Use of hydrate slurry for transport of associated gas

Abstract

A system and method for the transport of the natural gas associated with the production of crude oil from offshore wells includes transforming the associated gas into a solid gas hydrate. The solid gas hydrate is mixed with liquid crude oil to form a pumpable slurry. The conversion of the gas into a solid gas hydrate and the mixing of the pumpable slurry may take place on a floating offshore storage vessel or on a shuttle tanker. The pumpable slurry is transported on a shuttle tanker to an onshore location where the solid gas hydrate may be extracted from the oil and the associated gas transformed from a solid gas hydrate back into a gaseous state.

Description

FIELD

This invention relates to a system and method for transporting associated natural gas produced at an offshore location to an onshore location; more particularly, this invention relates to the transport of associated natural gas from an offshore production or storage facility to an onshore storage facility or a refinery using shuttle tankers.

BACKGROUND

In locations such as the Gulf of Mexico, liquid and gaseous hydrocarbons produced at offshore locations are transported from offshore locations to onshore facilities using pipelines laid along the ocean floor. However, the cost of laying a pipeline on the ocean floor to remote deep water locations can become prohibitively expensive. Accordingly, the operators of offshore production platforms located in remote deep water locations must use other means of transport to move liquid and gaseous hydrocarbons to an onshore location for storage or further processing. One such system involves the use of shuttle tankers. Shuttle tankers have been used in the North Sea for many years to transport hydrocarbons produced from offshore wells to onshore locations.

The transportation of gaseous hydrocarbons is particularly problematic. A variety of techniques have been tried from transporting gaseous hydrocarbons in a highly compressed state to transporting the gaseous hydrocarbons in liquid form. Both such techniques require special equipment to either compress or liquify the natural gas. Once compressed or liquified, special containment facilities are needed to maintain the natural gas in a compressed or liquified state. The containment facilities must be made of specialty steels and other expensive materials to withstand the extremely cold temperatures of which compressed or liquified natural gas must be transported. Accordingly, the cost of the equipment to transport natural gas from offshore production platforms to onshore locations is so high that many operators of offshore production platforms simply prefer to flare or burn off the natural gas associated with the production of crude oil, if allowed to do so by host governments.

Accordingly, a need remains in the art for a system and method for inexpensive transport of natural gas from offshore production platforms to onshore locations without the need for special equipment dedicated solely to the transport of natural gas.

SUMMARY

The system and method of the present invention provides for the inexpensive transport of natural gas from offshore production platforms to onshore locations without the need for special equipment dedicated solely to the transport of natural gas.

The system and method for the transport of associated gas from an offshore production platform to an onshore facility of the present invention includes a plant for transforming the associated gas into solid gas hydrates. This solid gas hydrates are then mixed with liquid crude oil to create a pumpable slurry. The pumpable slurry is transported to an onshore location using a shuttle tanker. At the onshore location, the solid gas hydrates are separated from the liquid crude oil and transformed back into a gaseous state for storage or further processing.

The plant for transforming the associated gas into solid gas hydrates may be located either on a floating offshore storage vessel or on a shuttle tanker calling on the onshore location. If the plant for transforming associated gas into solid gas hydrates is located on the floating offshore vessel, the solid gas hydrates are mixed with liquid oil on the floating offshore storage vessel to form a pumpable slurry. The pumpable slurry is then transferred to a shuttle tanker. Once the shuttle tanker reaches the onshore location, the pumpable slurry is offloaded from the shuttle tanker. At the onshore location, the solid gas hydrate and the liquid crude oil are separated so that the solid gas hydrate can be transformed back to a gaseous state.

If the plant for transforming an associated gas into solid gas hydrates is located on the shuttle tanker, the solid gas hydrates are mixed with liquid crude on the shuttle tanker to form a pumpable slurry.

