GB2403963A

GB2403963A - Drill cuttings settlement tank

Info

Publication number: GB2403963A
Application number: GB0316330A
Authority: GB
Inventors: Michael F Crawley
Original assignee: MACTENN SYSTEMS Ltd
Current assignee: MACTENN SYSTEMS Ltd
Priority date: 2003-07-12
Filing date: 2003-07-12
Publication date: 2005-01-19
Also published as: GB0316330D0

Abstract

A non-pressure rated silo (1) for temporary storage of drill cuttings which comprise a mixture of solids and liquids has a discharge opening (40) for solids and a liquid discharge pipe (9) which is open near the top of the silo (1), so that liquid from which solid material has settled out can be drained. The solids in the bottom of the silo (1) are agitated with paddles (6) prior to discharge and the breaking up of the solids may be enhanced by the use of water (11) or air jets (12, fig 2). After leaving the silo (1) the solids are transported by use of a dense phase pneumatic machine (16) which fluidises the cuttings and moves them through a discharge pipe (44).

Description

Handling Drill Cuttings This invention relates to a method of handling, in

particular storing and discharging, drill cuttings from offshore well drilling operations, and to a silo arranged to perform the method.

Drill cuttings is the term used in the offshore industry to describe a mixture of sea bed, sea water and cutting oils and chemical additives, that have been brought up to a drill platform from the drill bit when the bit is drilling into the sea bed. Regulations require that drill cuttings are not returned to the sea but are taken ashore for safe disposal on land.

As a result of these regulations, it is necessary to provided a storage facility on the drilling platform where cuttings can be stored awaiting the availability of a vessel to transport them ashore.

There is thus a need to reliably empty drill cuttings from a storage silo after a period of storage during which the cuttings have changed their homogeneity. The material when delivered into the silo comprises various mixtures of seabed, oil, water and sedimentary earths, which have been somewhat, mixed into a live condition from the process of delivery to and transfer from a dense-phase pneumatic conveying arrangement into the silo.

Over a period of time of approximately two weeks or so, the drill cuttings in the silo segregate to generally - 2 - different components, which are water and earth. The heavier earth material sinks into the lower portion of the silo and the lighter more liquid components rise to the higher parts of the silo. The effect of this migration is to cause the more solid and heavier materials to form a dense plug above the outlet of the silo preventing reliable emptying under the sole influence of gravity.

According to the invention, there is provided a method of handling drill cuttings on an offshore drilling platform, the invention comprising the steps of pumping the cuttings into a non-pressure rated silo, allowing the cuttings to remain in the silo until facilities are available for transporting the cuttings to a remote disposal point, and when the facilities are available, discharging the silo contents into a transport vessel, wherein the step of discharging the cuttings comprises mechanical agitation of the cuttings in the silo, removal of supernatant liquid through a liquid discharge pipe and subsequent opening of a discharge outlet at the bottom of the silo to allow the cuttings to pass into a transport vessel.

The invention also provides a silo for storing material which is a mixture of solids and liquids, the silo having an inlet at the top for material to be stored, an outlet at the bottom for discharging solid material, an agitator paddle arrangement within the silo and a liquid discharge pipe extending the height of the silo and arranged to allow liquid within the silo to be discharged through a route which is independent of the solid material outlet. - 3 -

The material will be introduced into the silo as a sludge or slurry, ie as a flowable mixture of solids and liquids.

During the time for which the material is held in the silo, the solid material will settle to some extent, leaving a volume of liquid above the solids. Although reference is made here to a solids outlet, the material discharged through this outlet will still contain some liquid.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical cross section through a silo in accordance with the invention; Figure 2 is an elevation of the lower section of the silo of Figure 1; Figure 3 is a schematic plan view of the top of the silo; and Figure 4 is a schematic vertical cross section through the upper section of the silo.

A circular storage silo 1 of a suitable capacity with an inverted conical lower end 19 is supported by legs 20.

Material to be stored (eg drill cuttings) is fed into the top of the silo through a pipe 22 by a pneumatic conveying device. Entry into the silo is by way of the pipe 22 into an end diverter 2. The silo is designed to be able to - 4 - store the material for a period of time and is fitted with a vacuum and pressure vent valve 13 which will open to relieve internal pressure in the event that this builds up to an unacceptable level. Any internal pressure will be at a relatively low level, and the silo is not designed as a pressure vessel, but just as a (non-pressure rated) storage vessel.

The material is introduced into the silo until the level inside the silo reaches a high-level probe 3. The material then remains in the silo for storage for a period of time.

During this period there will be settlement of the contents into a liquid phase at 24 and a solid or sludge phase at 26. The liquid phase will rise to an upper level 4 and the heavier material will sink to the bottom of the silo.

