DANISH OPERATORS
Offshore Oil and Gas Operators in Denmark
Use of Nitrogen Purge
in Flare and Vent Systems
7 September 2009
Esplanaden 50 6 1263 Copenhagen K 6 Denmark
Telephone: +45 3363 4097 6 E-mail: info@danishoperators.com
1. Title of Initiative
Use of Nitrogen Purge in Flare and Vent Systems.
2. Description of Initiative
The offshore installations flare and atmospheric vent headers are required to be purged in order to
prevent oxygen ingress to the flare and atmospheric vent systems. This is required in order to avoid
the formation of explosive mixtures in the headers, which could lead to explosions if ignited. Fuel
gas or nitrogen can be used as purge gas. The purge gas is injected at different locations in the
systems in order to maintain a positive pressure in the flare headers thus preventing air ingress.
Cold vents (atmospheric vent headers) are used to vent hydrocarbon gas from low pressure sources
where insufficient pressure is available to allow the gas to be flared. Under normal operating
conditions the volume of gas vented via the cold vent is minimal.
The use of fuel gas in flare and vent headers for purging purposes results in environmental
emissions. These can be in the form of CO2 or NOx when the fuel gas used in the HP and LP flare
headers is burnt or in the form of CH4 and other species present in the atmospheric vent header
purge gas when this is cold vented. The green house effect associated with the CH4 is around 23
times worse than that for the CO2 emissions.
The replacement of the use of fuel gas with nitrogen for purging the flare and atmospheric vent
headers is one of the options currently being investigated in order to reduce environmental impact.
The use of nitrogen will eliminate the environmental emissions described in the above paragraph. It
should be noted that, when replacing purge fuel gas with nitrogen for Atmospheric Vent headers,
the NOx emissions increase. This is due to the NOx emissions produced in the gas turbines when
generating the necessary power for N2 generation. However, the environmental impact of cold
venting in terms of CO2 emissions is seen as much higher than that of the increased NOx emissions
for nitrogen generation.
This initiative is applicable to the DUC Facilities only. Nitrogen is currently being used on the
Dong Energy Siri facilities for purging the flare system. A flare recovery system is planned to be
installed on Hess South Arne Facility which will eliminate the need to purge the flare headers with
nitrogen.
In the case that Flare Gas Recovery is installed on any of the DUC platforms, it will not be
necessary to replace the use of fuel gas with nitrogen for purging purposes as the purge fuel gas
would be recovered and sent back to the process. If the pay back time for changing from fuel gas to
nitrogen purge is significantly less than an expected implementation time for a flare gas recovery
system, nitrogen purge should be considered.
Nitrogen will still be required in order to purge the flare stack, downstream of the Fast Opening
Valves that are normally installed in the main headers as part of flare recovery projects. This is
considered to be a project requirement and therefore considered to be outside the scope of this
report.
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3. Potential for Reduction of Environmental Emissions
Table 1 below summarises the potential for environmental emissions reduction as well as an
estimate of the total investment required to replace the use of fuel gas with nitrogen for the purpose
of purging the DUC Facilities Flare Header. The total potential reduction in fuel gas usage is around
0,06MMSCFD. The figures below exclude the Gorm HP, LP and Vent headers given that the
platform does not have sufficient nitrogen generation capacity to supply the required flow rate. It is
not considered feasible at this stage to proceed with the installation of a new nitrogen generation
unit for this purpose.
Table 1
Net CO2 Emissions
Reduction, tonnes/year
(Notes 1, 2 and 3)
4000
NOx Emissions
Estimated
Total DKK/(ton/year of CO2
Reduction, kg/year Investment (Note 8) reduction)
(Note 4)
(Note 5)
MM DKK
457
50
12500
Notes:
1. Figure takes into account CO2 emissions generated when combusting FG in the Gas Turbines for
generating the power necessary to produce purge nitrogen.
2. Figure represents approx 0,2% of the total DUC CO2 emissions for 2008.
3. Value includes both the burnt and unburnt fractions of fuel gas used for purging the HP/LP flare and
atmospheric vent headers.
4. When replacing purge fuel gas with nitrogen for Atmospheric Vent headers, the NOx emissions increase.
This is due to the NOx emissions produced in the gas turbines when generating the necessary power for N2
generation. However, these will be small as compared to those generated in the flare tips when burning the
purge FG and as a result a net reduction is achieved for all the categories.
