Device for pumping slurry

The present invention relates to a device for pumping slurry, for example the waste slurry produced by drilling operations.

When drilling for oil, the drill cutting head is frequently driven by a mud motor, which is powered by mud pumped down the drill shaft and which returns to the surface up the drilled bore. The mud motor is commonly of the progressing cavity type in which a helical rotor rotates and orbits inside a stator having a helical bore of the same pitch as the stator but one more, or one less, starts. The returning mud or slurry is normally discharged into a tank and contains a large proportion of foreign bodies and tramp materials which settle out in the tank. The foreign bodies in the slurry are of a wide variety, including rock, bits broken from the cutting head, pieces of polymer broken from the stator of the mud motor and often rubbish from the drilling rig such as nuts, bolts and spanners. Once the slurry tank is full it is taken away and used as landfill. For off-shore drilling rigs, the slurry tank is commonly provided on a barge which is taken to a lagoon where the tanks are emptied.

Because of the high proportion of hard and abrasive foreign bodies in the slurry it is a particularly difficult medium to pump and it is particularly difficult fully to empty the slurry tank, since the proportion of foreign bodies increases towards the bottom as they settle out. It has been proposed to use a centrifugal pump protected by a static screen for pumping slurry bit this was found to be unsatisfactory.

According to the present invention there is provided a device for pumping slurry, the device comprising:

• a screen adapted to pass only particles of less than a predetermined size and disposed generally horizontally with an inlet in its lower surface and an outlet in its upper surface;
• a positive displacement pump for pumping said slurry, said pump being disposed above said screen and having an inlet; and
• chamber connecting the outlet of said screen to the inlet of said pump.

A positive displacement pump is one which delivers the same volume of product for each revolution or stroke regardless of pumping speed. A rotodynamic pump, such as a centrifugal pump, requires a certain speed to create the centrifugal effect. It has been found that a positive displacement pump, particularly when operating at a low speed, has less effect on the rheological properties of the slurry and so pumps more efficiently. A centrifugal pump was found to cause the slurry to thicken. The pump is preferably of the progressing cavity, or Moineau, type and may have a single start rotor, with a two start stator.

The screen prevents blockages and damage to the pump by rejecting oversize particles. The screen is preferably a macerator, or combined screen and grinder, which reduces those particles which are too large to pass through the pump, rather than simply rejecting them thereby necessitating their removal by other means. In some embodiments however, a simple screen, e.g. a bar or mesh screen may suffice. Such a screen may be static or agitated to prevent blockages.

A presently preferred embodiment of the invention consists of a device for pumping slurry, the device comprising:

• a macerator having a macerating chamber having sidewalls and open upper and lower faces, two parallel, substantially horizontally disposed, shafts extending between opposite sidewalls of said macerating chamber, alternating cutter wheels and spacers disposed on said shafts, said cutter wheels having a greater diameter than said spacers and said cutter wheels on one of said shafts being interleaved with said cutter wheels on the other of said shafts, and means for driving said shafts to counter-rotate;
• a funnel chamber having upper and lower openings, said lower opening of said funnel chamber being larger than said upper opening thereof and communicating with said open upper face of said macerating chamber; and
• a pump of the progressing cavity type comprising a helical rotor of N starts and a stator having a helical bore therethrough complimentary to the rotor and having N ± 1 starts, said stator being generally vertically arranged, and said pump having an inlet end in communication with the upper opening of said funnel chamber;
• whereby in use said pump is operated to pump slurry out of said funnel chamber and thereby draw slurry in through said macerator, and said shafts of said macerator are counter-rotated so as to pass particles of less than a predetermined size and to break up particles of a greater than said predetermined size, said predetermined size being a size acceptable to said pump.

