GB2160852A

GB2160852A - Off-loading crude oil

Info

Publication number: GB2160852A
Application number: GB08416180A
Authority: GB
Inventors: John Frederick Ridley
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-06-25
Filing date: 1984-06-25
Publication date: 1986-01-02
Also published as: GB8416180D0; GB2160852B

Abstract

A method of off-loading crude oil from a cargo vessel (10) having a plurality of tanks (11) containing crude oil, delivery apparatus for pumping oil from the tanks to deck level for discharge ashore, and ducting (12) interconnecting the tanks and the delivery apparatus, comprises introducing water into the ducting and delivery apparatus and discharging it therethrough in order to displace residual oil remaining therein after the tanks have been drained as far as possible using the delivery apparatus. <IMAGE>

Description

SPECIFICATION Off-loading crude oil This invention relates to a method of off-loading crude oil from a cargo vessel.

The invention provides a method of off-loading crude oil from a cargo vessel of the kind having a plurality of tanks for containing crude oil, delivery apparatus for pumping oil from the tanks to deck level for discharge from the vessel and ducts interconnecting the tanks and the said apparatus, which method comprises the steps of: draining the oil from the tanks as far as possible using the delivery apparatus, introducing water into the ducts and delivery apparatus, and then discharging the water through the delivery apparatus in order to displace the oil which was left therein after the initial step of draining the oil from the tanks.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawing which is a diagrammatic perspective view showing a typical cargo vessel.

The drawing illustrates a typical cargo vessel 10 with a plurality of tanks, one of which is designated 11, which contains crude oil. There can be as many as 30 such tanks in some vessels. The vessel 10 is equipped with a pipeline system 12 which connects each of the tanks 11 with a pumping station 13 at the aft end of the vessel. The pumping station 13 pumps the oil from the tanks via the pipeline system 12 up to deck level and to discharge manifolds 14 which are usually situated amidships. Oil is then pumped ashore using flexible pipes, or chicksans, which are coupled up to the discharge manifolds 14, from whence the oil is distributed into storage tanks ashore.

Conventionally, discharging of the oil is stopped once air starts to appear at the outlet cock of each chicksan, and a vacuum breaker situated about half way along each chicksan is opened. At this, the oil remaining in the discharging system up to the vacuum breaker simply drains back into the vessel.

This, together with a residue of oil which is left in each tank, is known as the remaining on board (ROB) quantity. The ROB quantity typically varies between about 0.15 and 0.26% ofthe total cargo which, when cargoes of as much as 250 000 tons are carried, is a significant amount of oil. Since payment is generally made by a purchaser on the basis of the amount of oil shown on the bill of lading, it is clearly desirable to be able to reduce the ROB quantity to a minimum.

The cargo tanks themselves are hardly ever emptied completely of oil, although the residue of oil which is left in the tanks is kept to a minimum by draining the tanks using a positive displacement pump, which is part of the so-called stripping equipment on the vessel.

Nevertheless, there still remains the fact that oil is left in the discharging pipeline system itself after the tanks have been drained as far as possible. The present invention proposes displacing the oil which is left in the discharging pipeline system by pumping water therethrough after discharging of the oil cargo, thereby achieving a lower retention of ROB.

The method is as follows: The tank which is furthest from the pumping station, i.e. the one designated 11 in the drawing, and which therefore has the longest section of connecting pipework, is preferably emptied first (this is usually the foremost tank in the vessel, and will be referred to as number 1 centre tank). When oil has been discharged from number 1 centre tank and the tank has been drained as far as possible using the stripping equipment, water, say, between 250 and 500 tons, is introduced into the tank.This can be done in two ways: using the water supply system on the deck of the vessel (the so-called fire/butterworth pipeline) connected with a long flexible hose which must reach the bottom of the tank to avoid the risk of build up of static electricity associated with a free fall of water; or using the ballast system of the vessel if the system is one in which the cargo tanks can be connected with the ballast pump and associated pipework.

When oil has been discharged from all the tanks and the tanks have been drained as far as possible using the stripping equipment, water from number 1 centre tank is pumped through the discharging pipeline system, displacing ahead of it the oil which was left in the system. The shore personnel can continue with distributing oil to the storage tanks ashore up to the point when water starts to appear at the jetty sampling point. There is a small amount of mixed water and oil, and this is merely drained into a slop tank ashore and left to separate. Now, when the vacuum breaker is opened, the discharging system will be full of water, rather than oil, and this water is simply allowed to drain back into the vessel's slop tank.

The method can be expected to save about 60% of the usual ROB quantity. Of course, more oil could be saved if the operation were to be repeated in more of the tanks of the vessel and hence for more of the pipeline system. However the amount of additional oil that could be recovered thereby would become progressively less and it would take some considerable time, which is not desirable as docking time is expensive. Because docking time is expensive, it is preferable that number 1 centre tank be emptied of oil first so that it can be filled with water whilst the other tanks are being emptied of oil. If the method is used properly, it need not add more than an hour to the discharging time for the vessel. The method works equally well whether the shore facilities permit off-loading via a single deck pipeline or twin pipeline or more.

