transient-analysis--pigging-(pipeline).pdf

7/29/2019 transient-analysis--pigging-(pipeline).pdf

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KCP-GNS-FAS-DRP-0006Revision: 02

Project Title: Kingsnorth Carbon Capture & Storage Project Page 1 of 25

Document Title: Transient Analysis Pigging (Pipeline)

Kingsnorth CCS Demonstrat ion ProjectThe information contained in this document (the Information) is provided in good faith.E.ON UK plc, i ts subcontractors, subsidiaries, aff i l iates, employees, advisers, and the Department of Energy and Climate Change (DECC)make no representat ion or warranty as to the accuracy, rel iabil i ty or completeness of the Information and neither E.ON UK plc nor any of i tssubcontractors, subsidiaries, aff i l iates, employees, advisers or DECC shall have any l iabil i ty whatsoever for any direct or indirect losshowsoever arising from the use of the Information by any party.

Transient Analysis Pigging (Pipeline)

Table of Contents1. Executive Summary .......................................................................................................... 3

1.1. Scope of Work ......................................................................................................... 31.2. Intelligent Pigging .................................................................................................... 31.3. Air-Driven Pigging ................................................................................................... 31.4. Blowdown of Air-Filled Pipeline ............................................................................... 41.5. Recommendations .................................................................................................. 5

2. Scope of Work and Basis of Design ................................................................................. 62.1. Scope of Work ......................................................................................................... 62.2. Basis of Design and Assumptions ........................................................................... 6

2.2.1. Reservoir Pressure ...................................................................................... 62.2.2. Pig Model within OLGA ................................................................................ 62.2.3. Air Composition ............................................................................................ 62.2.4. Blowdown Equipment................................................................................... 72.2.5. Air Flowrates for Air-Driven Pigging ............................................................. 7

3. Intelligent Pigging ............................................................................................................. 84. Air-Driven Pigging ............................................................................................................ 9

4.1. Introduction ............................................................................................................. 94.2. Required Pigging Durations ..................................................................................... 94.3. CO2 Leakage ......................................................................................................... 104.4. Sensitivity of Leakage to Pig Diameter .................................................................. 124.5. Liquid Formation behind Pig .................................................................................. 13

4.6. Pig Velocity ........................................................................................................... 144.7. Flowrate surges ..................................................................................................... 154.8. Recommendations ................................................................................................ 17

5. Blowdown of Air-Filled Pipeline ...................................................................................... 185.1. Introduction ........................................................................................................... 185.2. Results .................................................................................................................. 18

5.2.1. Depressurisation Times ............................................................................. 185.2.2. Peak Air Flow Rate .................................................................................... 195.2.3. Minimum Temperatures during Blowdown ................................................. 19

5.3. CO2 Slippage ......................................................................................................... 196. Supporting References ................................................................................................... 207. Appendix A Thermodynamics of Air and Carbon Dioxide Mixtures ................................. 21

7.1.1. Heat of Mixing ............................................................................................ 217.1.2. Phase Envelope ......................................................................................... 22

