THE CSIRO’S HYDRATES FLOW LOOP AS A TOOL TO INVESTIGATE HYDRATE BEHAVIOUR IN GAS DOMINANT FLOWS

Mauricio Di Lorenzo , Yutaek Seo and Gerardo Sanchez Soto CSIRO Earth Science and Resources Engineering

Kensington, WA 6151AUSTRALIA

ABSTRACTCSIRO has recently built a pilot size test facility to investigate natural gas hydrate behaviour in

gas pipelines. It consists of a 1” diameter, 40 meters long one pass flow loop in which compressed gas can be circulated in contact with water at pressure and temperature conditions in which most natural gas hydrates form. Hydrodynamic conditions can be set to achieve wavy and annular flow regimes typical of gas producing pipes. CSIRO’s Hydrates Flow Loop has been designed to mimic as close as possible the conditions found at Australian offshore gas producing pipelines with less than 10% liquid load. In this work this test facility is described in detail. The

temperature and pressure profiles along the test section of the flow loop are provided at different conditions and operational procedures to conduct steady state and transient tests are described. Some preliminary results obtained during the standard tests are presented and possible areas of application for the oil and gas industry are highlighted.

Keywords: gas hydrates, flow loop, gas dominant flow

Corresponding author: Phone: +61 (0)8 6436 8800 Fax +61 (0)8 6436 8555 E-mail: Mauricio.Dilorenzo@csiro.au

NOMENCLATUREd pipe diameter [m]f friction factorP pressure [bar]

s coat thicknessT temperature [ºC]

v gas velocity [m/s]

P/l pressure loss [bar/m]

gas viscosity [Pa s]

gas density [kg/m3]

INTRODUCTIONIt has been recognized that a new flow assurance approach is required to reduce the cost of hydrate mitigation strategies for the development of offshore deep water gas fields [1]. The concept of a risk based management of hydrates in pipelines is gaining acceptance in the O&G production

community, based on the observation that the presence of hydrates does not always lead to

pipeline blockages. Such approach must be necessarily built upon a sound knowledge of the mechanisms that govern hydrate formationkinetics and pipe clogging in gas production flow

lines. Most of the research on hydrates behavior in flow lines has been limited to oil dominated systems in which the prevailing phase is black-oil or condensate. On the contrary, in gas flow lines, liquids (water and hydrocarbons) are usually present in small amounts which may increase with

the production time.Even if gas dominated systems have some documented field data for hydrate blockage [2],the authors are not aware of laboratory studies performed in realistic conditions. These should be preferably conducted using dedicated flow loops

designed to handle gas-water/oil systems at low temperatures and high pressures at hydrodynamic conditions representative of production scenarios.In Table 1 flow loops used for hydrates studies are listed with their main specifications. It is indicated

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

that most of the available data from these facilitieshave been obtained at liquid dominant flowconditions.

Flow loop Geometry/m (*)

P/bar T/°C

Liquid loading

Velocitym/s

Exxon’s F.L. [3]

ID: 0.097L: 83.8

P: 6−124 T: -6.7−37.8

83% 0.5−2.5

Texaco’s F. L. [4]

ID: 0.049 L: 31.7

P: 138 T: 3.3−66

Liquid dominant

6.7

Tulsa Univ. F. L. [5]

ID: 0.076L: 48.8

P: 138 T: -6.7−7.8

50% 0.18 − 2.2

Korea Gas Corp. [6]

ID: 0.015 L: 4.0

P: 150T: 0 − 17 °C

51% 0.28 − 0.78

Intertek Westport Tech. Center [7]

ID: 0.009.3 L: 85.3

P: 68.9 T: -21 − 3

100% 0.3 −0.6

Lyre F. L. [8]-[9]

ID: 0.049L: 140.0

P: 1-100T: 3.3−66

2% -100%

Gas: 1 − 6 Liquid: 0.2 − 3

Table 1. Flow loops applied to hydrate research(*) ID: internal diameter, L: length

To the authors’ knowledge only the Lyre flow loop at the Institut Francaise du Petrole has produced preliminary unpublished results showing that natural gas hydrates can be formed and transported at high volume fractions of gas (up to

98%) and high velocity. The CSIRO’s Hydrate Flow Loop has been recently commissioned as an experimental facility for investigation of natural gas hydrate formation and flow behavior in gas production pipelines. Inthis work this test rig is described and some

preliminary results are presented to highlight its potential applications.

