GB2174945A

GB2174945A - Electrochemical machining of subsea pipe

Info

Publication number: GB2174945A
Application number: GB08605925A
Authority: GB
Inventors: Tony James Sidney Green; Colin Seward
Original assignee: OILFIELD INSPECTION SERVICES
Current assignee: OILFIELD INSPECTION SERVICES
Priority date: 1985-03-11
Filing date: 1986-03-11
Publication date: 1986-11-19
Also published as: GB2174946A; GB8605925D0; GB8506255D0; GB2174946B; GB8605926D0

Abstract

There is provided a method of forming a hole in a subsea pipe (13) in which a hollow cylindrical tool electrode (10) having an open end (11) lying in a plane normal to its axis (12) is disposed so that said end is adjacent to the pipe wall and said axis is transverse to the pipe. The tool electrode (10) is rotated on said axis (12), an electric voltage is applied between the tool electrode and the pipe to cause electrochemical machining of the pipe wall, and the tool electrode is moved along said axis (12) to maintain a predetermined gap between the nearest parts of electrode and the pipe wall as machining proceeds. The electrode has an outside diameter which is a significant proportion of the pipe's diameter. Preferably, the electrode has an outside diameter at least equal to one-half of the inside diameter of the pipe. The operative face end of the tool electrode (10) is coated with non-conductive particles to maintain the necessary electrical gap between it and the pipe. <IMAGE>

Description

SPECIFICATION Electrochemical machining This invention relatesto electrochemical machining.

In electrochemical machining, a DC electric current is passed between a tool electrode and a workpiece which are separated by a liquid electrolyte so that material is removed from the workpiece. Many proposals for electrochemical machining have been made. In U.K. Patent Specification No.1338221,itis indicated that a rotating tubular electrode may be used to drill holes in metal submerged in sea-water, such as a ship's hull or a pipe line. The sea-water constitutes the electrolyte. U.K. Patent Specification No.2096518A indicates that a method involving a combination of electrochemical machining and electro discharge (spark) machining was expected to be useful for undersea cutting purposes and refersto such applications as hole drilling and trepanning. As made clear inthe priorart proposals, the size ofthe gap between the tool electrode and the workpiece is critical tothesuccessofthe machining process. In U.K.Patent Specification No.1338221, the rotating tubulartool electrode is illustrated as being used with a su bstantial Iy flat workpiece, so that there is a gap of constant size around the whole ofthe end ofthetool between the tool and the workpiece. The holes contemplated in the prior art proposals have therefore been of small diameter compared with the radiusofcurvature(ifany)oftheworkpiece.

Subsea pipes such as are used in the oil and gas industries are of large diameter (e.g.24") and substantial thickness (e.g. up to 4"). Frequently, it is necessary to cuta large hole in a pipe. Forthis purpose, mechanical and thermal cutting equipment has been used. The use of such equipment in a subsea environment is difficult and costly.

The present invention is based on the surprising discovery that it is possible to form large holes of good standard in subsea pipes by electrochemical machining.

According to the present invention, there is provided a method offorming a hole in a subsea pipe in which a hollow cylindrical tool electrode having an open end lying in a plane normal to the cylindrical axis is disposed so that said end is adjacent to the pipewallandsaidaxisistransversetothepipe,the tool electrode is rotated on said axis, an electric voltage is applied between the tool electrodes a nd the pipeto cause electrochemical machining ofthe pipe wall, and the tool electrode is moved along said axis to maintain a predetermined gap between the nearest parts of electrode and the pipe wall as machining proceeds, the electrode having an outside diameter such that material is removed from the pipe at different positionswhich are increasinglyspaced apart from one another circumferentially of the pipe during the course of electrode movement.

For most purposes, the electrode will have an outsidediameterat least equal to one-halfofthe inside diameter of the pipe. If desired, it may have an outside diameter substantially equal tithe inside diameter of the pipe.

To maintain the aforesaid predetermined gap, the end of the tool electrode may be coated with non-conductive particles, e.g. diamond or borazon, which engage the pipe, while the electrode is continuously urged towards the pipe wall, e.g. bya compression spring.

Sea water may be pumped into the interior of the tool electrode at an adjustable rate so that it emerges through the gap between the electrode and the pipe.

The invention also provides apparatusforcarrying out the method setoutabove.

