U.S. patent application number 11/477887 was filed with the patent office on 2008-01-17 for enhanced cargo venting system.
Invention is credited to Jos Bronneberg, Dirk de Vos.
Application Number | 20080011219 11/477887 |
Document ID | / |
Family ID | 38947960 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080011219 |
Kind Code |
A1 |
Bronneberg; Jos ; et
al. |
January 17, 2008 |
Enhanced cargo venting system
Abstract
A vessel (14) that stores crude oil in tanks (12A-12H) in the
vessel hull, maintains oxygen-free gas in the tank spaces (32) that
lie above the crude oil. This is achieved by flowing in
hydrocarbon-inert gas (gas that does not react with hydrocarbons),
when crude oil is removed so the crude oil level drops, and by
flowing out vent gas that includes the flowed-in hydrocarbon-inert
gas and gaseous hydrocarbons that come from the crude oil, when
crude oil flows into the tank and the oil level rises. A vacuum is
applied to the vent pipe by connecting the vent pipe to an eductor
(102) through which pressured gas is flowed. The hydrocarbon-inert
gas is a fuel gas, which is a gas such as propane that is stored
and used to fuel engines on the vessel.
Inventors: |
Bronneberg; Jos; (US)
; Vos; Dirk de; (Eze sur Mer, FR) |
Correspondence
Address: |
FREILICH, HORNBAKER & ROSEN;PROFESSIONAL CORPORATION
SUITE 1220, 10960 WILSHIRE BOULEVARD
LOS ANGELES
CA
90024-3702
US
|
Family ID: |
38947960 |
Appl. No.: |
11/477887 |
Filed: |
June 29, 2006 |
Current U.S.
Class: |
114/74R |
Current CPC
Class: |
B63B 25/12 20130101;
B63B 25/08 20130101 |
Class at
Publication: |
114/74.R |
International
Class: |
B63B 25/08 20060101
B63B025/08 |
Claims
1. A method for assuring maintenance of oxygen-free gas in the
empty spaces of crude oil-holding tanks in a vessel, which includes
passing inert gas that is inert to hydrocarbons, through a gas pipe
into upper tank portions that lie above crude oil in the tanks at
least when the crude oil levels decrease, and withdrawing vent gas
that lies in said upper portions of the tanks through a vent gas
pipe at least when the tank crude oil levels increase, wherein:
said step of passing inert gas into upper tank portions, includes
storing gaseous hydrocarbons which are gaseous at 10.degree. C. and
flowing said gaseous hydrocarbons into said upper tank
portions.
2. The method described in claim 1 wherein: said step of storing
gaseous hydrocarbon includes flowing some of said stored gaseous
hydrocarbons to an engine and using the stored gaseous hydrocarbons
as a fuel for said engine.
3. The method described in claim 1 including: recovering at least
some of the gaseous hydrocarbons that flow through said vent gas
pipe and returning the recovered gaseous hydrocarbons to a gas
storage tank.
4. The method described in claim 1 including: passing said gaseous
hydrocarbons under pressure through an eductor while also flowing
said gas from said vent gas pipe in a downstream direction into the
eductor, and recovering at least some of the gaseous hydrocarbons
flowing through the eductor and storing the recovered gaseous
hydrocarbons.
5. A vessel that includes a hull that is moored at a mooring
location, the vessel having a plurality of oil tanks that each is
designed to hold crude oil, and a system for keeping oxygen out of
tank upper spaces that lie above crude oil in the oil tanks,
comprising at least one inert gas pipe having a plurality of
outlets connected to said tank upper spaces to supply inert gas
thereto when oil is withdrawn from the tanks, and at least one vent
gas pipe having inlets connected to said tank upper spaces to take
away gas from said tank upper spaces when oil is flowed into the
tanks, including: a gas storage tank that contains gaseous
hydrocarbon which are gaseous at 10.degree. C. and atmospheric
pressure, said gas storage tank connected to said inert gas pipe to
flow said gaseous hydrocarbon therealong and into said outlets.
