NO328852B1 - Gas Process and System - Google Patents

Abstract

The present invention relates to a natural gas treatment system comprising an LNG vessel (11) having a cargo handling system, LNG tanks and a propulsion system, wherein the LNG vessel receives the natural gas from an offshore plant or a land-based plant, and wherein the system further comprises a LNG processing plant (10) mounted on the LNG vessel (11).

Description

FIELD OF THE INVENTION

This invention relates to a method and system for treating gas. More precisely, the invention relates to a method and system for condensing, storing and transporting LNG in an efficient manner.

BACKGROUND OF THE INVENTION

Production of hydrocarbons consists of, among other things of separation and stabilization of various hydrocarbon components which, together with crude oil, are part of the feed stream from the wellhead. When the feed stream consists of a high percentage of light hydrocarbons, the stock of which is known as natural gas, the field is often classified as an "associated gas field". The natural gas from such fields must be treated in one way or another. The simplest and most common way is to burn the gas using a burner at or near the production facility. However, such burning represents lost energy, possibly lost profits and increased emissions to the atmosphere.

In addition to burning, there are two well-known ways of treating associated gas. The first is to bring the gas back to the source/reservoir, and the second is to bring the gas in a gaseous state to the market through a gas pipe. In cases where the feed stream has a high percentage of natural gas, the sales potential becomes significant if the gas can be sold instead of burned or returned.

The publication US 2008 0202158 shows a system for cooling natural gas using a Brayton cycle with three stages of compression and one stage of expansion.

The publication WO 02/095284 shows an offshore regasification system for condensed natural gas comprising a mobile floating platform with a regasification unit.

The purpose of the invention is to provide an alternative way of treating natural gas, by condensing the gas LNG (liquefied natural gas) and transporting LNG to the market. Furthermore, the purpose of the invention relates to the use of LNG ships for condensing, storing and transporting condensed natural gas from the production facility to the market.

SUMMARY OF THE INVENTION

The present invention relates to a system for processing natural gas, comprising: an LNG ship, the cooling circuit unit comprising a cargo handling system, LNG tanks and a propulsion system, where the LNG ship receives the natural gas from an offshore facility or a land-based facility, where the system further comprises: a LNG process plant cooling circuit unit provided on the LNG ship the cooling circuit unit comprising: a gas expander unit the cooling circuit unit for recovering work from natural gas and thereby providing initial cooling of the natural gas; a heat exchanger unit for cooling and condensing the natural gas; a cooling unit to provide additional cooling and to facilitate condensation and subcooling of the natural gas; an expansion valve for final cooling through reduction of the pressure of the condensed gas before storage in the LNG tanks by means of the cargo handling system; where the cooling unit consists of a closed Brayton cooling circuit unit comprising compressor units and at least one expander unit, where the compressor and expander units are connected to a gearbox to thereby form a single compander unit.

In one aspect of the invention, power recovered by at least one shaft of the gas expander unit is used to drive either an electrical unit or contribute mechanical power to the compander unit.

In one aspect of the invention, the gas expander unit is mechanically connected to the gearbox of the compander unit.

In one aspect of the invention, the pressure of the gas received from the offshore oil production facility is above 7500 kPa.

In one aspect of the invention, the gas pressure after the gas expander unit is approx. 2000 - 300 kPa, normally 1000 kPa.

In one aspect of the invention, the heat exchanger consists of plate-fin heat exchangers.

In one aspect of the invention, the heat exchanger unit is insulated.

In one aspect of the invention, the cooling fluid in the Brayton cooling circuit unit is nitrogen gas.

In one aspect of the invention, the Brayton refrigeration circuit unit comprises compressor units with water-cooled intercoolers and an aftercooler.

In one aspect of the invention, the compander unit has an output shaft for each of the 3 compressor units and the expander unit respectively.

In one aspect of the invention, the compander unit has an input shaft.

In one aspect of the invention, the input shaft is mechanically connected to the shaft of the gas expander unit.

In one aspect of the invention, the input shaft of the compander unit is driven by a steam turbine supplied with steam from the boilers of the ship's propulsion system.

In one aspect of the invention, the compander unit consists of three compressor units and two expander units.

In one aspect of the invention, the compander unit receives cooling water from a cooling water system at the LNG ship.

The present invention also relates to a method for treating natural gas, comprising the following steps: providing an LNG ship, which has a cargo handling system, LNG tanks and a propulsion system, where the LNG ship receives natural gas from an offshore facility or a land-based facility, and where the method comprises the following steps: expanding the gas using a gas expander unit for recovering work from natural gas and thereby providing initial cooling of the natural gas; cooling, condensing and subcooling the natural gas using a heat exchanger unit; further cooling by means of a refrigeration unit for providing additional cooling and for facilitating the condensation and subcooling of natural gas; reducing the gas pressure through an expansion valve; storage of liquefied gas in the LNG ship's tanks using the cargo handling system.

