Note: Descriptions are shown in the official language in which they were submitted.
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Method and system for transporting a flow of fluid
hydrocarbons containing water
This is a divisional application of Canadian
Patent Application No. 2,346,905 filed on
September 21, 1999.
FIELD OF THE INVENTION
The present invention relates to a method and a
system for transporting a flow of fluid (i.e. liquid or
gaseous) hydrocarbons containing water. In the method said
flow is transported through a treatment and transportation
system including a pipeline.
BACKGROUND OF THE INVENTION
The search for new oil or gas resources has now
reached a stage where it is moving away from relatively
easily accessible continental waters, and towards deeper
waters. This trend is currently most visible in the Gulf of
Mexico, but also offshore Norway, any large oil or gas
discoveries in the future are primarily expected in deep
waters (>-4-500 m). This development gives rise to several
technological challenges. However, solutions based on sub-
sea installations and long. distance transport to already
existing production and processing facilities have already
been in use for some time in the North Sea, especially in
connection with economically marginal fields in the vicinity
of older platforms. This technology will become steadily
more prevalent in new field developments for deep water, but
also in the increasing number of smaller projects in already
developed areas.
Traditionally, in the North Sea, use of sub-sea
templates and pipeline transport of the well-stream in
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multiphase pipelines has been restricted to a few tens of
kilometers. However, better simulation and design tools,
better equipment for partial separation, as well as pumping
and boosting, has now led to solutions of this kind being
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used with transfer distances of up to 110 km in the Gulf of
Mexico.
The single most challenging problem for these
future trends in oil and gas exploration, is the presence of
natural gas hydrates in transport pipelines and equipment.
Natural gas hydrate is an ice-like compound consisting of
light hydrocarbon molecules encapsulated in an otherwise
unstable water crystal structure. These hydrates form at
high pressures and low temperatures wherever a suitable gas
and free water are present. These crystals can deposit on
pipeline walls and in equipment, and in the worst case lead
to complete plugging of the system. Costly and time-
consuming procedures may be needed to restore flow again.
In addition to the mere economic consequences, there are
also numerous hazards connected to hydrate formation and
removal, and there are known instances of pipeline ruptures
and loss of human lives due to gas hydrates in pipelines.
Although hydrate is generally thought of as a problem mostly
for gas production, there is now ample evidence that it is
also a significant problem for condensate and oil production
systems.
There are several available methods for dealing
with hydrate problems. So far, the usual philosophy has
been to take steps to avoid any hydrate formation at all.
This can be achieved by keeping pressures low (often not
possible from flow considerations), keeping temperatures
high (usually by insulating - which does not protect against
shutdowns or long distances), removing the water completely
(costly equipment and difficult), or by adding chemicals
that suppress hydrate formation thermodynamically.
Insulation is very often used, but is not sufficient alone.
Chemical addition, specifically methanol (MeOH) or ethylene
glycol (EG), is therefore the most widespread hydrate
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control mechanism in the industry today. These antifreezes
expand the pressure-temperature-area of safe operation, but
are needed in large quantities - 500 of the total liquid
fraction is not unusual in water-rich production. The use
of MeOH in the North Sea may approach 3 kg per 1000 Sm3 of
gas extracted. The need for such large amounts places
severe demands on logistics of transportation, storage and
injection in offshore facilities with a deficiency of space.
The transport and injection processes for MeOH in
particular, are also plagued with numerous leakages and
spills.
Inhibitor chemicals of different types are not
only used in the pipeline transport and processing areas,
but also extensively in drilling operations and wells.
Partly due to the huge amounts and large costs
involved in using traditional inhibitors like MeOH, there
has over the last decade been extensive efforts devoted to
finding chemicals which may be effective at controlling
hydrates at much lower concentrations.
Many oil companies and research institutes have
contributed to this effort, and at present, the results are
divided into three main categories: kinetic inhibitors,
dispersants, and modificators. Kinetic inhibitors have an
affinity for the crystal surface, and thereby can be used to
prevent hydrate crystal growth. Dispersants act as
emulsifiers, dispersing water as small droplets in the
hydrocarbon liquid phase. This limits the possibilities for
hydrate particles to grow large or to accumulate. The
modificators are to a certain extent a combination of the
two other methods, attaching to the crystal surface, but
also functioning as a dispersant in the liquid hydrocarbon
phase. These methods have been somewhat successful,
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although there are practical drawbacks to most of them. The
most significant problem, however, seems to be that all the
best chemical additives thus far produced have significant
negative environmental effects, and that no solution to this
problem seems imminent - at least in the open literature.
