5 Sep 2006 14. NORSOK P-100, Process Systems, Rev.03, Feb 2010 15. .NO-HLD-10-AET-1-002000, Material Selection and Corrosion Protection Report 16. P09010-MTA-0044 - METOCEAN REPORT DRAFT FROM ARGOSS 17. NORSOK L-002, Piping system layout, design and structural analysis, edition 3, July 2009

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APPENDIX B - NEW*S DOCUMENTATION

38 www.cepmagazine.org September 2002 CEP

Safety

his article presents the minimum require-ments for performing proper depressuriza-tion and fire-relief calculations togetherwith a procedure for sizing depressuriza-

tion and relief systems for pressurized systems exposedto fire. An engineering approach for modeling geomet-rically complex process segments is detailed. This ap-proach excludes the necessity of describing the totalsegment geometry in detail. A fire model is describedwith its required input parameters. The parameters willvary for different fire characteristics.

Minimum requirements for calculationsSeveral simulation tools are available for sizing de-

pressurization orifices and relief valves. Most lack thenecessary physical modeling required. The list belowsummarizes the minimum requirements for the designof depressurization and relief devices for pressurizedsystems exposed to fire:

rigorous thermodynamics (multicomponent fluidmodel and use of equation of state)

fire modeling (emissivities, absorptivity, tempera-ture, convection, initial flux, duration and size)

segment (vessel, pipe) material properties, i.e., ten-sile (rupture) strength, heat capacity, conductivity all are temperature-dependent

heat-transfer coefficients (boiling, radiation, con-vection and conduction)

mass transfer between the fluid phases (boilingand condensation)

fluid flow, i.e., flow-regime calculations (laminaror turbulent) that are input into heat-transfer and pres-sure-drop calculations

modeling of the process-segment geometry (sys-tem volume, system outer and inner wall areas, weight,wall thickness, liquid and gas volumes)

insulation (thickness and conductivity) stress or strain, depending upon the rupture criteria

used, to which all pipes and equipment are exposed.

The fireModeling of fires for engineering purposes requires

simplifications compared to the more thorough turbu-lent-combustion models used in computational fluiddynamics (CFD). Nonetheless, large-scale tests (3)have verified that an engineering approach to fire mod-eling gives wall- and fluid-temperature profiles that areclose to measured ones when choosing the appropriateinput parameters for Eq. 1.

Before proceeding with the fire model, some termi-nology needs to be defined:

A global fire is a large fire that engulfs the entireor a significant part of the process segment. A localfire exposes a small (local) area of the process seg-ment to the fire peak heat-flux. A jet fire is an ignitedrelease of pressurized, flammable fluids. A pool fire

Piping and equipment must withstand fires without rupturing. This can be accomplished by properly designing relief and depressurization systems and using passive fire protection, when needed.

Size Depressurization andRelief Devices for PressurizedSegments Exposed to Fire

TPer Salater, Sverre J. Overaa and Elisabeth Kjensjord,Norsk Hydro ASA

This article is extracted from the Norsk Hydro Best Practice on Depressurization and FireRelief Design. This Best Practice is a major basis for the international guide, Guidelinesfor the Design and Protection of Pressure Systems to Withstand Severe Fires (1) (to beissued shortly) and the Norwegian guide, Guidelines for Protection of Pressurized Sys-tems Exposed to Fire (2).

CEP September 2002 www.cepmagazine.org 39

is the combustion of flammable or com-bustible fluids spilled and retained on asurface. The ventilation- and fuel-con-trolled fires are related to the stoichiomet-ric ratio of air-to-fuel (Figure 1).

Figure 1 is general for both a jet and a poolfire; the difference being a higher flux for thejet fire. For a pool fire, the API fire (4) is illus-trated as the lower, dashed line to the right.Note that in the equation API RP 521 uses, in-creasing the area reduces the flux. The dashedlines represent the average heat flux. Howev-er, when studying the total volume of a fire,any point on the continuous curve will befound. A ventilation-controlled fire is to theleft of the peak heat-flux in Figure 1 at a stoi-chiometry of < 1. The fuel-controlled fire is tothe right, i.e., the stoichiometry is > 1.

