GB2222424A

GB2222424A - Removing material from the seabed by dredging

Info

Publication number: GB2222424A
Application number: GB8919908A
Authority: GB
Inventors: Ian James Murray
Original assignee: MURRAY IAN ENGINEERING Ltd
Current assignee: MURRAY IAN ENGINEERING Ltd
Priority date: 1988-09-03
Filing date: 1989-09-04
Publication date: 1990-03-07
Anticipated expiration: 2009-09-04
Also published as: GB8821885D0; GB8919908D0; GB2222424B

Abstract

Removal of material from the seabed is described in which waste material (46) is drawn from below a marine structure (42) into a pumping chamber (26a, 26b, 26c) by pumping apparatus (12). The chamber is then evacuated and the material (46) discharged through a conduit to deposit the material (46) in a location of the seabed spaced from the structure (42). <IMAGE>

Description

Subsea Waste Removal This invention relates to a method of and apparatus for removing waste material from the seabed in the vicinity of a marine structure such as an oil well drilling or production platform.

When drilling an offshore oil well, cuttings separated from the drilling fluid are usually dumped down a chute, and accumulate on the seabed below. Where the rig is supported on legs passing into the seabed, it has been found that the accumulated mass constitutes a serious environmental and operational problem. The mound of cuttings can be in excess of 20 metres in height, burying structural members of the rig and preventing its proper removal on completion of the well. The cuttings can also give rise to chemical or anaerobic attack on the structure, affecting its integrity.

The cuttings are normally oil-bearing and sticky, and present extremely difficult working conditions for divers.

Hitherto, attempts have been made to clear this material by means of a diver operating a suction pump powered electrically or hydraulically from the surface, but this is extremely difficult and slow, typically clearing only 1 m3/hour.

Accordingly, a primary object of the present invention is to provide an improved method of and apparatus for removing material of the type discussed.

The invention in one aspect resides in a method of removing material from the seabed, in which a pump assembly operated by hydrostatic pressure is lowered from the surface to contact said material, and the pump assembly is operated to pump the material to a remote location.

In a preferred form of the invention, the material to be removed is an accumulation of waste material beneath a structure fixed to the seabed. The pump assembly may be suspended from the structure. The material may suitably be discharged onto the seabed at a location spaced from the structure.

In another aspect, the invention provides apparatus for use in the foregoing method, the apparatus comprising a pump assembly adapted to be lowered from the surface to rest upon the seabed, the pump assembly comprising at least one pumping chamber, an inlet pipe extending downwardly from the pumping chamber so as to be immersed, in use, in the material of the seabed, an outlet pipe extending from a position within the pumping chamber and means for cyclically connecting the interior of the pumping chamber with a source of compressed air and with atmosphere.

Preferably there is a plurality of pumping chambers, suitably three in number, having their outlet pipes feeding a common outlet conduit.

Preferably also, the apparatus further comprises a flexible hose connected between said outlet conduit and a discharge tube which is adapted to be suspended in a position to discharge the dredqed material onto the seabed at the location spaced from the structure.

An embodiment of the invention will now be described by way of example only, with reference to the drawings, in which: Fig. 1 is a side view of a drilling jacket with accumulated cuttings material; Fig. 2 is a similar view illustrating the invention in operation; Fiq. 3 is a schematic side view of a pump assembly used in the system of Fig. 2; and Fiqs. 4A, 4B and 4C illustrate the operation of the pump.

Referring to Fiq. 1, an oilwell drilling jacket 10 is supported on leqs 42. Drill cuttings dumped via a chute 44 form a mound 46 enveloping the parts of the legs 42 adjacent the seabed and cross tubes between the leqs.

As seen in Fig. 2, the invention makes use of an unmanned pump assembly 12 which is suspended from the working platform 16 of the jacket 10, for example by crane 48. The assembly 12 operates, as will be described, to pump the cuttings away, thus clearinq the mound. The cuttings may be pumped to the working platform 16 and thence discharged to a barge. Alternatively, as shown, the cuttings may be pumped via a flexible pipe 18 to be directly discharged from a discharge tube 14, suspended from the surface, onto an area of seabed clear of the jacket 10.

Turninq to Fiq. 3, the pump assembly 12 comprises three pumping chambers 26a, b and c secured together in triangular formation. Each pumping chamber has a downwardlyprojecting intake pipe 28. Outlet pipes 30 extending from the chambers 26 join together to feed into the flexible pipe 18. The pump assembly 12 is actuated by compressed air from a compressor on the working platform via a supply hose 32, distributor valve 34, and conduits 36.

The operation of one chamber 26 is shown in Fig. 4. The pump is allowed to sit on the seabed so that the open end of the intake pipe 28 is immersed in waste material. Owing to the hydrostatic pressure acting at the seabed, the waste material flows into the chamber 26 via intake pipe 28 and check valve 38, as seen in Fig. 4A. Air displaced by this inflow is vented via conduit 36 and distributor valve 34.

When the chamber 26 is full (Fig. 4B) the intake valve 38 automatically closes under its own weight and that of the contents. At this point, compressed air is supplied via the distributor valve 34 and conduit 36 to force the chamber contents out through the outlet pipe 30 via an outlet check valve 40 (Fig. 4C). When the chamber 26 is almost empty, the distributor valve changes to allow the compressed air within the chamber 26 to be discharged to atmosphere. As soon as the internal pressure has been released, the cycle restarts.

The use of a pump assembly 12 having three chambers 26 is preferred, since this allows virtual continuous operation, but any number of chambers from one upwards may be used.

The pump may suitably be operated at about 3 cycles/minute.

It will be noted that there are no moving parts in the underwater assembly apart from the check valves 38 and 40, which may be simple rubber members. The pump is therefore highly reliable in the hostile environment of a sea water/abrasive mixture.

It is anticipated that the present invention can be used in water of considerable depth, typically 150-200 metres, and that with an 8 inch (19.3 millimetre) pipe size could remove cuttings at the rate of approximately 120 m3/hour. The power consumption is low, since most of the energy used is derived from the hydrostatic pressure at the seabed.

Modifications and improvements may be made to the foregoing embodiment within the scope of the invention.

Claims

1. A method of removing material from the seabed comprising lowering a pump assembly, operated by hydrostatic pressure, from the surface to contact the material, and operating the pump assembly to pump the material to a remote location.

2. A method according to Claim 1, wherein the material to be removed is an accumulation of waste material adjacent a structure.

3. A method according to Claim 2, wherein the waste material is beneath the structure.

4. A method according to any of the preceding claims, wherein the remote location is an area of the seabed spaced from the structure.

5. Apparatus for removing material from the seabed comprising a pump assembly adapted to be lowered from the surface to rest upon the seabed, the pump assembly comprising at least one pumping chamber, an inlet pipe extending downwardly from the pumping chamber so as to be immersed, in use, in the material of the seabed, an outlet pipe extending from a position within the pumping chamber and means for cyclically connecting the interior of the pumping chamber with a source of compressed air and with atmosphere to transfer the material from the seabed to the at at least one pumping chamber and then transfer the material from the pumping chamber to a remote location.

6. Apparatus according to Claim 5, wherein there are at least two pumping chambers having their outlet pipes feeding a common outlet conduit.

7. Apparatus according to Claim 5, further comprising a flexible hose connected between the outlet conduit and a discharge tube which is adapted to be suspended in a position to be removed to discharge the material removed onto an area of the seabed spaced from the original position of the material.

8. Apparatus according to any of Claims 5 to 7, wherein there are three pumping chambers.

9. Apparatus for removing material from the seabed substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of removing material from the seabed, substantially as hereinbefore described with reference to the accompanying drawings.