SLOP TANK DESIGN FOR IMPROVED LOAD-ON-TOP

Robert J. Fiocco Exxon Research and Engineering Company

Florham Park, New Jersey

and

Vincent W. Ridley Exxon International Company

New York, New York

ABSTRACT

Slop tanks are the focal point of the Load-On-Top system used on crude oil tankers to prevent pollution of the sea. Design of these tanks and their operating procedures strongly affect the degree of oil-water separation achieved. This paper presents the results of an investigation undertaken to define designs and procedures for improving separation and minimizing oil discharge to sea. The pro-gram was funded in part by the U.S. Maritime Administration.

Based on tanker experience and laboratory tests with tank models, guidelines on capacity, structure, inlets, outlets, system design, and wastewater handling, procedures were developed. The guidelines aim at assuring successful Load-On-Top operations by (1) providing tanker operational flexibility for handling oily water, (2) minimizing the degree of oil-water mixing, (3) avoiding re-dispersion of separated oil during feeding and discharging operations, and (4) eliminating the possibility of accidental oil contamination.

This investigation provides a basis for future large-scale or ship-board studies to improve the performance of slop tanks on existing tankers as well as on future tankers.

INTRODUCTION

Over the past several years, a considerable effort has been ex-pended by the oil and maritime industries to aid in developing technology and regulations to avoid pollution of the sea. The urgency for this work was spurred on by the greatly expanded needs to transport oil by sea.

One of the major steps to control pollution was introduced in 1964 with the adoption of the Load-On-Top (LOT) system for crude tankers. The system involves the use of slop tanks to separate oil-water dispersions generated during the ballast voyage. After setting, water is discharged to sea. At the loading port, crude cargo is loaded on top of the oil, and water is retained in the slop tank. The slop tank contents are ultimately discharged with the rest of the cargo refineries or other receiving facilities. There are economic incentives to minimize the amount of oily water retained with slop oil since water reduces cargo payload space and can lead to severe processing problems at refineries.

Pollution regulations aimed at controlling the amount of oily water discharged are established by the Inter-government Maritime Consulative Organization (IMCO). The most recent 1973 IMCO convention prohibits discharge of oily water unless the tanker is en route and more than 50 miles from land. These discharges cannot exceed a maximum of 60 liters per nautical mile, and the total quantity of oil discharged cannot exceed 1/15,000 DWT. More

stringent regulations and segregated ballast requirements have been proposed for new tanker construction.

There are many important facets to the problem of controlling oil discharge by tankers, including enforcement, monitoring, and equipment limitations. This paper relates to improvement of equip-ment, specifically the design of slop tanks used in LOT procedures.

Program description

A program was carried out to define slop tank designs and operat-ing procedures which enhance oil-water separation and thereby re-duce the oil content in water discharged to sea [1]. The guidelines developed in the investigation are intended to provide a basis for further efforts by tanker operators and builders to upgrade the per-formance of slop tank systems and to assure that present and future tankers can comply with pollution requirements.

The guidelines are based on a review of tanker operating ex-perience and studies with laboratory models. The laboratory pro-gram provided a means for analyzing separation phenomena and evaluating alternate designs. Plexiglass models representing the bottom sections of center and wing tanks (figures 1 and 2) were evaluated under static and rolling conditions. For ease of handling, a product oil, Isopar M, was used as the dispersed oil phase. Tests were also carried out with Kuwait crude oil.

Slop tank operations and design considerations

Slop tanks are the focal point of Load-On-Top operations. At various times during the ballast voyage, oily water generated by line flushing, tank washing, and dirty ballasting is pumped to the slop tank, and settled water is discharged from this tank to sea. Effective separation of oil and water must be achieved before discharge of water to sea.

Typical LOT operations commence after discharge of crude oil cargo. Tanker piping lines containing oil are drained to the slop tank and the tanker takes on ballast water in its dirty tanks before head-ing to sea. Tank compartments which are to be used to carry clean ballast are then washed with seawater. Other tanks may also be washed for general maintenance purposes. Oily wash water is con-tinuously stripped from the tank(s) being washed and collected in the slop tank to allow separation of oil from the water.

On inerted tankers which recirculate wash water, the slop system must separate oil and water under continuous flow conditions. A schematic of a two-tank continuous flow slop system used for this is shown in figure 3. Both tanks are initially filled with clean water. During tank cleaning, oily wash water is stripped to the primary slop

195

196 CONFERENCE ON PREVENTION AND CONTROL OF OIL POLLUTION

Figure 1. Photograph of center tank model

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,# \-;;'

Figure 2. Photograph of wing tank model

tank in which most of the oil collects. Water continuously transfers to the secondary tank via a balance line for further separation. This settled water is recycled from the secondary tank back to the clean-ing machines and is also used to drive an eductor which strips wash water from the tank being cleaned.

