Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008),
Vancouver, British Columbia, CANADA, July 6-10, 2008.
Experimental Investigation of Deposition and Wall Growth in Water
Saturated Hydrocarbon Pipelines in the Absence of Free Water
Joseph W. Nicholas1, Laura E. Dieker1, Lee Nuebling2, Bob Horn2, Helen He2,
Carolyn A. Koh1, and E. Dendy Sloan1*
1
Center for Hydrate Research
Department of Chemical Engineering
Colorado School of Mines
1500 Illinois Street
Golden, CO 80401
USA
2
Intertek Westport Technology Center
6700 Portwest Drive
Houston, TX 77024
USA
ABSTRACT
Using a combination of micromechanical force and flowloop measurements, hydrate deposition
on a pipe wall surface was investigated for ‘dry’ hydrates formed in the bulk phase and for
hydrates growing on the pipe surface.
Cyclopentane ‘dry’ hydrates (without a free water phase) were used to predict whether hydrates,
formed in a bulk condensate phase, would adhere to a pipe wall. Adhesion forces between
cyclopentane hydrates and steel were measured using a micro-mechanical force apparatus. The
average force of adhesion was measured to be very small, less than 0.01 N/m. This force was
used in a particle force balance, predicting that hydrates formed in the bulk phase would not
deposit on the pipe wall.
It was hypothesized than in the presence of a water saturated hydrocarbon, hydrates would grow
on the pipe wall as the fluid cooled below its equilibrium temperature. This hypothesis was
confirmed using a single pass condensate flowloop. Water was continuously dissolved into the
flowloop inlet stream as water deposited in the flowloop test section, resulting in both a pressure
drop and fluid temperature increase. This work illustrates the need for a hydrate wall growth
model.
Keywords: hydrates, adhesion force, flow assurance, flowloop, wall growth, deposition
NOMENCLATURE
CS Carbon steel INTRODUCTION
CyC5 Cyclopentane hydrate Clathrate hydrates have hindered the oil and
FA Adhesion force gas industry since 1934, when
FD Drag force Hammerschmidt discovered hydrates were
FL Lift force capable of plugging pipelines. Clathrate
MD External moment of surface stress hydrates are crystalline inclusion
compounds wherein hydrogen-bonded water
molecules form cages containing guest
* Corresponding author: Phone: +1 303 273 3723 Fax: +1 303 273 3730 E-mail: elsoan@mines.edu
molecules[1]. Hydrates form at high sample was contacted with the hydrate
pressures and low temperatures, similar to particle and then displaced until the hydrate
conditions on the seafloor; because particle was removed from the steel surface.
hydrocarbons act as guest molecules, Adhesive force was calculated via Hooke’s
offshore pipelines are prime candidates for Law, by taking the product of the spring
hydrate formation and plugging. In addition constant and the displacement of the glass
to lost production and revenue, hydrates fiber (Figure 1D), required to remove the
pose a significant safety hazard. hydrate particle from steel. A sample
measurement is shown in Figure 1.
In 1994 Linglem et al suggested that
hydrates may deposit on the pipe wall, 200 μm
similar to a freezing water pipeline[2].
Further evidence of a deposition mechanism A B C D
was also shown in a 2002 field study,
Figure 1. Hydrate-steel force measurement
conducted in the Werner Bolley field in
(reproduced from Taylor[4]). A) Hydrate
Wyoming . A steady pressure drop was particle and steel before measurement. B)
observed, which was then followed by a Applying contact force to hydrate with steel
rapid pressure drop decrease, indicating sample. C) Applying removal force to hydrate
hydrate was building up in the pipeline and particle by moving steel. D) Final displacement
then being released downstream[3]. of glass cantilever after the particle was removed
from the steel.
Before developing a predictive hydrate
deposition model, it was necessary to
determine what conditions result in hydrate Carbon steel 318 (CS) samples were
depositing on the pipe wall. This work prepared in the CSM metallurgy laboratory
investigates the feasibility of deposition by polishing 1 cm2 pieces of steel and
from hydrates formed in the bulk phase and cutting small test samples using a high
on the wall itself. precision saw. Sample surfaces were
polished using either 1 micron diamond
EXPERIMENTAL polish or 240 grit sandpaper (corresponding
to a surface roughness of approximately 59
Micromechanical force measurements microns).