The present invention further provides a boom assembly comprising:

• a swingable boom; and
• a pumping unit mounted on the distal end of said boom and immersible in a slurry, said pumping unit comprising:
• a screen adapted to pass only particles of less than a predetermined size and disposed generally horizontally with an inlet in its lower surface and an outlet in its upper surface;
• a positive displacement pump for pumping said slurry, said pump being disposed above said screen and having an inlet; and
• chamber connecting the outlet of said screen to the inlet of said pump.

Such boom assembly can conveniently used to remove slurry from, e.g., a barge by immersing the pumping unit in the slurry in the barge and sweeping it about to ensure all parts of the barge are reached. The boom assembly can be mounted on a dockside or another vessel as desired.

The invention will be described further below with reference to an exemplary embodiment and the accompanying drawings, in which:

• Figure 1 is a partly cross-sectioned schematic side-view of a first embodiment of the present invention;
• Figure 2 is a schematic enlarged plan-view of the grinding screen of the first embodiment;
• Figure 3 is a further enlarged perspective view of part of the counter-rotating shafts of the grinding screen of Figure 2; and
• Figure 4 is a schematic view of the first embodiment of the invention mounted on a boom.

In the drawings, like parts are indicated by like references.

Figure 1 shows a pumping device 1 according to a first embodiment of the present invention which is adapted to pump a drill cuttings slurry 3 out of a tank that may be provided on a barge or similar. At the suction end of the pumping device 1, a screening device 4 is provided. This lets through particles 2 in the slurry that are small enough to be pumped by the pumping element 12 and breaks up any larger particles 5. The pumping element 12 is, in this embodiment, of the progressing cavity type with a single-start helical rotor and a two-start helical bore in the stator. It is driven by motor 12a via drive-shaft 12b and pumps slurry from funnel shaped chamber 4a above the grinding screen 4 into the drive-shaft chamber 12c from whence it is discharged via suitable conduits (not shown) to its ultimate destination.

Pumping device 1 is also provided with impellors 6 which are disposed adjacent to and on a level with the grinding screen 4. The impellors 6 serve to agitate the drill cuttings slurry and keep the solid particles therein suspended.

Figure 1 shows only the grinding screen 4 and impellors 6 submersed in the slurry, however the entire device can optionally be made submersible by provision of appropriate proofing. The pumping device 1 can be provided on the end of a boom or suspended from a crane, so that it can be lowered into the slurry tank and swept across the bottom to remove all of the slurry and foreign bodies.

The grinding screen 4 is shown enlarged and from below in Figure 2. As can there be seen, the grinding screen 4 comprises two counter-rotating shafts which are driven to rotate via motor 4b and gears 4c. The shafts are horizontally disposed and extend between side walls of the screen, leaving only two narrow apertures 14 around the outside, and a centre aperture 13 between them. The sense of rotation indicated by arrows in the Figure, is such as to attract particles to the centre aperture 13 rather than the outside apertures 14.

Each shaft consists of a series of cutter wheels 9 held apart by cylindrical spacers 10. The spacers 10 are axially thicker than the cutter wheels 9 which are of greater diameter and are interleaved, so as to create gaps 11 through which small solids 2 can pass. Each cutter wheel 9 has at least one tooth 9a and preferably a plurality, e.g. seven as in this embodiment. The size of the gaps 11 will be determined by the size of solids that the pumping element 12 can accept and in a preferred embodiment of the invention is of the order of 8mm axially. Solid particles 5 that are too large to pass through the gaps 11 are broken up by the cutter wheels 9 until they are small enough to pass.

Figure 4 shows pumping device 1 mounted on the end of a swingable boom 20 so that it can be lowered into the slurry 3 contained in a barge 30. As the slurry is pumped out of the barge 30, the pumping unit can be lowered so that it is always immersed in the slurry to an appropriate depth. The pumping unit can also be moved around the barge to ensure that slurry is removed from all parts, particularly where the barge is compartmented or has ribs, e.g. stiffeners projecting upwardly from its bottom forming dead spaces in the slurry tank. The boom 20 may be mounted on a dockside or another vessel. A flexible pipe 40 takes the slurry to a desired location.