Title of Invention	SYSTEM AND METHOD FOR USING NATURAL GAS TO GENERATE ELECTRICITY OFFSHORE
Abstract	A system for using natural gas to safely provide large amounts (at least 30 megawatts) of electricity to consumers is disclosed. The system comprises a floating generating vessel (110) that lies offshore and carries one or more turbine-generator sets (114, 116) that use natural gas as fuel and whose electricity output is delivered though a power line (120) that extends at least partially in the sea to a consumer. One consumer is a process vessel (112) that processes natural gas and that has transfer facilities (68) that transfer liquified gas to or from a tanker (106) that holds over 10,000 tons of liquified gas. Some of the gas is transferred from the process vessel through a conduit (136) in the sea to the generating vessel (110) to provide fuel. Another consumer is an onshore facility (52). The generating and process vessels (110, 112) are widely separated (e.g. at least 0.2 kilometer) to protect personnel in the event of a gas explosion or fire. The separate generating and process vessels enable rapid acquisition of the vessels, which is especially useful to quickly supply large quantities of electricity in newly developed areas.

Title of Invention

SYSTEM AND METHOD FOR USING NATURAL GAS TO GENERATE ELECTRICITY OFFSHORE

Abstract

A system for using natural gas to safely provide large amounts (at least 30 megawatts) of electricity to consumers is disclosed. The system comprises a floating generating vessel (110) that lies offshore and carries one or more turbine-generator sets (114, 116) that use natural gas as fuel and whose electricity output is delivered though a power line (120) that extends at least partially in the sea to a consumer. One consumer is a process vessel (112) that processes natural gas and that has transfer facilities (68) that transfer liquified gas to or from a tanker (106) that holds over 10,000 tons of liquified gas. Some of the gas is transferred from the process vessel through a conduit (136) in the sea to the generating vessel (110) to provide fuel. Another consumer is an onshore facility (52). The generating and process vessels (110, 112) are widely separated (e.g. at least 0.2 kilometer) to protect personnel in the event of a gas explosion or fire. The separate generating and process vessels enable rapid acquisition of the vessels, which is especially useful to quickly supply large quantities of electricity in newly developed areas.