Thefollowing is a description, byway of example only, of an embodimentofthe invention, reference being made to the accompanying schematic drawings, in which Figures 1 and 2 illustrate respectively plan and side views of apparatus forming a circular hole with a rotating electrode.

Figures 1 and 2 show a rotating circular cutter 10 in theform of an axially movable steel cylinder having an open end 11 lying in a plane normal to a radial line 12from a steel pipe 13to be cut. Electrical stand offis provided by borazon or diamond-coated pa rticles 14 embedded on the cutting face, i.e. the open end ofthe electrode, to maintain a gap (suitably of 0.003" to 0.005") across which electrolysis occurs when a direct electric currentflows between the cutterand the pipe.Fresh sea-water is pumped into the cutter and outthroughthegap (e.g. at 80 litres/minute) by rn,aintaining a positive pressuresoastowashaway the products of electrolysis and keep fresh electrolyte in the gap. As shown, the pipe is of 24" outside diameter and 1"wall thickness andthe cutter is of 20" diameter. Different dimenions may of course be used.

Contact pressure between the cutter and the pipe is maintained bya linear compression spring. Itwill be seen thatasthe cutter is advancedtowardsthe axis of the pipe, initially metal is removed from two positions spaced apart along the pipe by a distance equal tothe diameter of the cutter. As soon as the cutter penetrates inside of the pipe, metal is removed from four different positions spaced apart lengthwise and circumferentially of the pipe. The circumferential spacing on the pipe ofthe cutting positions gradually increases as cutting proceeds and the lengthwise spacing decreases until metal is removed onlyfrom two circumferentially spaced positions nearthe end of the cutting operation.

The electriccurrentis DC and may bevariedfrom 0- 600 amps. Higher currents, e.g. up to 800 amps may be used. The electrolyte flow mayfor example be varied from 0 -300 litres per minute. The pipes shown in the drawings have an outside diameter of 24" and a wall thickness of 1 ", but holes may be cut in pipes having other dimensions, e.g. with a wall thickness of up to 4".