8. Appendix B Results for Depressurisation of Air-Filled Pipeline ....................................... 24


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Table of TablesTable 1-1 Pipeline Operating Scenarios ................................................................................................. 3Table 1-2 Time to Depressurise to 50% and 25% of Pipeline Design Pressure .................................... 4Table 2-1 Assumed Plug Properties ....................................................................................................... 6Table 5-1 Time to Depressurise to 50% and 25% of Pipeline Design Pressure .................................. 18Table of FiguresFigure 3-1 Pig Velocity for routine pigging base case and full flow scenarios ..................................... 8Figure 4-1 Distance Travelled by Pig through Pipeline vs. Time .......................................................... 10Figure 4-2 Example CO2 and N2 Molar Composition Profile ................................................................. 11Figure 4-3 Leaked CO2 Behind Pig at End of Pig Run ......................................................................... 12Figure 4-4 Molar CO2 Concentrations behind Pig Sensitivity to Pig Diameter .................................. 13Figure 4-5 Example of Liquid Formation behind Pig ............................................................................. 14Figure 4-6 Pigging Velocities during Air-Driven Pigging ....................................................................... 15Figure 4-7 Expanded View of Pigging Velocity Trend .......................................................................... 15Figure 4-8 Mass Flow Rate at Offshore Kingsnorth Platform during Air-Driven Pigging ...................... 16Figure 4-9 Expanded View of Mass Flow Rate at Offshore Kingsnorth Platform during Air-DrivenPigging .................................................................................................................................................. 16Figure 7-1 Temperature Drop due to Heat of Mixing of Air/CO2 Mixtures ............................................ 21Figure 7-2 Heat of Mixing of Air/CO2 Mixtures ...................................................................................... 22Figure 7-3 Phase Envelopes for Air/CO2 Mixtures ................................................................................ 23Figure 8-1 Fluid and Wall Temperatures during Blowdown of Air-Filled Pipeline ................................. 24Figure 8-2 Blowdown Rate and Upstream Pressure ............................................................................ 24Figure 8-3 Pipeline Liquid Content during Blowdown of Air-Filled Pipeline .......................................... 25Figure 8-4 Mass Flow through Pipeline during Blowdown .................................................................... 25List of Abbreviations

CCS Carbon Capture and storage

HP High pressure

HYSYS Process plant simulation software

LP Low pressure

OLGA Transient Flow Assurance Simulation Software

SPT SPT Group (owners of OLGA)


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1. Executive Summary

1.1. Scope of Work

The pigging analysis takes into account two operating scenarios agreed previously:

Table 1-1 Pipeline Operating Scenarios

Property Base Case Full Flow

Vapour Density LP Vapour HP Dense

Power Plant Capacity (without Capture) 400 MW 1600 MW

CO2 Flowrate 6600 t/d 26,400 t/d

The base case and full flow scenarios are considered in this pigging analysis. The maximumcapacity scenario examined in the steady state analysis is not considered in the transientstudies due to lack of definition.

This study examines the following pigging scenarios for the cases described in Table 1-1above using OLGA:

intelligent pigging

air-driven pigging to displace CO2 from the pipeline

Intelligent pigging may be required periodically for inspection purposes. In this scenario a pigwould be launched into the pipeline during normal production and received at the offshore

Kingsnorth platform (using CO2 to drive the pig). Pigging to displace CO2 from the pipeline ismore complicated in that the pig would be launched and driven by a dry air source to the pigreceiver at the platform. When the pig has been received, the air source would be shut offand the air in the pipeline vented to atmosphere via the Kingsnorth CCS plant vent system.

1.2. Intelligent Pigging

It was found that the velocity of the pig was within or near the optimum range for intelligentpigging (typically 1.5 3 m/s) throughout the duration of vapour phase operation. However,for dense phase operation the volume flow rates and hence velocities are lower, such that thevelocity of the pig is barely above the minimum acceptable velocity for intelligent pigging(typically 0.5 m/s). If it is required to use an intelligent pig for inspection purposes during

dense phase operation the vendor should be consulted to confirm that the low velocity isacceptable.

1.3. Air-Driven Pigging

Air-driven pigging was investigated for dense phase operation. The time for the pig to travelbetween the pig launcher and receiver was found to be 119 hours (5 days) assuming that thenormal pipeline velocity was maintained. If the velocity of the pig was increased by a factor of3 to a more typical pigging velocity of c. 1.5 m/s then the pig travel time was reduced to 51hours (c. 2 days). However it is unlikely that adequate heating could be maintained on theplatform (to sustain single phase fluid in the wellbore) within the available heater duty at this

increased flowrate.


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Significant leakage of CO2 behind the pig due to slippage was found to occur during thesimulation. This results in a substantial volume of CO2 contained within the air behind the pig,which must then be blown down via the Kingsnorth CCS plant vent system. The results of asensitivity study on the effect of pig diameter on CO2 slippage indicated that the degree ofslippage does not increase with the assumed gap between the pig and the pipe wall. As thisresult was contrary to expectations this issue was raised with OLGA support; the slippageresults will be updated if necessary following feedback from SPT.

