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

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

Kingsnorth Carbon Capture & Storage Project

Document Title:

Transient Analysis – Pigging (Pipeline)

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Transient Analysis – Pigging (Pipeline) Table of Contents 1. Executive Summary.................. ......... ................... ................... .................. ................... ................... .................. ................... ................... .................. ............. .... 3 1.1. Scope Scope of Work ................................ ................................................. .................................. ................................. .................................. ....................... ..... 3 1.2. Intelligent Pigging .................. ......... ................... ................... .................. ................... ................... .................. ................... ................... ................ ....... 3 1.3. Air-Driven Pigging ................... ......... ................... ................... ................... .................. ................... ................... .................. ................... ............... ..... 3 1.4. Blowdown Blowdown of Air-Filled Pipeline ................... .......... ................... ................... .................. ................... ................... .................. ............. .... 4 1.5. Recommendations Recommendations ................... .......... ................... ................... ................... ................... .................. ................... ................... .................. ............. .... 5 2. Scope of Work and Basis of Design Design .................. ......... .................. ................... ................... .................. ................... ................... ................ ....... 6 2.1. Scope Scope of Work ................................ ................................................. .................................. ................................. .................................. ....................... ..... 6 2.2. Basis of Design and Assumptions .................. ......... ................... ................... .................. ................... ................... .................. ........... 6 2.2.1. Reservoir Pressure .................. ......... ................... ................... .................. ................... ................... ................... ................... ........... .. 6 2.2.2. Pig Model within OLGA ................... ......... ................... .................. ................... ................... .................. ................... ............... ..... 6 2.2.3. Air Composition.................. ......... .................. ................... ................... ................... ................... .................. ................... .................. ........ 6 2.2.4. Blowdown Equipment.............. .................. ......... ................... ................... ................... ................... .................. ............. .... 7 2.2.5. Air Flowrates for Air-Driven Pigging .................. ......... ................... ................... .................. ................... ............... ..... 7 3. Intelligent Pigging .................. ......... ................... ................... .................. .................. ................... ................... .................. ................... ................... ................ ....... 8 4. Air-Driven Air-Driven Pigging ................... ......... ................... .................. ................... ................... .................. ................... ................... .................. .................. ............... ...... 9 4.1. Introducti Introduction on ........................... ............................................. .................................. ................................. .................................. ............................... .............. 9 4.2. Required Pigging Durations Durations ................... ......... ................... .................. ................... ................... ................... ................... ................... .......... 9 4.3. CO2 Leakage Leakage............................... ................................................ .................................. .................................. .................................. ....................... ...... 10 4.4. Sensitivity of Leakage to Pig Diameter ................... .......... ................... ................... .................. ................... ................... ......... 12 4.5. Liquid Formation behind Pig .................. ........ ................... .................. ................... ................... ................... ................... ................. ........ 13 4.6. Pig Velocity Velocity ......................... .......................................... ................................. ................................. .................................. ................................ ............... 14 4.7. Flowrate surges.................. ......... ................... ................... .................. ................... ................... .................. ................... ................... ................. ........ 15 4.8. Recommendations Recommendations ................... .......... ................... ................... ................... ................... .................. ................... ................... .................. ........... .. 17 5. Blowdown of Air-Filled Pipeline ................... ......... ................... .................. ................... ................... ................... ................... .................. ........... .. 18 5.1. Introducti Introduction on ........................... ............................................. .................................. ................................. .................................. ............................. ............ 18 5.2. Results Results ................................ ................................................. .................................. ................................. ................................. ................................ ............... 18 18 5.2.1. Depressurisation Depressurisation Times ................... .......... ................... ................... .................. ................... ................... .................. ........... .. 18 5.2.2. Peak Air Flow Rate .................. ......... ................... ................... .................. ................... ................... ................... ................... ......... 19 5.2.3. Minimum Temperatures during Blowdown Blowdown ................... .......... ................... ................... ................... ............ 19 5.3. CO2 Slippage Slippage............................... ................................................ .................................. .................................. .................................. ....................... ...... 19 6. Supporting References ................... .......... ................... ................... .................. ................... ................... .................. ................... ................... .............. ..... 20 7. Appendix A Thermodynamics Thermodynamics of Air and Carbon Dioxide Dioxide Mixtures ................... .......... .................. .............. ..... 21 7.1.1. Heat of Mixing .................. ......... ................... ................... ................... ................... .................. ................... ................... ................. ........ 21 7.1.2. Phase Envelope................... ......... ................... .................. ................... ................... .................. ................... ................... .............. ..... 22 8. Appendix B Results Results for Depressurisation Depressurisation of Air-Filled Air-Filled Pipeline Pipeline ................. ................... .......... ............ ... 24

Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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Table of Tables Table 1-1 Pipeline Operating Scenarios ................................................................................................. ................................................................................................. 3 Table 1-2 Time to Depressurise to 50% and 25% of Pipeline Design Pr essure .................................... 4 Table 2-1 Assumed Plug Properties ....................................................................................................... ....................................................................................................... 6 Table 5-1 Time to Depressurise to 50% and 25% of Pipeline Design Pressure .................................. 18

Table of Figures Figure 3-1 Pig Velocity for routine pigging – pigging – base case and full flow scenarios .......... ..................................... ........................... 8 Figure 4-1 Distance Travelled b y Pig through Pipeline vs. T ime .......................................................... 10 Figure 4-2 Example CO 2 and N2 Molar Composition Profile .............................................................. ................................................................. ... 11 Figure 4-3 Leaked CO 2 Behind Pig at End of Pig Run ......................................................................... ......................................................................... 12 Figure 4-4 Molar CO 2 Concentrations behind Pig – Pig – Sensitivity to Pig Diameter .................................. 13 Figure 4-5 Example of Liquid Formation Form ation behind Pig ............................................................... ............................................................................. .............. 14 Figure 4-6 Pigging Velocities during Air-Driven Air- Driven Pigging ....................................................................... ....................................................................... 15 Figure 4-7 Expan ded View of Pigging Velocity Trend ............................................................ .......................................................................... .............. 15 Figure 4-8 Mass Flow Rate at Of fshore Kingsnorth Platform Pla tform during Air -Driven Pigging ...................... 16 Figure 4-9 Expanded View of Mass Flow Rate at Offshore Kingsnorth Platform during Air-Driven Pigging .............................................................. .................................................................................................................................... .................................................................................... .............. 16 Figure 7-1 Temperature Drop due to Heat of Mixing of Air/CO 2 Mixtures ............................................ 21 Figure 7-2 Heat of Mixing of Air/CO 2 Mixtures ...................................................................................... 22 Figure 7-3 Phase Envelopes for Air/CO 2 Mixtures................................................................................23 Figure 8-1 Fluid and W all Temperatures during Blowdown of Air-Filled Pipe line................................. 24 Figure 8-2 Blowdown Rate and Upstream Pressure ............................................................................ 24 Figure 8-3 Pipeline Liquid Content during Blowdown of Air-Filled Pipeline .......................................... 25 Figure 8-4 Mass Flow through Pipeline during Blowdown .................................................................... .................................................................... 25

List 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 Vapour Density Power Plant Capacity (without Capture) CO2 Flowrate

Base Case LP Vapour 400 MW 6600 t/d

Full Flow HP Dense 1600 MW 26,400 t/d

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

This study examines the following pigging scenarios for the cases described in Table 1-1 above using OLGA: intelligent pigging air-driven pigging to displace CO 2 from the pipeline Intelligent pigging may be required periodically for inspection purposes. In this scenario a pig would be launched into the pipeline during normal production and received at the offshore Kingsnorth platform (using CO 2 to drive the pig). Pigging to displace CO 2 from the pipeline is more complicated in that the pig would be launched and driven by a dry air source to the pig receiver at the platform. When the pig has been received, the air source would be shut off and 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 intelligent pigging (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 the velocity 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 is acceptable.

1.3. Air-Driven Pigging Air-driven pigging was investigated for dense phase operation. The time for the pig to travel between the pig launcher and receiver was found to be 119 hours (5 days) assuming that the normal pipeline velocity was maintained. If the velocity of the pig was increased by a factor of 3 to a more typical pigging velocity of c. 1.5 m/s then the pig travel time was reduced to 51 hours (c. 2 days). However it is unlikely that adequate heating could be maintained on the platform (to sustain single phase fluid in the wellbore) within the available heater duty at this increased flowrate. Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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Significant leakage of CO 2 behind the pig due to slippage was found to occur during the simulation. This results in a substantial volume of CO 2 contained within the air behind the pig, which must then be blown down via the Kingsnorth CCS plant vent system. The results of a sensitivity study on the effect of pig diameter on CO 2 slippage indicated that the degree of slippage does not increase with the assumed gap between the pig and the pipe wall. As this result was contrary to expectations this issue was raised with OLGA support; the slippage results will be updated if necessary following feedback from SPT.