After the tanker takes on clean ballast in the washed tanks, water from the dirty ballast tanks is discharged to sea. Discharge from dirty ballast tanks is stopped with several feet of water plus floating oil still remaining in these tanks. At this point, settled water in the slop tanks is discharged to sea to provide capacity for the residues in

Other Inlet , To Slop Tank

m,

Primary Slop Tank

Balance Line -m I X

Secondary Slop Tank

(~l 0'1

Water

T

Tank Being Cleaned

Discharge Lines From Slop Tanks

_L / Eductor For Stripping ■ H ' Tank Being Cleaned

Figure 3. Schematic of closed cycle washing system on inerted VLCCs

the dirty ballast tanks. Dirty ballast residues are then pumped to the slop tank, and the lines are flushed in preparation for clean ballast discharge.

After allowing time for oil-water separation, settled water in the slop tank(s) is again discharged to sea. If there is more than one slop tank, all residues are transferred to the primary tank. Stripping lines are then flushed into the primary slop tank. Additional time for oil-water separation is allowed and water is again discharged to sea, retaining the oil layer and water with high oil content in the slop tank.

Upon arrival at the crude oil loading port, the tanker discharges all clean ballast. After crude is loaded into all tanks, including the slop tank, the tanker proceeds to the discharge point.

Each operation should be considered from the viewpoint of promoting oil-water separation in the slop tank and minimizing oil content of water discharged to sea. The following general criteria for slop tank design and operation were used as bases for specific guidelines discussed in subsequent sections of this paper.

1. Operational flexibility for handling oily water should be pro-vided to assure that adequate settling time is available.

2. The degree of mixing energy imparted to the oil-water mixtures being transferred to the slop tank should be mini-mized.

3. During transfers and discharging operations, redispersion of oil which has already separated in the slop tank should be avoided.

4. Accidental oil discharge should be prevented.

Slop tank capacity

Slop tank capacity should be adequate to allow tanker opera-tional flexibility with regard to oily water handling. Therefore, the recommended slop tank capacity is based on the total amount of oily water to be handled including:

a. Oily water generated during tank washing. On VLCCs with closed cycle washing, a minimum one-hour water holdup in the secondary tank insures a sufficient head for the cargo pump and also allows for separation during the washing cycle.

b. Retained dirty ballast water. c. Water from flushing and drainage of lines and bilges. d. Allowance for operational flexibility and avoidance of over-

filling. These quantities are considered to be additive since under some

conditions, such as short voyages or bad weather, vessels may not be able to partially decant slop tanks prior to these operations. Tanker operations should be aimed at limiting the amount of oily water generated, for example, by minimizing dirty ballast tankage and tank washing. Quantities of dirty ballast residue and flushing water will be affected by factors such as sea conditions and length

PREVENTION 197

and complexity of piping to be flushed. In view of the wide variation in tanker configurations and operating conditions, the desirable capacity for slops receipt is in the range of 2.5 to 6% of the total oil carrying capacity.

As an example, for a 240,000 DWT inerted VLCC with closed cycle washing, the estimated minimum slop tank capacity is as fol-lows:

water for washing and eductor drive 3-ft water innage from dirty ballast

tank flush and drainage of lines and bilges

Subtotal 20% allowance for operational

flexibility

Total

Slop tank location and shape

5,000 tons

4,000 tons 2,000 tons

11,000 tons

2,200 tons

13,200 tons or 4.4% oil carrying capacity

To reduce the length of piping involved in slop handling, slop tanks should be located as close as possible to the pumproom in the aft part of the vessel. This location will also facilitate the trimming of the tanker by stern (i.e., aft end of tanker lower than forward end) for proper drainage during tank washing.

Properly outfitted cargo tanks can be used as slop tanks; how-ever, wing tanks are preferred because of their sloping bottom shape. Laboratory tests with models of center and wing tanks indicate that drainage of water without reentrainment of the settled oil layer is most complete with the wing tank shape under both stationary and rolling conditions. As shown in figure 4, for comparable drainage conditions, water retention in the wing tank model was generally less than 20% of that in the center tank model. The limitation im-posed by rolling motion is due to remixing of the oil and water layers as the interface reaches the height of the tallest bottom longitudinal structure, e.g., main girders. The mixing action results as the water layer flows back and forth over the bottom structure.