There is growing understanding in the oil and gas
industry that hydrate particles in a flow situation are not
necessarily a problem per se. If the particles do not
deposit on walls or equipment, and do not have a large
impact on flow characteristics (i.e. their concentration is
not too large), they simply flow with the rest of the
fluids, without creating a problem situation. The challenge
will therefore be to achieve this situation in a controlled
manner, and making sure that hydrate formation does not take
place randomly throughout the flow system.
Another aspect which will definitely be affected
by the present invention, is corrosion in sub-sea pipelines.
Huge sums of money and large resources in material and time
are involved in protecting pipelines from corrosion, e.g.
through conservative design (pipeline wall thickness, steel
quality) and through the use of corrosion inhibitors. Not
necessarily used in the same amounts per pipeline as the
hydrate inhibitors, the total amounts of chemicals
(sometimes with environmentally highly adverse effects) are
huge, as they are used in such a great number of pipelines.
Much of this corrosion is connected with free water, and
successful results of the present invention may reduce this
problem significantly.
BRIEF SUMMARY OF THE PRESENT INVENTION
In one aspect of,the present invention, as
described in the parent application, there is provided a
method for transporting a flow of fluid hydrocarbons
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containing water through a treatment and transportation
system including a pipeline, the method comprising the
following steps: (a) introducing the flow of fluid
hydrocarbons into a reactor, wherein the flow of fluid
hydrocarbons contains water; (b) introducing a cold fluid
flow of hydrocarbons, containing particles of gas hydrates
acting as a hydrophilic agent, into the reactor where it is
mixed with the flow of fluid hydrocarbons containing water;
(c) cooling an effluent flow of hydrocarbons from the
reactor in a heat exchanger to ensure that free water
present therein attains the form of gas hydrates; (d)
treating the cooled effluent flow in a separator to separate
the flow into a first flow and a second flow, wherein the
first flow has a content of gas hydrate; (e) recycling the
first flow to the reactor to provide the particles of gas
hydrate; and (f) conveying the second flow to a pipeline to
be transported to a destination.
In a further aspect of the present invention, as
described in the present divisional application, there is
provided a system for treatment and transportation of a flow
of fluid hydrocarbons containing water, which system
includes the following elements listed in the flow direction
and connected with each other: connection to a hydrocarbon
source, a first heat exchanger, a reactor, a second heat
exchanger, a separator, and a pipeline; and in addition a
line which leads from the separator to the reactor and is
provided with a pump adapted to recycle material from the
separator back to the reactor.
DESCRIPTION OF THE FIGURES
Embodiments of the invention will be discussed
with reference to the following Figures:
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Figure 1 displays a schematic view of a system for
treatment and transportation of a flow of fluid
hydrocarbons.
Figure 2 displays a schematic view of a system for
treatment and transportation of a flow of fluid
hydrocarbons, the system comprising an additional
separator (14).
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method for
transporting a flow of fluid hydrocarbons containing water
through a treatment and transportation system including a
pipeline. According to the invention the flow of fluid
hydrocarbons is introduced into a reactor where it is mixed
with particles of gas hydrates which are also introduced
into said reactor, the effluent flow of hydrocarbons from
the reactor is cooled in a heat exchanger to ensure that all
water present therein is in
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the form of gas hydrates, said flow is then treated in a separator to be
separated
into a first flow and a second flow, said first flow having a content of gas
hydrates
is recycled to the reactor to provide the particles of gas hydrates mentioned
above, and said second flow is conveyed to a pipeline to be transported to its
destination.
Said flow of fluid hydrocarbons will normally come from a drilling hole well
and will be relatively warm and will be under pressure. It is generally
preferred to
cool the flow of fluid hydrocarbons in a first heat exchanger before
introducing said
flow into the above-mentioned reactor.
It is sometimes desirable to add certain chemicals to the flow upstream to
the reactor.
Before the flow enters the reactor it may advantageously be subjected to a
mixing operation in order to disperse the water present as droplets in the
fluid
hydrocarbon phase.
~s The second flow from the separator may be mixed with wet gas in a mixing
vessel before the flow is conveyed to the pipeline for further transport.
The method is particularly applicable in those cases where transportation
takes place at a relatively low temperature, both on land in a cool climate
and at
the sea bottom.