FLOW LOOP DESCRIPTIONCSIRO’s hydrates flow loop allows a mixture of gas and liquids at high pressure to be circulated

within a stainless steel 1 inch flow line, 40 m long, connected to a gas circulation compressor and injection liquid pumps. A simplified layout of the rig is presented in Figure 1. The flow loop can handle non-corrosive gases, pure water and brines

and model oils with water or oil-soluble additives. The loop can be pressurized using high pressure gas cylinders or the natural gas storage system. This consists of six gas cylinders in which the gas

from the city network can be pressurized using two gas compressors. The flow rates of the gas and liquids in the loop can be adjusted to obtain liquid volume fractions from up to 10% in the flow line. Here the fluids mixture can be cooled by means of a chiller unit circulating liquid coolant through a

4” pipe-in-pipe system. This flow loop operates at temperature and pressure conditions where most natural gas hydrates form. In Table 2 the main technical specifications of the flow loop are summarized.

Figure 1. Simplified layout of the CSIRO’s Hydrates Flow Loop

The mixture of fluids and hydrates transported in

the flow line is collected in a two phase separator tank at the flow line outlet where the mixture is separated into a gas and a liquid phase. The gas leaves the separator and flows through the gas

compressor before being injected into the flow line, while the liquid collected in the separator is discharged into the liquid storage tank A “U” shaped by-pass deviation, 2 meters long and 0.9 meter deep, is available to simulate a low point along the flow line.

The pressure and temperature at different points in the flow line (PT-1 to PT-6 in Figure 1) are measured using RTD sensors and pressure

transmitters with accuracies of 0.15 °C and 0.3 bar respectively. The measurement thermowells in the test section are approximately 6 m apart. The gas and liquid flow rates are measured using a

turbine gas flow meter and a positive displacement

liquid flow meter with accuracies of 0.3% and 1% respectively. The pressure, temperature and flow rate readings are transmitted to the data acquisition system and stored in a PC. The fluids inside the loop can be inspected and video

recorded from high pressure visualization windows at four different locations in the flow line (VW1 to VW4 in Figure 1) using high speed video cameras with x10 magnification lenses. All the

videos are taken in a transmitted light configuration.

Temperature range -8.0 to +30 ºC

Pressure range 1 to 117.2 bar (1700 psi)

Pressure drop < 13.8 bar (200 psi )

Liquid flow rate 0.06 to 0.5 m3/h

(sup. velocity < 0.4 m/s)

Gas flow rate 500 to 1000 Sm3/h

(sup. velocity < 8.5 m/s)

Gas volume fraction > 90%

Phases Water (brine)/model oils/

natural gas

Inner diameter 0.021 m

Test section length 40 m

Material 316 Stainless Steel

Total volume 160 L

Test section volume 15 L

Table 2. CSIRO’s Hydrates Flow Loop technical specifications

Further details can be found in Ref. [9].

EXPERIMENTAL PROCEDURESTwo types of tests have been performed so far in the flow loop. One test is referred to as “continuous flow test” for studies under steady

state flow conditions, similar to those found in a producing gas pipe. The second type is referred to as “restart test” to investigate transient flowconditions, simulating pipeline restart operations after a shut-down period. The experimental procedures are as follows.

Continuous flow testsInitially all liquids are cleared from the tests section.The flow loop is pressurized to the test pressure.

The test section is cooled down to the test temperature. In these tests the gas is circulated at a constant flow rate of 6.1 acfm (0.17 m3/min) until a steady-state gas temperature profile is established along the test section. Water is injected at a constant flow rate of 2 l/min in these tests.

Temperature and pressure along the test section and the flow rates of gas and water are measured and logged into the PC during the experimental time (around 10 minutes)

High speed videos at 1500 frames/second arerecorded at the viewing windows.

Restart testsThe restart tests are performed using the U-bend

section of the flow loop, where liquids can be accumulated and cooled down at static conditions and maintained at low temperature for a cooling-down time period, before restarting the gas flow.The test section is first cleared from all liquids and the low point is filled with water (700 mL in these

tests). This corresponds to an in-situ water volume fraction of 70% (0.44% volume fraction over the whole loop).The cooling-down period is initiated by cooling the flow loop from ambient temperature to the target value for the test. The cooling rate is around

0.8 C/min. During this period the fluids in the low point are visually inspected through the viewing window and snap-shots are taken using the video camera located at the low point window. Once the required cooling-down period haselapsed, six hours for these tests, the compressor is

turned on and the gas is forced to flow through the low point of the flow line at a constant flow rate. High speed videos at 1500 frames/second are recorded for further analysis of the fast process occurring at restart. During the test the gas flow rate, pressure and temperature along the flow loop

are measured and the readings stored in the PC.

