Note: Descriptions are shown in the official language in which they were submitted.
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GAS OFFLOADING SYSTEM
BACKGROUND OF THE INVENTION
Hydrocarbons that are in a gaseous state at atmospheric pressure and room
temperature (e.g. 200 C), are often transported as cold hydrocarbons, as by
ship
in liquid form such as LNG (liquified natural gas), at atmospheric pressure
and -
160 C. Another form of cold gaseous hydrocarbons that are ship-transported
are
hydrates (gas entrapped in ice). At the ship's destination, the LNG (or other
gas)
may be heated and flowed to an onshore distribution facility. Proposed prior
art
offloading stations have included a fixed platform extending up from the
seafloor
to a height above the sea surface and with a regas unit on the platform for
heating
the LNG. Because of fire dangers in dealing with LNG, rigid platforms, which
minimize flexing joints, have previously been proposed for offloading LNG from
a
tanker and heating it to gasify it.
The cost of a fixed platform is high even at moderate depths, and at
increasing depths (e.g. over 50 meters) the costs of fixed platforms increase
dramatically. In addition, if the platform lies in an open sea it is difficult
to moor a
tanker to the platform because the tanker shifts position and heading with
changing
winds, waves and currents. An offshore LNG offloading and regas station which
avoided the use of fixed platforms, and which provided the high reliability
demanded in LNG offloading, heating and storage, would lower the cost of such
stations and allow them to be used in situations where they previously were
uneconomical.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, a relatively
low-cost system is provided for offloading cold hydrocarbons, and especially
LNG
(Iiquified natural gas), and transporting the gas to an onshore gas
distribution
station. The system includes a floating structure such as a barge at the sea
surface
that is moored so it weathervanes. A tanker carrying LNG attaches itself to
the
floating structure so they weathervane together. A regas unit which heats the
LNG,
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usually by transferring heat from sea water, transforms the LNG into gas that
can
be more easily passed through moderate cost hoses or pipes and eventually to
the
onshore distribution station.
A new tanker arrives at the floating structure perhaps everyweek, and efforts
are made to offload the tanker as fast as possible, perhaps in one day. To
provide
a steady flow of gas to the onshore distribution station, much of the rapidly-
offloaded and regassed LNG is stored in an underground (and usually undersea)
cavern. The gas is slowly flowed from the cavern along a seafloor pipeline to
the
onshore distribution station, to provide a steady gas supply without requiring
a
large gas storage facility at the onshore station.
The regas unit and pumps for pressurizing gas, are preferably electrically
energized for safety and convenience. Electric power on the order of 60
megawatts
may be required. Such electrical energy can be obtained from a power generator
apparatus on the floating structure that uses gas from the tanker for fuel.
The
regas unit may require electric power only part of the time, such as one day
per
week when LNG is being offloaded and regassed. The rest of the time (e.g.
several
days per week) electric power from the power generator apparatus is passed
through a seafloor electric power line to an onshore electric distribution
facility. The
generation of electric power at the floating structure is economical because
the gas
fuel is already available and because a large amount of expensive land is not
required to isolate the power generation apparatus from onshore homes and
businesses for safety.
Electric power instead can be obtained from an onshore electric power
distribution facility. In that case, an electric power line extends from the
onshore
facility and along the seafloor and up to the floating structure.
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According to another aspect of the present
invention, there is provided an offshore gas unloading
system that lies in a sea having a sea surface and a
seafloor, wherein a tanker unloads liquified cold
hydrocarbons that are gaseous at room temperature,
comprising: a floating structure that lies at the sea
surface and that is moored so it weathervanes; a regas unit
on said floating structure, that heats at least some of the
cold hydrocarbons that were received from the tanker; a
seafloor platform that lies at the seafloor; a riser that
extends from said floating structure to said seafloor
platform to carry hydrocarbons from one to the other; said
floating structure being connected to said tanker to form a
combination of said floating structure and said tanker that
weathervane together; at least one mooring line that extends
from the seafloor to said combination to moor the
combination and allow it to weathervane.
