WO2015110443A2

WO2015110443A2 - Coastal liquefaction

Info

Publication number: WO2015110443A2
Application number: PCT/EP2015/051058
Authority: WO
Inventors: Tor Christensen; Pål Leo ECKBO
Original assignee: GLOBAL LNG SERVICES Ltd
Current assignee: GLOBAL LNG SERVICES Ltd
Priority date: 2014-01-22
Filing date: 2015-01-21
Publication date: 2015-07-30
Anticipated expiration: 2016-07-22
Also published as: WO2015110443A3

Abstract

A method for production of liquefied natural gas (LNG) from a natural gas source, the method comprising introduction of the natural gas into a gas terminal where the gas is pre-treated, where the pre-treated natural gas is led via subsea pipelines to floating liquid natural gas production units (FLNG) offshore, each comprising one or more LNG production unit(s) where the natural gas is liquefied to give LNG and where the LNG is transferred to LNG tankers for transport to markets, wherein the pre-treatment of the gas before transfer to the FLNG comprises separation of liquefied Petroleum Gas (LPG) from the natural gas to give a LNG ready natural gas comprising > 85 % by volume methane, and < 100 ppm C5+ hydrocarbons, where LNG ready natural gas is compressed to a pressure of 140 bar or more in the gas terminal, where the gas is transported in subsea pipelines to the FLNG at this high pressure, and where the pressure of the natural gas is reduced to a working pressure for LNG liquefaction onboard the FLNG, before the LNG ready natural gas is introduced into the LNG production unit(s) for liquefaction, and where the LNG ready natural gas is cooled in a heat exchanger against a heat medium, is described. A system for LNG production using the present method is also described.

Description

COASTAL LIQUEFACTION

Technical Field

[0001] The present invention relates to improvements in methods and plants for liquefaction of natural gas to provide Liquefied Natural Gas (LNG) with improved economics and a reduction of the environmental impact including the elimination of the water intake of today's floating liquefaction plants. More specifically, the present invention relates to a method and plant for LNG production environmentally suited to locations offshore, or for locations near coastlines.

Background Art

[0002] Natural gas is becoming more important as the world's energy demand increases as well as its concerns about air and water emissions' increase. Natural gas is readily available, in particular with the new technologies to utilize shale gas. It is much cleaner-burning than oil and coal, and does not have the hazard or waste deposition problems associated with nuclear power. The emission of greenhouse gases is lower than for oil, and only about one third of such emissions from coal.

[0003] There is substantial international trade in natural gas. However, its price differs significantly in different parts of the world. A large fraction of this trade is in the form of liquefied natural gas (LNG). LNG is produced using two major processing steps. The first step is gas pre-treatment to remove components that can solidify when cooled to cryogenic temperatures, mainly sour components and water. Trace elements, mainly mercury which can form amalgams - in particular with aluminum process components - are also removed from the gas. Heavy hydrocarbon fractions or Natural Gas Liquids (NGL) may be removed from the gas in the first or second of the two LNG processing steps. The second processing step is mainly liquefaction of the purified gas, which then comprises mainly methane. This methane, with small amounts of heavier components, is liquid at atmospheric pressure and about - 163°C. The LNG is shipped to the destination and re-gasified. [0004] Processing of natural gas to produce LNG has traditionally been done in large land based facilities which include the two steps of pre-treatment and liquefaction in the same location. Recent developments in technology and markets have enabled construction of LNG plants on floating structures, a development that has inspired movement of a substantial portion of LNG processing facilities offshore to Floating Liquefied Natural Gas facilities (FLNG) to exploit large offshore gas reservoirs. The FLNGs are typically designed to be located at a distance from a coast and are connected to natural gas reservoirs through sub-sea systems. The FLNGs typically are also designed to serve as buffer storage and as terminals for loading of LNG tankers that are used for transport of the LNG to the markets.

[0005] The recent development towards FLNGs has made offshore natural gas resources more available to the market relative to piping the gas to shore for liquefaction, and has resulted in a reduction of capital cost for establishing a LNG plant. Other key drivers include reduction of onshore environmental impacts; reduction of land use issues for equipment and infrastructure; and reduced likelihood of opposition from local

communities. The entire FLNG plant can be built in a shipyard, which is efficient and improves quality control, cost control and reduces

construction time. FLNG's are also mobile and can be transferred to alternative locations if required.