Due to the slippage of CO2 behind the pig, there is a region behind the pig where there is arelatively high CO2 concentration in the air. At the operating conditions of the pipeline,retrograde liquid condensation is predicted by OLGA. Where there is liquid formed behind the

pig there is a temperature rise associated with the liquid formation due to the enthalpy ofcondensation. This temperature increase predicted by OLGA is not in agreement withHYSYS analysis, which suggests that a significant temperature drop should be expected, dueto endothermic heat of mixing. It is recommended that this be investigated in more detail inthe next phase of the design project.

When the CO2 in front of the pig arrives at the offshore Kingsnorth platform, it is to be injectedinto the reservoir via the topsides heaters. The flow rate arriving at the platform is subject tofluctuation and surges and so it should be confirmed that the heater design flow rates can beexceeded for short durations without resulting in damage. As the heater will likely be unableto achieve its setpoint during periods of fluctuating flow it would be necessary to eitherincrease the heater setpoint to be sure of an adequate outlet temperature or alternatively to

leave the setpoint unaltered but allow the wells to operate with temporary excursions into thetwo phase region for the duration of the pigging if this is an emergency case to clear the lineof CO2.

1.4. Blowdown of Air-Filled Pipeline

The times required to depressurise the air-filled pipeline to 50% and 25% of the pipelinedesign pressure via a 6 blowdown orifice at the Kingsnorth CCS plant are tabulated below,for design pressures of 150 bar(g) (current basis of design) and 120 bar(g) (sensitivity casefor flow assurance studies).

Table 1-2 Time to Depressurise to 50% and 25% of Pipeline Design Pressure

Design pressure (bar(g)) Time to reach 50% (h) Time to reach 25% (h)

150 0.1 10.4

120 2.4 14.2

The time to depressurise to 0 bar(g) was found to be 83 hours (3.5 days). Larger orifice sizeswould reduce the blowdown time however the vent system at the Kingsnorth CCS plant iscurrently presumed to be limited to 6 (though it should be noted that this may be reducedfurther to 4) and so this may not be possible).


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The time taken to pig the line with an air-driven pig and subsequently depressurise the air

from the pipeline is less than that required to depressurise the pipeline of CO2 by blowdownvia the Kingsnorth CCS plant vent system (RefS1). The total time required to pig the flowline(119 hours) and then blow down the air (83 hours) is 202 hours, compared to 281 hours toblow down the CO2-filled pipeline. Depressurising the pipeline utilising air-driven pigging istherefore a method that could be investigated further as the project progresses.

The minimum temperatures in the system and peak mass flow rates through the vent systemare less severe than for depressurisation of a CO2-filled pipeline. The minimum fluidtemperatures in the pipeline and vent system are ambient temperature and -21Crespectively. It should be noted that the extent of CO2 slippage behind the pig into the air willhave only a minor effect on the results.

1.5. Recommendations

There are significant uncertainties associated with the modelling of air-driven pigging ofcarbon dioxide systems. It is recommended that the following be investigated in more detailprior to the development of the pigging procedures:

Extent of CO2 leakage behind pig and potential to model this with sufficient accuracy withinOLGA.

Potential for CO2 / air mixtures behind pig to reach low temperatures due to heat of mixingeffects.

Accuracy of OLGA results with respect to liquid condensation and temperature increase of

CO2 / air mixtures vs. temperature decrease suggested by HYSYS analysis and experimentaldata.

The issue of mixing CO2 with air (or nitrogen) giving rise to very low temperatures is notrestricted to pigging. Any operation where CO2 is sent to a vessel or pipe initially pressurisedwith air or nitrogen may give rise to low temperatures due to this effect.


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2. Scope of Work and Basis of Design

2.1. Scope of Work

There are two types of pigging that may be required through the life of the pipeline:

intelligent pigging

air-driven pigging to displace CO2 from the pipeline

Intelligent pigging (CO2-driven) is considered for vapour and dense phase operation while air-driven pigging prior to pipeline depressurisation is considered for the more onerous densephase operation scenario only. Also simulated is the blowdown of an air-filled pipelinefollowing air-driven pigging.

