GB2169835A

GB2169835A - Electrochemical machining

Info

Publication number: GB2169835A
Application number: GB08432196A
Authority: GB
Inventors: Tony James Sidney Green
Original assignee: OILFIELD INSPECTION SERVICES
Current assignee: OILFIELD INSPECTION SERVICES
Priority date: 1984-12-20
Filing date: 1984-12-20
Publication date: 1986-07-23
Also published as: GB8432196D0

Abstract

Apparatus for the electrochemical machining of subsea structures and particularly for reprofiling welds and grinding surface cracks comprises a tool electrode (which is preferably elongated in one direction so that it can be disposed parallel to a crack or weld in the structure), means for applying an electric potential difference between the tool electrode and the structure to remove material from the structure, means 17,19,21,22 supporting the electrode for movement in directions normal to and parallel to the surface of the structure and operating means 15,20 for selectively moving the electrode in said directions. The apparatus has electromagnetic fixings 23. <IMAGE>

Description

SPECIFICATION Electrochemical machining This invention is concerned with electrochemical machining of subsea structures.

It is known to machine workpieces by electro-chemical machining. In electrochemical machining, a D.C. voltage is applied across electrolyte, e.g. sea water, in a gap between a tool electrode and a metal workpiece so that metal is removed from the workpiece.

It is an object of the invention to provide improved means for machining subsea structures and particularly for reprofiling welds and grinding surface cracks on subsea structures.

According to the invention, apparatus for the electrochemical machining of subsea structures comprises a tool electrode, means for applying an electric potential difference between the tool electrode and the structure to remove material from the structure, means supporting the electrode for movement in directions normal to and parallel to the surface of the structure and operating means for selectively moving the electrode in said directions.

The electrode is preferably static, i.e. it is stationary in use except when moved by said operating means. Preferably, the electrode is elongated in one direction so that it can be disposed parallel to a crack or weld in the structure and moved transversely across the crack or weld.

Preferably, control means are provided for maintaining a predetermined distance between the electrode and the surface of the structure.

Preferably, means are provided conditioned by signals from said operating means to show the profile of the machined profile at a distant location.

The supporting and operating means may comprise a robotic arm or a combination of slides and lead-screws. The supporting means will normally be secured to said structure but may be carried by an adjacent body.

The following is a description, by way of example, of one embodiment of the invention, reference being made to the accompanying schematic drawings, in which Figure 1 is a plan view of the apparatus, Figure 2 is an end view of part of the apparatus, and Figure 3 is a sectional view of a crack before machining.

A static tool electrode 10 is carried by a shaft 11 axially movable to and fro in the direction of the arrows Z (Fig. 2) in a workhead 12 by a servo motor/gear unit. The workhead 12 is carried on a leadscrew 13 and parallel guide 14 so as to be movable in the direction of the arrows Y (Fig. 1) by rotation of the leadscrew by a servo motor 15 driving the leadscrew 13. The leadscrew 13 and the guide 14 (on which the workhead is slidable) are connected between a support 16 slidable on a guide 17 and carrying the motor 15 and a support 18 mounted on a leadscrew 19. Rotation of the leadscrew 19 by a servo motor 20 causes movement of the supports 16 and 18 in the direction of the arrows X (Fig. 1). The guide 17 and leadscrew 19 connect cross-pieces 21, 22, one of which carries the motor 20.Electro-magnetic fixings 23, for securing the apparatus to a subsea structure, are provided on the cross-pieces 21, 22.

In use, the apparatus is secured to the subsea structure, and the electrode is positioned adjacent and parallel to a surface crack or weld to be ground or profiled. A suitable D.C.

voltage is applied between the structure and the electrode, which is positioned at a predetermined standoff (e.g. 0.005") from the structure. The standoff is controlled and maintained by the servo motor on the workhead 12 and is measured by electricai resistance. The electrode is moved (continuously or step by step) transversely across the crack or weld to provide the required profile. The transverse movement and any desired longitudinal movement of the electrode in the directions X and Y is obtained by appropriate operation of servo motors 20, 15. Resolvers are attached to the leadscrews, enabling a plotter to record profile shape, position, etc. at a location remote from the work area. Fig. 3 shows in full lines a crack profile before relief grinding as hereinbefore described and in broken lines the profile after grinding, which is also shown in Fig. 2.

Other forms of apparatus may be used. For example, a robotic arm may be used to move a static electrode over the area to be machined while maintaining a pre-determined standoff and with feedback so that an accurate profile can be determined.

1. Apparatus for the electrochemical machining or subsea structures comprising a tool electrode, means for applying an electric potential difference between the tool electrode and the structure to remove material from the structure, means supporting the electrode for movement in directions normal to and parallel to the surface of the structure and operating means for selectively moving the electrode in said directions.

2. Apparatus according to claim 1 in which the electrode is elongated in one direction so that it can be disposed parallel to a crack or weld in the structure and moved transversely across the crack or weld.