Taylor measured adhesive forces between
two THF hydrate particles and conducted Ice particles used in the ice-carbon steel
initial force measurements between THF (CS) measurements were formed by placing
hydrate and stainless steel using a a water droplet on the end of the glass fiber
micromechanical force (MMF) apparatus[4]. and quenching the droplet in liquid nitrogen.
Taylor’s hydrate-steel forces were The ice particle was then placed in the MMF
completed as a portion of a larger scoping cooling chamber, containing n-decane.
study. This work combined Taylor’s Cyclopentane hydrate (sII) particles were
experimental procedure with a more refined formed using a similar procedure, except
sample preparation method. cyclopentane (CyC5) was used as a bulk
fluid instead of n-decane. The quenched ice
The MMF apparatus was used to measure particles were placed in the cell with the
the adhesive force by bringing a hydrate or CyC5 fluid below 0°C. The CyC5 was then
ice particle in contact with a steel surface. slowly raised above freezing, allowing the
Adhesive force was measured by first ice particles to convert to cyclopentane
placing a hydrate particle onto a thin glass hydrate over 30 – 60 minutes.
fiber, approximately 30 microns in diameter,
with a known spring constant. The steel
Single pass flowloop this laboratory [4]. It is important to note
that the adhesive forces are normalized
Figure 2 shows the schematic of the single using the mean harmonic radius [4].
pass flowloop. 100%
90%
80%
60’ cooling 70%
Cumulative probability
Inlet 280’ test
section section
Moisture 60%
Probe
60’ reheat 50%
Water Outlet
section
injection Moisture 40%
system Probe 240 grit-ice, CF=1.37uN, T=-7.0C
30%
1um-ice, CF=1.7uN, T=-7.0
20% 240 grit-ice, CF=1.86uN, T=-4.2C
1um-ice, CF=1.37uN, T=-4.3C
Two phase 10% 240 grit-ice, CF=2.77uN, T=-1.4C
separator 1um-ice, CF=2.56uN, T=-1.4
0%
Figure 2. Schematic of single pass flowloop. 0 0.001 0.002 0.003 0.004 0.005
Force (N/m)
The flowloop consisted of a cooling section, Figure 3. Cumulative probability distributions of
test section, and reheat section. The cooling measured adhesive forces between carbon steel
and ice.
and reheat sections were each 60 ft in
length, while the test section was 280 ft in Cumulative force distributions shown in
length, parsed into seven 40 ft sections. The Figure 3 indicate the steel surface roughness
flowloop was constructed using a series of did not affect forces within the range tested.
20 ft long, coiled, pipe-in-pipe heat
Figure 4 compares cyclopentane (CyC5)
exchangers with an inner diameter of 0.37
hydrate-CS forces with ice-CS and CyC5-
inches.
CyC5 measurements. Measurements were
completed at various subcoolings for each
The water concentration was measured prior
system, where subcooling is defined as the
to the cooling section and after the reheat
melting temperature minus the experimental
section using GE moisture probes. Moisture
temperature.
content was maintained throughout the
course of the experiment using an ultrasonic
0.0100
mixing system upstream of the separator. 0.0090
CyC5-CyC5
1um CS - ice
The flowloop was operated using 100% 0.0080 240 grit CS - ice
liquid loading and a flow rate of 0.75 0.0070
1um CS - CyC5
240 grit CS - CyC5
Force (N/m)
gallons per minute. 0.0060
0.0050
0.0040
0.0030
RESULTS 0.0020
0.0010
0.0000
MMF measurements 0.0 1.0 2.0 3.0 4.0 5.0
Subcooling (°C)
6.0 7.0 8.0
Initial experiments measured adhesive Figure 4. Adhesive force comparisons for
forces between ice and carbon steel. various subcoolings. CyC5 represents
Measurements were conducted using both cyclopentane hydrates and CS corresponds to
the 1 micron and 240 grit polished samples. carbon steel with varying surface roughness.
Error bars show the range of data measured (0%
Each steel sample was measured at three
and 100% in Figure 3).
different temperatures, using a new particle
for each measurement. The contact force Figure 4 shows that CyC5-CyC5 forces are
between the ice particle and the steel sample substantially higher than either ice or
(Figure 1B) ranged from 1.37 – 2.77mN. hydrate forces with carbon steel.