The use of smooth bottom tanks to avoid mixing during rolling can cause enhanced sloshing movement of the liquid and, as a result, offer no effective advantage for drainage. Enhanced sloshing action has been noted with smooth bottom tanks on oil-bulk-ore carriers. When smooth slop tanks are being designed, analyses of wave motions encountered in full size tanks at sea conditions should be carried out to minimize sloshing action.

Cascade tank systems

On inerted VLCCs using closed cycle washing, it is desirable to minimize oil content in the recirculated water to the tank cleaning machines and eductors. For continuous flow systems such as these, laboratory settling tests indicated that two or three tanks in a

9 ?

Center Tank Limit During Rolling:

Wing Tank Limi During Roll in; -HUuring Rolliingl ■, -H

Center Tank Model

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4 6 8 WATER DRAINAGE RATE (G PM)

series allow better separation than a single tank with the same total volume (table 1). For these tests, a standardized 1,125 ppm Isopar oil-in-water dispersion was fed to the primary tank and the effluent from the last settling tank was averaged over a one-hour period. A 3' long x 2' wide x 2-1/2' high tank was partitioned lengthwise to give the various settling volume ratios. The inlet to each tank was positioned near the top of the water phase and directed downward.

Unequal settling tanks (2:1 and 1:2 volume ratio) were found to be slightly more effective than tanks with a 1:1 ratio and an equiva-lent total volume. Separation with three equal size tanks was com-parable to two unequal tanks. In practice, a two tank system is preferable to a three tank cascade because operations are simplified. Also, a smaller primary tank offers advantages in segregation of slop oil and final decantation of water.

Table 1. Oil separation in cascade settling systems

Number of Settling

1

2

2

2

3

Sections Volume Ratio of Settling Sections

-1:1

1:2

2:1

1:1:1

Oil Content in Effluent

ppm

151

127

113

117

117

Figure 4. Water holdup in center and wing tank models at onset of entrainment of oil layer during discharge at various rates

Inlets

Slop tank inlets should be designed to avoid redispersion of separated oil and to minimize disturbances which hinder oil droplet settling. Redispersion of the oil layer by the jet action of incoming water not only creates additional oil contamination in the water phase, but also generates water-in-oil emulsions. Depending on the type of crude oil, these emulsions can be stable and form very ir-regular "froth" zones at the oil-water interface. Emulsion layers are undesirable since they lower the oil-water interface level and in-crease the amount of water which must be retained on board ship.

The inlet jet of water can also remix oil droplets which have partially settled from the lower portion of the water phase. In a con-tinuous flow system, such as used in closed cycle washing on inerted VLCCs, these droplets can be entrained in the recirculated washed water. Another situation to be avoided in continuous flow systems is directing the inlet jet toward the discharge line and, in effect, causing partial short-circuiting.

Laboratory tests were carried out to investigate the tendency of various inlet configurations to redisperse oil which has already separated in the slop tank. The tests were conducted by feeding a stream of clean water into a tank containing water with a layer of oil on the surface. The following inlets did not redisperse the oil layer:

1. nozzles positioned within the water phase directed horizon-tally with adequate clearance to the oil layer

2. nozzles positioned within the water phase directed downward with adequate clearance to the oil layer and to the bottom of the tank

3. nozzles directed upward from below the oil layer and, if close, baffled to deflect the flow away from the oil layer.

Clearances for the various inlets required in lab tests with an Isopar oil layer and an inlet velocity of 8 feet per second are shown in figure 5. For the horizontal inlet, the clearance of 10 pipe diame-ters to the oil-water interface allowed for expansion of the inlet jet; this could be decreased to 5 pipe diameters when an expanded inlet nozzle (twice the pipe diameter) was used. For the downward-directed nozzle, very little clearance below the interface was re-quired; however, to avoid disturbance by the reflected jet, a minimum height of 10 pipe diameters for the oil-water interface above the bottom of the tank was required. Upward-directed nozzles required 50 pipe diameters distance to the interface; this could be decreased to 4 diameters using a flat deflector baffle over the nozzle directed away from the wall.

198 CONFERENCE ON PREVENTION AND CONTROL OF OIL POLLUTION

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0.1.