The fire equationThe heat flux absorbed by a segment from a fire, qabsorbed

(kW/m2), can be modeled as:

The absorbed heat flux will be reduced with increasingsegment temperature, and a steady-state segment tempera-ture will be reached when the heat influx from the fireequals the heat outflux from the segment.

The view factor, which is not included in Eq. 1, is ascaling factor for the radiative terms. The view factor is 1.0. It equals 1.0 when the segments that absorb radiationsee nothing but an optical, thick flame. Calculation ofview factors is difficult and a conservative assumption in-volves use of a view factor of 1.0, which results in Eq. 1.

The incident heat flux is calculated by setting segment =1.0 and disregarding the segment emissivity term. Theinitial incident heat flux from a fire iscalculated by setting segment = 1.0 andTsegment equal to the normal operating tem-perature (of the cold segment).

Input to the fire equationThe different terms in the fire equation

are combined to achieve the required ini-tial heat fluxes. Tables 1, 2 and 3 suggestvalues to be used. The segment absorptivi-ty and emissivity in Eq. 1 are normallyequal and depend upon the nature of thesurface. Typical values are 0.70.9. Avalue of about 0.8 is typical for oxidizedsurfaces. The value will change as more

and more soot attaches to the surface. For more on absorp-tivity and emissivity, see Ref. 5.

By combining the suggested highest or lowest typical val-ues into the fire equation, the heat fluxes toward a cold seg-ment are found (Table 4 (6)). Typical heat fluxes measured inlarge-scale jet fire and pool-fire tests are within the maximumand minimum values in Table 4. Norsok (7) recommendsusing the initial incident heat fluxes as specified in Table 5.

Sizing proceduresThe fire-relief and depressurization calculations determine: size of the relief valves and depressurization orifices requirements for passive fire protection (PFP) size of the pipes downstream from the relief and de-

pressurization valves (if any)

q T T

h T T

absorbed segment fire fire segment segment

gas segment

= ( ) + ( )

4 4

(1)

Local fire

Global fuel-controlledGlobal-ventilation-controlled

Open-fuel-controlledpool fire

1.0

Stochiometric Ratio

Hea

t Flu

x

Figure 1. The heat flux from a fire and its relation to the stoichiometric ratio.

Table 1. Typical flame emissivities for globaland local fires.

Type of Fire Global Fire Local Firefire fire

Ventilation-controlled pool fire 0.60.75 0.70.9Fuel-controlled pool fire 0.60.75 0.70.8Jet fire 0.50.75 0.60.75

Table 2. Typical temperatures and convective heat-transfercoefficients for a global fire.

Type of Fire Tfire, C Tgas, C h, W/m2KVentilation-controlled pool fire 1,0001,050 850950 1530Fuel-controlled pool fire 9501,000 800900 1530Jet fire 1,0001,150 9501,050 50125

Table 3. Typical temperatures and convective heat-transfercoefficients for a local fire.

Type of Fire Tfire, C Tgas, C h, W/m2KVentilation-controlled pool fire 1,0501,125 Equal to Tfire 2030Fuel-controlled pool fire 1,0001,050 Equal to Tfire 2030Jet fire 1,0001,150 Equal to Tfire 100150

optimum location of the systemssectionalization valves

minimum design temperature forthe flare system (if any) and for thepressurized segment.

The minimum design temperature mayinfluence the materials selection of thesystem under evaluation. The design usu-ally begins by considering carbon steel orstainless steel as the material of construc-tion. However, temperature calculationsmay result in the need to use a different grade of steel, forexample, replacing a normal carbon steel with one suitedfor low temperatures.

Depressurization-orifice-sizing procedurePrior to running depressurization calculations, the fol-

lowing must be established: the fire scenarios (jet fire, pool fire, local fire, global

fire, etc.); define the initial heat flux, the duration and thesize (extent) of the fire(s)

the criteria for unacceptable rupture, which are usuallyone or more of the rupture pressures, the releasedflammable/toxic fluid at rupture, and the time to rupture

the time from the start of a fire until depressuriza-tion is initiated

the physical properties ultimate tensile strength(UTS), heat capacity and thermal conductivity at elevat-ed temperatures (up to 8001,000C) for the materials ofconstruction used in the depressurization segments

the depressurization segment geometry (system vol-ume, wall area, weight, etc.).