5. Required investment to reduce CO2 emission by 1 tonne per year. Figure represents the average for all
DUC Facilities. The individual values for each particular flare/vent header ranges from 243 DKK for the Dan
FG vent header (most attractive option) to 122541 DKK for the Tyra East LP flare header (less attractive
option). The values for all headers are shown in Table 2 below and are to be used when prioritising any
future works.
6. Given that the nitrogen purity currently generated offshore is not completely pure (purity> 93%) some
oxygen will be introduced into the flare headers. However, the Upper Flammability Limit (UFL) of natural
gas in oxygen is around 61% in volume (assuming pure methane). The volume fraction of gas during normal
operation in all flare headers will be above 99.9%, which is well above the UFL. The normal flaring rates
will have to be reduced to 0,0009 MMSCFD or lower in order to create flammable mixture. Rates as low as
those are never experienced during operation. Therefore a flammable mixture is not predicted under any
circumstance.
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7. The flammability of all mixtures expected in the flare tips, resulting from the replacement of fuel gas with
nitrogen have been checked and found not to be a problem. This is due to the high hydrocarbon/nitrogen
ratio seen in the flare headers.
8. Includes engineering, equipment and installation costs.
Table 2 below shows the above values and other relevant information for each individual header.
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Table 2
Current
Flare/Vent Stack
Total FG
Estimated
ID
required
Equivalent
Net CO2
Opex Increase
Net
Cost
Emissions
(cost of N2
economic
Estimate for
Cost/(tonnes/year of
Required
Header
Comments
CO2 reduction)
N2
reduction
Generation)
benefit
modifications
CO2 emissions
inch
Nm3/h
kg/h
Nm3/h
tonnes/year
DKK/year
DKK/year
DKK
DKK/(tonne/year)
DAN FG-Vent
12,39/6,36
1,3185
68,5
1,1367
597,1
1892
63715
150000
251
N2 generation is sufficient to meet requirements. N2 purging facilities exist. N2 flowmeter to be installed.
TYW-A Vent
13,62
1,6614
86,4
1,4323
752,3
2383
80282
1000000
1329
N2 generation is sufficient to meet requirements. New N2 purging facilities are required (pipework + flow meter)
TYE Vent
13,62
1,6614
86,4
1,4323
752,3
2383
80282
1875000
2492
N2 generation is sufficient to meet requirements. New N2 purging facilities are required (pipework + flow meter)
DAN FG- HP
23,50
12,4584
28,2
10,4009
216,8
17308
141885
600000
2768
N2 purge facilities are installed. N2 generation system capacity is 150N/m3. Normal consumption is 0 according to Design Manual.
Halfdan Vent
6,36
0,1190
6,2
0,1026
53,9
171
5749
150000
2784
N2 generation is sufficient to meet requirements. New N2 purging facilities are required (pipework + flow meter)
HWA Vent
10,42
0,6577
34,2
0,5670
297,8
944
31782
1000000
3358
N2 generation is sufficient to meet requirements. New N2 purging facilities are required (pipework + flow meter)
HALFDAN HP
23,50
11,3464
25,6
9,4553
197,1
15735
129220
750000
3806
N2 purge facilities are installed. N2 is supplied by N2 Generation package HDAC-A-0801 with a design capacity of 320 Nm3/h. Consumption is 100Nm3/h giving a spare capacity of 220 Nm3/h.
PCV designed for 44 Nm3/h. Flow to glycol regen package discontinuous. Nitrogen line to HDC has been disconnected. PCV OK for required purge flow.
HALFDAN LP
13,62
1,6614
3,8
1,4323
28,7
2383
18829
150000
5224
As per Halfdan HP header above.
Dan F Vent
10,42
0,6577
34,2
0,5670
297,8
944
31782
2000000
6715
N2 generation is sufficient to meet requirements. N2 purging facilities exist. N2 flowmeter to be installed.
DAN FG LP
12,39
1,2330
2,8
1,0323
21,4
1718
14033
300000
14019
As per Dan FG HP header above.
13,62 / 17,62
10,9217
24,7
4,9234
201,9
8193
132370
5000000
24768
N2 Generation System produces 30Nm3/h. LP/IP Comp consumption is 20Nm3/h. There is sufficient spare capacity to meet the requirements. Piping mods required.