The silo is fitted with a central shaft 5 which drives a series of blades or paddles 6 positioned in the lower cone 19 of the silo. The shaft 5 is supported on a number of bearings 7 and the assembly is driven by a hydraulic gearbox 8. Hydraulic power to the gearbox is by a central hydraulic power pack (not shown) able to serve a series of storage silos since only one silo will be activated at any one time.

In the upper part of the silo, opposite the location of material entry, there is a liquid outlet 9. The outlet comprises a heavy duty mesh 28 (Figures 3 and 4) surrounding a perforated drain pipe 30. In a particularly suitable embodiment for use with drill cuttings, the mesh - 5 28 will have a mesh size of about 3 - 4 mm. The pipe 30 will have a diameter of 80 mm with vertical slits of width about 3 - 5 mm. the tube will be closed at the top so that the only entry into the tube will be through the slits. Liquid from the liquid phase region will flow through the mesh and through the perforations 32 into the pipe 30 from where the liquid can be taken to a liquid/water cleaning process.

The end diverter 2 is designed so that the material entering the silo is directed away from the outlet 9, with a spray pattern generally as indicated by the numeral 27.

The solid, sludge phase in the lower part of the silo will tend to compact during storage. To prevent this happening, a series of high pressure water jets 11 (not shown in Figure 2) are located in the wall of the silo cone 19. They are pressurized by a central high pressure water pump (not shown) and water from these jets can be directed under high pressure into the body of sludge material, thus keeping the solid material to a degree fluidised. The jets may be operated continuously, at regular time intervals during storage of material in the silo, or just when the silo in to be emptied.

Additionally a "cries of air jets (so-called Aireweeps) 12 (see Figure 2) are located also in the silo cone 19 and powered by a high pressure air source (not shown).

At the bottom of the silo, there is an isolating gate - 6 valve 14, a pressure tight filling valve 15, a dense-phase pneumatic conveying vessel 16 and a pressure tight outlet valve 18. The dense-phase conveying vessel 16 uses a controlled high pressure air source 42 through a series of control devices 17 to propel material past the outlet valve 18 to a destination.

Within the silo a conically formed set of agitator blades 6 are mounted on the central, vertical drive shaft 5 which is supported within the silo 1 on three bearing spiders 34. The spiders are designed not to influence the vertical movement of the material as it flows downward through the silo. The drive for the shaft is the hydraulically driven gearbox powered by a single hydraulic motor of approximately 30 hp. The drive provides approximately 30 revolutions per minute of the agitator blades 6. The agitator blades have a directional set causing the material to be directed either upwards or downwards as well as causing partial homogenization through a gentle mixing action.

In use, the silo will be filled with drill cuttings through the pipe 22, and the probe 3 will signal when the silo is full. The silo will normally be one of a series of similar silos, and when one is full, the feed of drill cuttings will be passed to the next silo and so on.

Emptying of the silo takes place in batches. Valves at the bottom of the cone are opened to allow material to pass into the dense-phase conveying vessel 16; once the vessel is full, the valves at the bottom of the silo are 7 - closed, the outlet valve 18 downstream of the vessel 16 is opened and the vessel is charged with air to discharge material through an outlet pipe 44. Once the vessel 16 is empty, this process is repeated.

In more detail, the following stages take place: First, the water mixture at the top of the silo is drained away by opening the bottom drain valve 38. This allows the dirty water to be channelled to the drilling rig central water cleaning process. This action also provides free space for a potential volume increase during the first moments of homogenization when water is directed under high pressure into the silo cone through the water jets.

In addition this draining of the water mixture will provide a net reduction of water discharged with the solids (which are subsequently to be transported to the shore).

After water draining and before opening the valves 14,15 at the outlet 40 at the bottom of the silo, the blades 6 are operated to direct the material upwards to relieve the plug of solid material at the silo outlet. During this period the high pressure water jets 11 set into the cone wall are operated. The water jets have a special nozzle design to provide a cutting action to divide up the mass of material and to provide lubrication to the solid particles. Partial movement of the material upwards under this influence will relieve the plug of solid material above the emptying outlet 40 of the silo. - 8 -

After a period of between 10 and 25 seconds the drive shaft 5 is stopped and the rotation direction reversed for very short periods of about 5 seconds while the upstream discharge valve 15 is open to allow material to flow through into the dense-phase vessel 16. Also during this time the air jets 12 are activated for the same short period to clean the cone walls and encourage homogenization of the mixture. The blade rotating action is stopped when the valve 15 is closed.