Once the above data are assembled, follow the itera-tive procedure in Figure 2. The goal of this depressuriza-tion design is to limit the use of passive fire protection(PFP) by depressurizing as fast as possible, while re-maining within the discharge capacity of the flare sys-tem. PFP should be avoided due to the risk of the conse-quences of undetected corrosion under insulation, andthe additional installation and maintenance costs in-curred by PFP systems.

When designing a new plant, it is not recommended toconsider using the entire flare-system capacity in the cal-

culations. This is to allow for future tie-ins and expectedproject growth. However, by increasing the design ca-pacity of flare system, less PFP will usually be required.

Although a local fire has a higher heat flux than aglobal fire, the global fire normally exposes the pressur-ized segment to the largest flux of heat energy, due to thelarger area encompassed by a global fire. Hence, theglobal-fire parameters determine the rupture pressure. Onthe other hand, the local fire has the highest heat flux, soits parameters determine the rupture temperature of theprocess segment.

Valves and flanges are not accounted for in the procedure.Also, it does not consider the mitigating effects of active fireprotection (such as a deluge). The sizing criteria are set toavoid an unacceptable rupture that could escalate the fire.

Step 1: Perform an initial estimate of the size of all de-

Safety


Nomenclatureh = convection heat-transfer coefficient of air/flame in contact

with segment, W/m2Kqabsorbed = absorbed heat flux from the fire, W/m2Tfire = flame temperature, KTgas = temperature of air/flame in contact with segment, KTsegment = segment temperature (time-dependent), K

Greek letters:segment = segment absorptivity, dimensionlessfire = flame emissivity, dimensionless segment = segment emissivity, dimensionless = Stefan-Boltzmann constant = 5.67 10-8 W/m2K4axial = longitudinal stress, MPahoop = hoop stress, MPaVon Mises = equivalent stress (Von Mises), MPa

Table 4. Minimum and maximum heat fluxes calculated by Eq. 1 for the suggested values of the input parameters toward a cold segment.

Type of Fire Maximum Minimum Maximum Minimum Incident Flux, Incident Flux, Absorbed Flux, Absorbed Flux, kW/m2 kW/m2 kW/m2 kW/m2

Global jet fire 326 121 306 98Local jet fire 367 187 347 160Global fuel-controlled pool fire 138 88 127 65 Local fuel-controlled pool fire 187 124 171 92Global ventilation-controlled pool fire 158 101 145 75Local ventilation-controlled pool fire 228 142 208 105

Table 5. Initial incident heat fluxes against "cold" segment (7).

Type of Fire Global Fire Local Fire (Average load) (Maximum point

load)

Pool fire, enclosed area, ventilation-controlled 130 kW/m2 200 kW/m2

Pool fire (crude), open or enclosed area, 100 kW/m2 150 kW/m2 fuel-controlled

Jet fire 250 kW/m2


Required information prior to blowdown Iteration- Description of the fire scenarios (type of fire, duration,

heat fluxes, size)- Blowdown section geometry (system volume, area, weight, etc.)- Ultimate tensile strength at elevated temperature of

materials in the blowdown section- Manual or automatic blowdown, i.e., time delay for start of

depressurization- Acceptance criteria for rupture

Acceptance criteria:- Pipe rupture pressure- Equipment rupture pressure- Released fluid at rupture- Time to rupture- No rupture

Step 1:Estimate the size of all orifices and calculate the pressure profile andflare rates for all segments. Use the fire with the largest heatinput (kW). No PFP in this initial iteration.

(Step 1) Evaluate to increase the blowdown rate, preferably for the most hazardous blowdown section.

Step 2:Add insulation if required. Calculate the process segment pressureprofile. Use the fire with the largest heat input (kW).Tip: Do several calculations with varying amounts of fire insulation.

