Gas hydrates are clathrate compounds in which gas molecules are physically entrapped or engaged in an expanded lattice of water molecules. Gas hydrates form when light hydrocarbons and other light gases such as N2 and CO2 contact water at suitably elevated pressures and/or low temperatures. The oil and gas industry has for many years sought to inhibit the formation of hydrates at production and transmission installations as solid or concentrated hydrate may block flow channels impeding oil and gas production and transmission.

Furthermore, the production of associated natural gas at an oil production facility where there is no convenient or cost effective method of transporting the gas to a consumer has long been considered a nuisance. Such associated gas is conventionally disposed of by flaring which is environmentally unfavourable and a waste of valuable natural resources.

However, as a unit volume of solid natural gas hydrate may contain 150 or more unit volumes of natural gas, the conversion of natural gas into hydrate is now being considered as a safe, convenient and cost effective way of transporting or storing fuel gas. However, to maintain the hydrate in a stable form during transport or storage to prevent dissociation of the fuel gas, the hydrate must be kept at a suitably low temperature and/or high pressure which can be expensive.

An example of a method of transporting hydrate is disclosed in US 3514274 in which natural gas is contacted with solid particles of propane hydrate under appropriate conditions to form crystals of natural gas hydrate in liquid propane. The natural gas hydrate and liquid propane are cooled to about -40°C and kept at that temperature during storage and transport to keep the hydrate stable. However, to cool the hydrate to such a temperature and keep it at that temperature requires a considerable amount of expensive equipment and involves high running costs.

In US 3975167, to maintain the transported hydrate in stable form, it is loaded into and transported in a vessel submerged at a suitable depth in the sea such that the sea provides the required elevated pressure and reduced temperature to maintain the hydrate in stable form. However, submersible vessels that present the surrounding pressure and temperature conditions to their cargo are expensive, will have a relatively small loading capacity and will be limited in their use to areas where water of a suitable depth is available.

According to a first aspect of the present invention there is provided a method of transporting hydrate comprising maintaining the hydrate at an elevated pressure between substantially 2 and 20 bar.

According to a second aspect of the present invention there is provided a method of storing hydrate comprising maintaining the hydrate at an elevated pressure between substantially 2 and 20 bar.

Containers able to support a considerable quantity of hydrate such as 150 m3 to 500 m3 or greater at a pressure up to about 20 bar are inexpensive and provide a convenient means to store or transport hydrate in stable form.

The hydrate is preferably in the form of a slurry of hydrate particles in liquid. A slurry is far more convenient to pump in and out of pressure containers in which the slurry is stored or transported than solid hydrate. Having the hydrate in the form of a slurry is particularly convenient when the hydrate is produced offshore on a platform for example where space is at a premium as being able to pump hydrate through pipes requires less space than conveyor belts, pneumatic conveyors etc. which are required to move solids. The slurry may be between substantially 40% and 75% liquid by volume but is preferably between 45% and 65% liquid. Increasing the liquid content increases the flowability of the slurry but reduces the proportion of hydrate in the slurry and vice versa.

The hydrate is preferably maintained above the temperature at which the liquid in the slurry solidifies or freezes, which for water is 0°C, so that the slurry is maintained in a pumpable form.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 diagrammatically shows an apparatus for preparing a hydrate slurry; Figure 2 shows a hydrate equilibrium curve; Figure 3 diagrammatically shows a container for storing and transporting hydrate; Figure 4 shows a plurality of containers loaded on to a ship; and Figure 5 shows an apparatus to store hydrate at an offshore oil and gas production facility.

Figure 1 shows a hydrate forming plant 10 which is preferably as shown in our earlier patent application No. WO 97/26494 comprising one or more pressure vessels. The plant 10 receives a supply of water 11 and a supply of hydrate forming gas 12. The plant 10 may be onshore or may be incorporated on any offshore production facility. The plant 10 could be on a fixed or floating production facility or adjacent to the production facility. The hydrate forming gas supplied may be associated natural gas.