Figure 3 shows that adhesive force increases Furthermore, CyC5-CS measurements are
with increasing temperature (decreasing substantially lower than ice-CS forces.
subcooling), as shown in previous work by CyC5-CS force are only measurable at low
subcoolings, near the dissociation The specific removal mechanism can be
temperature. CyC5-CS forces also appear to predicted using the force balance shown in
decrease with increasing CS surface Figure 5. The lifting mechanism occurs
roughness, which would decrease the when
contact area for capillary forces[4].
FL F A (1)
whereby the particle is removed in the
Applying MMF Measurements to a Force vertical direction. The sliding criterion,
Balance
FD ( F A FL ) (2)
This work assumes that hydrate particles, is derived from the force balance in the
formed from dissolved water, will have a horizontal direction, where is the static
diameter of approximately 10 microns or coefficient of friction between the particle
less and will be within the laminar sub- and surface. The criterion for rolling is,
boundary layer in the pipeline. The force
balance used in this section is similar to M D FD l1 Fl l 2 F A l 2 (3)
work conducted in the colloidal literature[5,
where l1 and l2 are the vertical and
6]
horizontal moment arms, respectively [5, 6].
A particle on a surface can be removed by
three different mechanisms: (1) rolling, (2) Force Balance Results
sliding, and (3) lifting. The first step in
predicting the removal mechanism is to Calculations using the force balance model
develop a force balance on the particle [5-8]. were completed using an adhesive force of
Figure 5 shows the force balance used by 0.002 N/m, which is slightly higher than any
Burdick et al. [5, 6]. forces measured between CyC5 and carbon
FL steel. A particle diameter of 10 microns was
used, ensuring the particle is within the
FA
laminar sub-boundary layer and Figure 5
remains accurate. Using these parameters,
FD MD Equations 1-6 predict hydrate particles will
roll off the surface at velocities of 3 ft/s and
l1
z l2 6 ft/s for condensate and methane systems,
respectively.
Point around The MMF measurements and subsequent
which rolling occurs
calculations predict that hydrates formed in
Figure 5. Force balance on a particle in the bulk fluid phase will not deposit on the
laminar fluid flow. pipe wall, in the absence of free water, under
normal operating conditions. This
prediction is also consistent with cold flow
Adhesion force (FA), drag force (FD), and lift technologies.
force (FL), all act on the particle of diameter,
It is important to note that this study only
d. Additionally, there is one external
pertains to pipelines with fully converted
moment of surface stresses (MD), acting
water. In presence of free water, which may
through the center of the particle. Predicting
occur when a water saturated gas stream
FA is typically a key parameter in the
cools, it is possible for hydrates to grow on
colloidal literature; however, this study
the pipe wall.
simply used the adhesive forces measured in
the micromechanical apparatus.
Pressure drop 40 ft Pressure drop 120 ft
Pressure drop 80 ft Pressure drop 160 ft
70 Pressure drop 200ft Pressure drop 240 ft 0.90
Flowloop results Pressure drop 280 ft Flow rate
60
0.85
Test section pressure drop (psi)
50
A liquid condensate containing dissolved 0.80
Flow rate (gpm)
water was circulated through the flowloop at 40
0.75
1000 psia. The inlet condensate temperature 30
was held at 15°C and the test section 20
0.70
temperature was maintained at -21°C. After 10
0.65
passing through the test section the
0 0.60
condensate was reheated to 15°C The test 12/5/07 12/6/07 12/7/07 12/8/07 12/9/07 12/10/07
8:00 8:00 8:00 8:00 8:00 8:00
section inlet water concentration was
Figure 7. Pressure drop across the test section in
maintained at approximately 25 parts per
40 ft increments and the corresponding flow
million by weight (ppmw) and the outlet rates.
concentration was in equilibrium with the
solid deposit on the flowloop wall The step changes in flow rate were a result
(~7ppmw). Figure 6 shows the water of increasing pump speed. As pressure drop
system originally in equilibrium, before increased, flow rate decreased and the pump
water injection was started, which is denoted speed was manually increased to maintain a
by the increase in the inlet moisture flow rate near 0.75 gpm. Figure 7 shows the
concentration. initial increase in pressure drop occurred
Outlet moisture
primarily in the first 40 ft of the test section.
30 70
Inlet Moisture But as time increased, the gap between the
Test section pressure drop
25
60 40 ft and 80 ft line began to broaden,
indicating hydrate/ice was propagating
Water concentration (ppmw)
50
Pressure drop (psi)
20
Test section 40
downstream. This was also observed in the
15
30
gap between 80 ft and 120 ft. The
10 explanation for this behavior is shown in
Probe error 20
Figure 8.