6. The vessel described in claim 6 including: means for recovering
gaseous hydrocarbons that flow through said at least one vent pipe,
and for flowing at least some of the recovered gaseous hydrocarbons
to said gas storage tank.
7. The vessel described in claim 6 wherein: said means for
recovering includes an eductor and a pump that pumps gaseous
hydrocarbons from said gas storage tank to said eductor, with said
at least one vent pipe also connected to said eductor to form a
combined stream, and means for recovering gaseous hydrocarbons from
said combined stream.
Description
BACKGROUND OF THE INVENTION
[0001] Crude oil such as that produced from an undersea hydrocarbon
reservoir, is a liquid that is usually stored in tanks in the
vessel hull. Components of the crude oil evaporate and constitute
gaseous components that fill the upper spaces of the tanks. It is
important to keep oxygen out of the tanks, to avoid a fire. When
crude oil is removed from the tanks so the level of crude oil
falls, hydrocarbon-inert gas (gas that does not react with
hydrocarbons) is flowed into the tanks to provide a gas blanket
over remaining crude oil, to prevent the entrance of air. When
crude oil flows into the tanks so the level of crude oil rises, gas
of the gas blanket is removed from the tanks. The removed, or
vented gas includes hydrocarbons evaporated from the crude oil
components stored in the tanks, in addition to the
hydrocarbon-inert gas that was earlier flowed into the tanks. It is
important to provide for the safe disposal of vented or vent gas
containing hydrocarbon gas.
SUMMARY OF THE INVENTION
[0002] In accordance with one embodiment of the invention,
applicant provides a system for maintaining oxygen-free gas in the
upper spaces of crude oil-containing tanks of a vessel, which is
versatile and safe. The system includes an inert gas pipe that
carries hydrocarbon inert gas (gas that does not react with
hydrocarbons) and that has outlets that open into each of the
tanks. The system also includes a vent gas pipe that has inlets
that open into each of the tanks. Whenever oil flows out of a tank
so the oil level falls, hydrocarbon-inert gas flows into the tank
to maintain a gas pressure therein that avoids the leaking in of
air and the consequent leaking in of oxygen that constitutes part
of air. Whenever oil flows into a tank so the oil level rises, some
of the vent gas that lies in the tank exits the tank and flows
though the vent pipe and may be merely released into the atmosphere
or burned at a flare, or may be recovered.
[0003] The hydrocarbon-inert gas that is flowed into the tanks when
oil is removed, can be a gaseous hydrocarbon such as propane, that
is stored on the vessel and that may be used as a fuel for
engine(s) on the vessel. The propane also can be pressurized to be
used in an inductor that helps move gas out of the tanks. The
propane can be recovered by cooling the output of the eductor, or
the vent gas can be merely stored and used to fuel the engine or
provide gas blankets for the tanks.
[0004] 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 DRAWINGS
[0005] FIG. 1 is an isometric view of a vessel that contains a
cargo venting system of the invention.
[0006] FIG. 2 is a sectional view of one of the tanks of the vessel
of FIG. 1.
[0007] FIG. 3 is a partial sectional view of a pair of tanks of the
vessel of FIG. 1.
[0008] FIG. 4 is an isometric view of apparatus for boosting the
pressure of vent gas to be burned in the system of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] FIG. 1 illustrates a system 10 which includes a floating
structure such as a vessel 14 that has a hull 15 with crude oil
tanks 12A-12H that store crude oil. The particular vessel 14 is a
barge of a FPSO type (floating, production, storage, and
offloading) that has a turret 16 that is anchored to the sea floor
by chains 20, although a spread moored or other type of mooring can
be used. Crude oil from an undersea reservoir passes up through
risers 22 to fluid swivels 24, from which the crude oil flows to a
crude processor 26. The processor separates out crude oil from
sand, gaseous hydrocarbons, etc. and passes the crude oil to the
tanks 12A-12H. A gas flare may be provided for the processor.
Gaseous hydrocarbons or only some of them, may be stored in a tank
28 (under pressure and/or cold temperature).