2.2. Basis of Design and Assumptions

2.2.1. Reservoir Pressure

The highest reservoir pressure cases were considered for vapour and dense phase operation,i.e. the 29.5 bar(g) and 157.5 bar(g) reservoir pressure cases respectively. Thecorresponding pipeline inlet pressures are 35 and 87 bar(g) respectively

2.2.2. Pig Model within OLGAThe pig was modelled using the plug function within OLGA. The pigtracking module was notutilised as this was intended for modelling the pigging of systems with liquid slugs. As no pigdata is available at this early stage of the design project the default OLGA pig data wasassumed, presented in Table 2-1. It may be appropriate to perform sensitivities on thesevariables or update with data provided by a pig manufacturer when the pigging proceduresare developed in more detail.

Table 2-1 Assumed Plug Properties

Property OLGA DefaultValue

Static force (N) 1000Wall friction (Ns/m) 1000

Linear friction (Ns/m) 10

Quadratic friction (Ns/m) 0

Mass (kg) 140

Leakage factor (-) 0

It was assumed that the pig would be launched into the pipeline at the normal carbon dioxidevolume flow rate, rather than optimising the velocity to that preferred for intelligent pigging orto achieve faster blowdown times during air-driven pigging.

2.2.3. Air Composition

For the pigging and depressurisation simulations a mixture of 79% nitrogen 21% oxygen wasassumed. No water or other minor components were added for the pigging simulation for


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simplicity; however the air was saturated with water at standard conditions for the blowdown

simulation.

2.2.4. Blowdown Equipment

For depressurisation of the air-filled pipeline following pigging a single blowdown orifice of 6was assumed, as this is currently the approximate size of the onshore vent system.

2.2.5. Air Flowrates for Air-Driven Pigging

The normal dense phase CO2 flow rate of 26,400 t/d CO2 was assumed prior to launching ofthe pig. Two air flow rates following the pig were considered:

Same volume flow as normal full flow dense phase operation (i.e. normal pipeline operatingvelocity).

Increased pigging velocity that might be used to clear the pipeline of CO 2 in an emergencysituation; this was assumed to be 3 x full flow velocity.

The mass flow rate of CO2 during normal dense phase operation is c. 305.6 kg/s,corresponding to a velocity of 0.56 m/s. The mass flow rate of air required to achieve asimilar velocity was found to be c. 40kg/s.


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3. Intelligent Pigging

Intelligent pigs typically operate at a velocity between 0.5 - 6 m/s, with an optimum velocityaround 1.5 - 3 m/s, although this will vary slightly by manufacturer and application. The pigvelocity is shown in Figure 3-1 below for both base case (6,600 t/d) and full flow (26,400 t/d)scenarios, assuming that the inlet flow of CO2 remains as per normal operation during thepigging process. The OLGA modelling of intelligent pigging was performed using the singlecomponent CO2 module.

Figure 3-1 Pig Velocity for routine pigging base case and full flow scenarios

The 29.5 bar(g) reservoir pressure case was considered for vapour phase operation, as thishas the lowest flowing velocity. The velocity of the pig was close to the optimum range,

though it should be noted that this is for the highest inlet pressure case. Earlier in field life themass flow rate will be identical but the operating pressure will be lower, resulting in a lowergas density and thus higher volume flow rate and velocity. Pro-rating the differences invelocity for normal operation, the pig velocity for the 2.1 bar(g) reservoir pressure case wouldbe approx 2 2.4 m/s, which lies within the optimum range for most pigs.

The 157.5 bar(g) reservoir pressure case was selected as a representative case for densephase operation. Due to the lower volumetric flowrate during dense phase operation, thevelocity of the pig was barely above the minimum acceptable velocity for intelligent pigging. Ifit is required to use an intelligent pig for inspection purposes at the full flow rate of 26,400 t/dthe vendor should be consulted to confirm that the low velocity is acceptable.

Base CaseFull Flow


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4. Air-Driven Pigging

4.1. Introduction

As an alternative to venting CO2 from the pipeline prior to performing maintenance oremergency depressurisation, it has been proposed that it may be more appropriate to sweepthe CO2 into the wells using an air-driven pig. The air in the pipeline would then be vented viathe Kingsnorth CCS plant vent system. The main advantages associated with depressurisingair after pigging are that CO2 that has been captured is not vented to atmosphere (althoughnote that some additional CO2 will be released to atmosphere in order to provide thecompressed, dehydrated air) and the issues with low temperatures exhibited duringdepressurisation of dense phase CO2 are largely avoided by blowing down air instead.