To assure that the solid gas hydrate remains in a solid form while in the pumpable slurry, it is necessary to cool the tank in which the pumpable slurry is stored, whether the tank is located onboard the floating offshore storage vessel or onboard the shuttle tanker.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A better understanding of the system and method of the present invention may be had by reference to the drawing figures wherein:

FIG. 1 is a flow chart of the system and method of the present invention wherein the associated gas is transformed into a solid gas hydrate onboard a floating storage vessel;

FIG. 2 is a flow chart of the system and method of the present invention wherein the associated gas is transformed into a solid gas hydrate onboard a shuttle tanker.

DESCRIPTION OF THE EMBODIMENTS

The system and method of the present invention 10, 110 are best understood by reference to FIG. 1 and FIG. 2 against the background of the need to transport the gaseous hydrocarbons associated with the liquid crude oil produced by an offshore platform from a subsea hydrocarbon reservoir.

The offshore platform is typically located at a location which is in relatively close proximity to the subsea reservoir from which crude oil and associated gas are extracted. The liquid crude oil and the associated gas are usually separated one from another at the offshore platform 12 and then further transferred from the platform to a floating storage vessel (FSO) 14 by a variety of different means. Such transfer means include a collection of conduits, pumps, valves, and connectors well known to those of ordinary skill in the art.

Onboard the floating storage vessel, the natural gas may be temporarily stored 16 in a compressed state.

Because the floating storage vessel remains in a relatively stationary location offshore, the liquid crude oil and the associated gas must be transported onshore to a storage facility or to a refinery. According to the disclosed system, this transfer is accomplished by moving the crude oil and the gas from the floating offshore storage vessel to a shuttle tanker 18. The shuttle tanker carries both the liquid and gas hydrocarbons to an onshore location.

More specifically, the term offshore production platform is used above to describe the starting point for the oil and gas removed from a subsea reservoir. Those of ordinary skill in the art will realize that the term offshore product platform encompasses a variety of different structures—floating, fixed, or some combination thereof. The offshore production platform is equipped to separate the liquid hydrocarbons, generally called crude oil, from the gaseous hydrocarbons, generally called natural gas.

Since most offshore production platforms do not include storage for either crude oil or natural gas, the crude oil and natural gas must be transferred into a system which conveys it away from the offshore production platform. One such system is a floating storage vessel. The floating storage vessel receives the crude oil from the offshore production platform. This oil is often called “dead crude oil.” The floating storage vessel also receives the associated natural gas separately 14.

According to the present invention, the associated natural gas is converted into a solid gas hydrate 20 (a form of snow) by a plant on the floating storage vessel. The plant employs a process of controlled reduction in temperature, controlled pressure, and controlled introduction of water. Such plants are well known to those of ordinary skill in the art. Once the natural gas is converted into a solid gas hydrate, it is mixed 22 with the liquid dead crude oil to form a pumpable slurry. It has been found that lighter weight dead crude oil is best used for creation of the pumpable slurry. Such lighter weight dead crude oil is typically found near the top of oil storage tanks.

Those with experience in creating a pumpable slurry of solid gas hydrate and dead crude oil have found that a gas-to-oil ratio in the range of about 200 scf/bbl to about 600 scf/bbl is acceptable.

As shown in FIG. 1, following the creation of the pumpable slurry onboard the floating storage vessel, it may then be transferred to a shuttle tanker 18 using a conventional conduit system. It has been found that if the tanks containing the pumpable slurry are kept cool; that is, in a temperature range of about 0° F. to about −10° F., the gas hydrates will remain in their solid form. In the temperature range of 0° F. to about −10° F., no special cryogenic materials are required. Specifically, shuttle tankers adapted for transport of the pumpable slurry are made from the same materials as other ships that might be used in cold weather climates, such as near the Arctic Circle.

The shuttle tankers transport the pumpable slurry to an onshore location. At the onshore location, the pumpable slurry is offloaded from the shuttle tanker 24. It is at the onshore location that the solid gas hydrate may be separated from the pumpable slurry and then further transformed back into natural gas 24.