DETAILED DESCRIPTION

Below, embodiments of this invention are described in detail with reference to the attached drawings, where: Fig. 1 is a schematic overview of different ways of handling natural gas from an oil and gas field;

Fig. 2 illustrates the first embodiment of the invention

Fig. 3 shows the details of the gas processing system of the LNG ship in fig. 2.

Fig. 4 shows the second embodiment of the invention; and

Fig. 5 shows the compressor/expander principle.

Reference is made to fig. 1, which shows an overview of different ways of treating gas produced in a process plant. The processing plant 1 can take many different forms, including a ship (called an FPSO), a rig (floating or bottom-placed) or other types of facilities including land-based facilities.

In the embodiments shown in fig. 1 the main product is oil which can be transported to the market either on a ship or through a pipeline. They took off

the hydrocarbon components, also known as natural gas, can be burned, transported to market through a pipeline or reinjected into the well.

An alternative to the above is to treat natural gas using a system and a method for treatment in accordance with the present invention and which is shown with dashed lines and marked A in fig. 1.

Reference is now made to fig. 2. Process plant 1, as in fig. 2 is shown as a floating production rig, is moored to the seabed above a submarine well 2, where a well stream of hydrocarbons, i.e. oil and associated gas, is led through pipes to production facility 1 from well 2 through a riser 3. At the production facility, the feed stream divided into suitable components such as crude oil, condensate, heavier gas components (LPG) and lighter gas components or natural gas. The natural gas must be treated in such a way that it is ready for cooling and condensation. This means that impurities, carbon dioxide and water have been removed or reduced to an acceptable level for direct condensation. In fig. 2, the crude oil is exported using pipelines on the seabed or to a shuttle tanker via a loading buoy.

In fig. 2 is a gas treatment system or an LNG process plant 10 built on the LNG ship 11. The LNG ship 11 is moored close to the oil production plant 1. The LNG ship has a cargo handling system and other auxiliary equipment, such as cooling water system and other auxiliary systems. The LNG process plant 10 receives natural gas from the production plant 1 through a pipeline 12.

Reference is now made to fig. 3, which roughly corresponds to the dashed lines marked A in fig. 1. In this drawing, the LNG process plant 10 is illustrated together with other relevant parts of an LNG ship. The LNG process plant 10 consists of a gas expander unit 20 for initial cooling of the gas by recovering mechanical work or power from it, a heat exchanger unit 30 for cooling, condensing and subcooling the gas and a cooling unit 40 which supplies the heat exchanger unit with a cold stream for further cooling and condensation of the gas.

The gas expander unit 20 is supplied with natural gas from the production plant 1. After the treatment process at the processing plant, the natural gas will usually have a high pressure, normally above 7500 kPa. In the present embodiment, the gas expander unit 20 will consist of two gas expanders 20a and 20b in series, as shown in fig. 3. The gas pressure downstream of the gas expander unit will be between 300 - 2000 kPa, normally 1000 kPa.

The expansion process in the gas expander unit 20 leads to a recovery of energy and gas temperature drop. The recovered energy available on the expander shafts can be used to drive a power generator or be used as drive power for the cooling unit 40, which will be described in more detail below.

The heat exchanger unit 30 consists of a plate-fin heat exchanger where the cooled gas from the gas expander unit is supplied. The heat exchanger unit is insulated to limit the supply of heat from the surroundings. In the embodiment shown, a 4-pass plate-fin heat exchanger unit is used.

The cooling unit 40 in the present embodiment consists of a closed type of cooling circuit such as a closed Brayton cooling circuit, where the cooling medium used is nitrogen gas. In the following description, the cooling unit 40 is also referred to as a Brayton cooling circuit unit. The Brayton refrigeration circuit unit has 3 compressor units; each of these defines a compressor stage with water-cooled intercoolers and aftercoolers. Furthermore, the Brayton refrigeration circuit unit also consists of an expander unit which defines an expansion stage.

The natural gas supplied to the heat exchanger unit 30 is divided into two streams: a main stream that is cooled and condensed to form the LNG product, and a secondary stream that is heated and used as fuel to generate the mechanical energy for the operation of the Brayton refrigeration circuit unit .

The two remaining passages in the heat exchange unit 30 are used to carry the cooling circuit's primary and secondary cooling flows (nitrogen gas).

In the embodiment shown, the three compressor stages and one expander stage in the Brayton cooling circuit unit are assembled into a so-called "compander unit" (compressor and expander unit).