The thermodynamics of air/CO2 mixtures are discussed in Appendix B. This is relevant forair-driven pigging as air / CO2 mixtures may form ahead or behind the pig due to slippage.This is also relevant for situations such as pressurising vessels with nitrogen or sweeping ofCO2 from vessels or piping using air where air / CO2 mixtures could form. There is thepotential for significant temperature drops to occur, depending on the operating conditions.For worst case conditions, temperature drops in the order of 50C could be expected.

Air-Driven Pigging was investigated for dense phase operation, which has longer residencetimes and larger inventories than base case operation. It was also shown to be a significantchallenge to depressurise the pipeline by blowdown via the Kingsnorth CCS plant ventsystem in a previous depressurisation and venting report (Ref S1). Air-driven pigging wasmodelled using the composition tracking module within OLGA.

4.2. Required Pigging Durations

The time for the pig to travel between the pig launcher and receiver was found to be 119hours (5 days) for normal pipeline velocity and 51 hours (c. 2 days) for approximately 3 xpipeline velocity (higher velocity assumed for faster depressurisation). The position of the pigwithin the pipeline vs. time is shown in Figure 4-1.


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Figure 4-1 Distance Travelled by Pig through Pipeline vs. Time

4.3. CO2 Leakage

Leakage of fluid across the pig can occur by two mechanisms:

Leakage due toP across the pig

Leakage due to slippage

Leakage due to the pressure difference across the pig would result in forward flow of air infront of the pig; this is the mechanism that gives rise to bypass jetting. Leakage due toslippage between the pig and the film around the pig would result in the backward flow of CO 2

behind the pig. The concentration profiles of CO2 and N2 in the pipeline are plotted in Figure4-2 at a time when the pig is approximately halfway along the pipeline to the offshoreplatform:

normal velocity

3 x velocity


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Figure 4-2 Example CO2and N2Molar Composition Profile

The step change in hold up from 0 to 100% corresponds to the pig position, as there is gasphase air behind it and dense liquid CO2 in front of it. It is observed that there is a smallvolume of nitrogen leakage in front of the pig; however this is negligible in comparison to thesignificant volume of CO2 leakage behind the pig. It is therefore concluded that slippage isthe main leakage mechanism in the simulation.

It should be noted that there is a significant volume of CO2 leaked behind the pig by the timethe pig has reached the receiver, illustrated in Figure 4-3.


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Figure 4-3 Leaked CO2Behind Pig at End of Pig Run

A visual inspection of the area under the CO2 mole fraction curve indicates that there is asubstantial volume of CO2 contained within the air behind the pig. This should be consideredwhen designing the vent system for subsequent blowdown of the air-filled pipeline; thepresence of a significant proportion of carbon dioxide within the air will result in lowerblowdown temperatures than would be expected for a near 100% air composition. This isdiscussed in section 5.3 CO2 Slippage.

4.4. Sensitivity of Leakage to Pig Diameter

From the results presented in section 4.3 (CO2 Leakage), it is clear that the simulation resultsindicate that a significant volume of CO2 will leak behind the pig due to slippage. A sensitivitystudy was performed whereby the pig diameter was varied to determine the impact of the gap

between the pig and pipe wall on the volume of CO2 leaked. The default gap specified byOLGA is twice the pipeline roughness (the pipeline roughness was assumed to be 0.05mm,typical for new carbon steel). Significant gaps of 2mm through 6 mm were specified and theresults compared. Although these are much larger gaps than the default model, it wasconsidered a more valid comparison than multiples of pipeline roughness, as this will give achange in input which is greater than the noise of the simulation. It was found that therewas no clear relationship between the gap between the pig and the pipe wall and the volumeof CO2 leaked behind the pig. The CO2 molar concentrations behind the pig as the pig isleaving the pipeline are compared in Figure 4-4 below.


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Figure 4-4 Molar CO2Concentrations behind Pig

Sensitivity to Pig Diameter

This issue has been raised with OLGA support; an update will be provided in a later revisionof this document.