In an alternate embodiment, and as shown in FIG. 2, the associated natural gas is not transformed into a solid gas hydrate onboard the floating offshore storage vessel but, instead, is transformed into a solid gas hydrate 120 and mixed into a pumpable slurry 122 onboard the shuttle tanker. In this case, the associated natural gas is received on the floating offshore storage vessel in its gaseous state and transferred to the shuttle tanker in its gaseous state where it may be temporarily stored 116. Alternatively, the associated natural as may be transferred directly to the shuttle tanker from the floating offshore storage vessel without any temporary storage.

It is well known that natural gas can be transformed into a fuel which can be used as an energy source for engines or power equipment. Such engines or power equipment transform the energy contained in the natural gas into mechanical or electrical power. According to the instant invention, a portion of the natural gas may be diverted from flow to the solid gas hydrate plant to be transformed into mechanical or electrical power for the floating storage vessel or the shuttle tanker 90.

The process for making a pumpable slurry of a solid gas hydrate and liquid crude oil is known in the art. Specifically, produced liquid oil and associated natural gas is conveyed to a separation module where the liquid oil is separated from the natural gas. The first stage of such separation modules operates at pressures above 750 psi.

The liquid crude oil, which is typically in an emulsion with water, is processed according to the following steps:

• • 1. The crude oil-water emulsion is treated with either chemical de-emulsifiers or electrostatic separation techniques.
  • 2. The water and the crude oil are separated one from another.

The water from the crude oil water emulsion is transferred to a slop water system. If needed, the water may also be treated to obtain a desired chemical profile. In some instances, the cleaned up water can be returned to the sea.

Fresh water is typically used with associated natural gas to form a hydrate slurry, as the salt in sea water inhibits the formation of solid hydrates.

The gas from the separation module enters an LPG separation process to remove any rich LPG constituents and any condensates. This step chills the associated natural gas to about −50° F. and separates any condensed liquids from the methane, butane, and propane components of the associated natural gas.

The remaining associated natural gas is conveyed to the plant which makes the solid gas hydrate. The gas is pumped into plant along with cold fresh water. The water is chilled to about 32° F. The combination of the water with the gas forms a snow-like substance. Excess water is removed from the plant.

The solid gas hydrate snow is combined with liquid crude oil to form a pumpable slurry. This pumpable slurry is maintained at a temperature from about 0° F. to about −10° F. at atmospheric pressure. Accordingly, if the pumpable slurry is to be maintained on a floating offshore storage vessel, the storage tanks must be made of materials suited to such temperatures, and a cooling system is required to maintain the pumpable slurry in a temperature range from about 0° F. to about −10° F. Similarly, if the pumpable slurry is to be maintained on a shuttle tanker, the storage tanks must be made of materials suited to such temperatures, and a cooling system is required to maintain the pumpable slurry in a temperature range from about 0° F. to about −10° F.

The advantage of the disclosed system and method is that a solid gas hydrate can be mixed with liquid crude oil to form a pumpable slurry that is easily and conveniently transported from an offshore production platform to an onshore location. The addition of a solid gas hydrate to liquid crude oil only slightly increases the volume of material transported. Only minimal changes are needed to convert the vessels used to transport the pumpable slurry of a solid gas hydrate and crude oil. No special natural gas transport equipment is required. If there is no natural gas to be transported, the shuttle tanker is simply used to transport liquid crude oil or other alternate uses falling within the capabilities of the shuttle tanker.

While the present invention has been disclosed according to its preferred embodiment, those of ordinary skill in the art will understand that other embodiments have also been enabled. Such other embodiments will be well understood by those of ordinary skill in the art.