According to still another aspect of the present
invention, there is provided a method for operating an
offshore facility that lies off shore, and that unloads cold
hydrocarbons from a tanker, for delivery of the hydrocarbons
after warming, to a shore station on the shore, comprising:
offloading cold hydrocarbons from the tanker to a floating
structure that has a regas unit and an injection unit,
passing said cold hydrocarbons through said regas unit to
produce warmed gaseous hydrocarbons and passing the gaseous
hydrocarbons through the injection unit to pressurize them;
flowing said gaseous hydrocarbons to said shore station;
said floating structure has a turret, and including mooring
said floating structure to the seafloor with a plurality of
mooring lines extending from said turret to the seafloor;
coupling said tanker to said floating structure to form a
combination that weathervanes as a combination.
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According to yet another aspect of the present
invention, there is provided an offshore gas unloading
system that lies in a sea that has a seafloor and a sea
surface, and that lies within about fifty kilometers of a
shore, for unloading cold hydrocarbons from a tanker,
comprising: a floating structure that lies at the sea
surface and that has a fluid coupling for receiving said
liquid cold hydrocarbons from said tanker; a cavern that
stores gas; a seafloor platform and at least one pipe that
extends from said seafloor platform to said cavern; at least
one riser that extends from said floating structure to said
seafloor platform and that is coupled to said pipe to carry
hydrocarbons between said sea-surface structure and said
cavern; said floating structure carrying an electrically
powered equipment including a regas unit; an electric power
facility that lies on the shore; a current carrying power
line that extends between said sea-surface structure and
said electric power facility on the shore, to carry
electricity between them.
According to a further aspect of the present
invention, there is provided a method for operating an
offshore facility that lies off shore, and that unloads cold
hydrocarbons from a tanker, for delivery of the hydrocarbons
after warming, to a shore station on the shore, comprising:
offloading cold hydrocarbons from the tanker to a floating
structure that has a regas unit, and passing said cold
hydrocarbons through said regas unit to produce warmed
gaseous hydrocarbons; flowing said warmed gaseous
hydrocarbons to said shore station; including energizing
said regas unit with electricity
The novel features of the invention are set forth
with particularity in the appended claims. The invention
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will be best understood from the following description when
read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a partially sectional side view of an
offshore gas offloading and
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transfer system of a first embodiment of the invention.
Fig. 1 A is a plan view of a portion of the system of Fig. 1.
Fig. 1 B is a plan view of a portion of a system that is a variation of Fig.
1A.
Fig. 2 is a partially sectional side view of an offshore gas offloading and
transfer system of another embodiment of the invention.
Fig. 3 is a partially sectional side view of an offshore gas offloading and
transfer system of another embodiment of the invention.
Fig. 4 is a partially sectional side view of an offshore gas offloading and
transfer system of another embodiment of the invention.
Fig. 5 is a partially sectional side view of an offshore gas offloading and
transfer system of another embodiment of the invention.
Fig. 6 is a top isometric view of an offshore gas offloading and transfer
system of another embodiment of the invention.
Fig. 7 is a sectional side view of the system of Fig. 6.
Fig. 8 is a sectional side view of an offshore gas offloading and transfer
system of another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 illustrates an offloading and transfer system 10 that includes a
weathervaning floating structure in the form of a single barge 12 (there could
be
more than one barge) that floats at the sea surface 15. The barge receives LNG
through a coupling 17 and a loading arm 11 extending from midship of a tanker
13.
The barge is moored to the seafloor 14 by chains 16 extending from a turret 20
mounted at the bow of the barge. The illustrated chains extend in catenary
curves
to the seafloor and along the seafloor to anchors. Preferably, the tanker is
moored
to the barge and they weathervane together. This allows the barge and tanker
to
move in unison and therefore remain close together in an open sea. A regas
unit
22 (for heating LNG to produce gas) and an injection unit 24 for pumping the
LNG
or gas to a high pressure, are both located on the barge, and are used for
injection
of gas into an underground cavern 30 that lies under the sea. The regas unit
usually transfers heat from seawater to the LNG to change it into gas. A
flexible
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riser 32 (there often can be two or more) extends up from a platform 34 on the
seafloor to the barge. The platform is connected through a pipe 36 to the
cavern
30 in which the pressured gas is stored, that results from heating LNG. A
pipeline
40 extends primarily along the seafloor to an onshore gas distribution station
42.