Hydrate slurry leaving plant 10 is generally at a temperature between substantially 2°C and 10°C and a pressure between substantially 20 bar and 200 bar, more usually between 45 bar and 100 bar. The proportion of water in the hydrate/water slurry leaving the plant 10 will depend upon the particular type of plant used but could vary from about 50% to substantially 99%. If a plant as shown in our earlier patent application No. WO 97/26494 is used, the proportion of water in the produced hydrate slurry is likely to be greater than 90%. In these circumstances it is preferable to perform some separation of water from the slurry to increase the hydrate content of the remaining slurry. Increasing the concentration of the hydrate in the slurry increases the amount of valuable fuel gas transported for the same volume of hydrate slurry.

The hydrate slurry from plant 10 may be passed to a separator 20 through conduit 21. The separator 20 may take any suitable form. It may for example be a hydrocyclone, a screen or a device for allowing the slurry to settle in a tank such that the less dense hydrate floats on top of the water and an arm or scraper is used to physically remove hydrate floating on the top of the water. The water removed from the slurry by the separator may be recycled by being delivered to the plant 10 to be combined with water delivered through supply 11. This recycled water may contain traces of hydrate which will enhance hydrate production in the plant 10.

Hydrate slurry is delivered to one or more hydrate storage or transportation tanks 30 through conduit 31 from plant 10 or more preferably from separator 20.

Figure 2 is an equilibrium curve for the temperature and pressure conditions at which natural gas hydrate is stable.

The hydrate is stable when at temperatures and pressures above the curve. The curve is slightly different for hydrates of other gases such as methane or if additives such as salt are included in the slurry. As can be seen from Figure 2, natural gas hydrate must be stored at substantially 7 bar or greater to remain stable whilst at a temperature above the freezing point of the water (273k) in the slurry.

Additives such as salt may be added to the slurry to lower the temperature at which the water in the slurry freezes.

By adding salt, the hydrate slurry may be stored at a temperature less than 273k. Since the hydrate slurry generally leaves the plant 10 at a higher pressure than that at which it is stored in containers 30, no pressure increasing means are generally required, even if some pressure is lost in the separator. However, since the temperature at which the hydrate slurry leaves the plant 10 may be higher than that required in the containers 30 a chiller (not shown) as is well known in the art may be used.

The chiller is preferably used to cool the hydrate slurry leaving the plant 10. If a separator 20 is used, the chiller is preferably positioned between the plant 10 and the separator 20 since slurry with a higher proportion of water is easier to cool.

The hydrate slurry is preferably stored or transported at a temperature just above 0°C, when the liquid in the slurry is water so that it does not freeze and at a pressure less than 20 bar, more preferably less than 10 bar, such as between 2 and 7 bar.

Figure 3 shows a container 30 for storing or transporting hydrate slurry. The container 30 preferably has a cylindrical structure with substantially semi-spherical or dome shaped end portions. In this example the container is made from approximately 2-3 cm thick carbon steel portions welded together to be able to withstand the elevated pressures up to 20 bar or more conveniently 10 bar. However, the container could be made from any suitable material such as a plastics material, composites, other metals such as aluminium or alloys such as stainless steel. The container could be internally coated or lined to prevent degradation through corrosion or other chemical attack. The container 30 is provided with a hydrate slurry input port 34, a hydrate slurry exit port 35 and a pressure relief valve 36 to vent the pressure within the container if it exceeds a predetermined value which is considered safe for that particular container. In this example, the container is provided with a chiller 37 which pumps a cooling fluid, in this case a mixture of glycol and water, around the container 30 to keep the hydrate slurry within the container cool. The cooling fluid is pumped through a conduit 38 which is in close thermal contact with the container 30, in this case by being wrapped around the container in coils 39. The container 30 is also provided with a layer of insulation 40.

The size and number of containers 30 depends upon the quantity of hydrate slurry to be stored and/or transported.

3m diameter containers in lengths of between 10m and 20m are very cost effective as they can be pre-fabricated and supplied directly to a storage location or to a suitable vehicle or vessel. However, larger or smaller containers may also be used.

In a preferred application hydrate slurry produced in a plant 10 and passed through separator 20 is stored in one or more containers 30 which may be located onshore or offshore on or adjacent to a platform for example. When desired the hydrate slurry is pumped from exit port (s) 35 of storage container (s) 30 to the input port (s) 34 of one or more similar containers 30 on a vehicle or a ship 50 as shown in Figure 4. The ship 50 shown in Figure 4 has a plurality of containers 30 laid end to end and stacked in the ship's hold.