MATERIALSDeionised water and gas from the city network

saturated with water was used in all these tests.

The dry gas composition is shown in Table 3.

Components Composition (mol%)

CH4 84.22

C2H6 6.79

C3H8 3.12

i-C4H10 0.41

n-C4H10 0.59

i-C5H12 0.04

n-C5H12 0.02

C6 0.01

CO2 2.19

N2 2.59

Table 3. City gas composition used in these tests

SII hydrates are formed with this gas. The hydrate dissociation curve, as obtained using Infochem Multiflash Vr.3.6.26, is shown in Figure 2.

Figure 2. Hydrates dissociation curve for the city

gas

RESULTS

Continuous flow experimentsThe results obtained from two continuous flow tests performed at 103 bar (1500 psi) at different temperatures are reported to show the hydrate flow characteristics as indicated by the pressure drop behavior.

First the results obtained in a test performed at a pressure of 103 bar and a temperature of 28 ºC in a liquid/gas point of the phase diagram are shown as a reference. In Figure 3 the pressure drop across the whole test section (P1-P6) is presented. In this test gas is continuously circulated. Water injection

started at t=0.5 min and stopped at t=10 min. In the same graph the values for the pressure drop calculated for a natural gas system and a gas-water system at the experimental conditions of this test are shown according to the Beggs and Brill’s model [10]. The input values for the model

parameters are listed in Table 4.

Gas flow rate (m3/min) 0.17

Water flow rate (l/min) 2.0

Gas density (kg/m3) 100

Water density (kg/m3) 1000

Gas viscosity (Pa s) 1.5 10-5

Water viscosity (Pa s) 1.0 10-3

Table 4. Parameters used for pressure drop

calculations in the liquid/gas system

At these conditions the model predicts a liquid holdup of 6% and a pressure loss of 0.037 bar/m.

The pressure gradient due to the gas only is 0.0175 bar/m, according to the Darcy-Weisbach equation.The distance between P1 and P6 is 33.4 m.

0

0.5

1

1.5

2

0 1 2 3 4 5 6 7 8 9 10 11

Time (min)

Lo

op

pre

ss

ure

dro

p P

1-P

6 (b

ar) Expimental data

Model calculations

Figure 3. Pressure drop across the loop in the gas/liquid test.

In Figures 4 to 7 the evolution of pressure drop at different sections of the flow line is presented. The

data correspond to an experimental time of about 10 minutes from the time when water started to be injected in the loop. The temperature profiles at the same sections are presented in Figure 8. The temperature values reported in the graphs in Figure 8 for each section refer to a mean value estimated

from the temperature data at steady-state. In Figures 9 to 12 the pressure drop data for a test conducted at a lower temperature profile and the same pressure are presented. The temperature data for this test are shown in Figure 13.

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10 11Time (min.)

Pre

ssu

re d

rop

P1-

P2

(bar

)

T=16.0 °C

Figure 4. Pressure drop P1-P2 as a function of

time at P=103 bar T=16 ºC

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10 11Time (min.)

Pre

ssu

re d

rop

P2-

P3

(b

ar)

T=13.0 °C

Figure 5. Pressure drop P2-P3 as a function of time at P=103 bar T=13 ºC

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10 11

Time (min.)

Pre

ssu

re d

rop

P3-

P4

(bar

)

T=10.0 °C

Figure 6. Pressure drop P3-P4 as a function of time at P=103 bar T=10 ºC

00.5

11.5

22.5

33.5

44.5

0 1 2 3 4 5 6 7 8 9 10 11

Time (min.)

Pre

ssu

re d

rop

P4

-P6

(ba

r) T=8.0 °C

Figure 7. Pressure drop P4-P6 as a function of

time at P=103 bar T=8 ºC

-2

0

2

4

6

8

10

12

14

16

18

20

0 1 2 3 4 5 6 7 8 9 10 11

Time (min.)

Te

mp

era

ture

(°C

)

T1 T2 T3 T4 T6

Figure 8. Temperature profiles in each section

during the first test at P=103 bar

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10

Time (min.)

Pre

ssu

re d

rop

P1-P

2 (

ba

r) T=11.0

Figure 9. Pressure drop P1-P2 as a function of time at P=103 bar T=11 ºC

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8 9 10

Time (min.)

