Method and apparatus for reducing oil cargo sludge in tankers

A method and apparatus for converting crude oil tank bottoms in the cargo tanks of a ship to a liquid state is provided. The method comprises subjecting crude oil sludge contained in the tanker cargo tank to a collection system to facilitate extracting the sludge mixture from the bottom of the tank; comminuting the sludge mixture to reduce the size of sludge globules contained therein to produce a homogeneous mixture; optionally adding crude oil from the cargo tank to increase the homogeneity of the mixture; optionally adding an additional quantity of cutter stock and/or other selected additives to the sludge mixture, either before, during or after comminution, to form a blend stock.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to reducing the accumulation of sludge in 
tanker ships and in particular to recovering the fuel value from crude oil 
tanker bottoms. 
2. Description of Related Art 
A particular problem arises when transporting crude oil and petroleum 
products in tanker ships in which sludge accumulates on the bottom of the 
cargo tanks and often accumulates to significant depths. Heavy crude oil 
often can contain as much as 3 to 5% of the crude oil volume in globular 
form. When this crude oil is put into the cargo tanks of a tanker ship, 
the globular oil settles to the bottom of the tank. As successive loads of 
crude oil are transitioned through the tank, a thick layer of the settled 
globular oil accumulates in the bottom of the tank as a heavy sludge. 
The accumulated tank bottoms decreases the usable volume of the tank and 
decreases the volume of delivered oil to the destination, resulting in an 
economic loss of cargo volume during transit from the loading port to the 
destination port. 
A normal method of disposing of the tank bottoms is to flush the cargo tank 
with sea water, often under high pressure, and to discharge the resultant 
mixture overboard after the ship is in open ocean waters. This is an 
economic loss, for that volume of tank bottoms discharged is lost crude 
oil volume. 
Periodically the ship is withdrawn from service and put into a shipyard for 
maintenance. At this time it is customary to remove remaining sludge in 
the cargo tanks to inspect and repair, if necessary, the walls of the 
tanks. The tank bottoms removed in this operation is usually incinerated 
or otherwise disposed of at a cost, with the complete loss of the economic 
value of the crude oil. 
There is presently no effective method in use to process the crude oil tank 
bottoms in the tanker so that the economic value of the crude oil sludge 
is retained for discharge at the destination terminal. 
SUMMARY OF THE INVENTION 
One object of the invention is to obtain substantial recovery of the value 
of the crude oil tank bottoms in cargo tanks. 
Another object of the invention is to provide a system design that can be 
installed in the tanker ship that can process the sludge to produce a 
pumpable crude oil stock. 
These and other objectives are achieved by providing: a method for 
converting crude oil tank bottoms to a liquid state comprising subjecting 
crude oil sludge contained in the tanker cargo tank to a collection system 
to facilitate extracting the sludge mixture from the bottom of the tank, 
comminuting the sludge mixture to reduce the size of sludge globules 
contained therein to produce a homogeneous mixture, optionally adding in 
crude oil from the cargo tank to increase the homogeneity of the mixture, 
optionally adding an additional quantity of cutter stock and/or other 
selected additives to the sludge mixture, either before, during or after 
comminution, to form a blend stock. 
In further accordance with the present invention, there is provided a 
system for extracting crude oil sludge from the bottom of a cargo tank, 
the system comprising a number of auger pumps with open tops at points 
along the length of the tubing to collect and pump sludge, injectors 
through which crude oil or other liquid is pumped to loosen the sludge 
collected at the bottom of the storage tank, a system of pipes to collect 
the sludge and transport it to a section of the ship in which processors 
can be operated, and processors which contain a comminution chamber 
including therein a means to reduce the size of sludge globules contained 
within the sludge mixture, and an outlet for a blend stock product. 
Additional objects, features and advantages of the invention will be set 
forth in the description which follows, and in part will be obvious from 
the description, or may be learned by practice of the invention. The 
objects and advantages of the invention may be realized and obtained by 
means of the instrumentalities and combinations particularly pointed out 
in the appended claims.

