Method for handling of free water in cold oil or condensate pipelines

The present invention relates to a method and a system for transporting a flow of fluid hydrocarbons (oil or condensate) containing water at temperatures below 0°C. In the method said fluid is transported through a treatment and transportation system including a pipeline.

Many of the world's oil and gas fields are found in remote areas like e.g. Alaska, Canada and Siberia. After minimum processing at the fields the oil/condensate is often transferred to port or market trough long transportation pipelines like e.g. the Trans-Alaska Pipeline System (TAPS) from the North Slope of Alaska to Valdez. Such oil or condensate often contains some free water, usually in the range 0.01 to 5.0 volume percent. This water may convert to ice at the pipe wall if the oil/condensate is cooled down to temperatures below 0 °C, and the pipeline may slowly be plugged. Or if free water is allowed to accumulate in parts of a pipeline before being converted to ice, the pipeline may be plugged through conversion of the water to ice. Costly and time-consuming procedures may be needed to restore flow again.

There are several available methods for dealing with ice problems. So far, the usual philosophy has been to take steps to avoid any ice formation at all. This can be achieved by keeping temperatures high (usually by only insulating - which does not protect against shutdowns or long distances or by insulating and heating, which is costly), removing the water completely (costly equipment and difficult), or by adding chemicals like methanol or glycols that suppress ice formation thermodynamically. If antifreeze chemicals are to be used they are often needed in quantities of 50% of the water fraction. This places severe demands on the logistics of transportation, storage and injection in remote facilities. The transport and injection processes for chemicals are also plagued with numerous leakages and spills, which have adverse environmental impacts.

There is a growing understanding in the oil and gas industry that 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 ice formation does not take place randomly throughout the flow system. GB2358640 describes a method for converting free water in a pipeline to dry gas hydrate particles. This solution however requires that light hydrocarbon molecules are present in the hydrocarbon flow and also high pressure conditions. Prior art does not describe any instances of preferentially promoting ice formation in order to avoid ice problems. Viewing parts of a hydrocarbon transport system as a possible ice crystallization reactor is an innovation which yields insight into hitherto unforeseen advantageous processes and effects, as is outlined in the present invention.

Another aspect which will be affected by the present invention is corrosion in 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. Much of this corrosion is connected with free water, and successful results of the present invention may reduce this problem significantly. Note that the expression "free water" in these instances means the water which can be converted under the prevailing conditions. A water phase containing e.g. salt (or other compounds which may impact the freezing behavior), may only be converted to a certain degree, until the concentration of the salt in the remaining liquid water is high enough to stop further ice formation. We do not include this remaining water with increased concentration of salts or other compounds in our definition of "free water".

The present invention provides a method for transporting a flow 1 of fluid hydrocarbons containing free water through a treatment and transportation system including an exit pipeline 9. According to the invention the flow of fluid hydrocarbons is introduced into a reactor 4 and mixed therein with a cold flow 10' of fluid hydrocarbons containing ice particles to obtain an exit flow 9' of fluid hydrocarbons where all free water is converted to ice crystals, said exit flow 9' being conveyed through the exit pipeline 9 to be transported to its destination.

The invention also provides a system for treatment and transportation of a flow of fluid hydrocarbons containing free water. The system includes the following elements listed in the flow direction and connected with each other: connection to a hydrocarbon source 1 , a reactor 4, and an exit pipeline 9; and in addition a line 10 which leads cold fluid flow 10' of hydrocarbons containing ice particles to the reactor 4.

Figure 1 shows a flow chart of a preferred embodiment of the invention. Figure 2 shows the essential parts of the invention.

Figure 3 shows an embodiment of the invention where recirculation is performed in series. Figure 4 shows an embodiment of the invention where recirculation is performed in parallel.

The effluent flow of hydrocarbons from the reactor 4 may be cooled in a heat exchanger to ensure that all free water present therein is in the form of ice particles.

Further the flow 9' of hydrocarbons containing ice crystals may be treated in a splitter 6 to be separated into a side exit flow and a main exit flow, said side exit flow is recycled through line 10 to the reactor 4 to provide the particles of ice mentioned above, and said main exit flow is conveyed to a pipeline to be transported to its destination. This situation is shown in Figure 1.

The flow 1 of fluid hydrocarbons will normally be minimum processed oil/condensate at a drilling well site and will be relatively warm. It is generally preferred to cool the flow 1 of fluid hydrocarbons in a first heat exchanger 3 before introducing said flow into the above-mentioned reactor 4.

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.

The method is particularly applicable in those cases where transportation takes place at temperatures below O ⁰ C, both on land in a cool climate and at the sea bottom.

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.

An embodiment of the invention is shown in figure 1. This system for treatment and transportation of a flow of fluid hydrocarbons containing water 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 Fig. 1 and serve as illustration only): connection to a hydrocarbon source 1 , means for adding chemicals 11 , a mixer 2, a first heat exchanger 3, a reactor 4, a second heat exchanger 5, a splitter 6, and an exit pipeline 9; and in addition a line 10 which leads from said splitter 6 to the reactor 4 and is provided with a pump 7 adapted to recycle material from the splitter 6 back to the reactor 4. The pump may be any kind of pump, but it may advantageously be of a type which crushes the ice particles into more and smaller particles with a larger total crystal surface.

The inside of the system, in particular the inside of the reactor 4 may be coated with a water repellent material. Tubing may also advantageously be provided with such a coating material.

The system preferably includes a mixer 2 upstream to the reactor 4.