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Figure 5. Clearance required to prevent drop tearaway from oil layer in laboratory tests

Clearance requirements depend strongly on inlet velocity and interface properties of the oil layer. Figure 6 shows clearance re-quirements as a function of inlet velocity for horizontal inlets with and without an expanded nozzle (diffuser) which promotes dissipa-tion of the jet. Clearance requirements increase with increasing inlet velocity, but are reduced with the use of an expanded nozzle. When testing with a Kuwait crude oil layer, the clearance requirements were approximately twice those needed for the Isopar oil tests. This was primarily due to the presence of loose oily particles which built up at the interface and were readily swept into the water phase by the inlet jet.

When the inlet is positioned either above, within, or too close to the oil layer, a wide range of oil droplet sizes can result depending on specific conditions. Finer, more persistent oil drops are generated as the clearance decreases, as inlet velocities increase, and as oil-water interfacial tension decreases.

Several inlet configurations were evaluated in the laboratory for continuous flow conditions such as used in closed cycle washing. A standardized dispersion of 1,125 ppm Isopar oil in water was fed to the 3' X 2' X 2-1/2' tank. Inlet velocity was 8 feet per second; effluent was averaged over a one-hour period.

These tests (table 2) indicated that horizontal and downward-directed inlets are equivalent in tanks with transverse structure between the inlet and outlet. In tanks without transverse structure, a downward nozzle allowed better separation than horizontal or upward-directed nozzles. For all configurations, there was a degree of short-circuiting of feed to the outlet due to the mixing currents set up by the inlet jet Poorest separation was obtained with an up-ward-pointed nozzle which caused a high-velocity stream just below the interface that swept oil droplets towards the outlet.

In summary, for feeding oily water to a partially-filled slop tank, a horizontal inlet with an expanded nozzle positioned near the bottom of the tank is preferable. This inlet avoids the need for baffles which are subject to erosion/corrosion problems and allows greater operational flexibility for transferring oily water into the tank without disturbing settled oil. For continuous flow systems, low horizontal or downward-directed inlets are suitable for tanks with transverse structure between the inlet and outlet. However, for specially-designed tanks without transverse structure, a downward-directed inlet is preferred.

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Figure 6. Clearance below oil-water interface required for horizontal inlets to avoid redispersion of oil layer in laboratory tests

Outlets

Outlets should be designed so that settled oil does not remix into the water phase during discharge of water. Remixing of oil into water increases the oil contamination in the discharged water and ultimately limits the amount of water which can be decanted from the slop tank.

As illustrated in figure 7, oil entrainment in the discharge water can occur when (1) oil is drawn down around the bellmouth due to the flowing water, (2) a vortex forms near the outlet at the oil-water interface, or (3) jetting of water through openings in structural mem-bers causes tearing of drops from the oil-water interface.

At low interface levels, the acceleration of the water flow around the bellmouth causes oil to be drawn down into the discharging water. Laboratory tests showed that the interface height at onset of oil entrainment depended on drain rate, presence of restrictive structure, and oil layer properties. Also, crude oils with higher densities and lower interfacial tension are more easily entrained. Lower drainage rates delay the onset of oil entrainment. Taller bottom longitudinal structures cause oil entrainment at higher interface heights. In addition, with crude oil, oily emulsion particles which arise from natural surfactants tend to collect at the interface and are readily entrained by the discharging water phase.

Oil drawdown around the bellmouth can occur prematurely due to a weiring effect when tank structures that divide the slop tank into subcompartments are present. For strength reasons, there are limitations in the bottom hole drainage area in these structural members. As a result, when the interface drops below the top of the structure, oil spills over, causing the interface to be lower in the outlet subcompartment than in the neighboring subcompartment (figure 8). Decreasing discharge rates reduce the weiring effect.

To delay the onset of oil entrainment, expanded downward-facing outlet nozzles (i.e., bellmouths) are recommended. It is desirable that the peripheral clearance area to the base of the tank be twice the cross-sectional area of the connecting pipe. The use of expanded outlet nozzles allows this ratio to be achieved while keeping bottom clearance to a minimum. Laboratory tests indicated that circular and elliptical-shaped bellmouths were equivalent. Special-design out-lets, such as elephant-foot bellmouths, which may permit decanta-tion at higher rates were not tested. However, since it is desirable to reduce drainage rates during the final stages of decantation to avoid accidental oil discharge, the shape of the bellmouths appears to be of secondary importance.