Full Text	BACKGROUND OF THE INVENTION The storage of large quantities of natural gas carries the danger of an explosion or great fire. Large quantities of natural gas are becoming available by transporting it as liquified gas (by cooling to liquify or hydrate it) by tankers each holding over 10,000 tons of gas, from distant hydrocarbon fields that produce large quantities of natural gas. Such gas (primarily gas with three or four carbon atoms per molecule) may be liquified by a production and processing vessel lying over an offshore hydrocarbon reservoir, and later heated to regas it as its destination. For both liquefaction by cooling and regas by heating, large amounts of electricity are used. It would be desirable if such system for processing gas and generating electricity could be readily acquired. It also would be desirable if a maximum portion of personnel were safeguarded from explosions or large gas fires. SUMMARY OF THE INVENTION In accordance with one embodiment of the invention, applicant provides an offshore system for flowing electricity to a power consumer such as processing equipment that liquefies natural gas so it can be sent by tanker to a distant location, or that regases liquified natural gas that is received from a tanker. The system includes an offshore process vessel that processes gas and that transfers liquified gas to or from a tanker, and also includes a separate generating vessel that contains an electricity generating set. The use of two vessels enables smaller vessels to be used, which enables more rapid acquisition of the vessels. The generating vessej is far (at least 0.2 km) from the process vessel to safeguard personnel on the vessels from an explosion or fire at the process vessel or tanker or at the generating vessel. Electricity also can be carried from a generating vessel to an onshore distribution facility. In that case, the generating vessel lies a sufficient distance from shore to avoid on shore damage from any gas fire or explosion on the generating or process vessels, but close enough to enable efficient passage of electricity from the vessel to the onshore distribution facility though a sea floor power cable. The distance is preferably sufficient so the vessels are not clearly visible from shore. The novel features of the invention are set forth with particularity in the appended claims.. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is an isometric view of a system for using natural gas to generate electricity, with the gas obtained from liquified natural gas brought to the vicinity by a tanker which is shown in phantom lines. Fig. 2 is a side elevation view of a system of another embodiment of the invention wherein the system includes a process vessel that produces gas from a reservoir and liquefies it for tanker transport, and a separate and spaced generating vessel that generates electricity for the process vessel. Fig. 3 is a side elevation view of a system of another embodiment of the invention which includes a process vessel for offloading, regasing and pressurizing liquid gas received from a tanker, and a generating vessel for generating electricity for the process vessel and for delivery to an onshore consumer. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates a system 10 for generating large amounts of electricity (at least 30 megawatts), using natural gas as a fuel, which includes a floating structure or vessel 12 that lies offshore (usually less than about 200 kilometers from shore 14). The vessel, such as a barge, has a hull 16 that supports a turret 20 at its bow end 22. The turret is moored by a mooring system such as catenary lines 32 that extend to the sea floor and along it. Risers 34 extend from a swivel 36 on the turret to a sea floor platform 40. The turret allows the vessel to weathervane, that is, to face in different directions with changing winds and waves, while the catenary lines allow the vessel to drift but only a limited distance, from a location 44 over the sea floor platform. Other mooring systems that can be used instead, including spread mooring. The vessel carries an electricity generating unit 42 that uses gas as a fuel to generate electricity. A preferred unit is a turbine-generator set wherein the turbine is powered by natural gas and the turbine spins a rotor of an electric generator. Such turbine-generator set is of light weight in proportion to the electrical power it generates, and the use of gas results in the generation of minimum polluting gases. The system includes a power cable 50 that extends from the vessel and that has a major portion 56 extending along the sea floor to an onshore facility 52. The facility distributes electricity to consumers such as residential, factory and office structures. The vessel is shown also carrying a second electricity generating unit 54. In the system of Fig. 1, natural gas that is the fuel, is obtained from a tanker 56 that gathers natural gas from a distant reservoir, liquefies it (by cooling below 0°C, and usually below -40°C for efficient transport, and unloads the gas to the vessel 12. The vessel has a regas unit 60 that heats the gas to make it liquid, and a pressurizing unit 62 that pumps the gas. Although tanks are usually provided in the vessel to store gas, either before it is liquified or afterwards, most of the gas is preferably stored in an underground cavern 64 such as an undersea cavern that has been formed out of a salt deposit. A gas-loaded tanker may, for example, come to the vessel in a once-a-week cycle and stay for only a day or two to offload, so gas must be stored during the rest of the week. The vessel has tanker mooring facilities such as capstans 66 for holding to lines that moor the tanker alongside the vessel 14 or that moor the tanker while it lies behind the vessel. In both cases, if the vessel weathervanes then the vessel 12 and tanker 56 weathervane together. The vessel also has transfer facilities 68 that transfer liquified gas between the tanker and vessel. A vessel that is moored offshore and that carries an electricity generating unit can serve a need for large amounts of electricity in an acceptable manner. Crude oil is more easily transported than gas, and has more uses than gas, so crude oil transported by tanker is expensive. Natural gas that is transported from distant locations by tanker, is difficult to unload on shore near developed areas because people are concerned about a possible fire or explosion of the large quantities of natural gas at a large unloading facility at the shore. If the unloading facility can be placed far from shore, which is at least about one kilometer and usually more, so people and property are protected from any fire or explosion, then the use of natural gas there is more acceptable. The vessel