In case ofany of the "ORs"

Step 3:Calculate the wall-temperature profile for all pipes and equipment.Use the local fire with the highest heat flux (kW/m2).

Step 4:Use the temperature profiles from Step 3 to calculate the rupture pressurefor all pipes and equipment. Compare with the pressure profile from Step 2 (Step 1 in the first iteration).

Step 7:Calculate the minimum design temperature (low-temperature designtemperature) of the blowdown section and the flare system tail pipe.

(Step 1)Is the flare system

capacity utilized (when adding all of the simultaneous

blowdown ratestogether)?

Step 5:Are the acceptancecriteria for rupture

met?

Is the minimum design

temperatureacceptable?

Reduce thesize of theorifice.

Step 6:Decide which pipe/equipment to fire insulateorIncrease orifice diameter if available capacity in the flare system, or reduce system volume by relocation of sectionalization valves or increase the flare system capacity or change material quality or increase wall thickness.

Start depressurization at a highertemperature (or change material).

(Step 1)Is the blowdown

rate less thanmaximuml-dP/dt l

No

No

No

Yes

Yes

Yes

No

Yes

The design of this section blowdown orificeand fire insulation requirements is finished.

Figure 2. Follow this sizing procedureto design depressurization orifices.

pressurization orifices, using the capacity of the flare sys-tem in the calculations. A recommended first estimate is anorifice diameter that takes the pressure below the unaccept-able rupture pressure within the typical time to rupture. Theinitial pressure should be the highest normal operating pres-sure or an equalization pressure (settle-out pressure) for acompression segment. A global fire should be used. The typi-cal time-to-rupture can be set at that interval it takes to reach a600800C wall-temperature, depending upon the wall thick-ness. A value of 510 min is typical for a dry wall exposed toa medium-heat-flux jet-fire, with no depressurization.

One way of improving the safety of the plant is increasethe rate of depressurization, as the hazardous aspects of thesegment increase. The total blowdown rate can be kept un-changed by increasing the depressurization times of theless hazardous segments. A segment containing largeamounts of light liquids (e.g., condensate or liquefiedpetroleum gas (LPG)), those that will result in boiling-liq-uid expanding-vapor explosions (BLEVEs) are regarded asa particularly hazardous section. In any case, there may belimitations on maximum pressure gradients for items suchas compressors or gaskets.

Step 2: Add insulation, if required, and simulate thepressure profile during depressurization when expos-ing the segment to the global fire. For the first iterationonly, omit this step and go to Step 3. A global fire will addheat to the fire-exposed area without PFP. The initial pres-sure in this calculation should be equal to the highest nor-mal operating or settle-out pressure. Credit for insulationshould be given only for PFP. Piping and equipment withinsulation used for purposes other than for PFP should beregarded as uninsulated.

Unrealistic backpressure in the flare system may resultin a too-rapid simulated depressurization. The orifice back-pressure should be based on the time-dependent simultane-ous depressurization rates.

If a depressurization segment is 100% fire-insulated,then the integrity of the insulation and supports usually de-termines the maximum allowable depressurization time,which is typically 3060 min. Account for the integrity ofthe insulation by extending the depressurization time for a100% fire-insulated section. The reduced depressurizationrate for this section is used to allow for the increase of therate from a most-hazardous depressurization-section. A re-duced depressurization rate may increase the fire duration,if a leak in this section is the source of the fire.

Step 3: Simulate the temperature profile for allpiping and equipment in the depressurization seg-ment when exposed to the local peak-heat flux. A jetfire is normally used in these calculations, but the localload for a pool fire should be used if the segment willnot be exposed to a jet fire. All piping means all pipeswith different diameters, pressure classes and/or materi-al qualities. The temperature profile for one particularpipe usually is rather insensitive to pressure changes

within a segment, i.e., the temperature profiles from thefirst iteration can be kept throughout the whole iterationprocedure. A final update of the temperature profilesmust be performed prior to the last iteration.

Step 4: Calculate whether or not rupture occurs. De-termine the stress or strain that all pipes and equipment areexposed to for the temperatures and pressures seen duringthe depressurization (Calculated in Steps 1 or 2, and Step3) and determine whether the segment will rupture.