[0010] FIGS. 2 and 3 illustrate two of the crude oil or oil tanks
12A and 12B that are each about half filled with liquid
hydrocarbons 30, which may be referred to as crude oil. The space
32 above the crude oil is initially filled with an inert gas, which
is herein defined as a gas that does not chemically react with
hydrocarbons. The inert gas may be nitrogen obtained by
liquefaction of air, or flue gas obtained from a boiler room 40
(FIG. 1) by burning hydrocarbons with air, in which case the flue
gas comprises nitrogen and carbon dioxide. The inert gas is
supplied to oil tanks during offloading of oil to fill the space
previously occupied by crude oil. During loading, the inert gas and
hydrocarbon gas in the space 32 is displaced and passes (though a
vent pipe 52) and may be released into the atmosphere.
[0011] Some of the hydrocarbons stored in the oil tanks (the more
volatile components) evaporate and lie in the space 32 above the
liquid hydrocarbons along with the inert gas. Air is prevented from
entering an oil tank when crude oil lies in the tank, to prevent
fire (which requires the oxygen in air to burn with hydrocarbons).
Air is allowed in an oil tank (by passing it in through a separate
air pipe 34 dedicated to that function) only during maintenance
when crude oil has been removed and personnel must enter an oil
tank. Each oil tank holds many thousands of gallons of crude oil,
and if the space above them should hold some air and ignite, there
could be a disaster.
[0012] A pair of pipes 50, 52 are provided that each extends along
at least about half of the length of the vessel. Pipe 50 is an
inert gas pipe that carries inert gas (inert to hydrocarbons). The
inert pipe 50 has a plurality of outlets 54 such as 54A and 54B
that each opens to the upper portion of an oil tank 12A-12H (and
other, auxiliary tanks) to supply inert gas to the space above the
oil, or liquid hydrocarbons. Vent pipe 52 has a plurality of inlets
56, such as 56A and 56B that each receives gas from the space 32 at
the top of a crude oil storage tank. As a result, there is a flow
of inert gas though an outlet 54 into each oil tank during
offloading of crude oil, and there is an outflow of gas though an
inlet 56 during the filling of each oil tank. The outflowing gas
includes some of the inert gas earlier admitted into the upper tank
space though an inert gas outlet 54, and also includes gaseous
hydrocarbons that have evaporated into the upper tank space.
[0013] Applicant prefers to provide check valves at 60 and 62 (FIG.
3) along the inert gas outlets and along the vent gas inlets, to
assure proper flow of gasses. The inert pipe check valves 60 assure
that gas flows only into an oil tank though an inert gas inlet 54,
and prevents the flow of vent gas (with entrained hydrocarbons)
into the inert gas pipe 50. The vent pipe check valves 62 assure
that gas flows only out of an oil tank through a vent gas outlet,
and prevents the flow of vent gas (with entrained hydrocarbons)
from flowing into a tank instead of inert gas flowing into the
tank. So long as inert gas is available to flow into the inert pipe
50 and vent gas can flow though and out of the vent pipe 52, during
the flow of crude oil into or out of the tanks, the cargo venting
system of the invention will operate properly. Applicant notes that
safety valves (not shown) are also connected to the oil tanks to
assure that the pressure in each tank is not more or less than 2.5
psi (1800 mm of water) different from atmospheric pressure.
[0014] Applicant may release vent gas into the environment though a
vent gas outlet 60 (FIG. 1). Under most conditions, there is a
breeze and the released vent gas dissipates. However, if the
weather is very calm so there are little if any winds, then there
is a possibility that the released vent gas will accumulate on the
deck 62 of the vessel. The hydrocarbons in the vent gas then could
catch fire. To prevent this, applicant provides a flare system
(which includes headers, and a drum and flare tip) which includes a
flare tip at 60. The flare tip is lit to burn the gaseous
hydrocarbons of the combination of inert gas and hydrocarbons that
has passed along the vent pipe 52. The outlet or flare 60 lies at
the stern of the weathervaning vessel, so any winds carry the gas
(burned or unburned) away from the vessel.