may be located at least three kilometers from shore so it can be hardly seen from shore, and is preferably no more than 200 km from shore to minimize electric power cable cost and electricity loss. Another advantage of generating electricity by a gas-fueled generator set on a far offshore vessel, is that the vessel may be produced on speculation or be available when no longer used at a distant location. Such a vessel with the heavy generator set and other large equipment already on board, can be moved rapidly and at low cost to a desired offshore location near where electricity in large amounts is required. In addition, the vessel sometimes can be obtained by using an existing hull, especially one used to produce or transport hydrocarbons, and converting it. The first generating unit 42 provides a considerable amount of power such as 30 to 500 megawatts. The second generating unit 54 is usually smaller, and can be added to produce more electricity if sufficient gas is available and extra electricity is needed, and can be removed and transferred to another vessel. Fig. 2 illustrates a system 100 where liquified gas is produced from a local hydrocarbon reservoir, or well, 102 by equipment on a production and processing vessel 112. The processing, or process vessel cools the natural gas (to -40°C or lower) to liquify it, and periodically (e.g. one or two days out of every seven) loads it onto a tanker 106. Applicant uses two vessels 110,112. A generating or generator vessel 110 carries an electricity generating unit 114 and possibly an additional but smaller one 116, and connects to a power line 120 that extends in the sea to the process vessel 112. The process vessel 112 carries a gas liquification unit 122 and tanks for storing gas (in cold liquid form), in addition to mooring equipment for mooring to the tanker 106. The liquified gas is offloaded to the tanker that carries it to a distant gas-consuming location, such as to the system shown in Figure 1. Preferably, the process vessel 112 can store at least 10,000 tons of liquified gas to store at least one load of LNG for the tanker 102. Liquified gas cannot be stored in a cavern. In Fig. 2, the generator vessel 110 is moored by a turret 130 and catenary lines 132, and the process vessel 112 is moored in a similar way by a turret 130A and lines 132A, and with the offloading tanker lying alongside the process vessel and moored tightly to it. The quiescent location of the vessel bow (its location in a calm sea) is the vessel location. Since both vessels drift under the same forces, the distance between their bows is usually about constant. In the system of Fig. 2, gas from the well or reservoir 102 that lies below the sea floor 133 is provided to both vessels through conduits, or seafloor pipes 134,136 (pipes lying at least partially on or in the sea floor). Electrical power from generator vessel 110, which uses gas for fuel, is supplied through the electric cable 120 that extends partially along the sea floor to the power consuming liquification barge 112, or process vessel. Such electrical power is needed to liquify the gas from the well 102. Additional electric power can be supplied to other facilities on shore or offshore. The advantage of the arrangement of Fig. 2 is that two smaller vessels 110 and 112 are used instead of one large one. It takes longer to find an open slot in a shipyard to build a large vessel than it does to build a small one. Also, the vessel 110 is useful to generate electricity where gas is available from local fields as in Fig. 2 (and is not to be liquified and transported elsewhere), or where gas is available by offloading it from a tanker. The two vessels 110,112 are preferably spaced far a part, such as at least 0.2 kilometer and preferably at least 0.5 km apart. This provides safety to personnel on one vessel in the event of an explosion or great fire at the other vessel 112. Of course, the greatest explosion and fire would occur at the process vessel 112. Thus, applicant prefers to use two (or more) separate vessels. One vessel such as 112 is used to process gas as to liquify or gasify it. Such vessel for processing gas contains large quantities (e.g. over 10,000 tons) of hydrocarbon at the sea surface and in its vicinity (in the vessel 112 and/or the adjacent tanker 106) leading to the danger of an explosion or great fire. The other vessel 110 is used to generate electricity and uses gas as fuel, but may contain substantial gas (e.g. over 200 tons). The two or more vessels help isolate a maximum number of personnel from equipment on the other vessel and enable each vessel to be provided at the site with less delay because it can be built in moderate size shipyards or built by converting an existing vessel. Fig. 3 illustrates a system 140 wherein liquified gas (e.g. at -40°C or lower) is brought by a tanker 142 to a process vessel 144. Processing equipment 146 on vessel 144 regases (heats) the liquified gas to above about 0°C (to avoid icing) and pumps it. Considerable electricity is used in the heating and pumping processes, even though sea water is used to provide heat. Some of the gas is delivered through a conduit, or seafloor pipe 150 to a generating vessel 152 that has a unit 153 that generates electricity. A considerable amount (at least 1MW) of electricity is delivered by the generating vessel along power cable portions 154,156 to the process vessel to supply its electricity needs. If it is desired to deliver large amounts of electricity to another consumer such as one on shore, then the generating vessel carries large electric generator sets and delivers at lot (e.g. 30 MW to 500 MW) of electricity through a large seafloor cable 162. If it is desired to deliver large amounts of natural gas to an onshore facility, then a lot is delivered through a sea floor pipeline 164. Thus, the invention provides a vessel that uses gas to produce large amounts of electricity; The electricity is delivered to one or more offshore consumers such as an offshore gas processing facility (vessel or platform) that liquefies gas or that gasifies liquified gas, and/or to an onshore distribution facility. An offshore gas processing facility that sometimes contains over 10,000 tons of gas, is separated (e.g. over 0.2 kilometer and preferably at least one kilometer) from a separate electricity generating vessel and from any on-shore location where people may be present. The use of a vessel that carries an electricity generating unit and other equipment for using gas as a fuel, facilitates rapid setup of the electricity-generating facility and reuse at other locations. The invention is especially useful to safely provide large amounts of electricity to remote and fast-developing regions (e.g. certain countries in Africa) without requiring extensive onshore infrastructure other than an electricity distribution network. Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents. WE CLAIM: 1. A system for safely using natural gas to generate electricity offshore for one or more electricity consumers (52, 112, 144) that are located on shore, comprising: a floating offshore structure (12, 110, 152) that lies in a sea at a distance from shore; said floating structure having an electricity generating unit (42, 114, 153) that uses gaseous hydrocarbons as a fuel and that generates electricity; an electricity power line (50, 120, 154) that extends from said floating structure and under water to said onshore consumer, said floating offshore structure lying at a distance of between 0.2 kilometer and 200 kilometers from shore, to isolate onshore persons and property from a disaster at the floating structure while providing an easily set up facility to generate electricity. 2. The system as claimed in claim 1 wherein said floating structure lies at least one kilometer from the shore (14). 3. The system as claimed in claim 1 comprising: a gas processing vessel (112, 144) that is moored to lie at a first location in the sea, hydrocarbon processing equipment (122, 146) on said processing vessel, and transfer facilities (68) for transferring natural gas between said processing vessel and a tanker; said floating offshore structure comprises a generator vessel (110, 152) that has an electricity generator unit (114, 153) that generates electricity; and comprising a gas conduit (136, 150) that extends between a location adjacent to said gas processing vessel and said generator vessel and that carries gas to said generator vessel. 4. The system as claimed in claim 3 comprising an undersea hydrocarbon reservoir (102) and a conduit that extends from said reservoir to said process vessel, wherein: said hydrocarbon processing equipment cools gas to produce liquefied gas, and said gas processing vessel has tanks that store at least 10,000 tons of liquefied gas. 5. The system as claimed in claim 4 comprising an underground cavern (102) that stores natural gas and having at least one conduit (134) extending between said floating structure and said cavern. 6. The system as claimed in claim 3 comprising an offshore undersea hydrocarbon gas reservoir (102) and having: a first conduit (134) that extends down from said processing vessel (112) to said reservoir and that carries hydrocarbon gas from said reservoir up to said processing vessel; a second conduit (136) for passing a portion of the hydrocarbon gas that passes from said reservoir to said electricity generating unit (114), said second conduit providing fuel that energizes said unit (114). 7. The system as claimed in claim 3 comprising an offshore undersea hydrocarbon gas reservoir (102) and having: a first conduit (134) extending down from said processing vessel (112) to said reservoir that carries hydrocarbon gas from said reservoir up to said floating structure; a second conduit (136) for passing a portion of the hydrocarbon gas passing from said reservoir to said electricity generating unit (114), said second conduit providing fuel that energizes said unit (114); hydrocarbon processing equipment on said floating structure that comprises gas liquefying equipment (122) that cools hydrocarbon gas from said reservoir, to a temperature at which it is liquid, and transfer facilities (68) constructed to transfer liquefied hydrocarbon gas to a tanker (106). 8. A method for using gaseous hydrocarbons as a fuel to provide electricity to an onshore distribution facility (52) for delivery to electricity consumers such as factories, offices, and residences, in a manner that isolates the consumer from any dangers arising in the handling of such gaseous hydrocarbons, comprising: anchoring to a sea floor at a distance of at least 0.5 kilometers from shore, a first floating structure (12, 110, 152) that lies in a sea and that carries a gas-fueled electricity generating unit (42, 114, 153); establishing an underwater electricity-carrying power line (50, 162) in extension between said first floating structure and said onshore distribution facility; supplying said gaseous hydrocarbons to said first floating structure; flowing said gaseous hydrocarbons to said generating unit of said first floating structure to generate electricity; and passing said generated electricity along said power line to said onshore distribution facility. 9. The method as claimed in claim 8 wherein: said method of supplying said gaseous hydrocarbons to said first floating structure comprises mooring a tanker (106, 142) that carries cold liquefied hydrocarbons, adjacent to second floating structure (112, 144), transferring said liquefied hydrocarbons between the tanker and the said second floating structure, and supplying gas to said generating unit from a sea floor conduit (136, 150) that extends from said second floating structure to said first floating structure; and carrying electricity from said first floating structure to said second floating structure through a power cable (120, 154) that extends in the sea. 10. The method as claimed in claim 8 wherein: said step of supplying said gaseous hydrocarbons comprises producing hydrocarbon gas from an undersea reservoir and flowing some of the produced gas to said electricity generating unit to provide fuel used by said unit. 11. The method as claimed in claim 10 comprising cooling some of the gas produced from said reservoir to liquefy it, and flowing said liquefied gas to a tanker at intervals. ABSTRACT SYSTEM AND METHOD FOR USING NATURAL GAS TO GENERATE ELECTRICITY OFFSHORE A system for using natural gas to safely provide large amounts (at least 30 megawatts) of electricity to consumers is disclosed. The system comprises a floating generating vessel (110) that lies offshore and carries one or more turbine-generator sets (114, 116) that use natural gas as fuel and whose electricity output is delivered though a power line (120) that extends at least partially in the sea to a consumer. One consumer is a process vessel (112) that processes natural gas and that has transfer facilities (68) that transfer liquified gas to or from a tanker (106) that holds over 10,000 tons of liquified gas. Some of the gas is transferred from the process vessel through a conduit (136) in the sea to the generating vessel (110) to provide fuel. Another consumer is an onshore facility (52). The generating and process vessels (110, 112) are widely separated (e.g. at least 0.2 kilometer) to protect personnel in the event of a gas explosion or fire. The separate generating and process vessels enable rapid acquisition of the vessels, which is especially useful to quickly supply large quantities of electricity in newly developed areas.