[0006] Numerous studies of FLNG technologies have been carried out over the last couple of decades. Currently, several projects are underway worldwide. Actual construction has started for three units only, the Shell Prelude project, the Exmar /Pacific Rubicales barge project, and the Petronas FLNG 1 project.

[0007] In these and other projects, both gas pre-processing and liquefaction will typically be located on the deck of the FLNG. Space below deck is used for LNG storage and marine-specific equipment. The area available on the FLNG deck is generally only about 20% of the area used for similar facilities onshore. This reduced process lay-out space presents safety issues, including proximity to living quarters and limited space for safety barriers. Significantly, it also limits the size of the processing plants and the possibilities to utilize economies of scale.

[0008] In addition to safety issues, the liquefaction process involves

environmental issues. The liquefaction process generates large amounts of heat which must be transferred to the environment. Large amounts of sea water are needed for cooling purposes onboard the FLNG, water that is subsequently being discharged at a higher temperature. The

mechanical stress in sea water pipes, pumps and fittings and the increased water temperature are harmful to marine life. Additionally, the use of toxic chemicals to prevent fouling leading the decreased cooling efficiency and finally clogging, is detrimental to marine life and will probably be prohibited in many coastal waters, such as in the state of Louisiana in the near future. Submerged cooling coils are not preferred as the performance of cooling coils is difficult, if even possible to predict, due to varying operating conditions as a result of variation in current, seawater temperature and fouling .

[0009] US6094937, corresponding to NO301792, to Norske Stats Oljeselskap, now Statoil ASA, relates to a method for liquefaction /conditioning of a compressed gas / condensate flow extracted from a petroleum deposit, for transport in liquid form as liquefied natural gas (LNG) and liquefied petroleum gas (LPG) in a transport vessel. A flow of natural gas and condensate compressed to a pressure of 20 to 500 bar after being pretreated by CO2 removal and drying, is transported through a subsea pipeline from a production platform or vessel, to a transport vessel where the gas is expanded, separated, and liquefied in two fractions to give LNG and LPG. The method according to US '937 uses a hydrocarbon refrigerant, which is highly flammable in case of an accident. LNG extraction onboard the liquefaction vessel adds to the inventory onboard the vessel of highly flammable components in addition to occupying valuable space onboard. Additionally, even though the gas is transferred to the liquefaction vessel at a high pressure, the operating pressure of the liquefaction process is decided by the presence of the NGL extraction process, and not the liquefaction process. Even though it is stated that the pressure after expansion of the incoming gas to the liquefaction unit may be as high as 70 bar, the refrigerant envelope, see fig. 3 attached hereto, clearly indicates that this is a pressure that is too high for an efficient liquefaction process using the method described therein. Efficient precipitation of liquids would be most efficient near the widest part of the envelope. Accordingly, efficient LPG separation which as illustrated in figure 3 requires pressures well below 70 bar, a pressure which is lower than optimum liquefaction pressure for LNG production.

[0010] A novel adaptation of FLNG is the Coastal Liquefaction, Storage and

Offloading (CLSO) facility. The CLSO adaptation addresses FLNG safety, environmental impact and processing capacity issues. The first processing step, gas pre-processing, is mainly performed on shore, on separate terminals or on dedicated floating systems, instead of occupying valuable space on the FLNG. Pre-processed gas is then piped to one or more floating CLSO's, which now have much more deck space available. Extra deck space on the CLSO, freed up by removing pre-processing, can be used for additional safety features. Furthermore, possibilities exist for greater liquefaction capacity which will confer additional economic advantages. The extra deck space also opens the possibility of using air cooling instead of seawater cooling, solving the seawater intake and associated environmental issue.

[001 1] Air coolers are less efficient and require much larger space compared to seawater cooling. This presents a design challenge even with the extra deck space available on a CLSO. Furthermore, air cooled heat

exchangers typically require air temperatures 10 to 15°C below the temperature of the fluid to be cooled. LNG refrigerants might need to be at 30 to 40°C after cooling, whereas the design air temperature in temperate areas might be up to 40°C. In normal situations, it would be desirable or required to cool / condense the refrigerants to about 30 to 40°C before the refrigerants are routed to the LNG exchangers. However, in temperate areas, design ambient air temperature may be relatively high such as 32°C (90°F) or higher, and it is anticipated that the approach temperature for the air cooled heat exchangers should be at least 10°C, preferably 15°C or more. In addition, the deck space on a FLNG is severely limited. Therefore, the footprint of the air cooled exchangers must be minimized.