The principle of the compander unit is shown in fig. 5. The compander unit consists of a common gearbox 52 which has output shafts for each of the compressor units and the expander unit and an input shaft 55.

As mentioned above, the compander unit consists of three compressor units and an expander unit for the Brayton cooling circuit unit mechanically connected to the common gearbox. The input shaft of the compander unit is driven by a drive unit 55 which is either driven by a steam turbine with steam from the LNG ship's steam boilers 66, which is part of the LNG ship's propulsion system, or an electric motor driven by electricity produced on board the LNG ship.

Furthermore, the expander units of the gas expander unit 20 can also be physically connected to the gearbox 52, see fig. 5. Note that the expander units of the gas expander unit 20 and the expander units of the cooling unit 40 together with the driver unit 55 supply energy to the common gearbox 52, while the compressor unit of the Brayton cooling circuit unit uses energy from the common gearbox 52. By designing an integrated compander unit, as described above, the unit will be compact, reduce costs and reduce the space required for installation on the LNG ship.

Mechanical limitations and speed limitations can make it difficult for the gas expander unit 20 to be connected to the gearbox 52. In this case, the gas expander unit 20 can be connected to a generator for the production of electrical current. This energy can be used on the ship and thus reduce gas and steam consumption. In this case, the compander unit will consist of the cooling unit 40 and the driver unit 55.1 In this case, the gas expander unit 20 thus becomes a separate unit and the compander unit, consisting of the cooling unit 40 and the driver unit 55, a separate unit.

Note that the Brayton cooling circuit unit is described in more detail in Norwegian patent NO 305525, which is hereby incorporated by reference.

As shown in fig. 3, cold nitrogen produced by the Brayton cooling circuit unit circulates between the heat exchanger unit 30 and the cooling unit 40. As a result of this circulation, the natural gas is further cooled down and condensed and finally subcooled by heat exchange against nitrogen. Downstream of the LNG processing plant 10, an expansion valve 50 is used to reduce the pressure on the condensed gas before it is transported further to the LNG tanks via the LNG ship's cargo handling system. Fig. 3 shows the transport of LNG from the valve 50 to the LNG tank 60 through pipe 62.

Furthermore, the cooling unit 40 is supplied with cooling water from a cooling water system 64 at the LNG ship.

The natural gas from production plant 1 can be used as fuel by the ship's steam boilers 66 for the production of steam that will power the driver unit 55 when the ship is in LNG production mode, as shown in fig. 2. The fuel pipe is marked with 68 in fig. 3. Thus, the ship's steam boiler 66 has generated the necessary mechanical work for the driver unit 55 in the compander unit.

In this way, a method for handling natural gas is indicated, with the following steps: - providing an LNG ship that has a cargo handling system, LNG tanks and a propulsion system, where the ship receives natural gas from a floating production facility or a land-based production facility, and where the procedure further comprises the following steps: - expansion of the gas using a gas expander unit for recovering energy from the natural gas and thereby providing initial cooling of the natural gas; - cooling, condensing and subcooling the natural gas using a heat exchange unit; - additional cooling by means of a cooling unit to provide additional cooling and contribute to the condensation and subcooling of the natural gas; - reduction of the gas pressure using an expansion valve; - storage of condensed gas in the LNG tanks using the ship's cargo handling system.

Recovered energy from the expander unit is utilized for the process plant's total energy balance or the LNG ship's energy balance.

According to the invention, an LNG ship with a process plant 10 on board can be used to handle, that is to say receive, condense and transport natural gas from plant 1. As described above, several of the systems of the LNG ship are reused. Most traditional LNG ships are equipped with a steam-driven propulsion system consisting of 2 gas-fired steam boilers that supply steam to a steam turbine that drives the LNG ship's propeller. When the LNG process plant is in operation, the steam is instead led to the steam turbine drive unit 55 on the compander unit's drive shaft. Alternatively, the drive unit 55 can be an electric motor that receives power from the LNG ship's power supply system. Such an option is possible for LNG ships that have installed an electric propulsion system powered by medium speed gas engines.

When the LNG cargo tanks are full, gas supply through pipeline 12 from the production plant stops. The LNG ship is then disconnected from the mooring and sails to an LNG receiving terminal for unloading LNG. The ship then travels back to the production facility, is moored and connected for another cycle of gas condensing, storage and transport. When the LNG ship is not connected to the pipeline 12, the gas at the production facility can either be burned or reinjected into the well.

A synergy effect has also been achieved by using parts of the seawater cooling system for the propulsion turbine condenser as part of the cooling system for plant 10.