4.5. Liquid Formation behind Pig

During air-driven pigging there is a region behind the pig whereby there is a relatively highCO2 concentration in the air due to slippage behind the pig. At the operating conditions of thepipeline, this can result in retrograde liquid condensation, as discussed in Appendix A. Where

there is liquid formed behind the pig there is a temperature rise associated with the liquidformation, as illustrated in Figure 4-5 where the increase in hold up is coincident with anincrease in temperature.

2 mm3 mm4 mm5 mm6 mm

5 mm3 mm 6 4

2


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Figure 4-5 Example of Liquid Formation behind Pig

The spikes in hold-up (shown as the black line in Figure 4-5) behind the pig correspond topools of liquid in pipeline low points. The temperature profile (shown as the blue line in Figure4-5) shows an increased temperature around the pools of liquid; this is presumed to be wherethe heat released from condensation has heated the liquid.

The temperature increases predicted by OLGA are not in agreement with the temperaturedecreases predicted by HYSYS. OLGA predicts the possibility of CO2 condensing in theregion behind the pig with a resultant increase in fluid temperature due to exothermic heat ofcondensation. Conversely the HYSYS analysis suggests that a significant temperature dropshould be expected, due to endothermic heat of mixing. It is recommended that this beinvestigated in more detail in the next phase of the design project.

4.6. Pig Velocity

It is suspected that the system instability is contributing to the majority of the slippage, ratherthan the specified gap between the pig and pipeline wall. The velocity of the pig is shown inFigure 4-6 and expanded in Figure 4-7 for both normal operating flow rate and 3x normal flowrate (i.e. typical optimum pigging velocity).

The flow rates (and thus velocities) in the system fluctuate significantly during air-drivenpigging. This is typical of the erratic velocities expected during pigging with dissimilar fluids.The pig velocity can be seen to fluctuate significantly and actually give negative velocities (i.e.reverse flow) on occasions. It is presumed that the fluctuations in velocity contribute to

slippage of CO2 behind the pig.


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Figure 4-6 Pigging Velocities during Air-Driven Pigging

Figure 4-7 Expanded View of Pigging Velocity Trend

4.7. Flowrate surges

While pigging the pipeline with an air-driven pig, there will be significant surges in the CO 2

flow rate arriving at the offshore Kingsnorth platform. This is illustrated below for both normaloperational flow rates in Figure 4-8 and expanded for the normal flow rate case in Figure 4-9:

normal velocity

3 x velocity


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Figure 4-8 Mass Flow Rate at Offshore Kingsnorth Platform during Air-Driven Pigging

Figure 4-9 Expanded View of Mass Flow Rate at Offshore Kingsnorth Platform during Air-Driven Pigging

It can be seen from the above figures that there are significant fluctuations in the flowrate ofCO2 arriving at the offshore Kingsnorth platform. For comparison the normal full flow densephase mass rate is 305.6 kg/s (26,400 t/d). This presents two issues; the first being that the

heater throughput may exceed the design flow rate, though it is possible that this may bepermitted for a short duration without resulting in damage to the topsides equipment.

normal velocity

3 x velocity


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Additionally, the CO2 is heated prior to injection into the wells, so this fluctuating rate will

require to flow through the heater1

. As it is unlikely that the heater will be able to respondquickly enough to fluctuating flowrate to achieve the desired setpoint, it will likely benecessary to either increase the heater setpoint to be sure of an adequate outlet temperatureor alternatively to leave the setpoint unaltered but allow the wells to operate in the two phaseregion for the duration of the pigging if this is an emergency case to clear the line of CO 2. Itshould be noted however that to do so would likely result in unstable flow in the wells.

4.8. Recommendations

There are significant uncertainties associated with the modelling of air-driven pigging ofcarbon dioxide systems. It is recommended that the following be investigated in more detailprior to the development of the pigging procedures:

Extent of CO2 leakage behind pig and potential to model this with sufficient accuracy withinOLGA.

Potential for CO2 / air mixtures behind pig to reach low temperatures due to heat of mixingeffects.