[0012] This problem can be solved by operating compressor inter-stage coolers at higher temperatures, and compressing the refrigerants, especially any refrigerant which shall condense, to higher pressure than normal. All cooling and condensation therefore takes place at higher temperatures, enabling efficient air cooling and higher Logarithmic Mean Temperature Difference (LMTD) and higher air cooler approach temperatures.

However, all of this significantly reduces the liquefaction efficiency, increases the energy demand and therefore increases the cooling duty, which partly defeats the intention of accomplishing air cooling in the first place, as indicated in Table 1.

[0013] The extra footprint required by air cooling may be minimized if the

temperature difference between the process fluid and the ambient air is large. This is illustrated in Figure 1 , which shows typical air cooled exchanger plot area (footprint) for 100 MW cooling of hydrocarbons, with an ambient temperature of 40°C. For the same duty, higher hydrocarbon outlet temperature reduces the required plot area significantly. Similarly, higher hydrocarbon inlet temperature also reduces the plot area. Cooling to temperatures below ambient temperature of 40°C is not feasible in this example. In actual practice, cooling to low temperatures is almost always more difficult with air cooling than with water cooling.

[0014] Liquefaction processes are powered by compressors with inter-coolers and after-coolers, as shown much simplified in Figure 2. Low pressure refrigerant enters the first compression stage, is compressed and cooled in an inter-cooler. The refrigerant is then further compressed in a next compression stage, and cooled in an after-cooler. The refrigerant now has high pressure and low enthalpy, and is returned to the liquefaction process. With air cooling, the coolers will have higher outlet temperatures. This increases compressor work by at least three mechanisms.

[0015] First, higher inter-stage temperatures results in higher suction volumes in the next compressor stage and therefore increased compressor duty, even if the refrigerant flow and pressure increase are constant. [0016] Second, in particular in cases where refrigerant is condensed in the compressor coolers, higher pressure is needed to accomplish

condensation at higher temperature, as shown in Figure 3. Figure 3 illustrates the effect of increasing the pressure from e.g. 35 to 52 bara on the condensation temperature for the gas. At 35 bara cooling below 40 °C is necessary for condensation as shown with line a), whereas the condensation is complete at 70 °C at a pressure of 52 bara, as shown by line b).

[0017] Third, when refrigerant is recycled to the liquefaction process with higher temperature, larger flow is needed to provide the same liquefaction capacity. Typically, all of these effects increase the compressor work by roughly 20% when air cooling is used instead of water cooling.

[0018] Numerous liquefaction processes have been developed and are known by people skilled in the art. All use refrigerant compressors and coolers. The liquefaction efficiency is measured as compressor work per kg gas liquefied. Typically, the efficiency of large base-load liquefaction processes with water cooling might be 0.3 kWh/kg LNG. The efficiency of smaller peak-shaving liquefaction processes might be 0.5 kWh/kg LNG. The efficiency depends on the process and on the refrigerant(s).

[0019] Table 1 shows a comparison of work and cooling duty for two liquefaction processes with water and air cooling. Liquefaction rate is 400 metric tons per hour, the feed gas is at 60 bara and 25°C, and consists of 98 mole% methane, 1.5 mole% ethane and 0.5 mole% propane:

Table 1

Comparison of work and cooling duty for two liquefaction processes [0020] Air cooling increases the overall cooling duty by 10 to 15%. The air cooler footprint increases by roughly the same amount. In addition, air coolers require power to drive air fans, typically about 2 to 3 MW for the examples in Table 1.

[0021] Nitrogen based LNG heat exchangers are preferred from a safety

perspective, especially onboard vessels, as the alternative heating medium comprising hydrocarbons adds to the fire hazard onboard.

However, LNG heat exchangers using nitrogen as heat medium are less efficient than hydrocarbon based systems. A nitrogen based water cooled LNG heat exchanger will have an efficiency of about 0.4 kWh/kg LNG, whereas the corresponding efficiency for a hydrocarbon based system is 0.3 kWh/kg LNG, as shown in table 1.

[0022] An object of the present invention is to provide a method and a system for generation of LNG from natural gas on Coastal Liquefaction, Storage and Offloading (CLSO) facilities that allows for maximum production using air coolers and with minimum air cooler footprint on the CLSO deck. Another object is to provide a method allowing the use of a nitrogen based LNG heat exchanger to reduce the fire hazard, which is considered particularly important. Other objects will be clear for the skilled person reading the present description and claims. Accordingly, an efficient base load liquefaction system should be employed, and as many other processes as possible should be located on separate platforms or floaters, or on shore.