23,50
10,9675
24,8
9,4553
189,6
15735
124302
5700000
30069
HWA HP 2 headers
Platform A: N2 Generation package supplies 60Nm3/h. LP comp consumes 2,4 Nm3/h. Capacity available will be sufficient to meet requirements. Platform E: N2 is supplied by N2 Gen Unit WEATYW HP
A-8501. Generation capacity is 80Nm3/h and consumption 44 Nm3/h. Spare capacity of 36Nm3/h will be sufficient. Only piping mods are required.
N2 generation system capacity is 40N/m3, consumption is 11Nm3/h, therefore there is sufficient capacity to meet the requirements. Two of the purging points are located on Platforms E and F.
TYE HP
23,50
12,0853
27,3
9,7738
210,8
16265
138203
11250000
53377
Nitrogen for these platforms is supplied by nitrogen bottles. It is not recommended to run nitrogen pipes across the bridges in order to replace FG purge with N2. Replacement is only to be applied to
Platform A.
N2 is available. Two N2 generation packages are available on Dan FC platform (A-0802 and A-0807) with a total combined capacity of 105Nm3/h. N2 from Dan FF is also available (A-0801) with
DAN FD - HP
17,62
8,0380
18,2
6,3475
140,6
10563
92213
13300000
94576
a capacity of 138Nm3/h. Consumption is not known but given the small flow rate required and the high generation capacity as compared with other platforms it will be assumed that there is
sufficient capacity available to meet the requirements.
TYE LP
13,62
1,6614
3,8
1,4323
28,7
2383
18829
3750000
130592
Capacity available will be sufficient to meet requirements. See TW HP Flare above . Only piping mods are required.
HWA LP
8,33
0,3847
0,9
0,3168
6,7
527
4389
1000000
149417
As per Harald HP header above.
TYW LP
10,42
0,7682
1,7
0,6005
13,5
999
8825
2300000
170899
Capacity available will be sufficient to meet requirements. See TW HP Flare above . Only piping mods are required.
GORM LP
10,42
1,4897
3,4
1,1440
26,2
1904
17152
-
-
Gorm F: N2 Generation System produces 10Nm3/h and supplies LP Compressor C-4201. LP comp consumption is 7 Nm3/h. , which makes the N2 Generator insufficient to supply the required N2
purge flow rates. Given the small gain to be obtained, is not considered feasible to install extra N2 generation capacity.
Gorm F: N2 Generation System produces 10Nm3/h and supplies LP Compressor C-4201. LP comp consumption is 7 Nm3/h. , which makes the N2 Generator insufficient to supply the required N2
GORM HP
23,50
15,9784
36,1
13,1277
278,1
21846
182331
-
purge flow rates. Given the small gain to be obtained, is not considered feasible to install extra N2 generation capacity.
Gorm F: N2 Generation System produces 10Nm3/h and supplies LP Compressor C-4201. LP comp consumption is 7 Nm3/h. , which makes the N2 Generator insufficient to supply the required N2
GORM Vent
10,42
0,6577
34,2
0,5670
297,8
944
31782
-
purge flow rates. Given the small gain to be obtained, is not considered feasible to install extra N2 generation capacity.
See next page for calculation methodology.
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Calculation Methodology
Purge gas rates calculation (applicable to both fuel gas and nitrogen):
Calculated based on API 521 (5th Edition, 2007) equation.
Q = 0,0035283* D^3,46 * K
Where,
Q is the purge gas rate, expressed in normal cubic metres per hour (standard cubic feet per hour);
D is the flare stack diameter, expressed in metres (inches);
K is a constant which depends on the purge gas composition. Different values are used for fuel gas and
nitrogen.
Equivalent CO2 emissions
Equivalent CO2 emissions are calculated as follows:
For burnt Fuel Gas
CO2 emissions (kg/h) = Fuel Gas normal volume flow rate (Nm3/h) x 2.26 kg CO2 / Nm3*
*Figure based on an Emission Factor of 57 kgCO2/GJ and a Heating Value of 39,6GJ/1000Nm3.
