WO2002027068A1

WO2002027068A1 - Corrosion preventive system and method for transporting fresh water

Info

Publication number: WO2002027068A1
Application number: PCT/US2001/030717
Authority: WO
Inventors: Jens Korsgaard
Original assignee: Jens Korsgaard
Priority date: 2000-09-27
Filing date: 2001-09-27
Publication date: 2002-04-04
Also published as: AU2002211336A1

Abstract

A method of transporting fresh water includes the steps of: filling a portion of at least one cargo tank with fresh water; and filling a remaining portion of the at least one cargo tank with a gas, the gas being non-reactive with a surface of the at least one cargo tank to thereby prevent oxidation of the surface of the at least one cargo tank. An alternative method of transporting fresh water includes the steps of: filling at least one cargo tank with a gas; filling the at least on cargo tank with fresh water and displacing a corresponding volume of the gas; and storing the displaced gas; wherein the gas in non-reactive with a surface of the at least one cargo tank to thereby prevent oxidation of the surface of the at least one cargo tank. A corrosion protection system includes: at least one cargo tank; an arrangement configured to fill a portion of the at least one cargo tank with fresh water; and an arrangement configured to fill a remaining portion of the at least one cargo tank with a gas, the gas being non-reactive with a surface of the at least one cargo tank to thereby prevent oxidation of the surface of the at least one cargo tank.

Description

CORROSION PREVENTIVE SYSTEM AND METHOD FOR TRANSPORTING FRESH WATER

Background of the Invention

Field of the Invention

The present invention relates generally to the transport of fresh water at sea by tanker vessels that have steel cargo tanks that are wholly or partly uncoated and not protected against corrosion by a paint system.

Background Art

In a number of areas in the World fresh water resources are being strained and exploited to a level that the natural rainfall cannot replenish the water that is being used. Among such areas today is Israel and neighboring countries, North Africa bordering the Mediterranean, Saudi Arabia, and many other areas in the World. In many cases abundant water resources are available a short sea voyage away, thus if the transport is sufficiently inexpensive such transport may be a partial or full answer to providing an adequate fresh water supply.

Other solutions to the lack of water are desalination of seawater or brackish water, recycling of sewage to for example agricultural use, and reduction of water consumption by means of conservation measures.

Fresh water is an extremely price sensitive commodity. Therefore a particularly attractive solution to transport of fresh water is the use of old crude oil tankers that must be phased out of crude oil transport due to pollution risk concerns. Many of these tankers are well maintained vessels in good condition that can safely serve as fresh water carriers for many years more provided they can be adequately cleaned and provided that they can be operated in a manner that maintains the quality of water in the transport process. A traditional method consists of sandblasting the tanks to remove all rust and old paint. Following the removal of the rust and old paint, a paint system is applied to the tank floor, walls, and ceiling. If an appropriate paint system is used then contamination of the transported water may be completely avoided. These processes are however very expensive and adds significantly to the cost of transporting the water.

This invention teaches a method of transporting fresh water in tanker vessels with uncoated or partially coated steel tanks in a manner which minimizes corrosion of the tanks and minimizes contamination of the transported water.

Summary of the Invention

Some fresh waters are very aggressive from a corrosion point of view and could therefore cause rapid corrosion of the bare walls of steel tanks. This action would also raise the level of iron dissolved in the water, which may be undesirable.

Traditionally the corrosion in the tanks is avoided by cathodic protection. Sacrificial anodes made of zinc and aluminum are not suitable because of the resulting solution of these metals in the water, therefore, impressed current type cathodic corrosion protection would typically be used in this service. This is known technology. During the discharge of the water the tanks would normally be filled with atmospheric air and in this condition subject to corrosion. It is this second source of corrosion that is avoided by the present invention.

The typical oil tanker has a so-called inert gas system aboard that may be used to replace the atmosphere in the tanks. The purpose of this system is to prevent the formation of explosive oxygen-hydrocarbon .mixtures in the tanks when the vessel is in crude oil service. The inert gas system typically comprises systems that cool and wash the flue gases from the engines and circulate this gas in the tanks. This gas typically has about 80% nitrogen, 17% carbon dioxide and 3% oxygen. The gas cannot support combustion, however, it is highly corrosive in the presence of water because of the combined content of carbon dioxide and oxygen and therefore not suitable as a protective gas when the tanker returns empty.

The present invention teaches the filling of the space being emptied of water when the tanker discharges with a gas that does not react chemically with the tank walls and thereby preventing corrosion. This gas may for example be pure nitrogen, methane, ethane, c3 hydrocarbons, c4 hydrocarbons, a number of gases used for refrigeration systems, or a mixture of these gases. When combining the filling of the gas space with a gas that prevents corrosion and using impressed current cathodic protection to protect the tank surfaces that are immersed in water corrosion would be limited to the small areas that are alternately immersed in oxygen rich water and alternately above the water surface. The corrosion of this much-reduced area may further be avoided by removing oxygen from the water prior to shipping.

