Enhanced cargo venting system

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.

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° 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° 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.