The process plant is installed and integrated into the LNG ship's auxiliary systems. This means that the most important auxiliary systems that are necessary for the operation of the process plant are provided from the existing ship systems. This includes, for example, steam power, electric power, cooling water and existing LNG pipe systems. This system integration can be done without affecting the LNG ship's normal functions such as passage at full speed, terminal access, mooring and unloading of cargo.

Reference is now made to fig. 4. The LNG ship 11 with the LNG process plant 10 shown in fig. 4 corresponds to the LNG ship 11 and the LNG processing plant 10 as described above. Fig. 4 shows the process plant 1 as a land-based gas export plant with a port. As shown, the LNG process plant 10 is connected to plant 1 via pipeline 12 A, for the transport of natural gas from the plant to the ship. Consequently, the present invention can be used for handling any type of natural gas, both associated gas from oil fields, for handling natural gas from gas fields or gas export facilities. When the LNG ship receives gas from a land-based facility, it is possible that the gas pressure is not high, but approximately atmospheric pressure. In this case, the gas expander unit in the process plant can be bypassed or replaced with a compressor.

It should be noted that the cooling unit 40 should be designed based on the technical requirements for which it is to be used. For example, the Brayton refrigeration circuit unit may consist of more or fewer than 3 compressors, more than one expander, etc. Furthermore, one of the intercoolers may be omitted. This will consequently affect the design of the compander unit.

Also, it should be noted that the steam boilers 66 can be replaced with gas engine generators, etc.

Further changes and variations will be apparent to one skilled in the art after reading the above description. The scope of the invention appears from the following claims and their equivalents.

Claims (16)

1. System for processing natural gas, comprising: an LNG ship (11) comprising a cargo handling system, LNG tanks and a propulsion system, where the LNG ship receives the natural gas from an offshore facility or a land-based facility, where the system further comprises: an LNG - process plant (10) provided on the LNG ship (11) comprising: a gas expander unit (20) for recovering work from natural gas and thereby providing initial cooling of the natural gas; a heat exchanger unit (30) for cooling and condensing the natural gas; a cooling unit (40) to provide additional cooling and to facilitate condensation and subcooling of the natural gas; an expansion valve (50) for final cooling through reduction of the pressure of the condensed gas prior to storage in the LNG tanks by means of the cargo handling system; where the cooling unit (40) consists of a closed Brayton cooling circuit unit comprising compressor units and at least one expander unit, where the compressor and expander units are connected to a gearbox to thereby form a single compander unit. 2. System in accordance with claim 1, where power recovered by at least one axle of the gas expander unit is used to drive either an electrical unit or contribute mechanical power to the compander unit. 3. System in accordance with claim 1 or 2, where the gas expander unit (20) is mechanically connected to the gearbox of the compander unit. 4. System in accordance with claim 1, where the pressure of the gas received from the offshore oil production facility is over 7500 kPa. 5. System in accordance with claim 4, where the gas pressure after the gas expander unit is approx. 2000 - 300 kPa, normally 1000 kPa. 6. System in accordance with claim 1, where the heat exchanger consists of plate-fin heat exchangers. 7. System according to claim 1, wherein the heat exchanger unit is insulated. 8. A system according to claim 1, wherein the cooling fluid in the Brayton cooling circuit unit is nitrogen gas. 9. A system according to claim 1, wherein the Brayton refrigeration circuit unit comprises three compressor units with water-cooled intercoolers and an aftercooler. 10. System in accordance with claim 9, where the compander unit has an output shaft for each of the 3 compressor units and the expander unit, respectively. 11. System according to claim 10, wherein the compander unit has an input shaft. 12. System according to claim 11, wherein the input shaft is mechanically connected to the shaft of the gas expander unit. 13. System according to claim 11 or 12, where the input shaft of the compander unit is driven by a steam turbine supplied with steam from the boilers of the ship's propulsion system. 14. System in accordance with claim 1, where the compander unit consists of three compressor units and two expander units. 15. System in accordance with claim 1, where the compander unit receives cooling water from a cooling water system at the LNG ship. 16. Method for processing natural gas, comprising the following steps: providing an LNG ship (11), which has a cargo handling system, LNG tanks and a propulsion system, where the LNG ship receives natural gas from an offshore facility or a land-based facility, and where the method comprising the following steps: expanding the gas by means of a gas expander unit (20) for recovering work from natural gas and thereby providing initial cooling of the natural gas; cooling, condensing and subcooling the natural gas by means of a heat exchanger unit (30); further cooling by means of a cooling unit (40) to provide additional cooling and to facilitate condensation and subcooling of natural gas; reducing the gas pressure through an expansion valve (50); storage of liquefied gas in the LNG ship's tanks using the cargo handling system.