Pre

ssu

re d

rop

P2-

P3

(bar

)

T=8.0 °C

Figure 10. Pressure drop P2-P3 as a function of

time at P=103 bar T=8 ºC

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10

Time (min.)P

res

sure

dro

p P

3-P

4 (b

ar) T=7 °C

Figure 11. Pressure drop P3-P4 as a function of time at P=103 bar T=7 ºC

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6 7 8 9 10

Time (min.)

Pre

ssu

re d

rop

P4-

P6

(bar

) T=4.0 °C

Figure 12. Pressure drop P4-P6 as a function of

time at P=103 bar T=4 ºC

-5

0

5

10

15

0 1 2 3 4 5 6 7 8 9 10

Time (min.)

Te

mp

era

ture

(°C

)

T1 T2 T3 T4 T6

Figure 13. Temperature profiles in each section during the second test at P=103 bar

Different flow regimes have been observed in the continuous flow experiments, which are represented in Figures 14, 15 and 16. In the

dispersed flow regime most of the water is immediately converted into hydrates and hydrate particles are pneumatically conveyed by the gas at high velocity. In Figure 14 at time T=0 the image from the window VW1, located 15 m downstream the injection point, shows some hydrates stuck at

the window. In the following snapshots hydrate particles, with a size of several millimeters, can be seen passing through the window from the right to the left. The time at which each photo is taken from the first one is also indicated below each snapshot. The velocity of these particles can be

estimated to be very close to the superficial velocity of the gas (8.5 m/s). The dispersed flow regime is observed at lower temperatures (7 ºC and below) compared to the slurry flow regime.

T=0 2mm T=0.67ms

T=2.01ms T=2.68ms

T=3.35ms T=4.02ms

Figure 14. Sequence of snapshots showing hydrate particles flowing through window VW1 (P=103 bar T=7 ºC - dispersed flow regime)

A picture of the hydrate slurry observed in the viewing window VW1 is shown in Figure 15, which was obtained at a temperature of 12 ºC. Small particles can be seen through the water film wetting the window.

Figure 15. Snapshot showing a hydrate slurry at viewing window VW1 (P=103 bar, T=12 ºC)

A webcam attached on the viewing window VW4, located at the test section outlet, was used to monitor the fluids and hydrates coming out loop. In these experiments no liquid water has been observed at this point of the flow line. In Figure 16 a sequence of snapshots is presented

where a hydrate slug can be seen passing through the window out of the test section. Figure 16-A exhibits the image of the window VW4 just before its arrival. Some hydrates particles can be seen at the test section outlet on the right. In Figure 16-B the hydrate slug is passing at the window and the

photo in Figure 16-C shows the window just after it went through.

-A- -B-

-C-

Figure 16. Sequence of photos showing a hydrate slug flowing out of the test section

Restart experimentsRestart tests have been performed at a pressure of

97 bar and a temperature of 3.51 ºC at two different in-situ water volume fractions of 70% and 100% (volume of water/volume of. the void horizontal section of the U-bend). Figure 17 shows the temperature profiles at two

points: T5 (in the horizontal section of the U-bend) and T6 (downstream the U-bend). The temperature quickly drops from 25 ºC (in the hydrate free region of the phase diagram) to a steady-state value between 4 and 2 ºC in 50 minutes before rising sharply at the restart time, (after about six

hours of cooling-down period. The oscillations in the steady-state temperature profile are due to the chiller cycling and the increase at restart is attributed mainly at the injection of hotter gas. Both restarts have been performed at the maximum gas flow rate of 6.1 acfm during a time

period varying from 10 to 50 minutes.

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400

Time (min.)

Tem

per

atu

re (

oC

)

T5

T6

Restart time

Figure 17. Temperature as a function of time at points T5 and T6 during the shut-down and restart test at P=97 bar, T=3.5 ºC.

The pressure drop values at restart across the U-

bend (P4-P6) are presented in Figure 18.As soon as the gas is restarted (at T ime: 1 minute) the pressure drop sharply rises to stabilize later on with a small increasing trend over the experimental time. The pressure drop at restart can be estimated to be around 1 bar for the 70% water

cut test and 1.5 bar for the 100% water cut.A sequence of snapshots taken at restart during the test at 100% water cut is presented in Figure 19. The time elapsed from the restart is indicated below each photo. At time T=0 the viewing window at the U-bend (VW3) can be seen fully

flooded with water, as the pipeline is. The following photos show how the gas bubbles break through the water upon restart as soon as the gas phase reaches the U-bend. As the turbulence

increases with time more bubbles are formed enhancing the gas-water interfacial area and the hydrate formation rate.

