GB2354726A

GB2354726A - Abrasive cutting nozzle

Info

Publication number: GB2354726A
Application number: GB9923124A
Authority: GB
Inventors: Graham Thomas Altham; Ian Mcfayden
Original assignee: UWG Ltd
Current assignee: UWG Ltd
Priority date: 1999-10-01
Filing date: 1999-10-01
Publication date: 2001-04-04
Anticipated expiration: 2019-10-01
Also published as: GB9923124D0; GB2354726B

Abstract

An abrasive cutting nozzle 16 for use underwater is fitted with a shroud 20. When the nozzle is operating, a jet of abrasive particles is driven through an opening 28 in the nozzle against the surface to be cut. At the same time, air is blown into the shroud at 30 and this expels water within the shroud so that the jet does not encounter water until it passes out of the open end of the shroud.

Description

2354726 Abrasive Cutting This invention relates to abrasive cutting, and

in particular to a nozzle and modifications thereto for emitting a jet of abrasive particles, for cutting hard materials under water.

The process of cutting metal or other hard materials by directing a jet of abrasive particles against a surface is we 11 known. For example the particles can be garnet particles carried in a slurry and emitted from a nozzle at high pressure to impinge against a local area of the metal surface. The surf ace is abraded in the local area to such an extent that the material is cut through. The nozzle is moved across the surface to continue the cutting action.

There are occasions when it is desirable to be able to cut through objects underwater. This is particularly the case in connection with subsea drilling structures or wells. It has however been found that conventional cutting using a jet of abrasive particles does not work efficiently underwater, or when the abrasive jet has to pass through other fluids because the energy of the jet is dispersed. The traditional solution to this problem is to increase the pressure at the nozzle. Operating at very high pressures is expensive and can be dangerous.

According to the invention, there is provided a method of operating an abrasive cutting nozzle underwater, wherein the nozzle is surrounded by a shroud with an open end, and water is expelled from the open-ended shroud during operation.

Preferably air is blown into the shroud to expel water.

The air then also co-mingles with the abrasive jet. The air pressure can be between 75-100 psi over ambient pressure, although other pressures may be suitable in particular circumstances.

The, invention also provides a shroud for an abrasive cutting nozzle, the shroud having one end adapted to fit over the nozzle, an opposite end which is open to allow an abrasive cutting jet to pass from the nozzle out of the shroud, and means for expelling water from the shroud during jet operation.

The means for expelling water can be an air inlet adapted for connection to a source of compressed air.

An internal wall is preferably fitted across the internal cross-section of the shroud to divide the shroud into a first chamber and an exit chamber, the wall having a central aperture through which the abrasive cutting jet can pass without obstruction, and secondary apertures through which water can be driven to empty the first chamber.

The secondary apertures preferably all lie in a part of the wall which, in use, will be at the bottom part of the shroud.

Another aspect of the invention provides an abrasive cutting nozzle which has an aperture through which a jet of abrasive particles can be emitted and a shroud surrounding the nozzle, the shroud encircling the nozzle, extending beyond the nozzle and having an open end, and means for removing water from the interior of the shroud whilst the jet is operating.

A shroud as described above can be retro-fitted to an existing abrasive cutting nozzle.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in 5 which:

Figure 1 is a schematic view of an application of the invention; Figure 2 shows a cross-section through a nozzle and shroud in accordance with the invention; and Figure 3 is an end view of the shroud shown in Figure 2.

Figure 1 shows three concentric casing tubes 10,12,14, all filled with or surrounded by water or other liquid, which are to be cut through in a plane generally at right angles to the tube axes. Such operations have to be completed under water, for example when severing subsea wellheads.

In order to do this cutting, an abrasive cutting nozzle 16 is shown lowered inside the inner tube 10 and oriented so that the nozzle is pointing at the inner surface of the tube wall 10. The nozzle is supported by a conventional support structure (not shown) and is fed with a slurry containing abrasive cutting particles through a pipe 18 and is surrounded by a shroud 20 which is fed with compressed air through a pipe 22. When the nozzle is working, a stream of abrasive particles is driven against the wall 10 to produce a cutting action.