The onshore station can be a gas grid that distributes the gas to users, can
be a
power plant that distributes the gas to gas turbines, etc..
The flexible riser 32 and connections 50, 52 at its opposite ends, can be
made highly reliable. In addition, reliable shutoff valves are present at 54
on the
platform and on the barge. During the past forty years or so, large numbers of
flexible risers have been designed, constructed and used in offshore
installations
to produce hydrocarbons (usually including gas and liquid) from undersea
reservoirs. Experience gained from such use has resulted in high reliability.
By
using such reliable flexible risers and shutoff valves in the present floating
offloading and injection station, applicant is able to achieve the same high
standards of reliability previously achieved with fixed platforms, but at far
lower
cost.
Fig. 1A shows a combination 62 of the tanker 13 and barge 12 held together
to weathervane together about the turret axis 56. Fig. 1 B shows another
combination 64 where the tanker moored to the barge by a hawser 60, so they
weathervane together.
Fig. 2 shows an offloading/injection system 70 similar to that of Fig. 1,
except
that two risers 72, 74 are shown. One riser 72 connects to a pipe 76 that
extends
to the cavern 30. The other riser 74 connects directly to a seafloor pipeline
80 that
extends to the onshore station 82. A break is indicated at 83 to indicate that
the
pipeline may be long (e.g. over one kilometer). A pressure boosting unit 84 on
the
barge 90 can pressurize gas that is pumped through the pipeline 80. Such
pressured gas is directed through valves in the onshore station 82 but the gas
does
not have to be pressurized by the onshore station. This keeps the pumps at 84
far
from any inhabited structures on shore.
During regasification of LNG on a vessel and offloading of gas from the
vessel, some of the offloaded gas is injected via riser 72 into the cavern 30
while
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other gas is transferred through riser 74 to the shore station. When no LNG is
being offloaded, gas is removed from the cavern via the riser 72, its pressure
is
boosted by pressure boosting unit 84, and sent to the shore station via riser
74.
Thus, riser 72 is used bi-directionafly.
Fig. 3 shows a system 100 in which the barge 102 injects LNG directly into
the cavern through a cryogenic pipeline or flexible pipe 104 that connects to
a
conduit 105. In the cavern 106 the LNG gradually changes into its gas phase.
Gas
is withdrawn through a separate pipe 112 leading from an upper portion of the
cavern to a seafloor pipeline 110 that extends to an onshore station 114.
In Fig. 4, all gas from the barge passes through a seafloor pipeline 120 to
an onshore station 122 that injects it into a cavern 124 that is directly
connected to
the onshore station.
In Fig. 5, cold LNG is pumped from the barge 130 through a cryogenic hose
or pipeline riser 132, and passes through a cryogenic seafloor pipeline 134
directly
into an onshore injector and regas unit 136 that connects through pipe 138 to
the
cavern 140. The injector 136 can inject LNG or can regas some or all of the
LNG
before injection, depending upon the expected rate of gas withdrawal and the
amount already stored in the cavern. Gas is removed from the cavern through a
separate pipe 142 leading to another onshore station 144.
Fig. 6 illustrate another offloading station 150 for offloading gaseous
hydrocarbons from a tanker 152. The tanker 152 carries the hydrocarbons as LNG
at -165 C and atmospheric pressure. The station includes a direct-attachment
floating structure 154. The direct-attachment floating structure includes a
buoyancy-adjusting floating system 160 and a propulsion system 162 that allows
the floating structure to lie low in the water, slowly propel itself until its
under-tanker
part 164 lies under the tanker, and then deballast itself (by emptying water
from
ballast tanks) until its parts 164, 166 engage the tanker. Such a structure
has been
previously used in offloading crude oil from tankers.
The particular floating structure 154 of Fig. 6 also includes a regas system
170 that warms the LNG so it becomes gaseous. The floating structure pumps the
gaseous hydrocarbons through a riser 172 into a subsea cavern and/or through a
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pipeline to a shore station. By regasing LNG, applicant avoid the need to
provide
a cryogenic riser which may be very expensive.