DETAILED DESCRIPTION 
The present invention provides a method for converting crude oil tank 
bottoms to a liquid state comprising subjecting crude oil sludge contained 
in the tanker cargo tank to a collection system to facilitate extracting 
the sludge mixture from the bottom of the tank, comminuting the sludge 
mixture to reduce the size of sludge globules contained therein to produce 
a homogeneous mixture, optionally adding in crude oil from the cargo tank 
to increase the homogeneity of the mixture, optionally adding an 
additional quantity of cutter stock and/or other selected additives to the 
sludge mixture, either before, during or after comminution, to form a 
blend stock. 
In addition, circulatory equipment designed to further comminute and 
homogenize the contents of the cargo tank can be optionally added to the 
system. Advantageously, a liquid mixture having a product specification 
essentially identical to the crude oil in the cargo tank is produced, and 
substantially all the fuel value of the crude oil sludge is recovered 
while maintaining the standardized product specification of the crude oil. 
In further accordance with the present invention, there is provided a 
system for extracting crude oil sludge from the bottom of a cargo tank, 
the system comprising a number of auger pumps with open tops at points 
along the length of the tubing to collect and pump sludge, injectors 
through which crude oil or other liquid is pumped to loosen the sludge 
collected at the bottom of the storage tank, a system of pipes to collect 
the sludge and transport it to a section of the ship in which processors 
can be operated, and processors which contain a comminution chamber 
including therein a means to reduce the size of sludge globules contained 
within the sludge mixture, and an outlet for a blend stock product. 
Advantageously, the apparatus will further comprise at least one diluent 
injection port which is connected to, and in fluid communication with the 
comminution chamber, the injection port being adapted to controllably 
inject a variable quantity of diluent into the sludge mixture so as to 
improve the flowability of the blend stock product. 
The invention further provides an optional system for circulating the oil 
and sludge in the cargo tank during the time the ship is in transit and 
shearing, mixing, and homogenizing the sludge with the crude oil using 
circulation units mounted in the cargo tanks that will create vertical 
circulation patterns in the tank, further mixing the sludge from the 
bottom of the tank through the action of the fluid motion through the 
circulation unit. 
A general description of a method and apparatus for recovering the fuel 
value of crude oil sludge is described in related copending application, 
U.S. Ser. No. 08/553,900, which is incorporated herein in its entirety by 
reference. 
With reference to the Figures, FIG. 1 shows three auger pumps 2 at the 
bottom of the cargo tank discharging the sludge/crude oil mixture to the 
outside of the cargo tank. Additional pumps move the mixture to the 
holding tanks of the processors. The processors may recirculate some of 
their output to the holding tank. The output is pumped through 
injectors/nozzles back into the cargo tank to continue to loosen and 
circulate sludge and/or is pumped to a pipe that discharges the mixture 
directly into the cargo tank. FIG. 2 illustrates the circulation unit 
consisting of a propeller inside a tube. One or more of the propeller/tube 
units would be mounted in the cargo tanks to circulate and homogenize the 
crude oil and sludges in the fluid mixture. FIG. 3 illustrates the 
circulation unit consisting of an impeller inside a perforated tube. One 
or more of the impeller/stator units would be mounted in the cargo tanks 
to circulate and homogenize the crude oil and sludges in the fluid 
mixture. 
The apparatus of the present invention generally includes three component 
units; a sludge recovery unit, a processing unit, and an optional 
circulation unit. The sludge recovery unit is a self contained apparatus 
that is mounted in the bottom of a cargo tank 1; the processing unit is 
located externally to the cargo tank in an area of the ship such as the 
pump room; and the circulation unit is mounted in the cargo tank. 
