Refrigeration – Storage of solidified or liquified gas – Liquified gas transferred as liquid
Reexamination Certificate
2001-05-23
2003-04-15
Capossela, Ronald (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Liquified gas transferred as liquid
C062S240000
Reexamination Certificate
active
06546739
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to liquefied natural gas regasification. More particularly, the invention relates to offshore LNG regasification.
BACKGROUND OF THE INVENTION
World natural gas consumption is rising faster than that of any other fossil fuel. About two-thirds of the increase in gas demand is in the industrial and power generation sectors, while the remaining one-third is in space heating of buildings and homes. Recent technological improvements in the design, efficiency, and operation of combined cycle gas turbines have tilted the economics of power generation in favor of natural gas. With the demand for electricity constantly increasing, the demand for natural gas can be expected to increase even further.
As is the case with oil, natural gas is unevenly distributed throughout the world. More than one-third of the world's gas reserves are in the territory of the Former Soviet Union. The second largest gas reserve is located in the Middle East. However, North America accounts for more than one-half of the world's gas consumption. The United States alone consumes about 2.4 tcf more natural gas per year than it produces. Germany and Japan also import large amounts of natural gas each year. Thus, natural gas frequently needs to be transported from its production locations to the consumption locations. However, the low density of natural gas makes it more expensive to transport than oil. A section of pipe in oil service can hold 15 times more energy than when used to transport high pressure gas. An alternative method of natural gas transportation is by ships. While natural gas can be piped in a gaseous state, it needs to be liquefied so that it may be economically transported by ships. When natural gas is supercooled to minus 162° C., it becomes liquid, and takes up only {fraction (1/600)}th as much space as gas. Liquefaction makes it practical to ship natural gas in large volumes, using specially designed ships that maintain the cargo's ultra-low temperature. Once liquefied natural gas (LNG) is transported to its destination, it is converted into gas at a regasification terminal before it is sent to the consuming end. Thus, regasification terminals are important links in the natural gas supply chain.
Most regasification units in operation are located at onshore terminals.
FIG. 1
illustrates one such terminal. As shown in
FIG. 1
, an onshore LNG regasification terminal typically consists of a pier or jetty
1
, storage tanks
2
, and regasification plants
3
. An LNG ship
5
comes and berths at the pier
1
, and off-loads its cargo of LNG to storage tanks
2
which keep the gas in the same liquid state as they are transported. LNG in the storage tank
2
is later regasified at the regasification plant
3
to produce natural gas which is then transferred to end users through pipelines (not shown).
The storage tanks
2
typically are double barrier tanks with an “interior” container installed inside an independently reinforced concrete caisson. Built of concrete and steel, the inner tanks typically are made of 9% nickel steel and the secondary containers are typically made of pre-stressed concrete with a steel liner. The regasification plant
3
(or regasification unit) typically consists of heat exchangers (vaporizers)
3
a
, pumps
3
b
, and compressors
3
c
. Regasifying or regasification means bringing the cold LNG to the gaseous state at the ambient temperature and proper pressure so that it can be exported and fed into the existing pipeline grid for sale and transport to the consuming end.
To date, most LNG regasification facilities have been built onshore. However, public concern about safety has caused the gas industry to look for remote sites for such facilities. One alternative is to build the regasifi cation facility offshore. Various offshore terminals with different configurations and combinations have been proposed. Most of these offshore designs are based on large floating barges installed to mooring systems. As shown in
FIG. 2
, an offshore regasification terminal typically includes a barge
16
with storage tanks
11
and means (not shown) for a vessel to approach, berth and offload its cargo. The barge
16
includes at least one regasification unit
12
and a connector
13
that is adapted to connect to an underwater pipeline
15
via a riser
14
. Offshore LNG regasification terminals offer potential advantages over their onshore counterparts because they are further removed from populated areas thus minimizing risk to neighboring areas and reducing ship traffic and minimizing ships traveling in restricted waterways.
In an offshore terminal, the storage tanks
11
are incorporated in a barge
16
that supports the tanks. The storage tanks
11
may be membrane or non-membrane (freestanding) tanks. The main difference between these two types of tanks is how they are insulated. Membrane tanks are typically made with an inner liner of, for example, stainless steel or a specialized alloy such as invar (35% nickel steel). Non-membrane (freestanding) tanks are either spherical or prismatic and are typically made of aluminum or 9% nickel steel. In membrane tanks, insulation is built outside the liner in a manner that allows circulation of an inert gas, usually liquid nitrogen, through the insulating material, in order to monitor the integrity of the barrier. In non-membrane tanks, whether spherical or prismatic, the insulation is built and applied to the outside surface of the tanks.
Both types of tanks, whether prismatic or spherical, and whether membrane insulated or not, have been proposed for use in offshore LNG regasification systems. However, prismatic tanks are preferable, because as in the ships they allow for a more rational use of the space available in the offshore barge. As is the case for onshore terminals, in order to export the gas into the pipeline system, the cold-stored LNG must be brought to ambient temperature and the corresponding pipeline pressure. This is accomplished at the regasification unit
12
fitted onboard the barge. The regasification unit
12
is usually built on top of the tanks
11
, in case of prismatic tanks, or around and between them, in case of spherical tanks (not shown).
The flow of gas from the barge
16
to the onshore pipeline system (not shown) may be accomplished through a riser
14
connected to the sea bottom where an underwater pipeline
15
receiving end exists. The riser
14
connection at the barge end may be made through a fixed point in the case where the barge
16
is spread-moored, with mooring lines directly attached to several points on the barge
16
. The riser
14
connection may also be through a turret system such as shown at
13
, that provides a common end for the moored lines
17
, and connects the riser
14
through a swivel (not shown), so that the barge
16
may weathervane due to change of direction of the environmental conditions while gas is flowing to the riser
14
. Instead of the turret system
13
, the barge
16
may also be moored to a CALM buoy (not shown), that also provides single point mooring, and thus weathervaning, with the mooring system attached to buoy itself and thus independent of the barge
16
. The preferred solution is for the barge to
16
weathervane through a connector such as a turret or CALM buoy system. This scheme allows the ships carrying LNG to approach and moor alongside the barge
16
thus allowing side-byside offloading the LNG cargo from the ships; side-by-side offloading is more convenient. However, in order to conveniently and safely moor the LNG ships alongside the barge
16
, the barge
16
has to be longer than any conventional LNG carrier.
U.S. Pat. No. 6,089,022, issued to Zednik et al., discloses a method to regasify LNG onboard an LNG tanker before transferring the gas to an onshore facility. This approach requires that each LNG tanker be equipped with a vaporizer. A specially designed FSRU (floating LNG storage and regasification unit) has also been designed based on a tanker type double-hulled vessel per
Cappoen Leo Florent Lucien
De Laender Johan Robert Karel
Frimm Fernando C.
Capossela Ronald
Exmar Offshore Company
Rosenthal & Osha L.L.P.
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