For cold vented Fuel Gas
Equivalent CO2 emissions (kg/h) = Normal volume flow rate (Nm3/h) x 2,26 kg CO2 / Nm3 x 23**
**Figure takes into account more harmful environmental effect of unburnt CH4 (advised by Production
Department)
Net CO2 Emissions reduction
Net CO2 Emissions reduction = Equivalent CO2 emissions - CO2 Emissions resulting from N2 generation
Net economic benefit
Net economic benefit =Additional Revenue (sales gas) + CO2 emissions reduction- Cost of fuel gas for N2
Generation
CO2 emissions reduction: according to Forudsætninger for samfundsøkonomiske analyser på energiområdet,
May 2009, the CO2 saving should be included in the economic assessment. A rate of 84,925 DKK/tonne is
used for the calculations.
NOx Emissions reduction / increase
For fuel gas burnt if flare tips: NOx emissions (mass units) = 0,0015 * flare gas mass flow rate
For fuel gas burnt in gas turbines = Fuel Gas normal volume flow rate (Nm3/h) x 0,0049 kg NOx / Nm3***
***Figure based on an Emission Factor of 124g NOX /GJ and a Heating Value of 39,6GJ/1000Nm3.
CAPEX:
Includes engineering, equipment and installations costs.
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4. Investments by Major Components and Years
As stated in Table 1 above the total required investment is DKK 50.3MM. The cost estimates for
the modifications required for each individual header are presented in Table 2 above.
The implementation of the modifications is to be prioritised according to the cost / (ton/year of CO2
reduction). Some of the modifications for a particular platform (e.g HP/LP/Vent headers) could be
combined in a single CFI package if it is decided to proceed with them.
The timeline for implementation is subject to the decision to proceed with the project.
5. Operating Costs and Revenues
5.1 Total Net Economic Benefit
The total net economic benefit that would be obtained if all the proposed projects were
implemented is given in Table 3 below.
Table 3:
Additional Revenue from increased gas sales
(purge FG sold).
Additional Revenue from CO2 emissions
reduction (CO2 quotas sold)
Cost of FG for N2 Generation
Net Economic Benefit
0.88 MMDKK/year
0.34 MMDKK/year
-0.10 MMDKK/year
1.12 MMDKK/year
*Gas price 32.6 DKK/GJ, Heating Value = 39.6 GJ/1000Nm3.
See Section 6 below for payback period calculation.
6. Operating Economic Assessment
A brief economic assessment based on payback period is present below for both the most attractive
option and for the implementation of all the options. Included in the assessment is the value of CO2
emission reduction.
According to Forudsætninger for samfundsøkonomiske analyser på
energiområdet, Februry 2009, the CO2 saving should be included in the economic assessment. A
rate of 84,925 DKK/tonne of CO2 is used for the assessment assessment.
Most attractive option:
The most attractive option in terms of Cost/(tonnes/year of CO2 reduction), is the installation of a
nitrogen flow meter on the existing nitrogen line to the Dan FG atmospheric vent header. The
following economic assessment is made for this option:
Required investment = DKK 150000 (includes cost of flowmeter, engineering and installation)
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Additional Revenue (FG Export) = 14890 DKK /year
CO2 Emission Reduction =
50706 DKK /year
Cost of FG for N2 Generation = - 1892 DKK /year
Net Economic Benefit =
63704 DKK /year
Payback period = 150000 / 63704 = 2,4 years
Assuming that all the proposed projects are implemented:
Required investment = MMDKK 50.3
Net Economic Benefit (Table 3) = 1.12 MMDKK /year
Payback period = 50.3 / 1.12 = 45 years
For the most attractive option the payback period of 2,4 years makes this worth pursuing and as
such should be progressed to better define the costs. For all other individual options, and the
implementation of all the options combined, the payback period exceeds 4 years such that the main
drive for this initiative is seen as not an economic one.
7. Possible Socio-Economic Calculation
Refer to Section 3 above.
8. Possible Socio-Economic Assessment
Refer to Section 3 above.
9. Recommendations
1. Decision to proceed with this initiative and scope of implementation to be confirmed.
2. The installation of flare gas recovery systems on the DUC platforms is currently being studied.
It is recommended to wait for the results of such studies before taking the decision to proceed
with the implementation of this initiative.
3. To check the fuel gas flow rates currently being used offshore in order to ensure that the correct
flows are in place.
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