The recovery unit may be of any configuration. An example of a suitable 
configuration is shown in FIG. 1, where a first unit described is a 
network of several auger pumps 2, each of which has an auger rotating 
inside a tube, the rotating motion moving the sludge down the length of 
the tube. Certain sections of the tube may be cut away so that the sludge 
can enter the tube from above. This preferred arrangement will permit the 
augers 2 to loosen solid globular sludge prior to being pumped inside the 
tube. The rotating augers act to break up the sludge as the sludge is 
moved along the length of the pipe. The auger pumps 2 can be powered by 
electric or hydraulic motors, the motors being attached to the augers or 
remotely mounted and connected to the augers by power transmission systems 
such as shafts, gears, or chains. 
Additionally, the sludge can be pumped out of the cargo tank by centrifugal 
pumps that are designed for viscous material such as dredging pumps 
manufactured by H & H Pump and Dredge Company of Holden, La. 
The recovery unit additionally is provided with at least one injector 8 
which serves to inject a hydrocarbon cutter stock 12 or crude oil that is 
part of the cargo or processed sludge directly into the cargo tank in the 
vicinity of the auger pump tube opening. It is contemplated that the 
injectors 8 are positioned such that the pressurized fluid is injected to 
loosen the surrounding sludge from the bottom of the cargo tank so that it 
will more easily move to the auger pump 2 tube opening. 
The extracted sludge mixture is moved through the pipe system by the auger 
pumps 2 in the tanks and an additional auger pump or pumps 3 and/or other 
types of positive displacement pump or pumps such a progressive cavity 
pumps. The extracted sludge may, and preferably is then fed to an 
intermediate reservoir or tank 4. The intermediate reservoir 4 will hold 
the liquid sludge prior to further treatment operations. The intermediate 
reservoir 4 may be provided with an inlet 6 for the addition of cutter 
stock and/or other additives. In addition, the intermediate reservoir 4 
can be equipped with magnetic barriers and a sump to collect metal 
fragments and other objects that may have progressed through the oil 
collection and tanker systems to prevent their being passed through the 
processors 5. The inclusion of an intermediate reservoir 4 further serves 
to create a continuous source of material to the processor 5 by acting as 
a surge accumulator for material being pumped from the extraction pump 2. 
At times when the flow from the extraction pump 2 is low, processed 
material can be recirculated 7 into the intermediate reservoir tank 4 to 
ensure material being available to the input of the processor. 
The extracted sludge is then transferred, either from the intermediate 
reservoir or directly from the cargo tank if no intermediate reservoir is 
included, to a processing unit 5 which includes a high shear mixer which 
is capable of blending, homogenizing, particle size reduction, and 
de-agglomeration. All material extracted from the cargo tank 1 by the 
extraction system 2 flows through the processor 5. The high shear mixer 
preferably includes a motor and an enclosed chamber with an inlet and an 
outlet. A suitable high shear mixer is described in detail in U.S. Ser. 
No. 08/553,900. Within the chamber, a work head is mounted on a spindle. 
The work head preferably is removable and several different 
interchangeable work heads preferably will be provided with the processing 
unit to allow for selective processing capabilities. The work head 
includes a stator about its outer periphery. The stator is provided with 
apertures or openings such that when the work head is subjected to high 
speed rotation, solid particles in the vicinity of the work head will be 
forced through the openings or apertures, thus reducing the particle size 
of the solids. The high speed rotation of the rotor blades within the work 
head exerts a powerful suction, drawing liquid and solid materials into 
the rotors. Centrifugal force then drives materials towards the periphery 
of the work head where they are subjected to a milling action. Hydraulic 
shear is then applied as the materials are forced, at high velocity, out 
through the perforations of the stator, then through the chamber outlet, 
exiting the mixing device. 
The processing machine 5 may require a positive displacement pump provided 
upstream from the processor 5 to ensure most efficient processing volumes. 
For example, a progressing cavity type pump may be employed if necessary. 
Downstream from the initial high shear mixer, additional holding tanks and 
processors may be provided to increase the breakdown of the solid globules 
to its smallest particle size as determined by the successively finer high 
shear mixing stators. The use of successively finer stators is dependent 
on the physical characteristics of the globules and how readily they break 
down. 