Summary of invention

[0023] According to the present invention the object indicated above is met by arranging as many processes as possible on a separate location, such as on shore, or on separate platforms of floaters. The intended processes to be arranged at separate locations are typically pretreatment as gas pre- treatment to remove components that can solidify when cooled to cryogenic temperatures, mainly sour components and water. Trace elements, mainly mercury which can form amalgams - in particular with aluminum process components - are also removed from the gas.

According to the present invention separation of heavier hydrocarbons to give a LNG ready natural gas, i.e. a natural gas comprising > 85 % by volume methane, and less than 100 ppm Cs+ hydrocarbons, is included in the pre-treatment arranged at separate locations. Additionally, the LNG ready natural gas is compressed to a pressure higher than a typical working pressure in a plant for LNG production, before leaving the pre- treatment utility arranged on a separate location, i.e. separate platform, onshore or the like. The pressurized natural gas is then transported in subsea gas lines to a floating LNG facility, such as a CLSO as mentioned above. The transport in the subsea gas line will inevitably cool gas having a temperature higher than the temperature of the sea. Onboard the LNG production plant the pressurized gas is expanded to a pressure that is typical as operating pressure throughout a LNG facility, before final pressure let-down of the liquefied gas. Due to the cooling in the sea and, if expander is employed, cooling caused by the expansion, this combination of steps reduces cooling requirement and thus the requirement for space, and power needed at the floating facility, substantially.

According to a first aspect, the present invention relates to a method for production of liquefied natural gas (LNG) from a natural gas source, the method comprising introduction of the natural gas into a gas terminal where the gas is pre-treated by removing or substantially reducing the content of acid gases therein, dehydration, and compression, where the pre-treated natural gas is led via subsea pipelines to floating liquid natural gas production units (FLNG) offshore, each comprising one or more LNG production unit(s) where the natural gas is liquefied to give LNG and where the LNG is transferred to LNG tankers for transport to markets, wherein the pre-treatment of the gas before transfer to the FLNG comprises separation of liquefied Petroleum Gas (LPG) from the natural gas to give a LNG ready natural gas comprising > 85 % by volume methane, and < 100 ppm C5+ hydrocarbons, where LNG ready natural gas is compressed to a pressure of 140 bar or more in the gas terminal, where the gas is transported in subsea pipelines to the FLNG at this high pressure, and where the pressure of the natural gas is reduced to a working pressure for LNG liquefaction onboard the FLNG, before the LNG ready natural gas is introduced into the LNG production unit(s) for liquefaction, and where the LNG ready natural gas is cooled in a heat exchanger against a heat medium.

[0025] Compression of the natural gas increases its temperature. The transport of the pressurized natural gas in subsea pipelines to the FLNG(s) cools the natural gas, or reduces the enthalpy thereof. By expanding the natural gas, the natural gas is cooled, and provided expanders are employed, the enthalpy is further reduced, thus reducing the cooling demand onboard the FLNG. This obtained reduction of the cooling demand for production of LNG makes it possible to reduce substantially the space and weight of the LNG production unit(s) onboard the FLNG.

[0026] The natural gas is further compressed to a pressure of more than 160, such as more than 220 bar before being transported in pipelines to the FLNG. The skilled person is able to calculate the pressure that is preferred from a technical / economical viewpoint based on the specifics of a specific plant and the teaching herein.

[0027] According to one embodiment, the heat medium is cooled in a

compression, cooling and expansion circuit where the cooling is performed in heat exchangers against air. The compression, cooling and expansion circuit, and the LNG heat exchanger are, with the exception of the offshore use of air coolers, i.e. cooling of the heat medium against air on a vessel, standard solutions according to the prior art. Air cooling have been regarded as too space and energy consuming for such plants, especially on floaters where space is limited and where it has been important to reduce the area. By means of the method according to the first aspect of the present invention, the energy demand onboard the FLNG for LNG production is substantially reduced, which makes it technically and economically possible to use air coolers and avoid the environmental problems associated with the use of cooling water.