Two failure (rupture) criteria are often used: the maximumstress or maximum strain (% elongation). The maximumstress criterion is usually the UTS. Rupture strain is a matterof definition. Strain calculations require finite-element mod-eling of the system, which is usually not performed duringthis step-wise method. Such calculations should be per-formed for verification purposes during the final design.

The suggested approach is to calculate the stressfrom the internal pressure and add extra stress (margins)when calculating the longitudinal stress. The stresses ofimportance for a pipe are the hoop stress caused by in-ternal pressure, and the longitudinal stress. The longitu-dinal stress is the sum of axial stresses due to pressure;the weight of the pipe, valves, fittings, branch pipes,etc; stress due to reaction forces exerted on the pipe bypressure; and stress due to thermal elongation of thepipe. The equivalent stress (von Mises) is the stress tobe compared with the temperature-dependent UTS todetermine whether rupture occurs. The hoop stress,hoop, is equal to:

The longitudinal stress, long, is given by:

long = 1/2hoop + x (3)

The equivalent stress is given by:

The term x in Eq. 3 represents all stress except for thatset up by the pressure. A piping engineer should be con-sulted when determining the value of x.

It is recommended that the UTS by reduced by 20% ormore, depending on the reliability of the UTS data. The20% is a safety margin. Reduce the wall thickness by ac-counting for the mill tolerance. It must be assumed that thelower mill tolerance is delivered. Reduce the strength byincluding the weld factor. Again, a piping engineer shouldbe consulted.

Step 5: Check the rupture pressure against the ac-ceptance criteria. If all piping and equipment in the de-

Von Mises hoop axial hoop axial_ = + 2 2 (4)

hoop =

Pressure Outer dia.2 Wall thickness

(2)

Safety


pressurization segment meet the acceptance criteria, thenthe fire insulation is completed. Go to Step 7 for low-tem-perature calculation, otherwise go to Step 6 and add in in-sulation. Alternatively, go back to Step 1 and increase thesize of the orifice or increase the flare system capacity.

Step 6: Decide which piping/equipment to fire-insu-late. If any run of piping or piece of equipment does notmeet the acceptance criteria, then add PFP to one or moreof these components. It is recommended to add PFP tothe corrosion-resistant pipe with the largest diameter. But,if there are pipes that are already insulated for reasonsother than PFP, these should be fire-insulated first.

The reasons for choosing the pipe with the largest di-ameter are it is the most critical with respect to reactionforces and pressure waves when it ruptures, and it will re-quire the largest amount of insulation per length. Largepipes are also cheaper to paint and insulate (per unit area)than smaller ones. The reason for insulating the corro-sion-resistant pipes first is to avoid insulation on metalsthat corrode more easily. When partially insulating pipes,it is preferable to add the covering on an area where thepossibility of a fire is largest and where inspection of theinsulation and pipe can be performed easily.

Step 7: Calculate the design low-temperature limita-tion of the depressurization segment (this is known asthe minimum design-temperature calculation) and in

the flare-system tail-pipe. This depres-surization calculation should be per-formed without fire input to the section.All planned types of insulation (notonly fire insulation) should be takeninto account. The initial temperatureshould be the minimum ambient tem-perature or minimum operating temper-ature, whichever is lower. The initialpressure is calculated from a cooldownof the system down to the start tempera-ture, prior to depressurization. Thecooldown calculation should be per-formed using the trip pressure from thehighest shutdown pressure. The mini-mum temperature in the flare tail-pipeshould be calculated with the depressur-ization segment as the only source tothe flare system.

Other considerationsSome key ones to note are: The loss of bolt pre-tensioning due

to bolt elongation as a result of increas-ing temperature is important whenstudying flange failure. The piping engi-neer should be consulted on this. Flangesare recommended to be fire-insulated.

Lines in the flare system havingno flow during a fire depressurization (e.g., downstreampressure-control and pressure-relief valves) are usuallyfire-insulated because: they are thin-walled and can heat uprapidly; the depressurization gas flowing in the flare sys-tem does not cool these pipes; and the flare system will bepressurized to a value near its design pressure, at the sametime that the pipe temperature is high.