Zo When the surroundings are rather cool, one or more of the heat exchangers
used may be an uninsulated pipe. When the surrounding temperature is
sufficiently low, this will provide satisfactory cooling without any further
cooling
medium.
The invention also provides a system for treatment and transportation of a
flow of fluid hydrocarbons containing water. The system includes the following
elements listed in the flow direction and connected with each other so that
the
hydrocarbons may pass through the entire system (the numerals in parenthesis
refer to the enclosed drawings which serve as illustration only):
connection to a hydrocarbon source (1),
3o a first heat exchanger (4),
a reactor (6),
a second heat exchanger (7),
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a separator (8), and
a pipeline (13);
and in addition a line (9) which leads from said separator (8) to the reactor
(6) and
is provided with a pump (10) adapted to recycle material from the separator
(8)
back to the reactor (6). The pump may be any kind of pump, but it may
advantageously be of a type which crushes the hydrate particles into more and
smaller particles with a larger total crystal surface.
The inside of the system, in particular the inside of the reactor may be
coated with a water repellent material. Tubing may also advantageously be
~o provided with such a coating material.
The system preferably includes a mixer or a choke (5) upstream to the
reactor (6).
In many cases it is advantageous to add different chemicals to the flow of
hydrocarbons, in particular during start-up and when changes are made in the
>> operation. The system accordingly contains for such purpose means for
adding
chemicals to the flow.
In the following the present method and system will be described in more
detail, again with reference to the drawings.
in a first embodiment (fig. 1 ) warm oil/condensate/hydrate-forming
Zo components and water under pressure (1 ) are mixed with any desired
chemicals
(2) in a mixing means (3). If much water is initially present, some of the
water is
preferably separated off before mixing said components and water with
chemicals.
The chemicals in question may be nucleating agents for hydrate, emulsion-
breakers/-formers, wax inhibitors or any type of chemical used for
Zs transportationlstorage of said fluid. The chemicals used should be
acceptable for
the environment and should generally be used during start-up only. in any case
the consumption of chemicals will be much lower during continuous operation
than
previous transportation/storage systems, and chemicals may even be left out
completely.
3o The fluid from the mixer (3) may be cooled to a temperature just above the
hydrate equilibrium curve of the fluid (the melting curve of hydrate) in a
heat
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exchanger (4). At the bottom of the ocean said heat exchanger may be an
uninsuiated tube, or it may be any type of cooler.
The fluid from the heat exchanger (4) is conveyed to a mixer (5) which may
be any type of mixer. The mixer distributes the water in the fluid
hydrocarbons as
droplets. It should be noted that the mixer is not strictly necessary. The
question
whether or not a mixing operation is necessary depends on the characteristics
of
the fluid, i.e. the ability of the fluid to distribute the water as droplets
in the fluid
without any other influence than the turbulence which occurs when the fluid
flows
through a pipe.
The fluid from the mixer (5) is conveyed into a reactor (6), where it is mixed
with cold (temperature below the melting temperature of the gas hydrate) fluid
from a separator (8) (see below). Said cold fluid from the separator (8)
contains
small particles of dry hydrate.
The water which is present in the fluid from the mixer (5) will moisten dry
hydrate from the separator (8) in the reactor (6). In the reactor (6) the
water which
moistens the dry hydrate will immediately be converted to hydrate. New hydrate
which is formed will accordingly increase the size of the hydrate particles
from the
separator (8) and also form new small hydrate particles when larger hydrate
particles break up. New hydrate seed may also be formed elsewhere in the
reactor (6).
Sub-cooling (the actual temperature being lower than the hydrate
equilibrium temperature) of the fluid is required to form hydrates. The
necessary
extent of sub-cooling for formation of hydrate in the reactor (6) is
accomplished by
adding sufficient cold fluid from the separator (8). Cooling may also come
from
2, the reactor walls of the reactor (6) or from separate cooling ribs in said
reactor.
Undesired fouling or formation of deposits in the reactor (6) may be avoided
by
coating all surfaces with a water-repellent coating.
From the reactor (6) the fluid is cooled down in a second heat exchanger
(7). At the bottom of the ocean said cooler may be an uninsulated pipe. The
heat
3o exchanger (7) may also be any type of cooler which even may be integrated
as a
part of the reactor (6).
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In the separator (8) some of the total amount of hydrate particles and
excess fluid are separated from the rest and conveyed out to a pipeline (13)
or
first through a mixing means (12) to be mixed with wet gas (11) before
entering
the pipeline (13).