Full Text

BACKGROUND OF THE INVENTION
The storage of large quantities of natural gas carries the danger of an
explosion or great fire. Large quantities of natural gas are becoming available
by transporting it as liquified gas (by cooling to liquify or hydrate it) by tankers
each holding over 10,000 tons of gas, from distant hydrocarbon fields that
produce large quantities of natural gas. Such gas (primarily gas with three or
four carbon atoms per molecule) may be liquified by a production and
processing vessel lying over an offshore hydrocarbon reservoir, and later heated
to regas it as its destination. For both liquefaction by cooling and regas by
heating, large amounts of electricity are used. It would be desirable if such
system for processing gas and generating electricity could be readily acquired.
It also would be desirable if a maximum portion of personnel were safeguarded
from explosions or large gas fires.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention, applicant provides
an offshore system for flowing electricity to a power consumer such as
processing equipment that liquefies natural gas so it can be sent by tanker to a
distant location, or that regases liquified natural gas that is received from a
tanker. The system includes an offshore process vessel that processes gas and
that transfers liquified gas to or from a tanker, and also includes a separate
generating vessel that contains an electricity generating set. The use of two
vessels enables smaller vessels to be used, which enables more rapid
acquisition of the vessels. The generating vessej is far (at least 0.2 km) from the
process vessel to safeguard personnel on the vessels from an explosion or fire
at the process vessel or tanker or at the generating vessel.
Electricity also can be carried from a generating vessel to an onshore
distribution facility. In that case, the generating vessel lies a sufficient distance
from shore to avoid on shore damage from any gas fire or explosion on the

generating or process vessels, but close enough to enable efficient passage of
electricity from the vessel to the onshore distribution facility though a sea floor
power cable. The distance is preferably sufficient so the vessels are not clearly
visible from shore.
The novel features of the invention are set forth with particularity in the
appended claims.. The invention will be best understood from the following
description when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 is an isometric view of a system for using natural gas to generate
electricity, with the gas obtained from liquified natural gas brought to the vicinity
by a tanker which is shown in phantom lines.
Fig. 2 is a side elevation view of a system of another embodiment of the
invention wherein the system includes a process vessel that produces gas from
a reservoir and liquefies it for tanker transport, and a separate and spaced
generating vessel that generates electricity for the process vessel.
Fig. 3 is a side elevation view of a system of another embodiment of the
invention which includes a process vessel for offloading, regasing and
pressurizing liquid gas received from a tanker, and a generating vessel for
generating electricity for the process vessel and for delivery to an onshore
consumer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 illustrates a system 10 for generating large amounts of electricity
(at least 30 megawatts), using natural gas as a fuel, which includes a floating
structure or vessel 12 that lies offshore (usually less than about 200 kilometers
from shore 14). The vessel, such as a barge, has a hull 16 that supports a turret
20 at its bow end 22. The turret is moored by a mooring system such as
catenary lines 32 that extend to the sea floor and along it. Risers 34 extend from
a swivel 36 on the turret to a sea floor platform 40. The turret allows the vessel
to weathervane, that is, to face in different directions with changing winds and