[0015] FIG. 4 illustrates a flare system 100 that is connected to
the vent pipe 52 that carries hydrocarbons resulting from
evaporations from stored crude oil and inert gas, and that includes
the flare 60. The flare system includes an eductor 102 that
receives inert pressured gas from a source 104. The eductor has a
region 106 of reduced diameter, where the pressure is reduced, and
to which the vent pipe is connected and points downstream. The
eductor boosts the pressure of gas supplied to the flare when the
vent gas is to be burned, because the flare working pressure is
between 1 and 10 bars (about one and ten atmospheres). The eductor
mixes the vent gas and the pressured inert gas from the source 104,
and delivers the combination to a flare drum 110 that separates
liquid from gas and drains away any liquids at 112. The gases are
burned at the flare 60.
[0016] In accordance with the present invention, applicant uses a
gaseous hydrocarbon, such as propane, as the hydrocarbon-inert gas
that is flowed into oil-holding tanks when oil is removed from the
tanks. Applicant provides a gas tank 120 (FIG. 1) that holds
gaseous hydrocarbons (in a liquid or gas state), which is here
defined as hydrocarbons that assume a gaseous state at an
environment temperature of 10.degree. C. Gaseous natural
hydrocarbons include four major components, including methane
(CH.sub.4), ethane (C.sub.2H.sub.6), propane (C.sub.3H.sub.8) and
butane (C.sub.4H.sub.10); their normal boiling point temperature
are, respectively, -162.degree. C., -89.degree. C., -42.degree. C.
and -12.degree. C.
[0017] Applicant usually obtains gaseous hydrocarbons from the
crude processor 26 and stores them in the gas tank 28. The gaseous
hydrocarbons are used as fuel for engines on the vessel, such as
engine 120 that drives a generator to generate electricity. Instead
of using the gaseous hydrocarbons to generate flue gas, applicant
can flow the gaseous hydrocarbons directly into the pipe 50 that
carries inert gas to the crude oil tanks 12A-12H. Of course,
gaseous hydrocarbons do not contain oxygen, and are inert to
evaporations from crude oil.
[0018] In FIG. 4, a source 104 of pressured inert gas flows through
the eductor 102 to draw in vent gas flowing along pipe 52 from the
oil tanks. The gases flowing at 52 and 104 combine and flow into a
drum 110. Where substantially all of the gas from the source 104
and the vent pipe 52 is hydrocarbon, the gas can be returned to the
gaseous hydrocarbon storage tank 28 and can be used to fuel engines
and can be used again to fill oil tanks from which crude oil is
being removed. To achieve this, application includes a pump 124
that is connected to the gas storage tank (28) to pump gaseous
hydrocarbons through the eductor.
[0019] Where a considerable percent (e.g. over 20%) of the gas
flowing into the drum is not hydrocarbon, then the drum can be
cooled to liquefy heavier hydrocarbons (propane and butane). The
rest of the gas can be flared (burned by flare 60). The heavier
gaseous hydrocarbons such as propane and butane are easier to store
in the gas tank 28, because they can be stored as LNG (liquefied
natural gas) at moderately cold temperatures and moderate
pressures. Lighter gaseous hydrocarbons such as methane and ethane
can be stored as gas in another tank and can be the first gas used
as fuel or as an oil tank blanket.
[0020] Thus, the invention provides a system for maintaining an
oxygen-free blanket over oil in crude oil tanks despite oil flowing
into or out of the tanks. The system includes an inert gas supply
pipe that carries hydrocarbon-inert gas to the tanks, and a
separate vent pipe that carries away gas from the tanks. The
hydrocarbon-inert gas can be a hydrocarbon that is gaseous at
10.degree. C., and that is stored for use as fuel for engines on
the vessel. Pressured hydrocarbon gas can be used in an inductor to
draw gas out through the vent pipe, and some or all of the
hydrocarbon gas passing through the eductor can be captured and
reused.
[0021] 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.
* * * * *