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30

Time (min.)

Pre

ss

ure

dro

p P

4-P

6 (

ba

r)

water vol. fract.=70%

water vol. fract.=100%

Figure 18. Pressure drop across the U-bend with the horizontal section 70% and 100% filled with water (P=97 bar, T=3.5 ºC)

T=0 ms T=90 ms

T=130 ms T=160 ms

T=190 ms T=290 ms

Figure 19. Sequence of snapshots showing the

water-gas mixing and hydrate formation at restart in the test at 100% water volume fraction, P=97 bar, T=3.5 ºC.

The results from two shut-down and restart tests

conducted at similar conditions are shown to explore the effect of different restart gas rates. In the lower velocity test, the gas flow rate was set at 4 acfm (gas velocity: 5.4 m/s) and the pressure and temperature at 110.3 bar and 1.5 ºC respectively.

In the higher velocity restart the maximum gas flow rate of 6.1 acfm (gas velocity: 8.2 m/s) was used and the conditions are: pressure P=110.3 bar and temperature T=2 ºC. In both tests the in-situ water volume fraction is 70%.Figure 20 shows the pressure drop (P4-P6) after

restart obtained as a function of time in these tests.The pressure drop is consistently higher during the first 30 minutes in the test performed at a lower restart rate.Finally a sequence of snapshots obtained after restart in the higher velocity test is presented in

Fig. 21. The time elapsed from the restart is indicated below each photo.

0

2

4

6

8

10

0 10 20 30 40 50

Time (min.)

Pre

ss

ure

dro

p (

P4

-P6

) (b

ar) gas velocity=8.2 m/s

gas velocity=5.4 m/s

Figure 20. Pressure drop across the U-bend for two tests at different restart rates.

The image taken at the T=0 ms exhibits the three

phases present after the system has been maintained at P=110.3 bar and T=2 ºC for six hours. The gas phase is on top, the liquid water phase at the bottom and a hydrate layer, about 1 mm thick, is extended over the gas-water interface. Hydrate dendrites also appear below the hydrate

layer protruding into the water phase. The following images show how the hydrate layer is disrupted by the incoming gas stream and the following strong mixing process taking place due to the turbulence in the flowing gas-liquid mixture.Fresh gas-liquid interface is generated during the

process at a high rate allowing for a continuousand fast hydrates formation. After only 1.3 seconds the window is completely covered with hydrates.

T=0 ms T=200 ms

T=240 ms T=250 ms

T=255 ms T=1260 ms

Figure 21. Sequence of snapshots taken upon a

high velocity restart at P=110.3 bar and T=2 ºC.

DISCUSSIONThe preliminary results obtained in these tests show that hydrates in pipelines can display a variety of behaviors even in a relatively short

experimental time under dynamic conditions as those simulated in this flow loop.At this moment a model to describe this complex behavior is not available. The main challenge here is to couple the kinetics of hydrate formation with mass and heat transfer processes determined by the

thermodynamic and hydrodynamic conditions in gas dominant flows at high velocity.A qualitative analysis of the data obtained can provide useful insights at the phenomenological level. The first striking evidence is that hydrates form and evolve under the experimental conditions

simulated in this flow loop at much shorter time scales (fraction of a second to several minutes) compared to oil dominated flow loops (hours to days).

The data obtained in the steady state tests indicate that the presence of hydrates can be detected from the pressure drop measurements. The values obtained in the short 6 meters sections (P1-P2, P2-P2, P3-P4) and in the long one (P4-P6) are well in excess compared to the values expected in absence

of hydrates (0.22 bar for the short and 0.5 bar for the long section). Furthermore the time evolution of the pressure drop in the presence of hydrates deviates from a constant trend, and exhibits a much complex behavior, including the presence of transient peaks or steady increase over time.