[0028] According one embodiment, the heat medium is nitrogen. Gaseous

nitrogen is used as heat medium. Nitrogen as a heat medium is less efficient than traditionally used hydrocarbon heat medium, as only the heat capacity of nitrogen gas us used for transfer of heat, and not the phase transition between gas and liquid as used for hydrocarbon heat medium. However, hydrocarbons are highly inflammable and adds to the fire hazard onboard a floater. Nitrogen, at the other side, is inert and is regarded as safe.

[0029] According to one embodiment, the working pressure for the LNG

liquefaction is 70 to 120 bar, such as 75 to 100 bar. High pressure reduces the gas enthalpy (assuming constant temperature) which in turn reduces the liquefaction work. The heat transfer becomes more efficient. The volume flow of gas and hence the pressure loss is reduced. The use of LNG ready gas ascertains that no hydrocarbon condensate is formed before cooling, liquefying and sub-cooling of the natural gas to LNG temperature of about -163 °C.

[0030] According to a second aspect, the invention relates to a system for

generation of LNG for transport with LNG tankers, the system comprising a gas terminal receiving natural gas from one or more natural gas source(s) via a natural gas line, where the terminal is adopted for pre- treatment of the natural gas by removing or substantially reducing the content of acid gases therein, dehydration, and compression of the gas, where one or more floaters each comprising one or more production units for producing liquefied natural gas (LNG) from natural gas, and pipelines arranged for transporting natural gas from the terminal to floating liquid natural gas production units (FLNG), wherein the gas terminal further a natural gas separation unit for separation of natural gas liquid (LGN) fraction for export from the terminal, and a liquefied natural gas (LNG) ready natural gas comprising >85 % by volume methane, and <100 ppm C5+ hydrocarbons, a compression unit for compression of the LNG ready natural gas before transport thereof at the gas terminal, where one or more expander(s) is (are) provided onboard to FLNG(s) to expand and thereby cool the gas from pipelines , where one or more LNG production unit(s) is (are) provided on the FLNG for liquefaction of LNG by heat exchanging against a heat medium.

[0031] According to one embodiment, air coolers are provided for cooling of

refrigerants for LNG cooling.

[0032] Brief description of drawings

[0033]

Figure 1 is a plot of air cooled exchanger plot area for obtaining a set hydrocarbon outlet temperature after cooling at different hydrocarbon inlet temperatures,

Figure 2 is an illustration of a compressor train comprising two

compression stages with air cooled inter-cooler and after-coolers,

Figure 3 is an illustration of the condensation temperature and pressure for an exemplary hydrocarbon refrigerant,

Figure 4 is an illustration of a gas terminal for gas pre-processing, sub-sea piping of the pre-processed gas to a CLSO, and liquefaction of the gas on the CLSO according to the prior art,

Figure 5 is an illustration of a gas terminal for gas pre-processing, compression of the gas to pressures above the pressure needed for transport and liquefaction, cooling of the gas after compression, sub-sea piping of the pre-processed and pressurized gas to a CLSO, expansion of the gas on the CLSO to optimum pressure for liquefaction, and liquefaction of the gas on the CLSO,

Figure 6 is an illustration of a gas terminal including one or more CLSOs according to the present invention,

Figure 7 shows the reduction in enthalpy when gas is liquefied, as function of feed gas temperature, at constant pressure 75 bar, and

Figure 8 shows specific compressor work for a liquefaction plant as function of feed gas temperature, at constant pressure 75 bar.

Detailed description of the invention

[0034] The term "natural gas" is in the present description and claims used to describe hydrocarbon gas as produced from a gas field, and which is gaseous at atmospheric pressure and at a temperature of 20 °C. "LNG ready natural gas" is in the present description and claims used to describe a natural gas comprising >85 % by weight methane, < 100 ppm C5+ hydrocarbons, such as > 90 or even > 95 % by weight methane. The term Natural Gas Liquid, or "NGL", refers to a gas mainly comprising ethane, propane, butanes and even some higher molecular weight hydrocarbons. The term Liquefied Petroleum gas, or "LPG", refers to a gas mainly comprising propane and butane.

[0035] LNG plants according to the present invention will be located on offshore floaters. The floaters will receive pre-treated and compressed natural gas from a remote location, which may be pre-treatment facilities on an offshore terminal, a barge or other floater, or land based facilities. Onboard the floater, the pretreated gas is partly expanded and liquefied by cooling from post-expansion temperature to about - 163°C.