Fire-relief-valve sizing procedureSizing of the fire-relief valves (Figure 3) should be

performed with the same minimum requirement as spec-ified at the beginning of the article. Also, the procedureclosely follows that for sizing depressurization orifices,with the exception, of course, that the pressure will in-crease until the relief valve opens. The size of the reliefvalve should be such that the minimum relief-rateequals the liquid boil-off and gas-expansion rates. Thisavoids a pressure increase above the set pressure of therelief valve.

A fire-relief valve will usually not protect a pressur-ized system against rupture if the gas-filled part of thesystem is exposed to fire. The fire-relief valve will nor-mally protect against rupture if the flame is exposed tothe wetted wall only when the boiling liquid on the in-side keeps the wall temperature at a reasonably lowvalue. For multicomponent mixtures, the temperature


Required information prior to relief-calculation iteration- Description of the fire scenarios (type of fire, duration,

heat fluxes, size)- Relief segment geometry (system volume, area, weight, etc.)- Ultimate tensile strength at elevated temperature of

materials in the relief section- Acceptance criteria for rupture

Acceptance criteria:- Pipe rupture pressure- Equipment rupture pressure- Released flammable fluid

at rupture- Time to rupture- No rupture

Add insulation if required. Calculate the process segment pressure profile. Use the fire with the largest heat input (kW).

Calculate the wall-temperature profile for all pipes and equipment.Use the local fire with the highest heat flux (kW/m2).

Use the temperature profile to calculate the rupture pressure forall pipes and equipment. Compare with the actual pressure.

Are theacceptance

criteria for rupturemet?

The design of this section fire-relief valveand fire-insulation requirements is finished.

Decide whichpipe/equipmentto fire insulate.

No

Figure 3. Sizing procedure for fire-relief valves is similar to that for orifices.

will increase as the lighter components evaporate, andthe wall will eventually reach the rupture temperature,even if it is liquid-filled.

Modeling process segmentsHere, we present an approach to modeling complex

depressurization and relief segments. The method modelsthe complete complex geometry by creating one hypo-thetical segment that represents the total system volumeand heat-transfer areas, and several sub-segments thatrepresent the real geometries of the segment.

The hypothetical segment is used for calculation of thesystem pressure during depressurization or relief. The sub-segments are used for calculation of the temperature re-sponse of each piece of piping and equipment within a pro-cess segment. The sub-segments are modeled with thesame geometric information that is required for a wall-tem-perature-response calculation namely, the sub-segmentwall thickness, segment or pipe diameter and inside fluid.

Hypothetical segment used for system pressureThe hypothetical segment is modeled with the real sys-

tem volume, system outer-area, with and without PFP, sys-tem inside-area in contact with the gas, system inside-areain contact with the liquid, and system weight (of pipingand equipment).

The hypothetical segment is modeled as a cylinder.This shape is specified with a diameter equal to the most-dominating (volume) diameter of any item of piping orequipment in the original segment. The associated wallthickness for this diameter should be used. The length ofthe cylinder is set such that its volume equals the volumeof the original segment. The liquid level is adjusted to ob-tain the actual liquid volume. The hypothetical cylindernow represents the correct system volume. However, theouter area of this cylinder must be corrected to the real-system outer area by subtracting or adding gas and liquidarea to the segment. Therefore, heat transfer with the sur-roundings (fire, ambient air) will be modeled over the real

Safety


Antisurge Valve

Sectionalization ValveEV or XV

BlowdownEV

BlowdownEV

BlowdownOrifice

BlowdownOrifice

LengthDi

To the Flare System

Flare System

Flare System

To the Flare System

PSV

PSV

PSV = Pressure Safety ValveEV = Emergency ValveXV = Sectionalization Valve

Sectionalization valves close in emergency situations.

Blowdown valves open in fire situations.

