US3746023A

US3746023A - Method for cleaning oil tanker holds

Info

Publication number: US3746023A
Application number: US00237669A
Authority: US
Inventors: B Smith
Original assignee: Gulf Oil Corp
Current assignee: Chevron USA Inc
Priority date: 1972-03-22
Filing date: 1972-03-22
Publication date: 1973-07-17
Anticipated expiration: 1990-07-17

Abstract

The hold in an oil tanker which has been emptied of its oil cargo is cleaned by first filling the hold with sea water ballast so that oil vapors are forced from the hold and a residual oil layer floats upon the sea water ballast. The floating oil layer is thereupon skimmed or otherwise removed from the aqueous ballast. The hold is then deballasted and washed with a jet of high pressure sea water.

Description

United States Patent [1 1 Smith 1 July 17,1973

[ METHOD FOR CLEANING OIL TANKER HOLDS [75] Inventor: Billy E. Smith, Darien, Conn.

[73] Assignee: Gulf Oil Corporation, Pittsburgh, Pa.

[22] Filed: Mar. 22, 1972 [21] Appl. No.: 237,669

Related US. Application Data [63] Continuation of Ser. No. 70,046, Sept. 8, 1970,

abandoned.

[52] US. Cl. 134/22 R, 134/24 [51] Int. Cl B63b 57/02, B08b 9/00 [58] Field'of Search 134/22 R, 10, 24,

[56] References Cited UNITED STATES PATENTS 1,806,740 5/1931 Butterworth 134/24 1,849,932. 3/1932 Johnson et a1... 134/24 X 1,891,592 12/1932 Fitzgerald 134/24 2,065,462 12/1936 Olsson 134/22 R 2,092,321 9/1937 McFadden 134/40 X Primary Examiner-Morris O. Wolk Assistant ExaminerD. G. Millman AttorneyMeyer Neishloss et al.

[5 7] ABSTRACT The hold in an oil tanker which has been emptied of its oil cargo is cleaned by first filling the'hold'with sea water ballast so that oil vapors are forced from the hold and a residual oil layer floats upon the sea water ballast. The floating oil layer is thereupon skimmed or otherwise removed from the aqueous ballast. The hold is then deballasted and washed with a jet of high pressure sea water.

5 Claims, 7 Drawing Figures PATEMED JUL 74975 sum 2 or 5 FIG. 2

BALLAST WA TER PATENIEDJUL 1 m I SHEEI 3 0F 6 METHOD FOR CLEANING OIL TANKER HOLDS This invention is a continuation of Ser. No. 70,046, filed Sept. 8, 1970, now abandoned.

This invention relates to a method for cleaning the hold of an oil tanker transporting crude oil or refined products which prevents occurrence of explosive conditions in the atmosphere of the hold.

In ocean-going crude oil tanker vessels, the total storage space is divided by bulkheads into a plurality of holds which are individually emptied of crude oil in port. Subsequently, either while still at port or on return voyage, before filling a hold with sea water ballast the hold is washed with one or several high pressure sea water jets which are vertically and rotatably maneuverable within the hold to remove the film of oil which adheres to the bulkheads and to remove the oil residue remaining on the bottom of the hold.

The washing operation involves considerable danger of explosion because of combustible vapors emanating from the filmof liquid oil remaining on the side bulkheads of the hold and from the small oil reservoir remaining on the bottom of the hold. Combustible vapors ordinarily emanate from this oil residue but the quantity of these vapors is enhanced by the atomization action of high pressure washing jets. In an emptied hold the mixture of oil vapor and air is generally sufficiently rich to be explosive even prior to sea water guncleaning but the condition becomes aggravated by the agitation of guncleaning. The water required for effective gunspray cleaning is generally more than 100 pounds per square inch in pressure and the static electricity generated by washing with jets of sea water at this pressure might be capable of inducing a static spark whereby the danger of explosion is considerable.

Prior art methods have attempted to alleviate the danger of explosion by searching for methods of washing which avoid induction of a spark. However, this approach not only results in inferior washing and prolongs the duration of the wash cycle but also frequently requires that the ship devoid of cargo remain in port during the washing operation and therefore involves a great economic waste.