[0036] The liquefaction plants will be based on known technology, preferably

efficient base-load systems, but less efficient peak-shaving systems may also be employed. Known refrigerants, such as hydrocarbons or nitrogen, will circulate in cooling systems, comprising compressors, air cooled heat exchangers, and LNG exchangers. Depending on the refrigeration system, refrigerants may or may not be condensed in the air coolers before being routed to the LNG exchangers.

[0037] Figure 4 illustrates a principle sketch of a gas terminal and CLSO

according to the prior art. Natural gas is introduced into a gas terminal 1 from a natural gas pipeline 2. The natural gas is pre-processed at the gas terminal 1 to prepare the gas for liquefaction.

[0038] The processing of the natural gas at a gas terminal 1 normally comprises:

• gas sweetening, i.e. removal of unwanted acid gases from the

natural gas,

• dehydration, i.e. removal of water that may otherwise cause formation of hydrates from the gas,

• Hg removal,

• full NGL processing, i.e. separation of the NGL from the gas, and any fractionation of the NGL into saleable products, typically ethane, propane, butane and a heavier C5+ fraction,

• and compression of the pre-processed natural gas.

[0039] Normally, the natural gas pre-treated as mentioned above is compressed to about 50 to 120 bar at the terminal 1 , mainly to facilitate pipeline transportation through a transfer pipeline 3 to a Floating Liquefied Natural Gas (FLNG), or Coastal Liquefaction, Storage and Offloading (CLSO) units 4, and to provide optimum pressure for the liquefaction process. After arriving at the vessel, the pressurized and pretreated natural gas is introduced into a LNG liquefying plant 5 as described above. The LNG produced in the LNG liquefying plant 5 is then stored and later loaded onto a not shown LNG tank vessel through a LNG export line 6.

[0040] Figure 5 illustrates an embodiment of the present invention, where the gas leaving the gas terminal 1 in line 3 is further compressed by means of a compressor 10, and is cooled in a cooler 1 1. The skilled person will understand that the compressor 10 may be one or more compressors, and that the cooler 1 1 may be one or more coolers. The gas is compressed by compressor 10 so that the pressure is 140 bar or higher, such as 160 to bar or higher, such as 200 bar or higher, and then cooled to a temperature of about 40°C by the cooler 1 1. The gas leaving the cooler 1 1 is withdrawn through a pretreated gas transfer line 12 through which the gas is transferred to a FLNG or CLSO unit 4. During the transport in the pretreated gas transfer line, the pretreated natural gas is cooled by the surrounding water due to the temperature difference between the gas and the surrounding water. The cooler 1 1 may be omitted, or the cooling duty thereof may be reduced if the cooling efficiency by the seawater of the gas in line 12 is high.

[0041] This compression and subsequent cooling reduces the enthalpy of the gas and reduces the cooling load on the FLNG or CLSO unit(s) 4. Onboard the FLNG or CLSO unit(s) 4 the pressurized gas is expanded in an expander 13 to a pressure in the range from 70 to 120 bar, such as e.g. 75 to 100 bar. The expansion cools the gas and also reduces the enthalpy. An expansion from typically 180 bar to 75 bar, results in a temperature reduction of the gas from typically about 20°C to about -30°C.

[0042] According to this first part of the present invention, the natural gas is

compressed not only for transportation and optimum liquefaction pressure, but with the intention of expanding the gas at the floater, thus reducing the gas enthalpy before liquefaction. By doing so the liquefaction energy requirement onboard the CLSO 4 is reduced and the liquefaction process thereon is made more efficient. This reduces the air cooler duty, potentially to the point where it does not exceed the cooling duty for efficient water cooled liquefaction systems. An additional benefit is the production of supplementary power on the CLSO by e.g. by connecting the expander 13 to a generator and using the energy of expansion to produce electrical energy.

[0043] Pressurizing the gas to the mentioned pressures requires more

compression energy at the gas terminal. The cooling in the pipeline does, however, reduce the gas enthalpy and therefore the energy demand for producing LNG from the incoming pre-treated natural gas. This in turn reduces the air cooler space requirement on-board the FLNG or CLSO unit. The expander further reduces the gas enthalpy and hence the liquefaction duty, or the enthalpy difference of the hydrocarbons before and after liquefaction.

[0044] The total cooling duty for the air coolers on the floater comprises two main components. These are the enthalpy difference between the

hydrocarbons at the inlet and at the outlet of the liquefaction plant, plus the energy supplied by the compressors. The enthalpy difference is independent of the liquefaction system efficiency. It is a function of the feed gas temperature and pressure. An example of this, for a gas with constant pressure of 75 bar, is shown in Figure 7.