Cooler

ProcessStream

Sectionalization ValveEV or XVSectionalization Valve

EV or XV

Process Stream

Process Stream

ScrubberCompressor

The real geometry of any type of process segment (e.g., the above system drawn in continous lines) is transformed into a hypothetical cylindrical vessel (the vessel below in continuous line).The hypothetical vessel is used in calculation of system pressure during depressurization and relief. The diameter and length are increased until the volume equals the volume of the original system (use the dominating diameter of the original system). The liquid level is adjusted to match the liquid volume of the original system, "Add or subtract" wet and dry areas to match the wet and dry area of the original system. Set a wall thickness equal to the dominating wall thickness of the original system and adjust the weight until the weight matches the original sysem weight.Hence, we have a hypothetical segment where the system area and weight do not fit the hypothetical cylinder, but do fit the original system.

Figure 4. Modelingthe real process segmentas a hypothetical onesimplifies sizing proce-dures.

area. The system outside and inside areas may be differentfor a high-pressure system (typically, 100150 bar), withlarge amount of small-diameter piping (usually, 3 in. andbelow), and should similarly be corrected to the real sys-tem inside area. This effect is largest for pipes with a cor-rosion allowance, that is, those made of carbon steel, gen-erally. The dominating (by weight) metal quality shouldbe specified.

Unique geometry used for wall-temperature calcula-tions. The temperature response of a pipe or piece ofequipment exposed to fire depends upon the metal wallthickness, metal properties (e.g., heat capacity and con-ductivity) and the thermal mass of the inside fluid. Eachcombination of these must be calculated individually todetermine the possible different wall-temperature respons-es of the system.

The exact inside and outside diameters (I.D.s andO.D.s) should be used. The I.D. will determine the ther-mal mass of the inside fluid. The O.D. will enable calcu-lation of the fire-exposed area. Any length can be used,since the sub-segment volume and mass are proportionalto the length. The actual thickness, minus a corrosion al-lowance, should be used. The actual thickness is thenominal wall thickness, minus the allowable tolerance ofthe pipe thickness (the mill tolerance). The corrosion al-lowance should be accounted for after reduction by themill tolerance. Typical values of the mill tolerance are1.53 mm for carbon steel, with the larger value being

more common. The mill tolerance is specified on thepipe data sheet. When this tolerance is not specified bythe pipe supplier, consult a piping engineer.

The correct metal or alloy composition must be used toobtain the correct material properties (i.e., heat capacity,conductivity and the UTS). If the UTS is not available atelevated temperatures, a preliminary tensile curve can bemade based on the UTS at 20C; this is usually available.When the strength at an elevated temperature is known fora material that is close in physical properties (the samefamily of materials, such as two different carbon steels),then the percentage difference in the UTS at 20C is keptat all temperatures. That is, the new tensile strength curveshould have the same shape as the known curve. When thetensile strength at elevated temperatures is not known for amaterial in the same family, then the percentage differencebetween the UTS and yield at 20C is reduced linearly be-tween 20 and 1,000C. The UTS should be released by atleast 30% (not 20%) in such an approximation. When theUTS is equal to the yield strength in the above calculation,then the UTS should be set equal to the yield strength athigher temperatures.

Concluding remarksWe believe that the calculation of longitudinal stress

represents the major challenge when performing depressur-ization and fire relief design. Modeling of time-dependentfire characteristics (the extent and heat flux) also representsa challenge, since the plant layout is usually unknown dur-ing the design stage. Yield and UTS data at temperaturesabove 400500C often do not exist for the materials usedin the system.


PER SALATER is a principal engineer in Norsk Hydro ASA (N-0246 Oslo,Norway; Phone: +47 22 53 76 91; Fax: +47 22 53 95 37; E-mail:[email protected]). He has ten years of experience as a process andsystem engineer for Norsk Hydros North Sea oil and gas facilities. His areasof expertise are system design, heat-exchanger thermal design and designof depressurization systems. He has with Sverre Overaa co-patented(PCT/NO99/00123, U.S. Patent 09/673467) a design that eliminates theflare system for oil-and-gas processing facilities and replaces it with ablowdown header connected to storage. He holds an MSc in mechanicalengineering from the Norwegian Institute of Technology, Trondheim.