Thus corrosion of the tanks may be completely avoided by the procedures taught in this invention.

The present invention furthermore teaches the use of flammable gases as the protective gas for the cargo tanks, the compression and storage of these gases when the cargo being loaded drives the gas out of the tanks and the use of these gases as fuel for propulsion of the vessel. This reduces the volume of compressed gas storage aboard the vessel that is required when using gas as a fuel for the propulsion of the ship because compressed gas storage capacity is then only required for a one way trip .

Brief Description of the Drawings

Figure 1 shows the general arrangement of cargo tanks in a typical oil tanker.

Figure 2 shows a first embodiment of the invention in which the tanks in the tanker are filled with a gas not reacting with the tank surfaces when the tanker is empty.

Figure 3 shows a second embodiment of the invention in which the tanks in the tanker are filled with a flammable gas when the tanker is empty. The tanker has in addition a pressurized storage for flammable gas and the flammable gas is used for fuel aboard the tanker.

Figure 4 shows schematically the operations of the second embodiment .

Figure 5 shows schematically the operations of a third embodiment similar to the second embodiment in which the flammable gas is compressed on shore rather than aboard the tanker.

Detailed Description of the Preferred Embodiments

Figure 1 shows a plan view of a typical oil tanker. The tanker vessel 10 is divided into a number of compartments below deck shown with dotted lines. The vessel has a number of cargo tanks 11 and ballast tanks 13. The ballast tanks 13 are usually filled with seawater when the tanks 11 are empty. At the aft of vessel 10 the compartment 12 contains the machinery (not shown) of vessel 10. The machinery (not shown) includes the pumps and piping (not shown for distributing and pumping the cargoes and the ballast water. On the deck of the vessel 10.there is a manifold 15,16 permitting the connection of the piping 17 and 18 to shore piping (not shown) when the vessel is in port. The connection 15 and piping 18 permit the transfer of the cargo of the vessel. The vessel 10 is shown to have a manifold 15, 16 at only the port side of the vessel, however, it would usually also have a corresponding manifold (not shown) on the starboard side of the vessel. The vessel 10 also has a connection 16 and piping 17 that permit the transfer of gases contained in the top of the tanks 11 and 13. Additional connections (not shown) may be placed at the keel of the vessel to permit transfer via a submersible mooring buoy of the type described in US patent 5305703.

Figure 2 shows a first embodiment of the invention. Figure 2 shows a cut-away view of the tanker 10 shown in figure 1. The tanker 10 is shown at the destination port floating in the sea with surface 19 during discharge of a cargo of water in tanks 11. The tanker 10 is shown in partly discharged condition. In a typical tank 11 the water cargo 20 is at the bottom of tank 11 and the top of tank 11 is filled with a gas 21. The water 20 and the gas 21 are separated by the water surface 22. The gas 21 is a gas that does not react with the surfaces of tank 11 and thereby prevents the oxidation of the surfaces of tank 11 which would occur if gas 21 contained oxygen it for example being atmospheric air. Gas 21 may be any one of a number of different gases such as nitrogen, carbon monoxide, methane, ethane, propane, butane, or a combination thereof. Other possible gases include common refrigeration gases such as CFC and inert gases such as argon or neon. During the cargo transfer the water 20 is discharged through piping 18 to shore piping (not shown) . The internal distribution piping and the ship's pumps are not shown. The gas 21 may be received from shore piping (not shown) and distributed to tanks 11 through piping 17. Figure 2 also shows the tanker 10 at the loading port. Only in this case the water cargo 20 is received from shore by means (not shown) through piping 18 and the gas 21 is discharged to shore through piping 17 to receiving means (not shown) on shore .

Figure 3 shows a second embodiment of the invention in which the gas filling the tanks 11 when they are empty is compressed and stored in a high-pressure storage tank 34 for carriage during the loaded voyage when the tanks 11 are filled. Figure 3 shows the tanker 10 at the loading port (not shown) where the tanker 10 receives the cargo 20 of water. When tanks 11 are filled with water 20 the gas 21 in the tanks 11 is displaced by the rising water surface 22. This gas 21 is conveyed through pipes 17 and 31 to compressor 30 that compresses the gas and discharges the compressed gas through pipe 32 and valve 33 into storage tank 34. Storage tank 34 is for clarity shown placed vertically on the forecastle of vessel 10. Tank 34 would ordinarily be comprised of multiple tanks placed within the hull of tanker 10 or placed horizontally on the deck of tanker 10. The maximum operating pressure of tank 34 would depend on the composition of gas 21. In the event that the gas 21 can be easily liquefied at ambient temperature such as for example butane and propane the maximum operating pressure of tank 34 may be in the range of 0.4 to 2.0 MPa gage. In the event gas 21 is a natural gas comprised for example of ' a mixture of methane, ethane, and propane then the maximum operating pressure of tank 34 may be in the range of 10 Mpa to 25 Mpa gage. In the event gas 21 is an inert gas such as nitrogen then the maximum operating pressure of tank 34 may be in the range of 5 to 20 Mpa gage.

