Abstract:
A vessel ( 14 ) that stores crude oil in tanks ( 12 A- 12 H) 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.

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  12 A- 12 H 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  12 A- 12 H. 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  12 A and  12 B 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  54 A and  54 B that each opens to the upper portion of an oil tank  12 A- 12 H (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  56 A and  56 B 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° C. Gaseous natural hydrocarbons include four major components, including methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ) and butane (C 4 H 10 ); their normal boiling point temperature are, respectively, −162° C., −89° C., −42° C. and −12° 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  12 A- 12 H. 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° 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.