Figures 2 and 3 show the nozzle 16 and shroud 20 on a larger scale. The shroud f its over the nozzle 16 and is typically held in place by screws passing through f ixing holes 24. The shroud has an internal wall 26 with a central aperture 28 through which a jet of abrasive slurry is projected for the nozzle. An air inlet 30 leads into the shroud behind the wall 26. The wall 26 has, in addition to the central aperture 28, a number of small holes 32, which here are shown in the lower half of the wall only.

For example, the length of the shroud can be between 75 and 250 mm and the diameter can be between 40 and 75 mm.

In operation, the nozzle and shroud assembly are positioned adjacent the surface to be cut. At this stage, the shroud will fill with water. The feed of abrasive slurry to the nozzle is switched on, and the feed of compressed air to the inlet 30 is switched on. The compressed air first purges the shroud chamber 34 behind the wall of water and then purges the exit chamber 36.

The abrasive slurry then passes through air as it travels through the shroud, before meeting a layer of water in front of the surf ace to be cut.

The presence of water would dissipate the energy of the jet so it is important that the air is present to expel the water. It also appears that the air is then carried by the jet and this helps to ease the flow path of the abrasive jet as it travels through the fluids in the casing beyond the end of the shroud.

The air feed to the shroud is typically maintained all the time cutting is taking place.

It has been found that the use of a shroud in this way 35 allows the abrasive cutting jet to penetrate through a body of water and to successfully abrasively cut a metal surface when cutting would not be possible if the jet from the nozzle encountered a body of water immediately on exiting the nozzle.

The location of the holes 32 in the lower half of the wall 26 ensures that the incoming compressed air which rises to the top of the chamber 34 has to push any water in the chamber out of the chamber before the air itself can exit.

The air is thus also introduced to the exit chamber 36 at the bottom to encourage it to drive any remaining water out of this chamber also. The air pressure required is not particularly great, for example a pressure of 75 - 100 psi over ambient pressure has been found adequate.

From tests, it appears that the use of a water-free shroud allows the jet to achieve the necessary power not only to cut the tube 10, but also to penetrate the water layers between the tubes 10 and 12 and cut the tube 12 and also to penetrate the water layer between the tubes 12 and 14 and to cut the tube 14. Successful operation is anticipated on tubes or sets of concentric/eccentric tubes which may have diameters of from 150 mm to 900 mm. However the invention is not limited to the cutting of cylindrical tubes, but has application for the underwater cutting of a variety of materials (metal and non-metal) in a variety of different forms.

Claims

1. A method of operating an abrasive cutting nozzle underwater, wherein the nozzle is surrounded by a shroud with an open end, and water is expelled from the openended shroud during operation.

2. A method as claimed in Claim 1, wherein air is blown into the shroud to expel water andlor co-mingle with the abrasive jet.

3. A method as claimed in Claim 2, wherein the air is blown in at a pressure of between 75-100 psi over a-mbient conditions.

4. A shroud for an abrasive cutting nozzle, the shroud having one end adapted to fit over the nozzle, an opposite end which is open to allow an abrasive cutting jet to pass from the nozzle out of the shroud, and means for expelling water from the shroud during jet operation.

5. A shroud as claimed in Claim 4, wherein the means f or expelling water is an air inlet adapted for connection to a source of compressed air.

6. A shroud as claimed in Claim 5, wherein an internal wall is fitted across the internal cross-section of the shroud to divide the shroud into a first chamber and an exit chamber, the wall having a central aperture through which the abrasive cutting jet can pass without obstruction, and secondary apertures through which water can be driven to empty the first chamber.

7. A shroud as claimed in Claim 6, wherein the secondary apertures all lie in a part of the wall which, in use, will be at the bottom part of the shroud.

8. An abrasive cutting nozzle which has an aperture through which a jet of abrasive particles can be emitted and a shroud surrounding the nozzle, the shroud encircling the nozzle, extending beyond the nozzle and having an open end, and means for removing water from the interior of the shroud whilst the jet is operating.

9. A shroud for an abrasive cutting nozzle substantially as herein described with reference to the accompanying drawing.

10. An abrasive cutting nozzle substantially as herein described with reference to the accompanying drawing.