In a preferred embodiment, at least one or as many as all of the individual 
components or tanks of the processing apparatus may be provided with flow 
measuring equipment which can monitor the quantity of extracted sludge, 
diluent, and blend stock at various stages of processing. 
The flow of the sludge mixture through the system can be controlled either 
manually or automatically. If the control is automatic, it is contemplated 
that the system will be provided with a means to manually override the 
automatic control in the event the operator determines a change in the 
treatment parameters is warranted. 
The system may also include one or more sifting screens and/or magnetic 
field generators upstream from the processing unit which will act to 
separate large or ferrous objects which would damage the processing 
components of the mixer. 
In a typical embodiment, the processed sludge end product blend stock is 
added directly to the cargo tank 1, either to the tank at a high or 
intermediate level 9, or additionally or wholly at a lower level through 
injectors 8 placed at the bottom of the tank to induce internal 
circulation in the cargo tank and/or to the injectors that are part of the 
recovery system. Additional pumps 10 and 11 may be necessary to provide 
additional pumping pressure. 
A circulation unit optionally can be utilized to increase the efficiency of 
homogenization of the crude oil with the tank sludge at such times that 
the cargo tank is substantially filled. The circulation unit can be of any 
design, but most expediently includes rotating propellers or impellers 
turning in a stator or a tube acting as a shroud. 
In one embodiment, as shown in FIG. 2, the propeller 13 rotating in a tube 
14 is shaped so that the outer edges of the propeller 13 follow closely 
the shape of the interior surface of the tube 14, with a uniform space 
between the edge of the propeller blades 13 and the inside of the tube 14. 
As the propeller 13 rotates at high speed, a powerful suction is created 
which draws liquid and solid material through the tube 14. Much of the 
liquid and solid material moves towards the outer edge of the propellers 
13 through centrifugal force where they are subject to mechanical and 
hydraulic shear forces in the space between the ends of the propeller 
blades 13 and the inside of the tube 14. The propeller 13 can be rotated 
in a direction to draw the liquid and sludge up from the bottom. A 
diverting cone or plate 15 optionally can be placed above the propeller 13 
at some distance below the surface of the liquid so that the circulation 
pattern remains an internal motion minimally incorporating air into the 
liquid. Optionally, the propeller 13 can be rotated in a direction to push 
the liquid downward to push settled sludge from the bottom of the tank 1. 
In another embodiment shown in FIG. 3, an impeller 16 rotates in a stator 
17, the stator 17 being a tube-like structure in which there is a uniform 
pattern of apertures or openings. Solid particles and globs of sludge in 
the vicinity of the impeller 16 will be forced through the openings or 
apertures, thereby reducing the particle size of the solids and breaking 
up the sludge. The high speed rotation of the impeller blades 16 within 
the stator 17 exerts a powerful suction, drawing liquid and solid 
materials into the impellers 16. Centrifugal force then drives materials 
towards the periphery of the stator 17 where they are subjected to milling 
action. Hydraulic shear is then applied as the materials are forced, at 
high velocity, out through the perforations of the stator 17. A plate 18 
can be placed over the stator 17 causing the flow to be up through the 
bottom of the stator 17 and out through the sides of the stator 17, or, as 
shown in FIG. 3, the plate 18 can be placed under the stator 17 causing 
the flow to be downward through the top of the stator 17 and out through 
the sides of the stator 17. 
The number of such circulation units and their placement in the cargo tanks 
1 will be a function of the size and shape of the cargo tanks 1. The units 
could be individually mounted or be mounted in a sub-frame in the cargo 
tank 1. The rotating parts can be powered by hydraulic motors although 
other methods of providing the necessary mechanical rotation are also 
acceptable. 
Additional advantages, features and modifications will readily occur to 
those skilled in the art. Therefore, the invention in its broader aspects 
is not limited to the specific details, and representative devices, shown 
and described herein. Accordingly, various modifications may be made 
without departing from the spirit or scope of the general inventive 
concept as defined by the appended claims and their equivalents.