1. A method of forming a hole in a subsea pipe in which a hollow cylindrical tool electrode having an open end lying in a plane normal to the cylindrical axisisdisposed sothatsaid end is adjacenttothe pipe wall and said axis istransversetothe pipe, the tool electrode is rotated on said axis, an electric voltage is applied between the tool electrode and the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electrochemical machining This invention relatesto electrochemical machining. In electrochemical machining, a DC electric current is passed between a tool electrode and a workpiece which are separated by a liquid electrolyte so that material is removed from the workpiece. Many proposals for electrochemical machining have been made. In U.K. Patent Specification No.1338221,itis indicated that a rotating tubular electrode may be used to drill holes in metal submerged in sea-water, such as a ship's hull or a pipe line. The sea-water constitutes the electrolyte. U.K. Patent Specification No.2096518A indicates that a method involving a combination of electrochemical machining and electro discharge (spark) machining was expected to be useful for undersea cutting purposes and refersto such applications as hole drilling and trepanning. As made clear inthe priorart proposals, the size ofthe gap between the tool electrode and the workpiece is critical tothesuccessofthe machining process. In U.K.Patent Specification No.1338221, the rotating tubulartool electrode is illustrated as being used with a su bstantial Iy flat workpiece, so that there is a gap of constant size around the whole ofthe end ofthetool between the tool and the workpiece. The holes contemplated in the prior art proposals have therefore been of small diameter compared with the radiusofcurvature(ifany)oftheworkpiece. Subsea pipes such as are used in the oil and gas industries are of large diameter (e.g.24") and substantial thickness (e.g. up to 4"). Frequently, it is necessary to cuta large hole in a pipe. Forthis purpose, mechanical and thermal cutting equipment has been used. The use of such equipment in a subsea environment is difficult and costly. The present invention is based on the surprising discovery that it is possible to form large holes of good standard in subsea pipes by electrochemical machining. According to the present invention, there is provided a method offorming a hole in a subsea pipe in which a hollow cylindrical tool electrode having an open end lying in a plane normal to the cylindrical axis is disposed so that said end is adjacent to the pipewallandsaidaxisistransversetothepipe,the tool electrode is rotated on said axis, an electric voltage is applied between the tool electrodes a nd the pipeto cause electrochemical machining ofthe pipe wall, and the tool electrode is moved along said axis to maintain a predetermined gap between the nearest parts of electrode and the pipe wall as machining proceeds, the electrode having an outside diameter such that material is removed from the pipe at different positionswhich are increasinglyspaced apart from one another circumferentially of the pipe during the course of electrode movement. For most purposes, the electrode will have an outsidediameterat least equal to one-halfofthe inside diameter of the pipe. If desired, it may have an outside diameter substantially equal tithe inside diameter of the pipe. To maintain the aforesaid predetermined gap, the end of the tool electrode may be coated with non-conductive particles, e.g. diamond or borazon, which engage the pipe, while the electrode is continuously urged towards the pipe wall, e.g. bya compression spring. Sea water may be pumped into the interior of the tool electrode at an adjustable rate so that it emerges through the gap between the electrode and the pipe. The invention also provides apparatusforcarrying out the method setoutabove. Thefollowing is a description, byway of example only, of an embodimentofthe invention, reference being made to the accompanying schematic drawings, in which Figures 1 and 2 illustrate respectively plan and side views of apparatus forming a circular hole with a rotating electrode. Figures 1 and 2 show a rotating circular cutter 10 in theform of an axially movable steel cylinder having an open end 11 lying in a plane normal to a radial line 12from a steel pipe 13to be cut. Electrical stand offis provided by borazon or diamond-coated pa rticles 14 embedded on the cutting face, i.e. the open end ofthe electrode, to maintain a gap (suitably of 0.003" to 0.005") across which electrolysis occurs when a direct electric currentflows between the cutterand the pipe.Fresh sea-water is pumped into the cutter and outthroughthegap (e.g. at 80 litres/minute) by rn,aintaining a positive pressuresoastowashaway the products of electrolysis and keep fresh electrolyte in the gap. As shown, the pipe is of 24" outside diameter and 1"wall thickness andthe cutter is of 20" diameter. Different dimenions may of course be used. Contact pressure between the cutter and the pipe is maintained bya linear compression spring. Itwill be seen thatasthe cutter is advancedtowardsthe axis of the pipe, initially metal is removed from two positions spaced apart along the pipe by a distance equal tothe diameter of the cutter. As soon as the cutter penetrates inside of the pipe, metal is removed from four different positions spaced apart lengthwise and circumferentially of the pipe. The circumferential spacing on the pipe ofthe cutting positions gradually increases as cutting proceeds and the lengthwise spacing decreases until metal is removed onlyfrom two circumferentially spaced positions nearthe end of the cutting operation. The electriccurrentis DC and may bevariedfrom 0- 600 amps. Higher currents, e.g. up to 800 amps may be used. The electrolyte flow mayfor example be varied from 0 -300 litres per minute. The pipes shown in the drawings have an outside diameter of 24" and a wall thickness of 1 ", but holes may be cut in pipes having other dimensions, e.g. with a wall thickness of up to 4". CLAIMS

1. A method of forming a hole in a subsea pipe in which a hollow cylindrical tool electrode having an open end lying in a plane normal to the cylindrical axisisdisposed sothatsaid end is adjacenttothe pipe wall and said axis istransversetothe pipe, the tool electrode is rotated on said axis, an electric voltage is applied between the tool electrode and the pipe to cause electrochemical machining ofthe pipe wall,and the tool electrode is moved along said axis to maintain a predetermined gap between the nearest parts of electrode and the pipe wall as machining proceeds,the electrode having an outside diameter such that material is removed from the pipe at different positions which are increasingly spaced apartfrom one another circumferentially of the pipe during the course of electrode movement.

2. Amethod accordingto claim 1 whereinthe electrode has an outside diameter at least equal to one-half of the inside diameter of the pipe.

3. Amethod accordingto claim 1 or claim 2 wherein the electrode has an outside diameter substantially equal to the inside diameter of the pipe.

4. A method according to any preceding claim wherein the end of the electrode is coated with non-conductive particles and the electrode is continuously urged towards the pipe wall.

5. A method offorming a hole in a subsea pipe substantially as hereinbefore described with reference to the accompanying drawings.

6. Apparatusforforming a hole in a subsea pipe substantially as hereinbefore described with reference to the accompanying drawings.