Due to the slippage of CO 2 behind the pig, there is a region behind the pig where there is a relatively high CO 2 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 of condensation. This temperature increase predicted by OLGA is not in agreement with HYSYS analysis, which suggests that a significant temperature drop should be expected, due to endothermic heat of mixing. It is recommended that this be investigated in more detail in the next phase of the design project.

When the CO 2 in front of the pig arrives at the offshore Kingsnorth platform, it is to be injected into the reservoir via the topsides heaters. The flow rate arriving at the platform is subject to fluctuation and surges and so it should be confirmed that the heater design flow rates can be exceeded for short durations without resulting in damage. As the heater will likely be unable to achieve its setpoint during periods of fluctuating flow it would be necessary to either increase 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 the two phase region for the duration of the pigging if this is an emergency case to clear the line of CO2.

1.4. Blowdown of Air-Filled Pipeline The times required to depressurise the air-filled pipeline to 50% and 25% of the pipeline design 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 case for flow assurance studies).

Table 1-2 Time to Depressurise to 50% and 25% of Pipeline Design Pressure Design pressure (bar(g)) 150 120

Time to reach 50% (h) 0.1 2.4

Time to reach 25% (h) 10.4 14.2

The time to depressurise to 0 bar(g) was found to be 83 hours (3.5 days). Larger orifice sizes would reduce the blowdown time however the vent system at the Kingsnorth CCS plant is currently presumed to be limited to 6” (though it should be noted that this may be reduced further 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 CO 2 by blowdown via the Kingsnorth CCS plant vent system (Ref S1). 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 to blow down the CO 2-filled pipeline. Depressurising the pipeline utilising air-driven pigging is therefore 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 system are less severe than for depressurisation of a CO 2-filled pipeline. The minimum fluid temperatures in the pipeline and vent system are ambient temperature and -21°C respectively. It should be noted that the extent of CO 2 slippage behind the pig into the air will have only a minor effect on the results. 1.5. Recommendations There are significant uncertainties associated with the modelling of air-driven pigging of carbon dioxide systems. It is recommended that the following be investigated in more detail prior to the development of the pigging procedures: Extent of CO 2 leakage behind pig and potential to model this with sufficient accuracy within OLGA. Potential for CO 2 / air mixtures behind pig to reach low temperatures due to heat of mixing effects. Accuracy of OLGA results with respect to liquid condensation and temperature increase of CO2 / air mixtures vs. temperature decrease suggested by HYSYS analysis and experimental data. The issue of mixing CO 2 with air (or nitrogen) giving rise to very low temperatures is not restricted to pigging. Any operation where CO 2 is sent to a vessel or pipe initially pressurised with 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 CO 2 from the pipeline

Intelligent pigging (CO 2-driven) is considered for vapour and dense phase operation while airdriven pigging prior to pipeline depressurisation is considered for the more onerous dense phase operation scenario only. Also simulated is the blowdown of an air-filled pipeline following 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. The corresponding pipeline inlet pressures are 35 and 87 bar(g) respectively

2.2.2. Pig Model within OLGA The pig was modelled using the plug function within OLGA. The pigtracking module was not utilised as this was intended for modelling the pigging of systems with liquid slugs. As no pig data is available at this early stage of the design project the default OLGA pig data was assumed, presented in Table 2-1. It may be appropriate to perform sensitivities on these variables or update with data provided by a pig manufacturer when the pigging procedures are developed in more detail.

Table 2-1 Assumed Plug Properties Property Static force (N) Wall friction (Ns/m) Linear friction (Ns/m) Quadratic friction (Ns/m²) Mass (kg) Leakage factor (-)

OLGA Default Value 1000 1000 10 0 140 0

It was assumed that the pig would be launched into the pipeline at the normal carbon dioxide volume flow rate, rather than optimising the velocity to that preferred for intelligent pigging or to 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 was assumed. No water or other minor components were added for the pigging simulation for Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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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 6” was 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 CO 2 flow rate of 26,400 t/d CO 2 was assumed prior to launching of the pig. Two air flow rates following the pig were considered: Same volume flow as normal “full flow” dense phase operation (i.e. normal pipeline operating velocity). Increased pigging velocity that might be used to clear the pipeline of CO 2 in an emergency situation; this was assumed to be 3 x full flow velocity. The mass flow rate of CO 2 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 a similar velocity was found to be c. 40kg/s.




