BLUE STREAM GAS-IN PROCEDURE
THE INJECTION OF NATURAL GAS THROUGH THE VACUUM SYSTEM
MINNETTI Giuseppe, Manager of Process (Snamprogetti S.p.A.) CARUSO Salvatore, Technical Director (Blue Stream Pipeline Company) BOROVIK Vladimir, Technical Director (Blue Stream Pipeline Company) MANSUETO Massimiliano, Process Engineer (Snamprogetti S.p.A.) TERENZI Alessandro, Process Engineer (Snamprogetti S.p.A.) FERRINI Francesco, Manager (SICURGAS)
1. INTRODUCTION
The Blue Stream Project represents a challenging work from a technological perspective, since it is the first sealine installed at more than 2000 m of water depth, in a sea environment having peculiar characteristics and presenting safety problems (high water pressure, landslides). The pipeline installation has been completed on 2002. The whole pipeline profile is shown in Figure 1.
This paper describes the gas-in operation relevant to one of the twin pipelines crossing the Black Sea, called W2.
The procedure developed to carry out the gas-in operation was conceived in order to satisfy the following requirements:
{ minimise the duration of the operations; { use of conventional or standard equipment; { minimise temporary pipeworks; { maximise the safety of the operations; { take care of risk of possible abort and operation restart; { minimise environment impact; { minimise consumable materials; { avoid any possible hydrate formation during gas-in.
The main problem related to the execution of such an operation is represented by the risk of flammable mixture. In fact, air and natural gas in the proper stechiometric proportions will ignite liberating heat. The temperature rise of the gases causes an increase of pressure and, under confinement, might result in an explosion. There are two composition limits of flammability for air and gaseous fuel under specified conditions. The lower limit corresponds to the minimum concentration of combustible gas that will support combustion, the higher limit to the maximum concentration of combustible gas which will be ignited by the corresponding amount of oxygen present in the air.
The influence of pressure on the flammability limits is showed in Figure 2 [1]. At low pressures, i.e. 6.7*103 Pa(abs) (67 mbar) (vacuum conditions), natural gas-air mixtures are not combustible. This peculiarity has been taken into consideration as safe operating condition for the gas-in of gas pipelines. Three methodologies were considered for the gas-in:
{ natural gas injection under pipelines vacuum conditions without use of pigs; { natural gas injection as propellant of a pig train (2) confining a nitrogen batch; { natural gas injection under pipelines pressure conditions without use of pigs
The vacuum injection method The vacuum injection method is based on the principle that natural gas does not produce a flammable mixture with air at pressures below 6.7*103 Pa (abs) (67 mbar).
Starting from pipeline stand-by filled by dry air at atmospheric pressure, the operation consists of a forced depressurisation by vacuum pump systems located both at Samsun terminal on the Turkish coast and at the opposite pipeline terminal at Beregovaya Compressor Station (BCS) on the Russian coast. Forced depressurisation is ended when the max pressure all along the pipeline, i.e. at the deepest sea bed locations, reaches the value around 6.0*103 Pa (abs) (60 mbar).
According to the flammability limits under vacuum conditions, it could be possible to inject natural gas into the pipeline without any risk of ignition, provided that, during the gas injection, vacuum conditions are maintained at the interface front all along the pipeline. However, due to the very small amount of air content, it is wise to displace in part or totally this residual air by nitrogen, in order to avoid any possible formation of gas-air mixture.
Therefore, a preliminary nitrogen injection is foreseen at Beregovaya terminal at a controlled flow rate which would permit to maintain all over the ultimate pressure profile reached at the end of air depressurisation. Vacuum pump systems remain in operation during nitrogen injection. The total nitrogen injected volume should be estimated to fill-up not less than 50% of the total pipeline volume.
Then, gas injection may follow immediately at the same flow rate as nitrogen and with vacuum pump systems in operation as well, up to the complete evacuation of dry air from the pipeline. This condition is assured by monitoring the oxygen content of the pipeline exhaust at the vacuum pump systems, by using an oxygen analyser probe.
Once assured that air has been completely displaced and pipeline is only filled by nitrogen and natural gas, the vacuum pump systems may be shut-down and the pipeline may start to be first pressurised up to something more than atmospheric pressure.