5
10
0 0 40 ft
-7 1
12/5/07 12/6/07 12/6/07 12/7/07 12/8/07 12/9/07 12/9/07 12/10/07 80 ft
8:00 2:00 20:00 14:00 8:00 2:00 20:00 14:00 120 ft
-9 Flow rate
0.9
Figure 6. Inlet and outlet moisture concentrations -11
and corresponding test section pressure drop. 40 ft
Flow rate (gpm)
Fluid Temp (oC)
0.8
-13
-15
As the condensate passed through the test 0.7
section, the fluid was cooled below the -17
80 ft
0.6
ice/hydrate equilibrium concentration. -19
120 ft
Consequently, dissolved water formed a -21 0.5
12/5/07 12/6/07 12/6/07 12/7/07 12/8/07 12/9/07 12/9/07 12/10/07
solid deposit on the flowloop wall and a 8:00 2:00 20:00 14:00 8:00 2:00 20:00 14:00
pressure drop increase was observed as
Figure 8. Condensate temperature profiles
illustrated in Figure 6. within the first 120 ft of the test section.
The deposit initially grew in the first 40 ft of Similar to the pressure drop, it is important
the test section, as shown in Figure 7. to compare the temperature profiles at
similar flow rates. Figure 8 shows the
temperature profile 40 ft into the test section
began to increase as solids deposited in the
first 40 ft of the test section (illustrated in
Figure 7). The temperature increase was a
result of the solid deposit acting as an
insulation. This temperature increase shifted ed. J.G. Speight. 2008: Taylor &
the equilibrium curve further downstream, Francis Group, LLC
which resulted in the solid deposit moving
downstream as illustrated in Figure 7. 2. Lingelem, M.N., A.I. Majeed, and
E. Stange, Industrial Experience in
This experiment validates the hypothesis of Evaluation of Hydrate Formation,
hydrate/ice forming on the pipe wall. A gas Inhibitionand Dissociation in
condensate fluid was cooled below the Pipeline Design and Operation. Int.
hydrate/ice saturation temperature and the Conf. on Nat Gas Hydrates, NYAS,
excess water deposited on the pipe wall. eds. Sloan, Happel, & Hnatow,
This work exhibits the need for a wall 1994. 715: p. 75.
growth model in water saturated 3. Hatton, G.J. and V.R. Kruka,
hydrocarbon systems. Such a model would Hydrate Blockage Formation -
also serve as a starting point for modeling Analysis of Werner Bolley Field
wall growth or deposition in a more Test Data, in DeepStar V Project
complicated system containing free water CTR 5209-1. 2002.
4. Taylor, C.J., Adhesion Force
Between Hydrate Particles and
CONCLUSIONS Macroscopic Investigation of
Hydrate Film Growth At the
Cyclopentane hydrates were used to Hydrocarbon/Water Interface, in
investigate whether hydrates, formed in a Chemical Engineering. 2006,
bulk condensate phase, would adhere to a Colorado School of Mines: Golden,
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cyclopentane hydrates and steel were 5. Burdick, G.M., N.S. Berman, and
measured using a micro-mechanical force S.P. Beaudoin, Describing
apparatus. The average force of adhesion hydrodynamic particle removal
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This force was used in a particle force Reynolds number. Journal of
balance, predicting ‘dry’ hydrates (without a Nanoparticle Research, 2001. 3: p.
free water) formed in the bulk phase would 455 - 467.
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S.P. Beaudoin, Hydrodynamic
It was hypothesized than in the presence of a Particle Removal from Surfaces.
water saturated hydrocarbon, hydrates Thin Film Solids, 2005. 488: p. 116
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cooled below its equilibrium temperature. 7. Sharma, M.M., et al., Factors
This hypothesis was confirmed using a Controlling the Hydrodynamic
single pass condensate flowloop. Water was Detachment of Particles from
continuously dissolved into the flowloop Surfaces. Journal of Colloid and
inlet stream as water deposited in the Interface Science, 1991. 149(1): p.
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ACKNOWLEDGEMENTS
This work was supported by Imerial Oil
Limited, ExxonMobil, ConocoPhillips, and
Shell. The author gratefully acknowledges
advice and support from: Alex Watson,
Glenn Cobb, Larry Talley, Doug Turner,
David Peters, Greg Hatton, Dan Crosby,
Aftab Khokhar, Nick Wolf, and Tom Baugh.