3. Apparatus according to claim 1 or claim 2 in which control means are provided for maintaining a predetermined distance between the electrode and the surface of the structure.

4. Apparatus according to any preceding claim in which means are provided conditioned by signals from said operating means to show

**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 is concerned with electrochemical machining of subsea structures. It is known to machine workpieces by electro-chemical machining. In electrochemical machining, a D.C. voltage is applied across electrolyte, e.g. sea water, in a gap between a tool electrode and a metal workpiece so that metal is removed from the workpiece. It is an object of the invention to provide improved means for machining subsea structures and particularly for reprofiling welds and grinding surface cracks on subsea structures. According to the invention, apparatus for the electrochemical machining of subsea structures comprises a tool electrode, means for applying an electric potential difference between the tool electrode and the structure to remove material from the structure, means supporting the electrode for movement in directions normal to and parallel to the surface of the structure and operating means for selectively moving the electrode in said directions. The electrode is preferably static, i.e. it is stationary in use except when moved by said operating means. Preferably, the electrode is elongated in one direction so that it can be disposed parallel to a crack or weld in the structure and moved transversely across the crack or weld. Preferably, control means are provided for maintaining a predetermined distance between the electrode and the surface of the structure. Preferably, means are provided conditioned by signals from said operating means to show the profile of the machined profile at a distant location. The supporting and operating means may comprise a robotic arm or a combination of slides and lead-screws. The supporting means will normally be secured to said structure but may be carried by an adjacent body. The following is a description, by way of example, of one embodiment of the invention, reference being made to the accompanying schematic drawings, in which Figure 1 is a plan view of the apparatus, Figure 2 is an end view of part of the apparatus, and Figure 3 is a sectional view of a crack before machining. A static tool electrode 10 is carried by a shaft 11 axially movable to and fro in the direction of the arrows Z (Fig. 2) in a workhead 12 by a servo motor/gear unit. The workhead 12 is carried on a leadscrew 13 and parallel guide 14 so as to be movable in the direction of the arrows Y (Fig. 1) by rotation of the leadscrew by a servo motor 15 driving the leadscrew 13. The leadscrew 13 and the guide 14 (on which the workhead is slidable) are connected between a support 16 slidable on a guide 17 and carrying the motor 15 and a support 18 mounted on a leadscrew 19. Rotation of the leadscrew 19 by a servo motor 20 causes movement of the supports 16 and 18 in the direction of the arrows X (Fig. 1). The guide 17 and leadscrew 19 connect cross-pieces 21, 22, one of which carries the motor 20.Electro-magnetic fixings 23, for securing the apparatus to a subsea structure, are provided on the cross-pieces 21, 22. In use, the apparatus is secured to the subsea structure, and the electrode is positioned adjacent and parallel to a surface crack or weld to be ground or profiled. A suitable D.C. voltage is applied between the structure and the electrode, which is positioned at a predetermined standoff (e.g. 0.005") from the structure. The standoff is controlled and maintained by the servo motor on the workhead 12 and is measured by electricai resistance. The electrode is moved (continuously or step by step) transversely across the crack or weld to provide the required profile. The transverse movement and any desired longitudinal movement of the electrode in the directions X and Y is obtained by appropriate operation of servo motors 20, 15. Resolvers are attached to the leadscrews, enabling a plotter to record profile shape, position, etc. at a location remote from the work area. Fig. 3 shows in full lines a crack profile before relief grinding as hereinbefore described and in broken lines the profile after grinding, which is also shown in Fig. 2. Other forms of apparatus may be used. For example, a robotic arm may be used to move a static electrode over the area to be machined while maintaining a pre-determined standoff and with feedback so that an accurate profile can be determined. CLAIMS

4. Apparatus according to any preceding claim in which means are provided conditioned by signals from said operating means to show the profile of the machined profile at a distant location.

5. Apparatus according to any preceding claim in which the supporting and operating means comprise a robotic arm.

6. Apparatus according to any of claims 1 to 4 in which the supporting and operating means comprise a combination of slides and lead-screws.

7. Apparatus according to claim 6 in which the supporting means is adapted to be secured to the structure.

8. Apparatus according to any preceding claim in which the supporting means suppot the electrode for movement in three di men- sions.

9; Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of reprofiling a weld or grinding a surface crack in a structure by electrochemical machining which comprises positioning a tool electrode adjacent to the weld or crack with an electrolyte therebetween, applying an electric potential difference between the tool electrode and the structure to remove material from the structure, moving said electrode in a direction normal to the surface of the structure to maintain a predetermined standoff of the electrode from the structure and moving the electrode relatively to the weld or crack to provide a required profile.

11. A method according to claim 10 in which the electrode is elongated and is moved transversely across the crack or weld.

12. A method according to claim 11 in which the position of the electrode is adjusted in its longitudinai direction.

13. A method of electrochemical machining substantially as hereinbefore described with reference to the accompanying drawings.