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3.

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Intelligent Pigging Intelligent pigs typically operate at a velocity between 0.5 - 6 m/s, with an optimum velocity around 1.5 - 3 m/s, although this will vary slightly by manufacturer and application. The pig velocity 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 CO 2 remains as per normal operation during the pigging process. The OLGA modelling of intelligent pigging was performed using the single component CO 2 module.

Base Case Full Flow

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 this has 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 the mass flow rate will be identical but the operating pressure will be lower, resulting in a lower gas density and thus higher volume flow rate and velocity. Pro-rating the differences in velocity for normal operation, the pig velocity for the 2.1 bar(g) reservoir pressure case would be 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 dense phase operation. Due to the lower volumetric flowrate during dense phase operation, the velocity of the pig was barely above the minimum acceptable velocity for intelligent pigging. If it is required to use an intelligent pig for inspection purposes at the full flow rate of 26,400 t/d the vendor should be consulted to confirm that the low velocity is acceptable.




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4. Air-Driven Pigging

4.1. Introduction As an alternative to venting CO 2 from the pipeline prior to performing maintenance or emergency depressurisation, it has been proposed that it may be more appropriate to sweep the CO2 into the wells using an air-driven pig. The air in the pipeline would then be vented via the Kingsnorth CCS plant vent system. The main advantages associated with depressurising air after pigging are that CO 2 that has been captured is not vented to atmosphere (although note that some additional CO 2 will be released to atmosphere in order to provide the compressed, dehydrated air) and the issues with low temperatures exhibited during depressurisation of dense phase CO 2 are largely avoided by blowing down air instead.

The thermodynamics of air/CO 2 mixtures are discussed in Appendix B. This is relevant for air-driven pigging as air / CO 2 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 of CO2 from vessels or piping using air where air / CO 2 mixtures could form. There is the potential for significant temperature drops to occur, depending on the operating conditions. For worst case conditions, temperature drops in the order of 50°C could be expected.

Air-Driven Pigging was investigated for dense phase operation, which has longer residence times and larger inventories than base case operation. It was also shown to be a significant challenge to depressurise the pipeline by blowdown via the Kingsnorth CCS plant vent system in a previous depressurisation and venting report (Ref S1). Air-driven pigging was modelled 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 119 hours (5 days) for normal pipeline velocity and 51 hours (c. 2 days) for approximately 3 x pipeline velocity (higher velocity assumed for faster depressurisation). The position of the pig within the pipeline vs. time is shown in Figure 4-1.




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normal velocity 3 x velocity

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 to ΔP across the pig Leakage due to slippage Leakage due to the pressure difference across the pig would result in forward flow of air in front of the pig; this is the mechanism that gives rise to bypass jetting. Leakage due to slippage 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 CO 2 and N2 in the pipeline are plotted in Figure 4-2 at a time when the pig is approximately halfway along the pipeline to the offshore platform:




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Figure 4-2 Example CO 2 and N 2 Molar Composition Profile

The step change in hold up from 0 to 100% corresponds to the pig position, as there is gas phase air behind it and dense liquid CO 2 in front of it. It is observed that there is a small volume of nitrogen leakage in front of the pig; however this is negligible in comparison to the significant volume of CO 2 leakage behind the pig. It is therefore concluded that slippage is the main leakage mechanism in the simulation.

It should be noted that there is a significant volume of CO 2 leaked behind the pig by the time the pig has reached the receiver, illustrated in Figure 4-3.