[0045] The second part, compressor work, is dependent on the liquefaction

efficiency. This efficiency is better when the feed gas is colder and has a lower enthalpy. Figure 8 shows an example where the efficiency is 0.36 kWh/kg when the feed gas temperature is 20°C and the pressure is 75 bara. 0.36 kWh/kg corresponds to the air cooled base load system shown in Table 1.

[0046] In addition to the improved liquefaction, the expansion of the gas produces energy, supplementing the energy supply on the CLSO as mentioned above.

[0047] Figure 6 is an illustration of a typical setup according to the present

invention where one terminal is arranged to perform the pretreatment of natural gas for two or more CLSO's 4, 4', 4", arranged at a distance from each other. [0048] Table 2 shows examples of terminal and CLSO operation without the expander, referring to Figure 4, and with the expander, referring to Figure 5. The examples are based on gas with composition about 91.0 mole% methane, about 4 mole% ethane and about 2 mole% propane, and the rest is butanes and some nitrogen. The gas flow is 400 metric tons per hour. For easy comparisons of enthalpies, the reference gas enthalpy is set at 0 kJ/kg at 85 bara and 40°C, which in all examples in Table 2 is the pressure and temperature of the gas from the pre-processing on the terminal. The compressor on the terminal and the expander on the CLSO both have adiabatic efficiencies of 85%. The pipeline from the terminal to the CLSO is assumed to be 25 km long, submerged in water which has temperature 20°C and a gas velocity of about 1.0 m/s.

[0049] The first column in Table 2 shows equipment reference numbers, which refer to Figures 4 and 5. The second column describes the equipment. The third and fourth columns show variables and units, respectively.

Results are shown for five cases. The first case, case 0, refers to Figure 4 and shows results without the expander system. The next four cases, cases 1 to 4, refer to Figure 5 and show results with the expander and with increasing compressor discharge pressure on the terminal.

[0050] The reference numerals used in the drawings are indicated in the first column in table 2. Accordingly, item 3 indicates the flow from the terminal pre-processing system, is the same for all cases, with pressure 85 bar, temperature 40°C and reference enthalpy 0 kJ/kg. Item 10 is the compressor duty, which is zero for case 0 and increasing for cases 1 to 4. Item 8 shows the compressor outlet conditions including the increasing pressure. Item 1 1 shows the compressor after-cooler duty, which increases when the compressor outlet pressure increases. Item 12 indicates the after-cooler outlet temperature, which is 40°C in each case. However, the gas enthalpy decreases as the pressure increases, which is a desirable effect. Item 12' indicates the sub-sea pipeline cooling duty, which increases slightly as the pressure increases. Pressure loss in the pipeline decreases as the operating pressure increases, as shown by items 12 and 12". Item 13 shows the expander duty, which increases with increasing inlet pressure. The expander reduces the gas enthalpy accordingly, as shown in Item 14. Item 5 shows the liquefaction efficiency, which improves as the item 10 compressor duty increases. Furthermore, item 5 shows that enthalpy change required to accomplish liquefaction is reduced as the item 10 compressor duty increases. The liquefaction enthalpy change is also shown as total cooling duty required in order to accomplishing the liquefaction, for the 400 metric tons per hour flow rate. It is 91.1 MW without the expander, and decreases to 67.8 MW as the expander is employed with increasing inlet pressures. Item 5 also shows the liquefaction system compressor duty, which decreases as the liquefaction efficiency is improved from case 0 to case 4.

Air cooler duty MW 230 222 212.4 205.4 199

6 Flow Pressure bara 1.0 1.0 1.0 1.0 1.0

Temperature °C -164 -164 -164 -164 -164

Enthalpy kJ/kg -822 -822 -822 -822 -822

Table 2

Examples, operation of terminal and CLSO without and with expander system

[0052] Finally, item 5 shows the total CLSO liquefaction systenn cooling duty, which is the sum of the total liquefaction enthalpy change and the liquefaction compressor duty. This reduced cooling duty shows a key benefit of the invention. It decreases from 235.1 MW without the expander to 200.2 MW in case 4, which is roughly a 15% reduction. Comparison with Table 1 shows that the cooling duty on the CLSO is now reduced to below the cooling duty needed for water cooling, assuming the same production rate of 400 metric tons LNG per hour and a base load liquefaction system. The main disadvantage with the air coolers on the CLSO, an increased cooling duty, has been removed, greatly facilitating environmentally friendly air cooling. Pre-processing has greatly reduced the complexity and in particular air cooler space requirement on the CLSO via what is in essence cooling assistance at a terminal remote from the CLSO. This advantage can be used to maximize the CLSO processing capacity, giving very significant economic benefits.