Residual amounts of the total amount of hydrate particles and residual fluid
from the separator (8) are recycled through a line (9) by means of a pump (10)
back to the reactor (6). The separator (8) may be any type of separator.
Similarly,
the pump (10) may be any type of pump, but it is important that it can handle
the
hydrate particles. It may advantageously be of a type which crushes the
hydrate
particles into more and smaller particles with a larger total crystal surface.
A
further cooler may be included in the line (9) either before or behind the
pump
(10).
Wet gas (11 ) under pressure may be mixed with the flow of fluid from the
separator (8) in a mixing means (12). Free water in the wet gas is absorbed by
the dry hydrate from the separator (8) in the mixing means (12). In the mixing
means (12) the water which moistens the dry hydrate will readily be converted
to
hydrate. The new hydrate formed will then increase the size of the hydrate
particles from the separator (8) and may also form new small hydrate particles
when larger hydrate particles are broken apart. New hydrate seed may also be
zc, formed elsewhere in the mixing means (12). At the outlet of the mixing
means
(12) connected to the pipeline (13) all free water has been converted to
hydrate.
At the beginning of the pipeline, either sub-sea at a wellhead template, or
onboard a minimum processing platform, water separation is expected to be
efficient enough so that after cooling and condensation, no more than 5-10
vol%
z~ water is present in the fluid stream.
After this separation stage, the fluids are cooled rapidly towards hydrate
stability temperatures in exposed (uninsulated) pipes of the necessary length.
The phases are also mixed, to provide a large interfacial surface area. Minute
amounts of chemicals may be needed at this stage, e.g. in connection with a
start-
3o up situation. A mixer will disperse the water as droplets. Upon next
entering the
hydrate reactor part of the system, hydrate particles and a cold fluid stream
are
mixed in from a downstream separator. Water wetting of the hydrate particles
will
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take place, and hydrate growth will therefore mainly be from existing
particles and
outwards. The hydrate formation process is thus aided by the addition of cold
fluid
(inside the stable hydrate pressure-temperature region), and - most important -
the
already present hydrate particles. Further cooling takes place through the
reactor.
According to a second embodiment (see fig. 2) the fluid hydrocarbon is
preferably a wet hydrocarbon gas. The method of this embodiment is
particularly
applicable at the sea bottom.
To a great extent the discussion of the first embodiment above will also
apply to this second embodiment. fn the following, particularly those features
which are more or less different will be discussed.
Warm hydrocarbon gas (1) under pressure is mixed with any desired
chemicals (2) in a mixing means (3). Chemicals may also be added to the system
in the reactor (6).
The flow from the mixer (3) may be cooled to a temperature just above the
hydrate equilibrium curve of the flow (the melting curve of hydrate) in a heat
exchanger (4) andlor through a choke (5) which may be a part of the reactor
(6).
At the bottom of the ocean said heat exchanger may be an uninsufated tube, or
it
may be any type of cooler.
The flow from the choke (5) is conveyed into the reactor (6), where it is
zo mixed with cold (temperature below the melting temperature of the gas
hydrate)
fluid from a second separator (8) (see below). Said cold fluid from the
separator
(8) contains small particles of dry hydrates.
Free water and water condensing from hydrocarbon gas in the flow from
the choke (5) will moisten dry hydrate from the separator (8) in the reactor
(6). In
zs the reactor (6) the water which moistens the dry hydrate will immediately
be
converted to hydrate. New hydrate which is formed will accordingly increase
the
size of the hydrate particles from the separator (8) and also form new small
hydrate particles when larger hydrate particles break up. New hydrate seed may
also be formed elsewhere in the reactor (6).
3o In a first separator (14) hydrocarbon gas is separated from the flow and
conveyed out to a pipeline (15). The separator (14) may be any type of
separator
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The rest of the flow is conveyed to the second separator (8) where some of
the total amount of hydrate particles and excess fluid are separated from the
rest
and conveyed out to a pipeline (13).
Residual amounts of the total amount of hydrate particles and residual fluid
from the separator (8) are recycled through a line (9) by means of a pump (10)
back to the reactor (6). The separator (8) may be any type of separator.
Similarly,
the pump (10) may be any type of pump, but it is important that it can handle
the
hydrate particles.
Additional cooled condensate under pressure may be added (16) to said
m recycled flow in order to dilute the hydrate particle concentration and as a
cooling
media. The addition may be made at any point between heat exchanger (7) and
reactor (6).
Hot hydrocarbon gas, either sub-sea at a wellhead template, or from a
minimum processing platform, is expected to be saturated with water vapour at
the beginning of the pipeline.