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Figure 4-3 Leaked CO 2 Behind Pig at End of Pig Run

A visual inspection of the area under the CO 2 mole fraction curve indicates that there is a substantial volume of CO 2 contained within the air behind the pig. This should be considered when designing the vent system for subsequent blowdown of the air-filled pipeline; the presence of a significant proportion of carbon dioxide within the air will result in lower blowdown temperatures than would be expected for a near 100% air composition. This is discussed 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 results indicate that a significant volume of CO 2 will leak behind the pig due to slippage. A sensitivity study 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 CO 2 leaked. The default gap specified by OLGA 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 the results compared. Although these are much larger gaps than the default model, it was considered a more valid comparison than multiples of pipeline roughness, as this will give a change in input which is greater than the “noise” of the simulation. It was found that there was no clear relationship between the gap between the pig and the pipe wall and the volume of CO2 leaked behind the pig. The CO2 molar concentrations behind the pig as the pig is leaving the pipeline are compared in Figure 4-4 below.




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2 mm 3 mm 4 mm 5 mm 6 mm

2 3 mm

6

4

5 mm

Figure 4-4 Molar CO 2 Concentrations behind Pig – Sensitivity to Pig Diameter

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

4.5. Liquid Formation behind Pig During air-driven pigging there is a region behind the pig whereby there is a relatively high CO2 concentration in the air due to slippage behind the pig. At the operating conditions of the pipeline, 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 liquid formation, as illustrated in Figure 4-5 where the increase in hold up is coincident with an increase in temperature.




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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 to pools of liquid in pipeline low points. The temperature profile (shown as the blue line in Figure 4-5) shows an increased temperature around the pools of liquid; this is presumed to be where the heat released from condensation has heated the liquid.

The temperature increases predicted by OLGA are not in agreement with the temperature decreases predicted by HYSYS. OLGA predicts the possibility of CO2 condensing in the region behind the pig with a resultant increase in fluid temperature due to exothermic heat of condensation. Conversely the HYSYS analysis suggests that a significant temperature drop should be expected, due to endothermic heat of mixing. It is recommended that this be investigated 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, rather than the specified gap between the pig and pipeline wall. The velocity of the pig is shown in Figure 4-6 and expanded in Figure 4-7 for both normal operating flow rate and 3x normal flow rate (i.e. typical optimum pigging velocity).

The flow rates (and thus velocities) in the system fluctuate significantly during air-driven pigging. 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 CO 2 behind the pig. Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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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 normal operational flow rates in Figure 4-8 and expanded for the normal flow rate case in Figure 4-9: Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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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 AirDriven Pigging

It can be seen from the above figures that there are significant fluctuations in the flowrate of CO2 arriving at the offshore Kingsnorth platform. For comparison the normal full flow dense phase 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 be permitted for a short duration without resulting in damage to the topsides equipment. Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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Additionally, the CO2 is heated prior to injection into the wells, so this fluctuating rate will 1 require to flow through the heater . As it is unlikely that the heater will be able to respond quickly enough to fluctuating flowrate to achieve the desired setpoint, it will likely be necessary to either increase the heater setpoint to be sure of an adequate outlet temperature or alternatively to leave the setpoint unaltered but allow the wells to operate in the two phase region for the duration of the pigging if this is an emergency case to clear the line of CO 2. It should 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 of carbon dioxide systems. It is recommended that the following be investigated in more detail prior to the development of the pigging procedures: Extent of CO 2 leakage behind pig and potential to model this with sufficient accuracy within OLGA. Potential for CO 2 / air mixtures behind pig to reach low temperatures due to heat of mixing effects. Accuracy of OLGA results with respect to liquid condensation and temperature increase of CO2 / air mixtures vs. temperature decrease suggested by HYSYS analysis and experimental data. If substantial slippage behind the pig is confirmed, it would be possible to mitigate against this by utilising a MEG sealing slug. The benefits of this may be evaluated at a later stage of the design.

1

It 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 required firstly to reduce the pressure of the pipeline prior to maintenance but also to remove the air from the system prior to resuming injection.

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

5.2. Results Key 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 are tabulated 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)) 150 120

Time to reach 50% (h) 5 mins 2.4

Time to reach 25% (h) 10.4 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 the Kingsnorth CCS plant is currently presumed to be limited to 6” (though it should be noted that this may be reduced further to 4”) and so this may not be possible. The size of the vent system from the pipeline is determined by the expected capacity of the Kingsnorth CCS plant vent system. If this is equivalent to the 6600 t/day throughput for the base case scenario this will likely result in a 4” orifice 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 sizing basis).

