BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of a vessel that contains a cargo venting system of the invention.
FIG. 2 is a sectional view of one of the tanks of the vessel of FIG. 1.
FIG. 3 is a partial sectional view of a pair of tanks of the vessel of FIG. 1.
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
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).
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.
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.
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.
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.
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.
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.
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° C. Gaseous natural hydrocarbons include four major components, including methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10); their normal boiling point temperature are, respectively, −162° C., −89° C., −42° C. and −12° C.
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.
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.
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.
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° 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.
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.
Patent Application
20080011219