The pig assisted injection method The pig assisted gas injection method is based on simultaneous displacement of dry air by natural gas injection assisted by an interface train of at least 2 pigs which includes a batch of nitrogen to prevent any possible formation of gas and air mixture. Due to the remarkably long track and to dry inner pipe conditions, the pig train is recommended to travel at a speed of approx. 0.5 m/s. Due to the remarkable pipeline elevation changes, the control of the pig travel speed should be assured by a back pressure at Samsun terminal of at least 5.0*105 Pa (abs) (5 bara). This value, remarkably higher than the common practice of 2 bar adopted for shorter on-shore pipeline tracks, is here recommended also for facing any possible pig stop, especially on the most sloping sections. The pipeline should firstly be pressurised by dry air up to 5.0*105 Pa (abs) (5 bara), by utilising the same equipment used for drying operation. Then, the first pig has to be launched by compressed nitrogen. The volume of nitrogen separation interface is evaluated on the basis of a predictive model of possible gas leak throughout the pig sealing cups. The second pig has to be launched directly by natural gas available on the Beregovaya CS by-pass line. The method might be performed with the use of more than two pigs. Although the separation efficiency is augmented, resulting in a lower nitrogen consumption, the operation reliability in this case decreases.
The method without pig assistance under pressure conditions This operation is based on the physical attitude to displace products from inside pipeline by accepting a confined interface volume expected under turbulent flow regime. Purging of air is carried out by using a nitrogen batch sized to avoid mixing between air and natural gas. The pipeline is assumed to be initially filled with dry air at least at 6.0*105 Pa (abs) (6 bara) due to the need to face an abort of gas-in. Then compressed nitrogen is pumped into the pipeline at a high flow rate to assure a minimum length of the air-nitrogen interface zone.
The final selection of the vacuum injection method was mainly dictated by safety considerations. In fact, this methodology does not involve any particular risk, even in case of an abort of the operations, when it would be sufficient to close both the inlet and the outlet points of the pipeline to maintain the achieved vacuum degree. On the other hand, the pig assisted gas injection method has several drawbacks due to the operation sensitivity to the flow control. In fact the pig must be controlled and monitored all over its travel along the pipeline. Moreover, in the event that the
operations during the gas-in phase must be stopped, it will be required to abort the entire operation by venting the gas at Beregovaya and allowing the Samsun terminal to dislodge back the pig train in order to restore the basic safety requirements. As for the method without the use of pigs under pressure conditions, the major disadvantages are related to the substantial amount of nitrogen which needs to be stored at site, regassified and compressed at high rates (approx. 30,000 Sm3/h) and at relatively high pressure (6 to 9*105 Pa (abs)). It is worth pointing out that the latter method is not referenced and tested by current practice. The following table sums up the key aspects of the three considered methods:
Vacuum injection Pig assisted Injection with no pigs Injection under pressure
- duration days 30 35 15 - dependence on external equipment -- high high high - move up time months 2.5 1 1 - upset risk level -- none high high - operation cost estimate -- comparable not evaluated - cost estimate or insurance for upset conditions -- none very high negligible
The whole operation was organised on the basis of an international cooperation between different partners, whose tasks are indicated herebelow:
{ BSPC (Blue Stream Pipeline Company) (Russia/Italy) General management of the system { Snamprogetti (Italy) Supervisor of operation { Kubangazprom (Russia) Responsible of operation in Russia { Sicurgas (Italy) Responsible of vacuum operation in Turkey { Botas (Turkey) Management of Turkish onshore section system
The time schedule of the whole operation is reported in Figure 3.
2. SYSTEM DESCRIPTION
The temporary equipment installed at both pipeline terminals and used to carry out the whole operation is described in outline in Figures 4 and 5.
Russian coast
3 { No.1 vacuum unit contracted on service base sized for 8,000 m /h (actual); 3 { No.1 nitrogen/natural gas depressurisation skid designed for max 3,000 Sm /h, during gas pressurisation up to 1.5*105 Pa(abs) (1.5 bara), and assembled at site using materials and devices suitable to withstand the maximum available pressure (8.0*106 Pa(abs) (80 bara)). The skid (see Figure 4) included mainly: • Gas heater in order to guarantee a gas temperature after reduction higher than +5°C; • Throttling valves for pressure reduction; • Piping instrumented with pressure gauges and flow meters. { No.1 nitrogen supply equipment (on local service base) with liquid nitrogen storage of 11,500 Sm3 and re-gassification apparatus for max 600 Sm3/h. Nitrogen characteristics are:
• Purity: not less that 95%
• Water Dew Point: -60°C
Turkish coast
3 { no.2 vacuum unit systems, contracted on service base, sized for 1,750 actual m /h each.
The installation location of said temporary equipment is foreseen near the SDV valve W2 in front of the receiving pig trap as shown in Figure 5.