PREVENTION 199

Table 2. Effect of inlet configuration on settling in a continuous flow system

1 . OIL LAYER IS DRAWN DOWN AROUND OUTLET

Inlet Nozzle Configuration

• Vertically downward, near top of water phase

• Vertically downward, near top of water phase, with transverse web frame

• Horizontal near bottom of water phase, with transverse web frame

• Vertically upward with deflector baffle, near top of water phase

• Horizontal near bottom of water phase

• Vertically upward near bottom of water phase

Oil Content In Effluent

ppm

151

173

290

Vortex formation is strongly dependent on flow geometry as well as drainage rate. The lab study indicated that bellmouth set between longitudinals have little tendency to form vortices since the longitudinals act as vortex breakers. Vortexing was noted when the longitudinals were removed. The vortex was initiated by a water jet at the top opening of the slot at the main longitudinal girder. Low drainage rates prevented the vortex.

In tanks with upward-facing bottom drains, which are sometimes used in double bottom tanks, a vortex readily formed above the outlet. For these cases the use of anti-vortex baffles is recommended.

Water jetting through openings in structural members causes oil entrainment in a manner similar to that observed for inlets. That is, oil drops tear away from the interface and are carried to the bell-mouth. If the interface in the outlet subcompartment is lower than in the neighboring subcompartment due to weiring, this remixing phenomenon is more severe. As in the case of jet-induced vortices, low drainage rates reduce the jet velocity and avoid drop tearing. Openings in structural members which compartmentalize slop tanks on tankers should be as low as possible to delay jet disturbances to the interface.

Design approaches to minimize or prevent weiring and jetting include (1) increasing bottom hole drainage area consistent with strength considerations, (2) avoiding structures which compartmen-talize slop tanks, or (3) providing bellmouths in each subcompart-ment. The first two approaches require more study because of the complexity of design factors involved. The use of multiple bell-mouths avoids problems with ship structural integrity. With this approach, piping and valving should be arranged to provide equal suction at each bellmouth. Guidelines on positioning of multiple outlets are follows:

In center slop tanks, the bellmouths should be located trans-versely in a manner compatible with the major longitudinal fram-ing in the tank. For example, at least one outlet on either side of the main longitudinal girders is desirable.

In wing slop tanks, bellmouths should be located longitudi-nally in the innermost longitudinal channel. The number of bell-mouths should be compatible with the number of transverse bays in the slop tank. Differences in water level over the outlets are minimized by operating with the ship trim by stern.

Pumps, piping, and valving

Reciprocating pumps are preferable to centrifugal pumps or eductors for stripping wash water from tanks being cleaned because they do not disperse the oil as finely. Shipboard data have been re-ported showing the slower separation rate of wash water stripped by centrifugal pumps compared to reciprocating pumps [2]. Educ-tors are frequently used for stripping tanks being cleaned because they provide good drainage required for effective tank cleaning. However, besides being very effective mixing devices, eductors also increase oily water flow to the slop tank. Therefore, the use of educ-tors for stripping wash water should be limited to closed cycle re-circulated washing systems.

For decanting slop tanks, flexibility in pumping rates is desirable for the various stages of decantation. Reciprocating pumps used for stripping are suitable for this service.

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i

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2. VORTEX FORMS AT OIL-WATER INTERFACE

1

fer^j k llliiii*illllll

k

3 . WATER JETS INTO OIL-WATER INTERFACE. INTERFACE MAY BE LOWER IN SUCTION COMPARTMENT DUE TO "WEIRING".

Transverse Frame

—Wifcg

Figure 7. Mechanisms of oil entrainment into discharging water

Flow Restricting Member With Inadequate Bottom Hole

Figure 8. Weiring is caused by structural member which restricts water flow to outlet. Oil layer becomes deeper in outlet section.

An independent stripping line should be provided from each slop tank. Such lines should connect the slop tank(s) directly to the pumproom and be used only for decantation operations. This would eliminate the need for using cargo stripping lines to decant the slop tank. When cargo stripping lines are used for decanting, additional oily water is generated in the final line flush, and there is a potential risk of contamination of the clean ballast water. If

200 CONFERENCE ON PREVENTION AND CONTROL OF OIL POLLUTION

independent stripping lines are not provided, master valves should be fitted just forward of the slop tanks to avoid contamination of the stripping lines not involved in decanting of the slop tanks.

For ease in operation, valves in the eductor piping and stripping delivery lines to the slop tank should be located on deck rather than at the pumproom.

The main and stripping suctions from the slop tank should always be supplied with two valves to prevent leaks, especially when dis-charging clean ballast at the loading port. One of these two valves should be a gate type.