Once the vessel 16 is full, the valve 15 is closed and the valve 18 opened. High pressure air fed into the vessel 16 forces the material, now in a fluidised state, through the valve 18 and along the discharge pipe 44. When all the material has been expelled from the vessel 16, the valve 18 is closed and the valve 15 opened so that another batch of material can pass into the vessel 16.

The process continues in sequence in response to a small PLC program until the silo is empty.

The emptying process will be controlled by system controls which detect when the silo is empty. The system controls will also monitor the power load requirements of the shaft driving the mixing blades. The information will show the higher load requirements at the beginning of the homogenization and this will reduce as the actions described achieve material mixing. The load information will be processed to automatically reduce or stop the need

for blade action and water introduction. - 9 -

By way of example, the silo discharge flange at 40 can be is 16 inches (400 mm) and is matched with a 16 inch (400 mm) inflatable seat valve 15 (such as an Inflatek valve - Inflatek is a Registered Trade mark). The valve 15 is the inlet valve to a MultiAshflo dense-phase vessel 16 that provides a transfer rate of 30 tons/hr. The MultiAshflo for thin application is a modified version of a well- proven method transferring ash and is used here for reasons of low cost and low profile.

The sequence for conveying requires the valve 18 on the discharge end of the vessel 16 to remain closed while the inlet valve 15 is open for filling. When the vessel 16 is full the inlet valve 15 closes and when automatically sealed, the transfer process begins to allow the material to pass beyond the outlet valve 18. Once the vessel 16 is empty, the outlet valve 18 closes and the vessel is vented to atmosphere. The conveying process along the pipe 44 continues from a conveying air supply introduced through an auxiliary air connection 46.

While conveying is continuing and the vessel 16 is vented to atmosphere the inlet valve 15 opens to allow refilling and remains available for the transfer cycle as soon as the material has reached its destination. Average total cycle time is 67 seconds based on a material of 2000 kgs/m3, and a distance of 200m and 30 tons/hr.

There are control interlocks for safe automatic and continuous operation, a fault diagnostic system. Controls are in accordance with Atex safety requirements.

Claims

Claims 1. A method of handling drill cuttings on an offshore drilling

platform, the invention comprising the steps of pumping the cuttings into a non- pressure rated silo, allowing the cuttings to remain in the silo until facilities are available for transporting the cuttings to a remote disposal point, and when the facilities are available, discharging the silo contents into a transport vessel, wherein the step of discharging the cuttings comprises mechanical agitation of the cuttings in the silo, removal of supernatant liquid through a liquid discharge pipe and subsequent opening of a discharge outlet at the bottom of the silo to allow the cuttings to pass into a transport vessel.
2. A method as claimed in Claim 1, wherein the cuttings are subjected to water jets from nozzles located in the silo walls, to assist in breaking up the solid aggregations of the cuttings.
3. A method as claimed in Claim 1 or Claim 2, wherein the cuttings are discharged from the silo to a dense phase pneumatic machine which fluidises the cuttings and expels them through a discharge pipe.
4. A silo for storing material which is a mixture of solids and liquids, the silo having an inlet at the top for material to be stored, an outlet at the bottom for discharging solid material, an agitator paddle arrangement within the silo and a liquid discharge pipe extending the height of the silo and arranged to allow liquid within the silo to be discharged through a route which is independent of the solid material outlet.
5. A silo as claimed in Claim 4, wherein the liquid discharge pipe has apertures through its walls to allow liquid to flow into the pipe while preventing solids from i entering.
6. A silo as claimed in Claim 5, wherein the part of the pipe with apertures through its wall is surrounded by a separate mesh guard to prevent large solid objects from reaching the wall of the pipe.
7. A silo as claimed in any one of Claims 4 to 6, wherein the solid material outlet is connected above a dense phase pneumatic machine arranged to receive solid material from the silo and to convey the material into an outlet pipe.
8. A silo as claimed in any one of Claims 4 to 7, wherein the solid material outlet is closed by an inflatable seat valve. Z
9. A silo as claimed in Claim 7, wherein the outlet from the dense phase pneumatic machine is closed by an inflatable seat valve.
10. A silo as claimed in any one of Claims 4 to 9, having a downwardly tapering lower section and water jet nozzles arranged in the walls of the lower section to direct water - 12 jest into the interior of the silo.
11. A silo as claimed in Claim 10, wherein air jet nozzles are arranged in the walls of the lower section to direct air under pressure along the internal surfaces of the lower section walls.
12. A silo as claimed in any preceding claim, wherein the inlet to the silo is arranged to direct incoming material into the silo interior in a direction away from the liquid discharge pipe.
13. A silo for storing drill cuttings, substantially as herein described with reference to the accompanying drawings.
14. A method of handling drill cuttings substantially as herein described with reference to the accompanying drawings.