The present invention approaches the washing problem in a totally different manner, i.e., it retains the existing washing method utilizing high pressure sea water jets which provides highly effective cleaning and permits the washing operation to be performed at sea. However, according to the present invention, combustible or explosive vapor conditions within the hold are avoided so that effective cleaning can proceed unhampered by concern of sparking or explosion. The danger of explosive conditions is obviated in accordance with the present invention by removing the oil residue 'remaining in the hold after emptying of the crude oil cargo by completely filling the hold following crude oil removal therefrom with an aqueous ballast, such as sea water, causing essentially the entire oil film and tank floor reservoir of oil to float on the top fo the ballast in the region of the top of the hold. Thereupon, the oil layer is substantially completely skimmed, siphoned or aspirated from the top of the hold so that upon subsequent emptying of the aqueous ballast, i.e., deballasting, the amount of oil coating the bulkheads and bottom of the hold is too low to form a combustible or explosive mixture with air, whereby effective high pressure guncleaning can proceed without concern about explosion, regardless of possible spark conditions. Although sparking is no longer a problem, the removal of oil as described permits the extra precaution of effective gun-cleaning with clean, cold sea or fresh water, while avoiding the high spark-inducing temperatures present during hot water or steam washing.

In accordance with the present invention, the removal of oil residue from an empty tank or hold, as described, results in removal of the source of combustible vapor. It. is emphasized that ballasting the tank with sea water without removing the resulting oil layer is not productive of the advantages of the present invention because upon removal of the ballast the unstripped oil will recoat the tank walls and redeposit on the tank bottom. Therefore, the bulkheads and internals are coated with oil and the tank bottom is the recipient of the residue as though ballasting had never occurred. By ballasting and then skimming or otherwise removing the oil from the top of the ballast tank three important advantages are realized. First, all gas in the tank is expelled when the ballast is pressed to deck level. Secondly, when the tank is deballasted no oil remains .on the water surface to recoat the tank and settle on the bottom to provide a source of gas regeneration. Thirdly, since the skimming operation results in an 80, 90 or even 95 percent or more removal of oil residue, the subsequent cleaning operation is much more efficient and of shorter duration due to the reduced quantity of oil to be removed.

In an actual test in a 130,000 barrel supertanker ship hold, the removal of crude oil cargo and filling of the empty hold with sea water ballast resulted in a 10 inch layer of crude oil on the ballast surface prior to skimming, which is approximately 1,000 barrels. The cargo was Kuwait crude having a gravity of about 34 A.P.I. Other tests showed oil residue in a hold is about 0.5 to 1 percent depending on tank size and configuration. However, the surface oil layer floating on the ballast was found to be considerably more viscous and had a gravity of about l4.7 A.P.I. Following siphoning off of the oil layer and subsequent deballasting, the holds were guncleaned with to pounds per square inch jets of sea water and made sludge free. Tests of the tank atmospheres during the water jet cleaning operation in accordance with this invention showed that the vapor concentrations were less than about 15 per cent of the lower explosive limit. In the absence of skimming or siphoning the supernatant oil layer, the vapor concentration in the tank atmospheres reached l00 percent of the lower explosive limit during similar sea water jet cleaning.

The present invention will be more completely understood by reference to the accompanying drawings wherein:

FIG. 1 is a cross-sectional view of a crude oil carrying vessel containing an apparatus of the present invention,

FIGS. 2, 3, 4 and 5 illustrate particular embodiments of oil skimming apparatus of the present invention, and

FIGS. 6 and 7 illustrate graphically the improvement in atmospheric conditions within the hold of a vessel when employing the process and apparatus of the present invention.

Referring to FIG. 1, vessel 10 is shown in crosssection containing a plurality of holds including central hold 12 demarked by

bulkheads

14 and 16. Central hold 12 is filled with ballast water and an oil layer floats on the ballast water in the upper region of the hold.

Also shown is a sea water jet apparatus 18 of which about three are ordinarily employed in a hold. Gunspray cleaning apparatus 18 comprises a shaft 20 internally through which motor 19 drives ball-like member 26 on which is mounted nozzle 24. Ball-like member 26 rotates in the general plane indicated by arrow 22 while nozzle 24 rotates in the general plane indicated by either arrow 28.

Arrows

22 and 28 indicate rotation on substantially transverse planes so that sea water spray 52 is directed throughout the length and breadth of hold 12 in the pattern of a figure 8, under a. pressure of 90-180 psi.