In cascade settling systems, the secondary slop tank should have an independent inlet similar to the primary slop tank. This allows washing of the primary slop tank and independent use of the secondary tank when the primary tank is unavailable. The balance line between slop tanks should draw from the bottom of the primary tank and discharge downward through an inverted U-loop at about midheight in the secondary tank. A valve should be fitted in the balance line to allow isolation of the tanks. The balance line should be designed to maintain a 3'-5' level differential between tanks.

Separation aids

Slop tanks should be provided with heating coils to speed up oil-water separation. These are particularly useful for providing a sharper oil-water interface and reducing water content in the oil layer. To allow flexibility, the heating coils should have at least two stages, one at low-tank level and the other at midtank level.

Oily water separators are desirable for use during the final stages of slop tank decantation. Present separator limitations in regard to capacity and effectiveness restrict their usefulness during the entire decantation process. However, in the final stages of decantation when flow rates are low and oil content is rapidly increasing, they can be useful and provide insurance against accidental oil spills.

Cleanliness of slop tanks

Particular attention must be given to the cleanliness of the slop tanks. Slop tanks free of sediment will facilitate oil-water separation and decantation. The slop tanks should have a protective paint coating and be supplied with fixed cleaning machines for easy cleaning and effective removal of settled sludge.

Transfer of residues to the slop tank

After the tank washing operation, the slop tank should have suffi-cient capacity to receive the volume of dirty ballast remaining in the tanks, line, and pump flush water, and the pumproom bilge contents. If it is necessary to discharge water from the slop tank to provide sufficient capacity, the water discharge should be stopped when the required capacity is reached in order to minimize disturbances to the settled oil layer during the final transfers to the slop tank.

Transfer of dirty ballast residues and the pumproom bilge con-tent to the slop tank should be carried out with reciprocating pumps operating at low rates. The stripping system should then be flushed if it is to be used in discharging the slop tank. Suction is taken simultaneously from the forwardmost clean ballast tanks with each dirty stripping pump. The lines and pumps are flushed to the slop tank to remove any oil accumulation in the stripping system.

Discharging the slop tank.

Discharge of water from the slop tank must be in accordance with international regulations. Therefore, the discharge operation must be carefully supervised to insure that it is stopped before a significant amount of water with high oil content is released.

It is recommended that before discharge of the slop tanks a minimum of 12-24 hours be allowed after the last operation trans-ferring oily water to the slop tank. Settling time requirements will depend on factors such as sea conditions and length of time the slop tank contents have settled prior to the last transfer operation.

Before beginning the discharge operation, it is important to get an accurate interface reading. If the interface between the oil and water cannot be determined with certainty, discharge should not take place and more time should be allowed for settling. In addi-tion, heating coils should be used to facilitate separation and give a sharper interface.

The slop tank should be drained either with one cargo pump at slow speed or by gravity, and the discharge should be monitored for oil content. Discharge should be stopped when the water innage (i.e., total innage less the oil and emulsion layers) is about 15% of the tank depth. This is normally 3 to 4 feet above the bottom transverse members. After allowing about an hour for any turbulence due to drainage of the tank to subside and taking another oil interface measurement to verify the water innage, slowly draw down the tank using one stripping pump at slow speed. If a separator is available, discharge through it at this time.

Discharge should be stopped when the oil content is high (e.g., before exceeding IMCO Convention limits), or when the oil-water interface reaches approximately 1 foot above the tallest bottom longitudinal member, whichever occurs first. If the discharge is stopped at a higher interface level due to high oil content, discharge can be repeated after allowing more settling time. Further decanta-tion should proceed only under nonrolling conditions and with dis-charge through a separator. Decantation should be stopped when oil content of the discharge is high or when a minimum of 1 foot of water below the interface remains in the tank.

CONCLUSION

This investigation provides a basis for future large-scale or ship-board studies. Slop tank designs and operating procedures have been described which can improve oil-water separation and assure compliance with international pollution conventions. Design im-provements can be implemented on existing tankers or in some cases, operations modified to account for design limitations. The guidelines should also aid in the evaluation of slop tank-system de-signs for new tanker construction.

REFERENCES

1. Fiocco, R.J.; Lanotte, V.X.; and Raffaelli, G. 1974. Investi-gation of slop tank designs to improve oil-water separation. U.S. Maritime Administration Report, 1974.

2. Shibata, K., and Hikita, K. 1971. Test report on distribution of oil in slop tanks. Japan Naval Architecture Society Trans-action, Vol. 129, June 1971.