waves, while the catenary lines allow the vessel to drift but only a limited
distance, from a location 44 over the sea floor platform. Other mooring systems
that can be used instead, including spread mooring.
The vessel carries an electricity generating unit 42 that uses gas as a
fuel to generate electricity. A preferred unit is a turbine-generator set wherein
the turbine is powered by natural gas and the turbine spins a rotor of an electric
generator. Such turbine-generator set is of light weight in proportion to the
electrical power it generates, and the use of gas results in the generation of
minimum polluting gases. The system includes a power cable 50 that extends
from the vessel and that has a major portion 56 extending along the sea floor to
an onshore facility 52. The facility distributes electricity to consumers such as
residential, factory and office structures. The vessel is shown also carrying a
second electricity generating unit 54.
In the system of Fig. 1, natural gas that is the fuel, is obtained from a
tanker 56 that gathers natural gas from a distant reservoir, liquefies it (by cooling
below 0°C, and usually below -40°C for efficient transport, and unloads the gas
to the vessel 12. The vessel has a regas unit 60 that heats the gas to make it
liquid, and a pressurizing unit 62 that pumps the gas. Although tanks are usually
provided in the vessel to store gas, either before it is liquified or afterwards, most
of the gas is preferably stored in an underground cavern 64 such as an
undersea cavern that has been formed out of a salt deposit. A gas-loaded
tanker may, for example, come to the vessel in a once-a-week cycle and stay
for only a day or two to offload, so gas must be stored during the rest of the
week. The vessel has tanker mooring facilities such as capstans 66 for holding
to lines that moor the tanker alongside the vessel 14 or that moor the tanker
while it lies behind the vessel. In both cases, if the vessel weathervanes then
the vessel 12 and tanker 56 weathervane together. The vessel also has transfer
facilities 68 that transfer liquified gas between the tanker and vessel.
A vessel that is moored offshore and that carries an electricity generating
unit can serve a need for large amounts of electricity in an acceptable manner.
Crude oil is more easily transported than gas, and has more uses than gas, so

crude oil transported by tanker is expensive. Natural gas that is transported
from distant locations by tanker, is difficult to unload on shore near developed
areas because people are concerned about a possible fire or explosion of the
large quantities of natural gas at a large unloading facility at the shore. If the
unloading facility can be placed far from shore, which is at least about one
kilometer and usually more, so people and property are protected from any fire
or explosion, then the use of natural gas there is more acceptable. The vessel
may be located at least three kilometers from shore so it can be hardly seen
from shore, and is preferably no more than 200 km from shore to minimize
electric power cable cost and electricity loss. Another advantage of generating
electricity by a gas-fueled generator set on a far offshore vessel, is that the
vessel may be produced on speculation or be available when no longer used at
a distant location. Such a vessel with the heavy generator set and other large
equipment already on board, can be moved rapidly and at low cost to a desired
offshore location near where electricity in large amounts is required. In addition,
the vessel sometimes can be obtained by using an existing hull, especially one
used to produce or transport hydrocarbons, and converting it.
The first generating unit 42 provides a considerable amount of power
such as 30 to 500 megawatts. The second generating unit 54 is usually smaller,
and can be added to produce more electricity if sufficient gas is available and
extra electricity is needed, and can be removed and transferred to another
vessel.
Fig. 2 illustrates a system 100 where liquified gas is produced from a local
hydrocarbon reservoir, or well, 102 by equipment on a production and
processing vessel 112. The processing, or process vessel cools the natural gas
(to -40°C or lower) to liquify it, and periodically (e.g. one or two days out of every
seven) loads it onto a tanker 106. Applicant uses two vessels 110,112. A
generating or generator vessel 110 carries an electricity generating unit 114 and
possibly an additional but smaller one 116, and connects to a power line 120
that extends in the sea to the process vessel 112. The process vessel 112
carries a gas liquification unit 122 and tanks for storing gas (in cold liquid form),

in addition to mooring equipment for mooring to the tanker 106. The liquified
gas is offloaded to the tanker that carries it to a distant gas-consuming location,
such as to the system shown in Figure 1. Preferably, the process vessel 112
can store at least 10,000 tons of liquified gas to store at least one load of LNG
for the tanker 102. Liquified gas cannot be stored in a cavern. In Fig. 2, the
generator vessel 110 is moored by a turret 130 and catenary lines 132, and the
process vessel 112 is moored in a similar way by a turret 130A and lines 132A,
and with the offloading tanker lying alongside the process vessel and moored
tightly to it. The quiescent location of the vessel bow (its location in a calm sea)
is the vessel location. Since both vessels drift under the same forces, the
distance between their bows is usually about constant.
In the system of Fig. 2, gas from the well or reservoir 102 that lies below
the sea floor 133 is provided to both vessels through conduits, or seafloor pipes
134,136 (pipes lying at least partially on or in the sea floor). Electrical power
from generator vessel 110, which uses gas for fuel, is supplied through the
electric cable 120 that extends partially along the sea floor to the power
consuming liquification barge 112, or process vessel. Such electrical power is
needed to liquify the gas from the well 102. Additional electric power can be
supplied to other facilities on shore or offshore.
The advantage of the arrangement of Fig. 2 is that two smaller vessels
110 and 112 are used instead of one large one. It takes longer to find an open
slot in a shipyard to build a large vessel than it does to build a small one. Also,
the vessel 110 is useful to generate electricity where gas is available from local
fields as in Fig. 2 (and is not to be liquified and transported elsewhere), or where
gas is available by offloading it from a tanker.
The two vessels 110,112 are preferably spaced far a part, such as at
least 0.2 kilometer and preferably at least 0.5 km apart. This provides safety to
personnel on one vessel in the event of an explosion or great fire at the other
vessel 112. Of course, the greatest explosion and fire would occur at the
process vessel 112. Thus, applicant prefers to use two (or more) separate
vessels. One vessel such as 112 is used to process gas as to liquify or gasify it.