The oil surface indicated at 30 is provided with a floating skimming device indicated generally at 32 and described more fully in FIG. 2. Skimming device 32 removes oil by siphoning through line 34 and pump 36.

Oil is initially pumped into hold 12 through port 42 by means of pump 38 and line 40. The oil can be removed through port 44 by means of pump 46 and line 48. Thereupon, hold 12 can be filled with ballast sea water through the aforementioned pump 38, line 40 and port 42. The ballast sea water forces out of the hold an atmosphere of vapors which may be above the lower explosive limit. An oil layer 30 forms and floats upon the ballast water and is skimmed or siphoned off by means of skimming apparatus 32 in the manner explained below. After the oil layer is removed, relatively oil-free ballast water is pumped out, water jet apparatus 18 is put into operation and the

bulkheads

14 and 16 as well as the base 50 of hold 12 are scrubbed clean of oil by means of a 90 to 180 pounds per square inch pressure jet of cold or warm sea water 52. During the water scrubbing operation the oil vapor level within hold 12 is well below the lower explosive limit.

Oil skimming apparatus 32 is illustrated more particularly in FIG. 2. As shown in FIG. 2, the skimming apparatus permits oil layer 30 to overflow into a funnel shaped fabric container 54, such as a canvas container, which container is supported by a steel ring 57 and small hooks 59. An air-inflated plastic tube 61 is attached to the funnel assembly by one or more clamps 58 in order to float the assembly at the oil surface. A plurality of steel supports 56 are attached at one end to steel ring 57 and on the other end to hinges 60 mounted on a base conduit 62 which feeds a suction hose 64 leading to pump 36, shown in FIG. 1. Pump 36 is provided with'suitable priming means, not shown.

In the operation of the apparatus of FIG. 2, oil in the layer 30 overflows into the canvas funnel 54 and flows downwardly between the canvas and tube 61 for removal by means of hose 64. As the oil layer 30 diminishes, the apparatus 32 progressively lowers until removal of sea water is observed at which time the siphoning operation is terminated and the apparatus 32 is removed in preparation for deballasting the hold.

FIG. 3 shows a modification of the siphoning apparatus of FIG. 2. In the apparatus of FIG. 3 the steel supports are omitted and a funnel shaped canvas 66 is clamped to a neoprene air-inflated circular tube 68,'or to other circular floatable tube means having a central opening, by means of a plurality of clamps 70. The clamps 70 form a plurality of passageway weirs 72 in the tube through which oil at surface 30 overflows into funnel 66.

FIG. 4 illustrates another apparatus for removing the oil layer from the ballast water. Also illustrated in FIG. 4 is an apparatus for establishing the depth of the oil layer. As indicated in FIG. 4, input of ballast water forces the oil layer flush against the bottom of deck 74 which is provided with an opening covered by cover plate 76 clamped securely into position by means of rotatable clamps 78. A central standpipe 80 projects from a central opening 82 in the cover plate ,while lateral standpipes 84 project from lateral openings 86 in the cover plate.

A dip pipe 88 extends through the cover plate and is provided with measurement markings 90. The dip pipe 88 is provided with an exhaust pump, not shown, which siphons liquid through sight glass 89 and which is provided with priming means. If dip pipe 88 were inserted through opening 82 and removed, it is apparent that no matter when length of pipe extended below plate 76 the entire portion projecting below plate 76 would be coated with crude oil upon removal and no determination of oil depth could be made. However, by inserting dip pipe 88 through the oil layer into the ballast water and siphoning out ballast water while gradually raising the dip pipe until oil is observed to be removed instead of water in sight glass 89, the marking indicia 90 will provide measurement of the depth of the oil layer. Similarly, if dip pipe 88 is gradually lowered through the oil layer while siphoning oil until water starts to be siphoned instead of oil, the depth of the oil layer can be measured by the position at which siphoning of water begins. 7

In the operation of the apparatus of FIG. 4 for removal of the oil layer, standpipes 84 are each provided with suction pumps, not shown. As the oil suction pumps remove the oil layer the void is continuously filled with sea water charged by any suitable pumping means to keep the oil layer flush against cover plate 76. Oil removal is complete when water is observed to be removed through pipes 84 instead of oil, assuming the ship is tilted so that the oil pool is disposed in the locality of cover plate 76, or assuming the vessel is in a relatively even position.