1. A method of off-loading crude oil from a cargo vessel of the kind having a plurality of tanks for containing crude oil, delivery apparatus for pumping oil from the tanks to deck level for discharge from the vessel and ducts interconnecting the tanks and the said apparatus, which method comprises the steps of: draining the oil from the tanks as far as possible using the delivery apparatus, introducing water into the ducts and delivery

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Off-loading crude oil This invention relates to a method of off-loading crude oil from a cargo vessel. The invention provides a method of off-loading crude oil from a cargo vessel of the kind having a plurality of tanks for containing crude oil, delivery apparatus for pumping oil from the tanks to deck level for discharge from the vessel and ducts interconnecting the tanks and the said apparatus, which method comprises the steps of: draining the oil from the tanks as far as possible using the delivery apparatus, introducing water into the ducts and delivery apparatus, and then discharging the water through the delivery apparatus in order to displace the oil which was left therein after the initial step of draining the oil from the tanks. By way of example, an embodiment of the invention will now be described with reference to the accompanying drawing which is a diagrammatic perspective view showing a typical cargo vessel. The drawing illustrates a typical cargo vessel 10 with a plurality of tanks, one of which is designated 11, which contains crude oil. There can be as many as 30 such tanks in some vessels. The vessel 10 is equipped with a pipeline system 12 which connects each of the tanks 11 with a pumping station 13 at the aft end of the vessel. The pumping station 13 pumps the oil from the tanks via the pipeline system 12 up to deck level and to discharge manifolds 14 which are usually situated amidships. Oil is then pumped ashore using flexible pipes, or chicksans, which are coupled up to the discharge manifolds 14, from whence the oil is distributed into storage tanks ashore. Conventionally, discharging of the oil is stopped once air starts to appear at the outlet cock of each chicksan, and a vacuum breaker situated about half way along each chicksan is opened. At this, the oil remaining in the discharging system up to the vacuum breaker simply drains back into the vessel. This, together with a residue of oil which is left in each tank, is known as the remaining on board (ROB) quantity. The ROB quantity typically varies between about 0.15 and 0.26% ofthe total cargo which, when cargoes of as much as 250 000 tons are carried, is a significant amount of oil. Since payment is generally made by a purchaser on the basis of the amount of oil shown on the bill of lading, it is clearly desirable to be able to reduce the ROB quantity to a minimum. The cargo tanks themselves are hardly ever emptied completely of oil, although the residue of oil which is left in the tanks is kept to a minimum by draining the tanks using a positive displacement pump, which is part of the so-called stripping equipment on the vessel. Nevertheless, there still remains the fact that oil is left in the discharging pipeline system itself after the tanks have been drained as far as possible. The present invention proposes displacing the oil which is left in the discharging pipeline system by pumping water therethrough after discharging of the oil cargo, thereby achieving a lower retention of ROB. The method is as follows: The tank which is furthest from the pumping station, i.e. the one designated 11 in the drawing, and which therefore has the longest section of connecting pipework, is preferably emptied first (this is usually the foremost tank in the vessel, and will be referred to as number 1 centre tank). When oil has been discharged from number 1 centre tank and the tank has been drained as far as possible using the stripping equipment, water, say, between 250 and 500 tons, is introduced into the tank.This can be done in two ways: using the water supply system on the deck of the vessel (the so-called fire/butterworth pipeline) connected with a long flexible hose which must reach the bottom of the tank to avoid the risk of build up of static electricity associated with a free fall of water; or using the ballast system of the vessel if the system is one in which the cargo tanks can be connected with the ballast pump and associated pipework. When oil has been discharged from all the tanks and the tanks have been drained as far as possible using the stripping equipment, water from number 1 centre tank is pumped through the discharging pipeline system, displacing ahead of it the oil which was left in the system. The shore personnel can continue with distributing oil to the storage tanks ashore up to the point when water starts to appear at the jetty sampling point. There is a small amount of mixed water and oil, and this is merely drained into a slop tank ashore and left to separate. Now, when the vacuum breaker is opened, the discharging system will be full of water, rather than oil, and this water is simply allowed to drain back into the vessel's slop tank. The method can be expected to save about 60% of the usual ROB quantity. Of course, more oil could be saved if the operation were to be repeated in more of the tanks of the vessel and hence for more of the pipeline system. However the amount of additional oil that could be recovered thereby would become progressively less and it would take some considerable time, which is not desirable as docking time is expensive. Because docking time is expensive, it is preferable that number 1 centre tank be emptied of oil first so that it can be filled with water whilst the other tanks are being emptied of oil. If the method is used properly, it need not add more than an hour to the discharging time for the vessel. The method works equally well whether the shore facilities permit off-loading via a single deck pipeline or twin pipeline or more. CLAIMS

1. A method of off-loading crude oil from a cargo vessel of the kind having a plurality of tanks for containing crude oil, delivery apparatus for pumping oil from the tanks to deck level for discharge from the vessel and ducts interconnecting the tanks and the said apparatus, which method comprises the steps of: draining the oil from the tanks as far as possible using the delivery apparatus, introducing water into the ducts and delivery apparatus, and then discharging the water th rough the delivery apparatus in order to displace the oil which was left therein after the initial step of draining the oil from the tanks.

2. A method as claimed in claim 1 wherein the step of introducing water into the ducts and delivery apparatus is carried out by introducing water into one of the tanks after that tank has been drained of oil.

3. A method as claimed in claim 2 wherein the said tank is emptied first in the initial step of discharging the oil from the vessel so that water can be introduced into the said tank while the remaining tanks are being emptied.

4. A method as claimed in claim 2 or claim 3 wherein the said tank is the one which is furthest from the delivery apparatus.

5. A method as claimed in claim 2, claim 3 or claim 4 wherein the water is introduced using the vessel's water supply system on deck.

6. A method as claimed in claim 2 or claim 3 wherein the water is introduced by connection with the vessel's ballast pump and pipeline system.

7. A method substantially as hereinbefore described with reference to the accompanying drawings.