These measurements, complemented by visual observations, suggest that, at the experimental conditions of these tests, the system evolves from a slurry flow regime where hydrate formation is limited, at temperatures between 16 and 10 ºC(subcooling < 8.7 ºC), to a dispersed flow regime

where most of the water is converted to hydrates,at temperatures below 8 ºC (subcooling > 10.8 ºC).It is considered that the increase in the pressure drop in gas dominant conditions is mainly due to a restriction of the flow area of the pipe. The presence of particle dispersed in the gas flow or in

the water phase will contribute to the pressure loss, but to a lesser extent. A reduction of the flow area in gas pipelines at hydrate forming conditions has been linked to the build up of a hydrate coat on the pipe wall (stenosis buildup) [11]. Depending on the mechanical properties of the coat this may or

may not withstand the shear imposed by the gas stream and parts of it can be peeled off and transported downstream (sloughing). The presence of the peaks in the pressure drop readings, that has been associated with hydrate formation in production pipelines [13, 14], could be explained

by such a mechanism. Using the Darcy-Weisbach equation:

d

vf

l

P 2

2

1

(1)

and the correlation for the friction factor in the turbulent regime:

25.0

0791.0

vdf , (2)

the pressure loss can be related to the hydrate coat thickness, s according to [14]

75.4

0

2

d

sd

l

p

l

p

h

(3)

Here subscript “0” refers to the hydrate free

pressure loss, and the subscript “h” to the pressure

loss in the presence of the hydrate coat.

Using: (p)0 = 0.22 bar and (p)0 = 0.5 bar for the

short and long sections and (p)h = 4 bar as a typical height value of the peaks in Figures 7 and 10, the thickness of the transient deposits in the pipe walls can be estimated to be 4.8 mm for the short section P2-P3 (Figure 10) and 3.7 mm for the long section P4-P6 (Figure 7). This highly idealized model assumes that the coat is equally

distributed along the pipe section, which may not be case, but these results indicate that the restriction to the flow should be quite substantial compared with the flow loop diameter. On the other hand, the slug flow behavior observed in Figure 16 could indicate that the hydrates

accumulate preferentially in localized sections of the flow line, building up a loose permeable plug until the rising pressure is enough to dislodge it and transport it downstream. In this case slug flow pneumatic conveying theories should be used to predict the pressure drop.

The shut-down and restart tests are more difficult to analyze. The visual observations suggest that hydrates are formed at a very high rate during the high velocity restart. Small fragments of hydrate particles and gas bubbles can be observed in the sub-cooled water phase within an extremely short

time frame (Figure 21). A slug develops and a hydrate-water-gas slurry starts to form under the intense mixing and agitation occurring in the low spot. This slurry is flushed downstream and appears to thicken with time sticking at the window and covering the glass completely.

Hydrates may accumulate at hot spots such as the pipe line bends downstream the U-bend section from P5 to P6 and form restrictions to the gas flow.In a previous work evidence has been presented that indicates that the pressure drop through the U-

bend can be related to the formation rate driven by high subcooling conditions [15]. In this work the transient tests have been performed at the close values of the subcooling (between 15 and 17 ºC). The results obtained with different amount of

water in the low spot indicate that the pressure drop increases with the water content in the pipe, which can be expected. Even if initially the

interfacial area for gas-water contact is smaller and hydrates do not appear to have formed during the cooling-down period from visual observation (see Figure 19), as soon as the gas flow is restarted,conditions for a high rate of hydrate formation should be achieved due to the intense mixing and

bubbling apparent from the snapshots. The results in Figure 20, showing a larger pressure drop obtained at a slower restart rate compared to the faster one, could be surprising. Even if the hydrate formation rate is expected to be higher at a faster restart due to a higher intensity of the

turbulence, the residence time of the subcooled water should be shorter in this case and a lesser amount of hydrates available to deposit at the pipe walls could be produced. More data and theoretical modeling is needed to elucidate theseimportant aspects of transient operations in the

presence of hydrates. It has been suggested that high velocity startups could be less prone to hydrate blockages than the low velocity ones [16], but the evidence is still scarce.

CONCLUSIONSPreliminary results obtained in the CSIRO’s Hydrate Flow Loop has been shown in this work to highlight the potential application of this facility for investigations on hydrates behavior in gas dominated pipelines. These results indicate a complex, fast evolution of the hydrates over time

occurring at a relatively short time scale at the conditions of the tests (high velocity, moderate to high subcooling). It has been demonstrated that useful information can be extracted from the pressure drop profiles together with visual observations and a preliminary description of the

phenomena taking place at steady-state and transient conditions has been provided as a first step to the implementation of a mathematical model. More data and further theoretical development are needed for such purpose.For practical flow assurance applications it is

envisaged that flow loop can be used to fine-tune hydrate inhibition strategies using chemicals at conditions representative of gas production pipelines, after pre-screening with conventional laboratory studies using autoclaves or rocking cells.

The CSIRO’s Hydrates Loop has been recently upgraded to allow for longer experimental times and a research program on natural gas hydrates transportability is underway.

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ACKNOWLEDGEMENTS

The authors thank CSIRO for permission to publish this paper