Accuracy of OLGA results with respect to liquid condensation and temperature increase ofCO2 / air mixtures vs. temperature decrease suggested by HYSYS analysis and experimentaldata.

If substantial slippage behind the pig is confirmed, it would be possible to mitigate against thisby utilising a MEG sealing slug. The benefits of this may be evaluated at a later stage of the

design.

1It should be noted that heating will only be required in early dense phase operation.


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5. Blowdown of Air-Filled Pipeline

5.1. Introduction

Following air-driven pigging, it will be necessary to depressurise the pipeline. This is requiredfirstly to reduce the pressure of the pipeline prior to maintenance but also to remove the airfrom the system prior to resuming injection.

Initial conditions of 80 bar(g) 4C were assumed. As the size of the vent system at theKingsnorth CCS plant is currently unknown, a 6 blowdown orifice was assumed.

5.2. ResultsKey findings are discussed below, supporting OLGA trend plots are shown in Appendix B.

5.2.1. Depressurisation Times

The times required to depressurise to 50% and 25% of the pipeline design pressure aretabulated below, for design pressures of 150 bar(g) (current basis of design) and 120 bar(g)(sensitivity case for flow assurance studies).

Table 5-1 Time to Depressurise to 50% and 25% of Pipeline Design Pressure

Design pressure (bar(g)) Time to reach 50% (h) Time to reach 25% (h)

150 5 mins 10.4

120 2.4 14.2

The time to depressurise to 0 bar(g) was found to be 83 hr (3.5 days).

Larger orifice sizes would reduce the blowdown time however the vent system at theKingsnorth CCS plant is currently presumed to be limited to 6 (though it should be noted thatthis may be reduced further to 4) and so this may not be possible. The size of the vent systemfrom the pipeline is determined by the expected capacity of the Kingsnorth CCS plant vent system. Ifthis is equivalent to the 6600 t/day throughput for the base case scenario this will likely result in a 4orifice for vapour phase operation (note that the requirement for dense phase operation is actually

smaller however it is assumed that the larger of the two requirements would be used as the sizingbasis).

It should be noted that the time taken to pig the line with an air-driven pig and subsequentlydepressurise the air from the pipeline is comparable to that required to depressurise thepipeline of CO2 by blowdown via the Kingsnorth CCS plant. The total length of time requiredto depressurise the pipeline through a 6 orifice at the Kingsnorth CCS plant was found to bec. 281 hours (RefS1). By comparison, the total time required to pig the flowline (119 hours)and then blow down the air (83 hours) is 202 hours. Therefore the blowdown process couldbe completed approximately 79 hours (3 days) faster using pigging rather than conventionalblowdown.


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5.2.2. Peak Air Flow Rate

The maximum flow rate through the orifice is c. 317 kg/s. This is only marginally larger thanthe normal dense phase flow rate of 306 kg/s (which the vent system is presumed to be ableto dipose of during a process upset situation) and so no issues associated with excessive flowrates are anticipated. The volume flow rates in the vent system would be of the same order ofmagnitude to those for routine full flow CO2 venting as the density of gaseous carbon dioxideand air at atmospheric pressure are of the same order of magnitude.

5.2.3. Minimum Temperatures during Blowdown

Unlike the blowdown of the CO2-filled pipeline, there is no liquid in the pipeline at thebeginning of the blowdown. There is therefore no heat of vaporisation absorbed from thefluid and so the fluid and pipe wall temperatures in the pipeline are not excessively low - i.e.

do not fall below the minimum ambient temperature of -6C. For comparison, the minimumfluid temperature in the pipeline for blowdown of a CO2-filled pipeline was found to be -18C(RefS1).

The minimum fluid temperature downstream of the orifice due to the Joule-Thomson effect is -21C. This is presumed to be far above the minimum design temperature of the vent system,which will be designed for the more significant cooling associated with blowdown of densephase carbon dioxide which results in a minimum CO2 temperature in the vent system of -79C.

It may therefore be preferable with respect to low temperatures and risk of solid formation todepressurise the pipeline when it is filled with air rather than carbon dioxide, as the operatingtemperatures are significantly higher.