Fig. 6 shows that a seafloor base 174 carries a fluid swivel 176. A hawser
180 that extends from a yoke 182 attached to the swivel, extends to the bow
184
of the tanker to moor the tanker so it weathervanes. The structure 154
weathervanes with the tanker.
Energy is required to power the propulsion and ballast systems, as well as
the regas systems. The regas system will use pumped seawater, as to warm an
intermediate liquid that warms LNG or even to directly warm the LNG to produce
hydrocarbons in a gaseous state. The hydrocarbons are pumped into a cavern 191
(Fig. 7) and/or a seafloor gas pipeline 190 that extends to an onshore gas
facility
192. Where the floating structure lies near shore (e.g. not much more than
fifty
kilometers from shore), power can be obtained from a power line 194 shown in
Fig.
7. The power line preferably extends parallel to the pipeline. The shore end
196
of the power line can be connected to an on shore electric power facility such
as
a utility electric line 200, or to a special shore based power station. The
floating
structure shown in Fig. 6 as well as Figs. 1-5, may consume on the order of
magnitude of 60 megawatts (e.g. up to 200 Mw) of electricity when unloading a
tanker. A power line to shore is most practical when the seafloor base lies
within
about fifty kilometers (less than 70km) of shore so there are only moderate
power
losses along the power line. The power line preferably lies partially on the
seafloor.
In most cases the floating structure lies at least 50 meters from shore in its
greatest
excursion, and the seafloor platform lies at least 50 meters from shore (high
tide).
It is also possible to provide a small power plant (e.g. 60 Mw), indicated at
201 in Fig. 7, which uses a small portion (much less than 50%) of the warmed
gas
as fuel to continually produce electric power. The power is used perhaps one
day
in five or seven primarily to pump sea water in the heat exchanger and to
pressurize gas. During the other 4 days out of 5 or 6 days out of 7, the power
is
sent to shore along the power line 194.
Fig. 8 illustrates a system 210 which includes a floating structure 212 that
is
moored through its turret 214 to the seafloor. A riser (one or more risers)
216
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carries gas to a seafloor reservoir 220 and to a pipeline 222 that extends
along the
seafloor to shore. An electric power line 224 that extends primarily along the
seafloor, extends from the turret and over a buoy 226 and along the seafloor
226
to a facility on shore. The floating structure carries a gas-powered generator
230
that generates electricity for energizing a unit 231 for regasing (heating)
LNG from
a tanker (not shown) as by pumping sea water through a heat exchanger, and for
pressurizing the gas. When not regasing or pumping, a switch arrangement 232
diverts the generated electric power through line 224 to an onshore facility
at P, as
to add to electricity generated by a local electric utility. Electricity can
instead be
transferred from a local utility to the power line to power equipment.
In environments that are subject to occasional harsh weather conditions
such as a heavy storm or hurricane, the riser can be constructed to be
disconnected from the floating structure, and laid down on the seafloor or
floated
in a submerged position. The floating structure can be disconnected from the
riser
and from its mooring system, and can be towed away, to be later reinstalled.
Thus, the invention provides a gas offloading and transfer system for
transferring gas from a tanker (wherein the gas is stored in a liquid-like
state such
as LNG) to an undersea or underground cavern and/or to the shore. The system
can be constructed at moderate cost even when it must lie in a sea of
considerable
depth. The system includes a floating structure such as a barge, which is
moored,
as by catenary chains, to the seafloor. In most cases the floating structure
is
moored so it weathervanes, to change direction so as to always face the sea in
the
direction of least resistance. A tanker that brings the gas to the barge is
moored
to weathervane with the floating structure, so the tanker and floating
structure can
remain attached to one another during offloading in the open sea. A
weathervaning
tanker could not be easily moored to a fixed platform in an open sea. In one
system, the floating structure is a weathervaning barge. In another system,
the
floating structure is a direct attachment floating structure that, by itself,
may not
have a bow end that turns to always faces upwind, but which attaches to a
tanker
that is moored and thereby weathervanes with the tanker. An electric current-
carrying power cable can extend between the floating structure and a shore-
based
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electric power structure, to deliver electric power to the floating structure
to energize
pumps and other equipment, or to carry electricity from a power plant on the
floating structure to shore when not used at the floating structure.