[0053] The plot area required for air cooled heat exchangers is quite large, about 1000 m2 per 100 MW of cooling duty. A typical CLSO might have a length of 350m and width of 60m. The deck space is therefore about 20000 m2. It is desirable to use the deck space for liquefaction equipment, safety barriers, accommodations, power generation and utilities.

For a person skilled in the art, and depending on permits and

environmental conditions, it would be possible to optimize the system by partial use of sea water for cooling, for example using a submerged pipe in which hot water is introduced, flows and is cooled by conduction of heat to the surrounding sea water, exits and is returned to the process for re-use as coolant, by using alternative liquefaction processes such as N2 refrigerant for smaller systems, and by water spray at the air intake to selected air coolers which would reduce the air temperature to the wet bulb temperature. In addition, power production for the CLSO may be partly done on the terminal and the power transferred to the CLSO via sub-sea cable. Air cooler efficiency may also be optimized such as by improving the finned tubes used therein, and the deck space required reduced by placing some or all coolers on a cantilever.

Claims

1. A method for production of liquefied natural gas (LNG) from a natural gas

source, the method comprising introduction of the natural gas into a gas terminal where the gas is pre-treated by removing or substantially reducing the content of acid gases therein, dehydration, and compression, where the pre- treated natural gas is led via subsea pipelines to floating liquid natural gas production units (FLNG) offshore, each comprising one or more LNG

production unit(s) where the natural gas is liquefied to give LNG and where the LNG is transferred to LNG tankers for transport to markets, wherein the pre- treatment of the gas before transfer to the FLNG comprises separation of Liquified Petroleum Gas (LPG) from the natural gas to give a LNG ready natural gas comprising > 85 % by volume methane, and < 100 ppm Cs+ hydrocarbons, where LNG ready natural gas is compressed to a pressure of 140 bar or more in the gas terminal, where the gas is transported in subsea pipelines to the FLNG at this high pressure, and where the pressure of the natural gas is reduced to a working pressure for LNG liquefaction onboard the FLNG, before the LNG ready natural gas is introduced into the LNG production unit(s) for liquefaction, and where the LNG ready natural gas is cooled in a heat exchanger against a heat medium.

2. The method of claim 1 , wherein the heat medium is cooled in a compression, cooling and expansion circuit where the cooling is performed in heat exchangers against air.

3. The method of claim 1 or 2, wherein the heat medium is nitrogen.

4. The method of any of the preceding claims, wherein the working pressure for the LNG liquefaction is 70 to 120 bar, such as 75 to 100 bar.

5. The method of any of the preceding claims, wherein the LNG ready natural gas comprises > 90 % by volume methane, such as more than 95 % by volume.

6. A system for generation of LNG for transport with LNG tankers, the system comprising a gas terminal (1 ) receiving natural gas from one or more natural gas source(s) via a natural gas line (2), where the terminal is adopted for pre- treatment of the natural gas by removing or substantially reducing the content of acid gases therein, dehydration, and compression of the gas, where one or more floaters (4, 4', 4") each comprising one or more production units (5) for producing liquefied natural gas (LNG) from natural gas, and pipelines (12, 12', 12") arranged for transporting natural gas from the ternninal (1 ) to floating liquid natural gas production units (FLNG) (4, 4', 4"), wherein the gas terminal further a natural gas separation unit for separation of the gas into a natural gas liquid (NGL) fraction for export from the terminal, and a liquefied natural gas (LNG) ready natural gas comprising >85 % by volume methane, and <100 ppm Cs+ hydrocarbons, a compression unit for compression of the LNG ready natural gas before transport thereof at the gas terminal, where one or more

expander(s) (13) is (are) provided onboard to FLNG(s) to expand and thereby cool and thus reduce the enthalpy of the gas from pipelines (12, 12', 12"), where one or more LNG production unit(s) is (are) provided on the FLNG for liquefaction of LNG by heat exchanging against a heat medium.

7. The system according to claim 6, wherein the system comprises a

compression, cooling and expansion circuit for cooling of the heat medium, where air coolers are provided for cooling in the circuit.