After the wellhead template or platform, the flow is cooled rapidly towards
hydrate stability temperature in exposed (uninsulated) pipes of the necessary
length or through a choke. Minute amounts of chemicals may be needed at this
stage, e.g. in connection with a start-up situation. Upon entering the hydrate
zn reactor part of the system, hydrate particles and cold fluid stream, are
mixed in
from a downstream separator. Water vapour from the hydrocarbon gas phase will
condensate and water wetting of the hydrate particles will take place. From
this
stage hydrate growth will therefore mainly take place from existing particles.
The
hydrate formation process is thus aided by the addition of cold fluid (inside
the
2; stable hydrate pressure-temperature region), and-most important - the
already
present hydrate particles. Further cooling takes place through the reactor.
Hydrocarbon fluid condensed from the cooled hydrocarbon gas will add to the
fluid
in the reactor.
A further, general discussion of the present invention is given in the
3o following.
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Free water in the pipeline proper will tend to act as a "bonding agent"
between hydrate and pipe walls. The inner surface of the hydrate reactor can
be
treated to become non-wetting with respect to water.
All of the water in the stream will be converted to dry hydrate particles by
the time it reaches the end of the hydrate reactor. Before the stream reaches
the
downstream separator it is cooled close to ambient temperature in exposed
(uninsulated) pipes of necessary length. In the separator some of the cold
hydrocarbon fluids and dry hydrate particles are taken out, and re-injected at
the
reactor inlet, as described above.
If injection of wet gas (from the initial separation stage) is desirable, it
may
take place after the separation/recirculation point (8), into the stream with
fully
converted hydrates. These fluids may then flow through a similar hydrate
reactor
to achieve full conversion before the main pipeline. However, no separation
and
recirculation is viewed as necessary for this stage.
~s The main pipeline starts immediately after the separator or the wet gas
hydrate reactor.
With the water being in hydrate form, and the hydrate particles being dry
(no excess water) it has been known experimentally in flow loops with both
model
systems and with real field fluids and pressures and temperatures, that the
zo resulting hydrate powder is easily transportable with the liquid flow.
These tests
also indicated that the particles will not aggregate or deposit on pipe walls
or
equipment - not even in the case of longtime shut-downs. This particular
phenomenon has been studied by the inventors for several years. It is also a
great advantage of the present invention that the absence of free water will
reduce
zs the risk of corrosion in' pipelines and other installations.
The hydrate powder will not melt back to free the water and natural gas
until temperatures rise or pressures become too low - which in reality will be
at the
end of the transport pipe, where the process will not be problematic. The
powder
can be mechanically separated from the bulk liquid phase by a sieve (unlike
3o dispersant-induced emulsions which are often difficult to break). Another
method
would be to melt the hydrates in a separator where the residence time is long
enough for the emerging water to separate out from the hydrocarbon liquids.
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Depending on the fluid system, the particle density may even deviate enough
from
the bulk liquid so that the particles may easily be separated off.
The present invention is expected to create considerable positive
environmental effects. The development of a safe and efficient way to
transport
free water in the form of hydrate particles will dramatically reduce the need
for a
host of different chemical additives which are used today, both hydrate and
corrosion inhibitors. This will impact all aspects of the hydrocarbon
production
process, from working conditions on production and processing facilities, to
the
effect on the environment through leaks, accidental discharges or injection
system
~o malfunctioning.
A secondary, but no less important, environmental effect will be the
improved safety aspects in pipeline operation: with the hydrate plugging and
corrosion risks minimized, the danger of pipeline ruptures and large-scale
blowouts will also be lowered. It should also be noted that a pipeline in
thermal
~; equilibrium with its surroundings will be safer with respect to melting of
hydrates in
the surrounding sediments which may induce instabilities (settling and
landslides).
This aspect is in addition to the fact that a cold fluid stream without
temperature-
induced changes in the fluid composition and properties makes the whole
pipeline
a more wet!-defined system to operate. This will not cause additional problems
in
Zo itself, as pipeline transport over any signifiicant distance will
eventually reach
ambient temperature also in traditional transport solutions.
The very limited use of chemicals according to the present invention also
has the effect that the flow of fluid hydrocarbons is more suitable for its
final use
than known from the prior art. Thus, e.g. antifreeze such as methanol may have
~> to be removed before the hydrocarbons are used in different processes, such
as
for polymerization purposes. Such removal is generally very costly.