It should be noted that the time taken to pig the line with an air-driven pig and subsequently depressurise the air from the pipeline is comparable to that required to depressurise the pipeline of CO2 by blowdown via the Kingsnorth CCS plant. The total length of time required to depressurise the pipeline through a 6” or ifice at the Kingsnorth CCS plant was found to be c. 281 hours (Ref S1). 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 could be completed approximately 79 hours (3 days) faster using pigging rather than conventional blowdown.




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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 than the normal dense phase flow rate of 306 kg/s (which the vent system is presumed to be able to dipose of during a process upset situation) and so no issues associated with excessive flow rates are anticipated. The volume flow rates in the vent system would be of the same order of magnitude to those for routine full flow CO 2 venting as the density of gaseous carbon dioxide and air at atmospheric pressure are of the same order of magnitude.

5.2.3. Minimum Temperatures during Blowdown Unlike the blowdown of the CO 2-filled pipeline, there is no liquid in the pipeline at the beginning of the blowdown. There is therefore no heat of vaporisation absorbed from the fluid 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 -6°C. For comparison, the minimum fluid temperature in the pipeline for blowdown of a CO 2-filled pipeline was found to be -18°C (Ref S1).

The minimum fluid temperature downstream of the orifice due to the Joule-Thomson effect is 21°C. 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 dense phase carbon dioxide which results in a minimum CO 2 temperature in the vent system of 79°C.

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

5.3. CO2 Slippage The simulation of the blowdown of the air-filled pipeline assumed that the pipeline would contain only air. Although it was found in the pigging simulations that there may be a substantial slippage of CO 2 behind the pig, this will not significantly affect the results. The blowdown orifice is at the Kingsnorth CCS plant, at which point the gas consists of pure air at the beginning of the blowdown. By the time any significant volume of CO 2 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 risk of solid formation in the vent system. It should be noted however that if the pipeline was depressurised via the platform vent system then there would be significant issues associated with the release of large volumes of high purity CO 2 at the beginning of the blowdown.




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6. Supporting References S1. KCP-GNS-FAS-DRP-0004 Revision 01 Transient Analysis – Depressurising and Venting (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 will be cooled. The temperature drop due to the mixing of liquid CO2 and dry air at 4°C was calculated using the Peng Robinson equation of state within HYSYS for a variety of pressures; this is illustrated in Figure 7-1 below:

Figure 7-1 Temperature Drop due to Heat of Mixing of Air/CO 2 Mixtures Therefore depending on the CO 2 concentration in the air/CO 2 mixture behind the pig, there is the potential for relatively low temperatures to be obtained. If the air used to drive the pig is not sufficiently dry there may be the potential to form ice or hydrates in the pipeline during pigging.

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




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Figure 7-2 Heat of Mixing of Air/CO 2 Mixtures

Comparing the above figures, it is observed that the maximum enthalpy change does not correspond to the maximum temperature change. For the same mass of air/CO2 mixture, a higher 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 from experimental data for N 2-CO2 mixtures (Ref S2). It was found that the HYSYS results were 2 generally in good agreement with the experimental data .

7.1.2. Phase Envelope The phase envelopes for a range of air/CO 2 mixtures are shown in Figure 7-3.

2

The HYSYS results are in reasonable agreement with the smoothed excess enthalpies presented in Table 2 of Ref S2, with the exception of those that were outside the experimental region i.e. high pressure low CO 2 concentration mixtures. Kingsnorth CCS Demonstration Project The information contained in this document (the Information) is provided in good faith. E.ON UK plc, its subcontractors, subsidiaries, affiliates, employees, advisers, and the Dep artment of Energy and Climate Change (DECC) make no representation or warranty as to the accuracy, reliability or completeness of the Information and neither E.ON UK plc nor any of its subcontractors, subsidiaries, affiliates, employees, advisers or DECC shall have an y liability whatsoever for any direct or indirect loss howsoever arising from the use of the Information by any party.



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Phase Envelopes for CO 2 / Air Mixtures 300 0% CO2 10% CO2 250

20% CO2 30% CO2 40% CO2

200 50% CO2 g r a b e r 150 u s s e r P

60% CO2 70% CO2 80% CO2 90% CO2

100

100% CO2

50

0 -200

-150

-100

-50

0

Temperature °C

Figure 7-3 Phase Envelopes for Air/CO 2 Mixtures

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


50



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

Increasing distance from Kingsnorth

Figure 8-4 Mass Flow through Pipeline during Blowdown


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

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