Manifolds (not shown) are provided to deliver hydrate slurry to the input ports 34 and withdraw hydrate from the exit ports 35. The hydrate slurry is then transported to its destination where it is pumped out of the exit port (s) 35 of the transporting container (s) 30 to the inlet port (s) 34 of one or more similar containers 30 to store the delivered hydrate slurry at its destination.

Alternatively, one or more containers 30 may only be provided on a vehicle or a ship and the vehicle or ship kept at the hydrate production plant 10 until a desired amount of hydrate slurry has been received in the container (s) 30. The vehicle or ship may then be transported to its destination and kept there whilst the hydrate slurry is gradually used up.

The present invention could be used to provide storage of fuel gas to accommodate variations in demand, for example. Hydrate slurry produced in plant 10 of Figure 1 is pumped to one or more hydrate storage containers 30 through conduit 31.

The hydrate in the slurry in container 30 is maintained in a stable condition by being maintained at an elevated pressure as in the earlier example. A separator 20 is preferably provided to separate some of the liquid from the hydrate slurry produced in the plant 10 before it is delivered to the storage container 30. Any liquid separated from the hydrate slurry by the separator may be recycled to the liquid input 11 of the plant 10. This recycled liquid may contain traces of hydrate which will enhance hydrate production in the plant 10. Additionally or alternatively a chiller may be used to cool the hydrate slurry produced in the plant 10 before it is delivered to container 30.

When demand for fuel gas is low, excess fuel gas may be delivered to input 12 of plant 10 and converted into hydrate slurry which is stored in container 30. When demand for fuel gas increases the hydrate slurry stored in container 30 may be converted back into fuel gas by being heated or by releasing the pressure in the container 30. Fuel gas may then be delivered to the source of demand.

This method is particularly suitable for use to accommodate diurnal variations in fuel gas demand.

The present invention may also be used to store fuel gas at or adjacent to an onshore or offshore oil and gas production facility. Figure 5 shows an offshore facility. The production facility illustrated in Figure 5 is an offshore platform 60 but this example is equally applicable to a floating facility. Produced oil and gas is delivered through a pipeline 61 to a tanker or an onshore receiving station by one or more pumps 62. The pumps 62 are generally powered by gas produced at the production facility.

However, during the life of a producing oil and gas well, the majority of the fuel gas is produced in the early life of the well, leaving little to power the pumps 62 in the latter life of the well. However, this problem may be overcome by storing excess fuel gas produced in the early life of the well in storage containers 30 as a hydrate slurry and converting the stored hydrate slurry back into fuel gas during the latter part of the life of the well to power the pumps 62. The production facility of Figure 6 has a hydrate forming plant 10 which pumps produced hydrate slurry to one or more containers 30 through conduit 41. If desired the hydrate slurry produced in plant 10 may be separated and/or chilled before being delivered to storage containers 30. In the example of Figure 5 the storage containers 30 are mounted below the surface of the sea so that the relatively high pressures (about 10 bar at a depth of about 90m) and low temperatures (typically 2-3°C) assist in maintaining the hydrate in the slurry in a stable condition. When fuel gas is required, the hydrate slurry in containers 30 is converted back into fuel gas by for example releasing the pressure in tanks 30 or pouring warm water into tanks 30. The fuel gas is delivered back to the platform 60 through conduit 63 to power pumps 62.

Many modifications may be made to the examples described above without departing from the scope of the invention.

For example the hydrate may be provided in a slurry in which the liquid component is essentially provided by oil or liquid hydrocarbons rather than water. Since oil and liquid hydrocarbons solidify at a lower temperature than that at which ice freezes, the slurry of hydrate in oil or liquid hydrocarbons in containers 30 may be maintained at a lower temperature and a consequently lower pressure determined by the hydrate equilibrium curve whilst still maintaining the slurry in a pumpable form.

The slurry of hydrate particles in oil or liquid hydrocarbons may be produced by supplying the hydrate forming plant 10 with a mixture of water, fuel gas and oil and/or liquid hydrocarbons. The water and fuel gas are consumed in the plant 10 to produce hydrate which leaves the plant 10 in the oil or liquid hydrocarbons as a slurry.