Figure 3 shows a pipe 41 between tank 34 and the machinery (not shown) in engine compartment 13 in vessel 10. If the gas 21 is a flammable gas such as natural gas then tank 34 may serve as the fuel supply for operating the tanker 10 by opening valve 40. This method of operating tanker 10 is particularly economical if the gas is available at the receiving point (not shown) of the water. In this case the tanks 11 may be filled with natural gas via the pipes 17 and the tank 34 may simultaneously be filled with compressed natural gas through compressor 30 when the cargo 20 is discharged from the tanker 10. The practicality of this method of operation may be illustrated by the following example. The tanker 10 may be a tanker with a capacity of 300,000 m3 of water, i.e., the combined volume of all tanks 11 is 300,000 m3. A tanker of this size would typically have a fuel consumption of 150 tonnes/day. The density of natural gas at atmospheric pressure and a temperature of 20 deg. C is approximately 1 kg/m3. Thus when the tanker 10 sails in ballast the tanks 11 are filled with approximately 300,000 kg of natural gas. The tank 34 may have a capacity of 300 tonnes of natural gas. Assume further that the one way voyage time is two days, then the fuel consumption one way matches the capacity and content of compressed gas tank 34. When receiving the cargo of water 20 at the loading port (not shown) the gas 21 is displaced by the cargo 20 and is transferred by compressor 30 to tank 34. Thus when all the tanks 11 have been filled the tank 34 will again contain sufficient fuel for a one-way voyage. The compressed gas tank 34 would typically cost approximately $15,000 per tonne of capacity of natural gas therefore this invention saves significant capital cost, saves fuel costs, and at the same time provides a non-corrosive protective gas in tanks 11 when they are empty.

Figure 4 shows schematically the operations of the first embodiment shown in figures 2 and 3.

Figure 5 shows schematically the operations of a third embodiment similar to the operation illustrated in figure 4. The means to compress the gas is in this embodiment located on shore and is not carried on the ship. Otherwise this embodiment is similar to the second embodiment.

The water that is carried by the tanker is usually oxygen rich with oxygen content on the order of 7 ppm. The surfaces of the tanks that are continually immersed in the water may be protected against corrosion by the known technology of impressed current cathodic protection. The cathodic protection is however not effective in preventing corrosion of the surfaces that are alternately immersed and above the water due to sloshing of the water in the tanks . Such corrosion may be avoided by removing the oxygen from the water before it being shipped. A range of known technologies not described herein may do this. Thus by combining the known technologies of cathodic protection, removal of oxygen from the water being shipped, and the maintenance of an inert atmosphere in the tanks as taught by this invention corrosion of the tanks in the tanker may be completely avoided.

Claims

WHAT IS CLAIMED IS:

1. A method of transporting fresh water, comprising the steps of:

(a) filling a portion of at least one cargo tank with fresh water; and

(b) filling a remaining portion of the at least one cargo tank with a gas, the gas being non-reactive with a surface of the at least one cargo tank to thereby prevent oxidation of the surface of the at least one cargo tank.

2. The method according to claim 1, wherein the gas includes at least one of nitrogen, carbon monoxide, methane, ethane, propane, butane, a refrigerant gas, a chlorofluorocarbon, an inert gas, argon and neon.

3. A method of transporting fresh water, comprising the steps of:

(a) filling at least one cargo tank with a gas;

(b) filling the at least one cargo tank with fresh water and displacing a corresponding volume of the gas; and

(c) storing the displaced gas; wherein the gas is ^' non-reactive with a surface of the at least one cargo tank to thereby prevent oxidation of the surface of the at least one cargo tank. .

4. The method according to claim 3, wherein the gas includes at least one of butane, propane, methane, ethane.

5. A corrosion protection system, comprising: at least one cargo tank; an arrangement configured to fill a portion of the at least one cargo tank with fresh water; and an arrangement configured to fill a remaining portion of the at least one cargo tank with a gas, the gas being non- reactive with a surface of the at least one cargo tank to thereby prevent oxidation of the surface of the at least one cargo tank.

6. The corrosion protection system according to claim 5, wherein the gas includes at least one of nitrogen, carbon monoxide, methane, ethane, propane, butane, a refrigerant gas, a chlorofluorocarbon, an inert gas, argon and neon.