GB2051178A

GB2051178A - Piston Unit for Use as a Transporter in Wells and Pipelines

Info

Publication number: GB2051178A
Application number: GB8020603A
Authority: GB
Original assignee: Webco Industrial Rubber Ltd
Current assignee: Webco Industrial Rubber Ltd
Priority date: 1979-06-13
Filing date: 1979-06-13
Publication date: 1981-01-14

Abstract

The invention relates to a piston unit for use in pump down operations for moving a tool string in and out of a well and pipeline the piston unit having a rigid tubular metallic core 2 and a cylindrical elastomeric body (1) formed with external spaced circumferential ridges (3) extending normal to the axis of the body. In operation the symmetrical nature of the ridges allows the piston to provide locomotion in either direction when pressure is applied to the opposite ends, it can be dimensioned to allow a chosen degree of leakage and the ridges can deform on meeting a restriction. <IMAGE>

Description

SPECIFICATION Piston Unit for Use as a Transporter in Wells and Pipelines The subject of this invention is a piston unit for use as a transporter in pumping operations in wells, particularly subsea oil wells and pipelines.

A necessary component of the equipment required for pumping in and out a tool string during such a pumping operation is a piston unit.

An object of the piston unit is to provide the motive power for pumping a tool string in and out of a well or pipeline. A piston unit must be capable of negotiating restrictions in a well or pipeline while being able to provide a degree of fit within the well or pipeline which allows a certain amount of fluid to leak past-the piston unit. The piston unit must thus be deformable to some extent but at the same time must be able to stand up to pressures of the order of several hundred bars.

Deformable piston units are already known.

The usual construction incorporates a stack of saucer-shaped members of elastomeric material.

The peripheral portions of the members form a series of spaced circular fins. Under pressure the fins perform like Bramah rings and expand radially outwards to seal against the inner wall of the well or pipeline. The known piston units constructed as described although providing good enough service under ideal conditions suffer from some disadvantages. Under high load conditions the comparatively weak flexible fins can turn inside out. Because of the severe distortion to which the everted rubber fins are subjected they often fail because of the high tension stresses imposed on them when this situation arises. Another disadvantage of the known type of piston unit is that since the saucer-shaped members seal when fluid is applied in one axial direction only the piston unit can provide locomotion in one axial direction only.Where locomotion in both axial directions is required it is necessary to provide two piston units with the saucer-shaped members orientated in opposite axial directions.

It is an object of the present invention to provide a piston unit which does not suffer from these disadvantages.

A piston unit according to the invention comprises a generally cylindrical body incorporating a rigid core and a coaxial tubular sleeve of elastomeric material encasing the core and attached to the core, said tubular sleeve being formed on its exterior surface with a series of spaced circumferential coaxial ridges, extending normally to the axis of the body.

In one form of piston unit according to the invention the ridges are of sinusoidal wave form when seen in section through a diametral plane of the piston unit, the pitch and amplitude of the wave being constant. Other forms may be employed e.g. the ridges may in section be of rectangular form or trapezoidal form.

The rigid core may be constituted by a metallic tube and the sleeve being bonded to the tube.

At least selected ridges may have in cross section the form of buttress screw threads i.e.

each ridge presents two sloping surfaces one of which is shorter than the other. Where the sleeve is formed with ridges in the form of buttress screw threads said ridges may be provided in two groups with equal numbers of ridges in each group and with the shorter surfaces of the ridges in the two groups facing in opposite axial directions.

Practical embodiments of the invention are illustrated in the accompanying drawings in which Fig. 1 is a section on a diametral plane of a piston unit having ridges of sinusoidal form, Fig. 2 is a section on a diametral plane of a piston unit having ridges of trapezoidal form in cross section and Fig. 3 is a section on a diametral plane of a piston unit having ridges in the form of buttress screw threads arranged in two groups with the shorter surfaces facing in opposite axial directions.

In the drawings 1 denotes an elastomeric sleeve bonded to a metallic tube 2 forming a rigid core. In the construction of Fig. 1 the elastomeric sleeve is formed with ridges 3 which when viewed on a diametral plane of the piston unit as illustrated are of sinusoidal wave form. In the construction of Fig. 2 the ridges 3 viewed on a diametral plane of the piston unit are in cross section of trapezoidal form. In the construction of Fig. 3 the ridges 3 viewed on a diametral plane of the piston unit have different forms in cross section.The ridges 3A are of trapezoidal form in cross section and the ridges 3B and 3C have in cross section the shape of buttress screw threads presenting sloping surfaces 4 and 5, the surface 4 being shorter than the surface 5, the ridges 3B and 3C being equal in number and being so orientated that the shorter surfaces face in opposite axial directions.

In practice, a piston unit to fit a particular internal diameter of well or pipeline is formed so that the diameter of the piston unit i.e. the diameter over the tips of the ridges is slightly less than the internal diameter of the well or pipeline.

The rigid core 2 prevents distortion of the elastomeric sleeve 1 while the succession of grooves presented between the ridges 3 provides the effect of a labyrinth packing so that although the piston unit is not an interference fit in the well or pipeline there is a pressure drop across the top of each ridge so that a proportion of fluid leaks past the piston unit. Because of the symmetry of the ridges the piston unit can be used to provide locomotion in either axial direction. In the event of the piston unit meeting a restriction in the well or pipeline where the internal diameter of the well or pipeline is reduced the ridges can flatten sufficiently to permit the piston unit to pass through beyond the restriction. Owing to the form of the ridges the main stresses therein are compressive so there is little likelihood of failure of the elastomeric sleeve so that the piston unit has a high rate of durability.

An additional advantage of the piston unit of the invention is that it or at least the elastomeric part may be moulded in one piece thus eliminating problems of assembly.

Claims

1. A piston unit particularly for use as a transporter in wells and pipelines comprises a generally cylindrical body incorporating a rigid core and a coaxial tubular sleeve of elastomeric material encasing the core and attached to the core, said tubular sleeve being formed on its exterior surface with a series of spaced circumferential coaxial ridges, extending normally to the axis of the body.

2. A piston unit as claimed in claim 1 in which the rigid core is constituted by a metallic tube, the sleeve being bonded to the tube.

3. A piston unit as claimed in claim 1 in which the ridges are of sinusoidal wave form when seen in section through a diametral plane of the piston unit, the pitch and amplitude of the wave form being constant.

4. A piston unit as claimed in claim 1 in which the ridges are of trapezoidal form in cross section.

5. A piston unit as claimed in claim 1 in which in cross section the ridges have a rectilinear form.

6. A piston unit as claimed in claim 1 in which at least selected ridges have in cross section the form of buttress screw threads, i.e. each presents two sloping surfaces one of which is shorter than the other, there being equal numbers of such ridges with their shorter surfaces facing in opposite axial directions.

7. A piston unit as claimed in claim 1 and substantially as described in Fig. 1 or Fig. 2 or Fig.

3 of the accompanying drawings.