5.3. CO2 Slippage

The simulation of the blowdown of the air-filled pipeline assumed that the pipeline wouldcontain only air. Although it was found in the pigging simulations that there may be asubstantial slippage of CO2 behind the pig, this will not significantly affect the results. Theblowdown orifice is at the Kingsnorth CCS plant, at which point the gas consists of pure air atthe beginning of the blowdown. By the time any significant volume of CO2 had travelled from

the offshore platform to the Kingsnorth CCS plant, the pressure in the line would be very low.There would therefore be minimal Joule-Thomson effect over the choke and thus minimal riskof solid formation in the vent system. It should be noted however that if the pipeline wasdepressurised via the platform vent system then there would be significant issues associatedwith the release of large volumes of high purity CO2 at the beginning of the blowdown.


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6. Supporting References

S1. KCP-GNS-FAS-DRP-0004 Revision 01 Transient Analysis Depressurising andVenting (Pipeline), September 2010

S2. The excess enthalpy of gaseous mixtures of nitrogen and carbon dioxide, Lee &Mather, Journal of Chemical Thermodynamics, 1970


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7. Appendix A Thermodynamics of Air and Carbon Dioxide Mixtures

7.1.1. Heat of Mixing

The mixing of air and carbon dioxide is an endothermic process, thus the resulting mixture willbe cooled. The temperature drop due to the mixing of liquid CO2 and dry air at 4C wascalculated using the Peng Robinson equation of state within HYSYS for a variety ofpressures; this is illustrated in Figure 7-1 below:

Figure 7-1 Temperature Drop due to Heat of Mixing of Air/CO2Mixtures

Therefore depending on the CO2 concentration in the air/CO2 mixture behind the pig, there isthe potential for relatively low temperatures to be obtained. If the air used to drive the pig isnot sufficiently dry there may be the potential to form ice or hydrates in the pipeline duringpigging.

The specific enthalpy of mixing was calculated for the same conditions and is presented inFigure 7-2 below:


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Figure 7-2 Heat of Mixing of Air/CO2Mixtures

Comparing the above figures, it is observed that the maximum enthalpy change does notcorrespond to the maximum temperature change. For the same mass of air/CO2 mixture, ahigher CO2 concentration has a higher heat capacity and hence a lower temperature change.

The enthalpies of mixing predicted by HYSYS were compared to those obtained fromexperimental data for N2-CO2 mixtures (RefS2). It was found that the HYSYS results weregenerally in good agreement with the experimental data

2.

7.1.2. Phase Envelope

The phase envelopes for a range of air/CO2 mixtures are shown in Figure 7-3.

2The HYSYS results are in reasonable agreement with the smoothed excess enthalpies

presented in Table 2 of RefS2, with the exception of those that were outside the experimentalregion i.e. high pressure low CO2 concentration mixtures.


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0

50

100

150

200

250

300

-200 -150 -100 -50 0 50

Pressureb

arg

Temperature C

Phase Envelopes for CO2 / Air Mixtures

0% CO2

10% CO2

20% CO2

30% CO2

40% CO2

50% CO2

60% CO2

70% CO2

80% CO2

90% CO2

100% CO2

Figure 7-3 Phase Envelopes for Air/CO2Mixtures

The critical points are shown for the envelopes for 50% through 90% CO2 compositions.Mixtures with CO2 concentrations of 40% or less only have a dew line, with no bubble line. Itcan be seen that there is a region whereby retrograde condensation is possible from the gasphase, i.e. drop in temperature or pressure will result in the dew line being crossed, howeverat the operating conditions of the pipeline this will only occur for CO 2 concentrations of c. 65%(molar) and above.


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8. Appendix B Results for Depressurisation of Air-Filled Pipeline

Figure 8-1 Fluid and Wall Temperatures during Blowdown of Air-Filled Pipeline

Figure 8-2 Blowdown Rate and Upstream Pressure


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Kingsnorth CCS Demonstrat ion ProjectThe information contained in this document (the Information) is provided in good faith.

Figure 8-3 Pipeline Liquid Content during Blowdown of Air-Filled Pipeline

Figure 8-4 Mass Flow through Pipeline during Blowdown

IncreasingdistancefromKingsnorth

transient-analysis--pigging-(pipeline).pdf

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