3. DESCRIPTION OF OPERATION
The operation was completed through the following stages:
{ Pipeline vacuumization { Nitrogen purging under vacuum conditions 5 { Gas filling up to 1.5*10 Pa(abs) (1.5 bara)
In total, three vacuum units were connected to the offshore pipeline, two in Turkey and one in Russia. Vacuum operation terminated after approx. 120 hours when the pressure stabilized around the values of approx. 8.4*103 Pa(abs) (84 mbar) and 3.8 *103 Pa(abs) (38 mbar), achieved at the Russian and Turkish sides respectively.
After vacuum, nitrogen injection was started still keeping the vacuum units in Turkey in operation, whereas the unit in Russia was shutdown. This in order to facilitate the air extraction and to reduce the amount of nitrogen needed for the operation. Two trucks were available at the Russian site to provide a storage capacity of liquid nitrogen corresponding to approx. 11,500 Sm3. Moreover, the gassification/depressurisation skid temporarily arranged close to the launcher trap D-003 was injecting an average flow rate of approx. 235 Sm3/h.
Once the amount of nitrogen (10,500 Sm3 were injected, leaving the remaining 1,000 Sm3 for any emergency/contingency) was pushed into the line, the gas-injection could begin. The upstream gas recipient (i.e. the compressor station with associated facilities as well as a 48” pipeline) fed the gas flow rate at the available pressure of around 5.5*106 Pa(abs) (55 bara).
A temporary dehydration unit, connected to the upstream gas feeding piping, assured the required drying level to the gas used for the operation. Additionally, the temporary depressurisation skid was used to heat the gas prior to drop its pressure down to the pipeline pressure.
The gas flow was increased stepwise up to a value of approx. 3,000 Sm3/h. At the Turkish terminal, the vacuum unit was maintained in operation as long as the oxygen content probe detected a value lower than 2%, i.e. when the air inside the pipeline was practically totally displaced by the batch of nitrogen. After, pipeline pressurisation continued up to reach a final value of 1.5*106 Pa(abs) (1.5 bara) monitored at the pipeline ends.
4. OPERATION RESULTS
The duration of the whole vacuum operation was 5 days. During this phase minor problems related to air ingress around the scraper trap area and electrical failure of one vacuum unit in Turkey were tackled without compromising the operation schedule. In Figure 6, a comparison is shown between the measured pressure time trend at both pipeline ends and the theoretical predictions made by the program OLGA2000 [2]. The simulation of OLGA2000 was carried out by imposing constant values of discharge actual flow rates at both pipeline ends. Due to the electrical failure of one unit at the Turkish side, a discrepancy is observed between the theoretical and measured values for a certain period of time; however the agreement between the two curves is quite good.
The duration of nitrogen injection operation was 45 hours. The volumetric pump used for nitrogen injection was intermittently operated in order to maintain approx. constant the vacuum pressure achieved inside the pipeline and avoid excessive consumption of nitrogen. Batches of nitrogen were continuously injected for 30 minutes, followed by one hour stand-by. The resulting average flow rate was 235 Sm3/h and the total amount of nitrogen pushed into the line was approx. 10,500 Sm3. Some evaluations may be made on the interface between air and nitrogen, where the two gases are mixed due to turbulent diffusion effects. If L is the interface length (considered from 2% to 98% of oxygen concentration) and x is the longitudinal coordinate along it, the oxygen concentration y may be represented by the following equation:
x 2 4 +1 1 e L −1 y(x) = + 0.156 (1) x 1.132 8 2 L ()e −1 e +1
During the gas-in operation, if it is assumed that the interface travels with a velocity v, the time trend of the oxygen concentration at the pipeline outlet y0(t) may be derived by the following equation (where y(x0) is the oxigen concentration at the beginning of the operation):
t dy dx y (t) = dt + y(x ) (2) 0 ∫ 0 0 dx dt
By replacing (1) in (2), the following expression is found for y0(t):
1 1 y (t) = − + y(x ) (3) 0 x x v 0 8 0 8 0 − t e L +1 e L L +1
where x0/L is the initial dimensionless coordinate of the interface at the pipeline outlet location (x0/L = 0.5 is the interface front). Based on the measured values of the oxygen concentration at the pipeline outlet, the following values of the interface parameters may be derived: x0/L = 0.3 v/L = 0.2 h-1 They mean that initially the interface front was outside the pipeline, i.e. the interface had partially gone out of the pipeline before the gas-in has commenced, and that the transit time of the interface was about 5 h. A comparison between measured values of oxygen concentration and equation (3) predictions, tuned by the measured values, is shown in Figure 7.
The duration of the gas filling operation was about 75 hours. The amount of the gas injected was approx. 158,000 Sm3, giving an average filling flow rate of 2,100 Sm3/h. The water dew point of the gas was continuously monitored and was found always below –18° C. Figure 6 shows the pressure time trend during filling both at the Russian and Turkish ends.