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 11 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 and water (flow 1) are mixed with any desired chemicals in a mixing means 2. The oil/condensate in line 1 may be at any pressure suitable for free water in the fluid to convert to ice at temperatures below 0 °C. The mixer 2 may be any type of mixer. Chemicals in question may be nucleating agents for ice, emulsion-breakers/-formers, wax inhibitors or any type of chemical used for transportation/storage 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.

The mixer 2 may also distribute 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 or pipe junctions.

The fluid from the mixer 2 may be cooled to a temperature just above 0 °C in a heat exchanger 3. The said heat exchanger may be an uninsulated pipe, or it may be any type of cooler. Said mixer 2 may be situated before or after said heat exchanger 3.

The fluid from the heat exchanger 3 is conveyed into a reactor 4, where it is mixed with cold (temperature below 0°C) fluid from a splitter 6 (see below). Said cold fluid from the splitter 6 contains particles of ice. Said reactor 4 may be an uninsulated pipe, or any suitable design for the purpose.

The water which is present in the fluid from the heat exchanger 3 will moisten ice particles from the splitter 6 in the reactor 4. In the reactor 4 the water which moistens the ice particles will be converted to new ice. New ice which is formed will accordingly increase the size of the ice particles from the splitter 6 and also form new small ice particles when larger particles break up. New ice seeds may also be formed elsewhere in the reactor 4.

Sub-cooling (the actual temperature being lower than ice equilibrium temperature) of the fluid is required to form ice. The necessary extent of sub-cooling for formation of ice in the reactor 4 is accomplished by adding sufficient cold fluid from the splitter 6 and/or by cooling which may also come from the walls of the reactor 4 or from separate cooling ribs in said reactor. The required level of sub-cooling is known to be smaller when using ice seed particles, than for systems without. Undesired fouling or formation of deposits in the reactor 4 may be avoided by coating all surfaces with a water-repellent coating.

From the reactor 4 the fluid is cooled down in a second heat exchanger 5, which may be an uninsulated pipe. The heat exchanger 5 may also be any type of cooler which even may be integrated as a part of the reactor 4.

In the splitter 6 some of the total amount of ice particles and excess fluid are separated from the rest and conveyed through an exit pipeline 9. Residual amounts of the total amount of ice particles and residual fluid from the splitter 6 are recycled through a line 10 by means of a pump 7 back to the reactor 4. The splitter 6 may be any type of separator/splitter and may be situated before or after said cooler 5. Similarly, the pump 7 may be any type of pump, but it is important that it can handle the ice particles. It may advantageously be of a type which crushes the ice particles into more and smaller particles with a larger total crystal

surface area. A further cooler 8 may be included in the line 10 either before or after the pump 7, or both.

In a second embodiment fluid flow 10' may come from a fluid flow 9' from a previous implementation of the first embodiment or from any other embodiment or process giving a cooled fluid flow of hydrocarbons containing ice particles (Figure 2 ).

In other embodiments of the invention several recirculations may be used in series or in parallel. In a series set-up (Fig. 3), a chain of implementations of the system (items 1 through 10), line 9 from the first implementation may become line 1 in the next implementation. This may help distribute cooling duty, and increase the reliability and ease of maintenance. In parallel set-ups (Fig. 4), two or more loops 10 with the associated pumps 7, coolers 8 may be placed in parallel to reactor 4 and splitter 6.

The basic and main embodiment of the present invention is the commingling in a reactor 4 of a hydrocarbon stream 1 containing free water, and a cold stream 10' with ice particles, to provide both sufficient cooling and seeding and/or growth sites to allow further water transport as suspended ice particles in an exit pipeline 9. All other system parts and alternatives described in the present embodiments and in the text are optional, and to be used only according to specific system needs, and in any desired combination. Variations are also not restricted to the specifics mentioned herein - persons skilled in this art will readily appreciate that various additional changes and modifications can be made without departing from the spirit of the invention as defined by the claims.

With the water being in ice form, and the particles being inert (no further growth of ice is possible) it has been known experimentally in flow loops with both model systems and with real hydrocarbon fluids and pressures and temperatures, that the resulting particles are easily transportable with the liquid flow.

It is also a great advantage of the present invention that the absence of free water will reduce the risk of corrosion in pipelines and other installations.

The ice particles will not melt back to free the water until the temperatures rise above about 0 °C - which in reality will be at the end of the transport pipe, where the process will not be problematic. The particles can be mechanically separated from the bulk liquid phase by a sieve (unlike dispersant-induced emulsions which are often difficult to break). Another method would be to melt the ice in a separator where the residence time is long enough for the emerging water to separate out from the hydrocarbon liquids. 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 without melting.

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 ice particles will dramatically reduce the need of chemical additives or insulating/heating which are used today.

EXAMPLE

In a specific set of tests of the present invention, crude oil and water (ranging in steps from about 5% to 30% of the liquid volume) were filled into a wheel-shaped pipe flow simulator which measures temperature, pressure and torque. The liquid filling of the wheel was about 40-50%, to allow the liquids to flow as a 'plug' in the bottom of the vertically mounted wheel when it was spinning, thus enabling the flow of the fluids through an 'endless ¹ pipe. The system was then cooled from 65°C and down to -3°C to -6°C, and warm water was injected into the subf reezing fluids.

During the tests, the torque measurements were closely monitored, as these give indications of wall deposition, and of rheological changes due to e.g. increasing particle content in a slurry. The results were unanimous in showing that after the first ice particle slurry was established, further injection of warm water (and subsequent cooling) resulted in no wall deposition of ice, but rather in an increasing amount of ice particles in the slurry, which continued to flow without problems. Even a shut-down at low temperature, lasting for 4 days, failed to result in any ice deposition on the walls of the pipe, and showed the flow quickly returning to pre-shut-in conditions after start-up.