Such vessel for processing gas contains large quantities (e.g. over 10,000 tons)
of hydrocarbon at the sea surface and in its vicinity (in the vessel 112 and/or the
adjacent tanker 106) leading to the danger of an explosion or great fire. The
other vessel 110 is used to generate electricity and uses gas as fuel, but may
contain substantial gas (e.g. over 200 tons). The two or more vessels help
isolate a maximum number of personnel from equipment on the other vessel
and enable each vessel to be provided at the site with less delay because it can
be built in moderate size shipyards or built by converting an existing vessel.
Fig. 3 illustrates a system 140 wherein liquified gas (e.g. at -40°C or
lower) is brought by a tanker 142 to a process vessel 144. Processing
equipment 146 on vessel 144 regases (heats) the liquified gas to above about
0°C (to avoid icing) and pumps it. Considerable electricity is used in the heating
and pumping processes, even though sea water is used to provide heat. Some
of the gas is delivered through a conduit, or seafloor pipe 150 to a generating
vessel 152 that has a unit 153 that generates electricity. A considerable amount
(at least 1MW) of electricity is delivered by the generating vessel along power
cable portions 154,156 to the process vessel to supply its electricity needs.
If it is desired to deliver large amounts of electricity to another consumer
such as one on shore, then the generating vessel carries large electric generator
sets and delivers at lot (e.g. 30 MW to 500 MW) of electricity through a large
seafloor cable 162. If it is desired to deliver large amounts of natural gas to an
onshore facility, then a lot is delivered through a sea floor pipeline 164.
Thus, the invention provides a vessel that uses gas to produce large
amounts of electricity; The electricity is delivered to one or more offshore
consumers such as an offshore gas processing facility (vessel or platform) that
liquefies gas or that gasifies liquified gas, and/or to an onshore distribution
facility. An offshore gas processing facility that sometimes contains over 10,000
tons of gas, is separated (e.g. over 0.2 kilometer and preferably at least one
kilometer) from a separate electricity generating vessel and from any on-shore
location where people may be present. The use of a vessel that carries an
electricity generating unit and other equipment for using gas as a fuel, facilitates

rapid setup of the electricity-generating facility and reuse at other locations. The
invention is especially useful to safely provide large amounts of electricity to
remote and fast-developing regions (e.g. certain countries in Africa) without
requiring extensive onshore infrastructure other than an electricity distribution
network.
Although particular embodiments of the invention have been described
and illustrated herein, it is recognized that modifications and variations may
readily occur to those skilled in the art, and consequently, it is intended that the
claims be interpreted to cover such modifications and equivalents.

WE CLAIM:
1. A system for safely using natural gas to generate electricity offshore for one or more
electricity consumers (52, 112, 144) that are located on shore, comprising:
a floating offshore structure (12, 110, 152) that lies in a sea at a distance from shore;
said floating structure having an electricity generating unit (42, 114, 153) that uses
gaseous hydrocarbons as a fuel and that generates electricity;
an electricity power line (50, 120, 154) that extends from said floating structure and
under water to said onshore consumer, said floating offshore structure lying at a distance of
between 0.2 kilometer and 200 kilometers from shore, to isolate onshore persons and
property from a disaster at the floating structure while providing an easily set up facility to
generate electricity.
2. The system as claimed in claim 1 wherein said floating structure lies at least one
kilometer from the shore (14).
3. The system as claimed in claim 1 comprising:
a gas processing vessel (112, 144) that is moored to lie at a first location in the sea,
hydrocarbon processing equipment (122, 146) on said processing vessel, and transfer
facilities (68) for transferring natural gas between said processing vessel and a tanker;
said floating offshore structure comprises a generator vessel (110, 152) that has an
electricity generator unit (114, 153) that generates electricity; and comprising
a gas conduit (136, 150) that extends between a location adjacent to said gas
processing vessel and said generator vessel and that carries gas to said generator vessel.
4. The system as claimed in claim 3 comprising an undersea hydrocarbon reservoir (102)
and a conduit that extends from said reservoir to said process vessel, wherein:
said hydrocarbon processing equipment cools gas to produce liquefied gas, and said
gas processing vessel has tanks that store at least 10,000 tons of liquefied gas.