Claims

1. A method for transporting the associated hydrocarbon gas produced at offshore production platforms to an onshore location, said method comprising the steps of: transferring the associated gas to a floating storage vessel; transforming the associated gas into a solid gas hydrate onboard said floating storage vessel; forming a pumpable slurry by mixing said solid gas hydrate with liquid oil onboard said floating storage vessel; temporarily storing said pumpable slurry on said floating storage vessel; transferring said pumpable slurry from said floating storage vessel to shuttle tankers; transporting said pumpable slurry to the onshore location.

2. The method as defined in claim 1 further including the step of holding said pumpable slurry in cooled tanks while aboard said shuttle tanker.

3. The method as defined in claim 1 wherein a portion of said associated gas is used as an energy source onboard said floating storage vessel before being transformed into a solid gas hydrate.

4. The method as defined in claim 1 wherein said pumpable slurry is offloaded from said shuttle tanker at the onshore location.

5. The method as defined in claim 1 wherein said solid gas hydrate is separated from said pumpable slurry at the onshore location and then transformed into gaseous hydrocarbons.

6. A method for transporting the associated hydrocarbon gas produced at offshore production platforms to an onshore location, said method comprising the steps of: transferring the associated gas to a floating storage vessel; transferring the associated gas from said floating storage vessel to a shuttle tanker; transforming the associated gas into a solid gas hydrate onboard said shuttle tanker; forming a pumpable slurry by mixing said solid gas hydrate with liquid oil onboard said shuttle tanker; transporting said pumpable slurry to the onshore location.

7. The method as defined in claim 1 wherein said pumpable slurry is held in cooled tanks while onboard said shuttle tanker.

8. The method as defined in claim 1 wherein a portion of said associated gas is used as an energy source onboard said shuttle tanker before being transformed into a solid gas hydrate.

9. The method as defined in claim 1 wherein said pumpable slurry is offloaded from said shuttle tanker at the onshore location.

10. The method as defined in claim 1 wherein said solid gas hydrate is separated from said pumpable slurry at the onshore location and then transformed into gaseous hydrocarbons.

11. A system for transporting associated hydrocarbon gas from an offshore production platform to an onshore location, said system comprising: a conduit for transferring gas from the offshore production platform to a floating storage vessel; a plant for transforming the associated gas into a solid gas hydrate onboard said floating storage vessel; a device for mixing said solid gas hydrate with liquid crude oil contained on said floating storage vessel to form a pumpable slurry; a cooled tank on said floating storage vessel for containing said pumpable slurry; a conduit for transferring said pumpable slurry from said floating storage vessel to a shuttle tanker calling on the onshore location.

12. The system as defined in claim 11 further including a cooled tank onboard said shuttle tanker for containing said pumpable slurry.

13. The system as defined in claim 11 further including a conduit for diverting a portion of the associated gas away from said plant and into an energy transformation system onboard said floating storage vessel.

14. The system as defined in claim 11 further including a conduit to unload said pumpable slurry at the onshore location.

15. The system as defined in claim 11 further including a plant to separate the solid gas hydrate from the pumpable slurry at the onshore location.

16. A system for transporting associated hydrocarbon gas from an offshore production platform to an onshore location, said system comprising: a conduit for transferring the associated gas from the offshore production platform to a floating storage vessel; a conduit for transferring the associated gas from said offshore production vessel to a shuttle tanker; a plant for transforming the associated gas into a solid gas hydrate onboard said shuttle tanker; a device for mixing said solid gas hydrate with liquid crude oil contained on said shuttle tanker to form a pumpable slurry; a cooled tank on said shuttle tanker for containing said pumpable slurry; a conduit for transferring said pumpable slurry from said shuttle tanker at the onshore location.

17. The system as defined in claim 16 further including a cooled tank onboard said floating storage vessel.

18. The system as defined in claim 16 further including a conduit for diverting a portion of the associated gas away from said plant and into an energy transformation system on said shuttle tanker.

19. The system as defined in claim 16 further including a plant to separate the solid gas hydrate from the pumpable slurry at the onshore location.