FIG. 5 shows an oil siphoning apparatus which is integral with and non-removable from the hold of a vessel. As shown in FIG. 5, 94 is the deck of a vessel and 96 is a tank bulkhead. A suction cradle assembly 92 is permanently attached to bulkhead 96. Cradle assembly 92 is comprised of enclosure 98 mounted on angle support 102. The oil-water interface is at the height of and can enter into enclosure 98. The suction device comprises a neoprene bouyant circular collar 100 which is open in the center and which is flexibly attached to an overhead high velocity eductor tube 104 to which a high speed water hose can be attached at 106 to accomplish suction or aspiration of oil through the collar. Eductor tube 104 must not be more than 28 feet above collar 100 and is flexibly attached to collar 100 by means of conduits 108 and 110 and flexible connectors 112 and 114, permitting collar 100 to freely move within the confines of cage 98 upon rolling of the vessel.

FIG. 6 is a graph showing the conditions of the atmosphere in the hold of a vessel from which the oil cargo has been removed when the method of skimming or siphoning in accordance with the present invention is not employed. The graph shows the oil vapor richness in the atmosphere in the hold expressed as per cent of the lower explosive limit. The graph shows various occurrences in the hold including blower venting, natural venting and sea water gun jet cleaning, and their effect upon the per cent of lower explosive limit of the hold atmosphere. As shown at the left of FIG. 6, during gunspray cleaning the hold reached the lower explosive limit and is removed from said limit by means of blower venting. Natural venting resulted in a trend back to the lower explosive limit. Subsequent periods of water jet cleaning utilizing water of 100-130 pounds per square inch pressure charged through three jets resulted in the rapid approach to or actual reaching of the lower explosive limit.

FIG. 7 illustrates the great improvement accomplished in accordance with the present invention. The graph of FIG. 7 shows the atmospheric condition during sea water gunspray cleaning in a hold which had been emptied of oil cargo and then filled with ballast sea water to form an overlying oil layer, which oil layer was removed in accordance with the present invention. Following removal of the oil layer, the ballast water was removed and the hold was cleaned by means of three jets of seawater at a pressure of l00-l30 per square inch gauge. FIG. 7 shows that in the atmosphere of the hold at no time during the water spray cleaning procedure of the present invention was even percent of the lower explosive limit reached. The graph of FIG. 7 is typical of samples taken at various depths and positions in the hold. This is in sharp contrast to the results shown in FIG. 6 wherein gunspray cleaning under similar conditions resulted in the rapid reaching of 100 percent of the lower explosive limit in the atmosphere in the hold.

Various changes and modifications may be made without departing from the spirit of this invention or the scope thereof as defined in the following claims.

I claim:

1. A process for cleaning a hold in an oil tanker which has been emptied of its oil cargo comprising filling said hold with aqueous ballast liquid before pressure spraying said hold so that oil vapors are forced upwardly from the hold and an oil liquid layer derived from oil on the walls and bottom of the hold floats upon the aqueous ballast liquid as said liquid moves upwardly towards the upper region of the hold, removing from the hold said oil liquid layer from the aqueous ballast liquid in the upper region of the hold, removing the aqueous ballast liquid after said removal of said oil liquid layer so that the removed liquid oil cannot recoat the walls and bottom of the hold, and cleaning by pressure liquid spraying said hold with water after said removal of the aqueous ballast liquid.

2. The process of claim 1 wherein said oil liquid layer is removed by skimming.

3. The process of claim 1 wherein said oil liquid layer is removed by siphoning.

4. The process of claim 1 wherein said oil liquid layer is removed by aspiration.

5. The process of claim 1 wherein the atmosphere in said hold is above the lower explosive limit prior to the aqueous ballast filling step and is below said limit as a result of said aqueous filling and oil removing steps.

Claims

2. The process of claim 1 wherein said oil liquid layer is removed by skimming.
3. The process of claim 1 wherein said oil liquid layer is removed by siphoning.
4. The process of claim 1 wherein said oil liquid layer is removed by aspiration.
5. The process of claim 1 wherein the atmosphere in said hold is above the lower explosive limit prior to the aqueous ballast filling step and is below said limit as a result of said aqueous filling and oil removing steps.