After 75 h the whole pipeline was pressurized at a value greater than 1.5*106 Pa(abs) (1.5 bara).
5. CONCLUSIONS
The whole gas-in operation up to 1.5*105 Pa(abs) (1.5 bara) was successfully completed within 10 days. The amount of approx. 158,000 Sm3 of gas with a water dew point better than (-)15°C at 5.5*106 Pa(abs) (55 bara) was injected into the pipeline.
When the pressure in the pipeline was stabilized at a value greater than 1.5*106 Pa(abs) (1.5 bara), the gas filling was continued under the BSPC direct supervision and the pipeline was pressurized at a level requested for gas sale in Turkey.
The whole operation has been carried out in close cooperation between the Russian and Turkish sites, highlighting a good coordination of the activities.
The times, costs and safety requirements have been fully satisfied according to the planned procedure.
The main aspects which may be highlighted from a lesson learnt perspective, are:
1) Vacuumization The vacuum could be stopped when the vacuum levels reached at the Russian and Turkish sides are approx. 9.0 *103 Pa(abs) (90 mbar) and 4.0*103 Pa(abs) (40 mbar), respectively. The recommended amount of Nitrogen for pipeline purging should be at least 11,000 Sm3.
2) Nitrogen filling The injection of Nitrogen at the Russian side should be such to maintain the achieved vacuum condition along the line, i.e. the pressures at both sides should remain approx. constant. The operation might be carried out through steps depending on the injection capacity of the system in Russia and the suction capacity of the vacuum unit in Turkey.
3) Gas filling at high flow Gas pressurisation can begin once the oxygen content starts decreasing at the Turkish terminal while the vacuum unit is still in operation in order to facilitate the air extraction. When the oxygen content is monitored to be lower than 2% for a certain time (say 5 hours), the vacuum machine in Turkey can be shutdown and gas can be injected into the pipeline at high flow keeping the outlet closed. Therefore, the oxygen content represents the key parameter to drive the completion of the operation and the gas flow measurement is not be strictly needed to be monitored.
6. REFERENCES
1. D.L. Katz et al. (1959). Handbook of natural Gas Engineering. McGraw-Hill, New York 2. K.H. Bendiksen, D.Malnes, R.Moe, S.Nuland (1990). The Dynamic Two-Fluid Model OLGA: Theory and Application. SPE paper n. 19541.
250
0
-250
-500
-750
-1000
-1250 Water Depth (m)
-1500
-1750
-2000
-2250 0 50 100 150 200 250 300 350 400
Beregovaya KP (km) Samsun
Seabed
Figure 1 – Blue Stream W2 offshore pipeline profile
Figure 2 – Effect of reduction of pressure below atmospheric on limits of flammability of a natural gas mixture [1].
Days Pos. Description 12345678 910 11 12
Vacuum Injection 1 W2 Offshore Pipeline
1.2 Forced depressurisation (vacumm system)
1.3 Nitrogen injection
1.4 Gas pressurisation up to 1.5 bar(abs)
1.5 Reporting
Figure 3 – Time schedule chart for the gas-in operation.
Figure 4 – Arrangement of the temporary equipment at the Russian side.
Figure 5 – Arrangement of the temporary equipment at the Turkish side.
1.2E+05
1.0E+05
8.0E+04 Inlet Pressure by OLGA2000 (Pa)
Inlet Pressure measured (Pa) 6.0E+04
Pressure (Pa) Pressure 4.0E+04
2.0E+04
0.0E+00 0 20 40 60 80 100 120 140 Time (h)
(a)
1.2E+05
1.0E+05
8.0E+04 Outlet Pressure by OLGA2000 (Pa)
Outlet Pressure measured (Pa) 6.0E+04
Pressure (Pa) Pressure 4.0E+04
2.0E+04
0.0E+00 0 20 40 60 80 100 120 140 Time (h)
(b)
Figure 6 – Pressure time trend during vacuumization. (a) Pipeline inlet. (b) Pipeline outlet.
0.8
0.7 Oxigen concentration measured 0.6 Oxygen concentration predicted by equation (3) 0.5
0.4
0.3
Oxygen concentration 0.2
0.1
0
00.511.522.533.544.55 Time (h)
Figure 7 – Oxygen concentration time trend during gas filling at pipeline outlet.
2.5E+05
2.0E+05
1.5E+05
1.0E+05
Pressure (Pa) Pressure
Inlet Pressure measured (Pa) 5.0E+04 Outlet Pressure measured (Pa)
0.0E+00 0 102030405060708090 Time (h)
Figure 8 – Pressure time trend during gas filling at pipeline ends.