SVERRE J. OVERAA is a principal engineer at Norsk Hydro ASA (Phone: +47 2253 81 00; Fax: +47 22 53 27 25; E-mail: [email protected]). He is amember of the GPA Technical Section F Research Committee. Overaa has 18years of experience as a process and systems engineer for Norsk HydrosNorth Sea oil-and-gas facilities and is currently head of technical systemsfor the Ormen Lange project under development by Norsk Hydro. His areasof expertise are simulation, fluid properties and system design. He has withPer Salater co-patented PCT/NO99/00123, U.S. Patent 09/673467.

ELISABETH KJENSJORD is a process engineer with Norsk Hydro ASA (Phone:+47 22 53 81 00; Fax: +47 22 53 27 25; E-mail: [email protected]). She is involved in process design of Norsk Hydros oil-and-gasfacilities and is presently engaged in the development of the Grane oil fieldlocated on the Norwegian Continental Shelf in the southern part of theNorth Sea. She holds an MSc in chemical engineering from the NorwegianUniv. of Science and Technology.

Literature Cited1. Institute of Petroleum, Guidelines for the Design and Protection

of Pressurized Systems to Withstand Severe Fires, Inst. ofPetroleum, U.K. (to be issued shortly).

2. Scandpower Risk Management AS, Guidelines for Protection ofPressurised Systems Exposed to Fire, Scandpower AS, Norway(www.scandpower.com/?CatID=1071) (May 13, 2002).

3. Stange, E., et al., Determination of Temperatures and FlareRates During Depressurization and Fire, paper presented at 72ndAnnual Gas Processors Association Convention, San Antonio, TX(1993).

4. American Petroleum Institute, Guide for Pressure-Relieving andDepressurizing Systems, API Recommended Practice (RP) 521,4th ed., API, Washington, DC (1997).

5. Incropera, F. P., and D. P. DeWitt, Fundamentals of Heat andMass Transfer, 4th ed., John Wiley, New York (1996).

6. Health and Safety Executive, Joint Industry Project on Blast andFire Engineering of Topside Structures, OTI 92 596/597/598,HSE, U.K.,(1991).

7. Norsok Standard, Technical Safety, S-001, Rev. 3, (www.nts.no/norsok) (2000).

Acknowledgment

We would like to thank Erik Odgaard, Jan A. Pappas and Geir Jo-hansen (Norsk Hydro, safety and piping discipline) for their help-ful discussions.

1Pipes exposed to medium sized jet fires - rupture conditions and models for predicting time to rupture

Author: Per Salater Co-author: Sverre J Overa Norsk Hydro ASA, Oslo, Norway

Presented at FABIG, London and Aberdeen, January 2004 and Houston, March 2004

IntroductionThis article presents data from experiments on pressurized steel pipes exposed to medium sized jet flames together with engineering models for predicting time to rupture.

In 1996 and 1997 Norsk Hydro, Norways second largest oil company, performed several medium scale jet-fire tests on pressurized pipes. Pipes were pressurized with nitrogen to approximately 85-90% of the design pressure of the pipes and then exposed to a 170 - 190 kW/m2jet-fire (incident heat flux). The pressure was kept constant during the tests, and internal wall and gas temperatures were measured until the pipes ruptured. Some of the test results are presented and discussed in this article.

The tests were performed to verify Norsk Hydros engineering methods /1/ for calculation of time to pipe rupture. The Norsk Hydro methods have later been included in the Institute of Petroleums Guidelines for the design and protection of pressurized systems to withstand severe fires /2/ and the Norwegian Guideline for Protection of Pressurized Systems Exposed to Fire /3/. The tests, together with several cold (without fire) depressurization tests of both large process systems and small vessels, have confirmed that Norsk Hydros process simulation tool NEW*S /4/ is an engineering tool that reproduces experimental depressurization and fire tests. Norsk Hydro has since 1992 used NEW*S in the design of depressurization orifices and fire relief valves on its oil and gas installations. NEW*S has also been used to determine the required amount of fire insulation and the minimum design temperatures for pipes, vessels and flare systems. At present NEW*S is used in the design of the Ormen Lange onshore facility, a gas plant (70 MSm3/day) to be built on the west cost of Norway with start up in 2007. NEW*S supports most of the re

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