5. The system as claimed in claim 4 comprising an underground cavern (102) that stores
natural gas and having at least one conduit (134) extending between said floating structure
and said cavern.
6. The system as claimed in claim 3 comprising an offshore undersea hydrocarbon gas
reservoir (102) and having:
a first conduit (134) that extends down from said processing vessel (112) to said
reservoir and that carries hydrocarbon gas from said reservoir up to said processing vessel;
a second conduit (136) for passing a portion of the hydrocarbon gas that passes from
said reservoir to said electricity generating unit (114), said second conduit providing fuel that
energizes said unit (114).
7. The system as claimed in claim 3 comprising an offshore undersea hydrocarbon gas
reservoir (102) and having:
a first conduit (134) extending down from said processing vessel (112) to said
reservoir that carries hydrocarbon gas from said reservoir up to said floating structure;
a second conduit (136) for passing a portion of the hydrocarbon gas passing from said
reservoir to said electricity generating unit (114), said second conduit providing fuel that
energizes said unit (114);
hydrocarbon processing equipment on said floating structure that comprises gas
liquefying equipment (122) that cools hydrocarbon gas from said reservoir, to a temperature
at which it is liquid, and transfer facilities (68) constructed to transfer liquefied hydrocarbon
gas to a tanker (106).
8. A method for using gaseous hydrocarbons as a fuel to provide electricity to an
onshore distribution facility (52) for delivery to electricity consumers such as factories,
offices, and residences, in a manner that isolates the consumer from any dangers arising in
the handling of such gaseous hydrocarbons, comprising:

anchoring to a sea floor at a distance of at least 0.5 kilometers from shore, a first
floating structure (12, 110, 152) that lies in a sea and that carries a gas-fueled electricity
generating unit (42, 114, 153);
establishing an underwater electricity-carrying power line (50, 162) in extension
between said first floating structure and said onshore distribution facility;
supplying said gaseous hydrocarbons to said first floating structure;
flowing said gaseous hydrocarbons to said generating unit of said first floating
structure to generate electricity; and
passing said generated electricity along said power line to said onshore distribution
facility.
9. The method as claimed in claim 8 wherein:
said method of supplying said gaseous hydrocarbons to said first floating structure
comprises mooring a tanker (106, 142) that carries cold liquefied hydrocarbons, adjacent to
second floating structure (112, 144), transferring said liquefied hydrocarbons between the
tanker and the said second floating structure, and supplying gas to said generating unit from a
sea floor conduit (136, 150) that extends from said second floating structure to said first
floating structure; and
carrying electricity from said first floating structure to said second floating structure
through a power cable (120, 154) that extends in the sea.
10. The method as claimed in claim 8 wherein:
said step of supplying said gaseous hydrocarbons comprises producing hydrocarbon
gas from an undersea reservoir and flowing some of the produced gas to said electricity
generating unit to provide fuel used by said unit.
11. The method as claimed in claim 10 comprising cooling some of the gas produced
from said reservoir to liquefy it, and flowing said liquefied gas to a tanker at intervals.

ABSTRACT

SYSTEM AND METHOD FOR USING NATURAL
GAS TO GENERATE ELECTRICITY OFFSHORE
A system for using natural gas to safely provide large amounts (at least 30 megawatts)
of electricity to consumers is disclosed. The system comprises a floating generating vessel
(110) that lies offshore and carries one or more turbine-generator sets (114, 116) that use
natural gas as fuel and whose electricity output is delivered though a power line (120) that
extends at least partially in the sea to a consumer. One consumer is a process vessel (112) that
processes natural gas and that has transfer facilities (68) that transfer liquified gas to or from
a tanker (106) that holds over 10,000 tons of liquified gas. Some of the gas is transferred
from the process vessel through a conduit (136) in the sea to the generating vessel (110) to
provide fuel. Another consumer is an onshore facility (52). The generating and process
vessels (110, 112) are widely separated (e.g. at least 0.2 kilometer) to protect personnel in the
event of a gas explosion or fire. The separate generating and process vessels enable rapid
acquisition of the vessels, which is especially useful to quickly supply large quantities of
electricity in newly developed areas.

SYSTEM AND METHOD FOR USING NATURAL GAS TO GENERATE ELECTRICITY OFFSHORE

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