This invention relates to tractors for movement along a pipeline within a fluid flow along the pipeline. The term “tractor” is used in this specification to denote any type of pig, tractor or other device for movement along pipeline within a fluid flow.
In the oil and gas exploration and extraction industry, there is widespread use of subsurface and subsea conduits, often of extended lengths. For example subsea pipelines may extend for many kilometres between subsea wellheads and support platforms, and between production platforms and onshore facilities. Over time, there tends to be build-up of material on the inner walls of such pipelines which may include wax, scale and various precipitates. Of course, such a build-up of material will reduce the flow capacity of the pipeline, and may result ultimately in the pipeline becoming blocked.
Conventionally such pipelines are periodically cleaned by passing a pig through the pipeline, with the pig being launched into the pipeline at an appropriate upstream point, which may require production to be temporarily stopped. A pig is typically of cylindrical form, of slightly smaller diameter than the pipeline, and is carried through the pipeline by the flow of fluid in the pipeline. Dislodged material therefore builds up in front of the pig, and it is accordingly not unusual for pigs to become stuck in pipelines, which may bring production to a halt. Retrieval of a stuck pig requires the pig to be located, the pipeline opened to remove the pig, and the pipeline reinstated, which is both an expensive and difficult undertaking.
It is an object of the invention to provide a tractor for movement along a pipeline within the fluid flow along the pipeline which renders it less likely that material will build up within the pipeline in such a manner as to block the passage of the tractor along the pipeline.
According to one aspect of the present invention there is provided a tractor for movement along a pipeline within a fluid flow along the pipeline, the tractor comprising propulsion means propelling the tractor along the pipeline in the direction of the fluid flow at a speed which is not directly related to the speed of the fluid flow.
According to another aspect of the invention there is provided a tractor for movement along a pipeline within a fluid flow along the pipeline, the tractor comprising speed regulated propulsion means for propelling the tractor along the pipeline in the direction of the fluid flow at a regulated speed.
According to another aspect of the invention there is provided a tractor for movement along a pipeline within a fluid flow along the pipeline, the tractor incorporating a geared braking mechanism for controlling the speed of the tractor along the pipeline in the direction of fluid flow.
In a pipeline containing gas or gas and fluid a conventional pig will travel along the pipeline at a variable speed in dependence on variations in the pressure of the gas and/or flow variations caused by slugging of a two-phase flow. By contrast the tractor of the invention travels along the pipeline at a constant and/or controllable speed, and this is beneficial in terms of allowing efficient cleaning of the pipeline (where the device is used for pipeline cleaning) or in regulating the speed of inspection operations (where the device is used for inspection of the pipeline). A tractor travelling at a constant and/or controlled speed is found to be more effective and to achieve better cleaning than a tractor which is propelled along the pipeline at the rate of fluid flow in the pipeline.
Further optional features of the invention are set out in the accompanying sub-claims.
In order that the invention may be more fully understood, several embodiments in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIGS. 1 to 3 schematically show three different embodiments in accordance with the invention within a pipeline; and
Various tractor arrangements for movement along a pipeline within a fluid flow are described in Patent Publications WO 98/06927, WO 00/73619 and WO 01/18351, and similar arrangements may be used with appropriate modifications in accordance with the invention. Such modifications involve regulation of the speed at which the tractor is propelled along the pipeline such that the tractor speed is not directly related to the speed of the fluid flow, and is preferably substantially independent of the speed of the fluid flow, as described in more detail below. It should be understood that the arrangements described in these prior publications are incorporated herein by reference.
In a first embodiment of the invention, shown diagrammatically in
The tractor module 10 is shown in more detail in
A speed governor is used to control the rotational speed of the drive shaft 21 or a traction wheel. For example a brake type centrifugal governor may be used to control the speed. In this case a brake shoe is used as the flyweight and restrained by a spring force. As the speed is increased the brake shoe comes into contact with the housing and slows the drive shaft down.
The traction module 10 includes a plurality of traction elements 22 each of which comprises a bush from which resilient fingers 50 extend, the bush being mounted on an off-axis bearing 48 such that rotation of the axial shaft 21 causes the fingers 50 to be oscillated backwards and forwards relative to the pipeline wall as described in WO 98/06927. Such oscillation of the fingers 50 varies the traction provided by the traction elements such that the tractor module 10 is propelled along the pipeline by the contact of the fingers 50 with the wall of the pipeline. By moving of the fingers 50 from one side of the bearing centre line to the other it is possible to reverse the direction of traction, to facilitate movement of the tractor in the opposite direction to the original drive direction.
In operation of such an arrangement the tractor is launched from the wellhead, or from an upstream position, into the pipeline in a similar manner to a conventional pig. However the power generation module 12 is driven by the fluid flow in the pipeline to generate power, and this power is then used to operate the tractor module 10 so that the tractor module 10 is positively driven in the direction of travel, rather than simply relying on pressure of the fluid flow to propel the tractor along the pipeline. Thus, because the tool is caused to travel along the pipeline at a controlled rate, the speed of propulsion is not dictated by the speed of fluid flow in the pipeline. Furthermore the power generated within the power generation module 12 may be used to operate auxiliary modules, such as the cleaning or inspection module 11, and may also enable power to be stored for alternative use. The speed of travel of the tractor module 10 is controlled by a speed governor or electronic speed control.
The traction elements of the tractor module 10 may alternatively be brush-type traction elements as used in the embodiments of
The embodiments of
It will be appreciated that the above described embodiments fall into two broad categories, namely (i) embodiments in which the traction elements (fingers, rollers or wheels) are overdriven by the power generation module and are simultaneously braked by a braking arrangement that can be controlled to vary the speed, and (ii) embodiments in which the traction elements (fingers, rollers or wheels) are driven by the power generation module by way of drive arrangement that is controllable to vary the speed. In the latter case the controllable drive arrangement may be in the form of turbine blades having angles that may be varied to change the rotational speed at which the turbine is driven in a fluid flow. In this case the arrangement includes a control system for actuating the turbine blades of the power generation module by axial displacement of a central actuating rod so as to alter the angle that all the blades make with the central axis to change the speed of turbine rotation independently of the fluid flow so as to vary the speed of movement of the tractor along the pipeline.
Alternately, in an arrangement such as that shown in
In a further non-illustrated embodiment the power generation module is coupled to the traction module by way of a series of epicyclic gear boxes and braking gear boxes which are switchable between different speed ratios by an associated control system.
The above described embodiments of the invention possess a number of advantages in practice in that they generate power from the fluid flow in the pipeline and use this power for controlled driving of the tractor independently of the pressure of the fluid imparting drive. This enables the speed of inspection operations to be regulated and/or more effective cleaning operation. The invention solves the problem of the intermittent travel of conventional pigs along a pipeline by using a tractor as a speed regulator so that the force of the fluid in the pipeline does not directly control the speed of travel. The pressure drop across the tool is significantly less than that across a conventional pig so that the pressure force pushing the tool is significantly less than in conventional pigging operation.
Various modifications of the above described embodiments are possible within the scope of the invention. For example the cleaning or inspection module may be provided with its own dedicated power unit for generating the power required for the module from the fluid flow. Furthermore, when the arrangement is used for cleaning, an additional scrubbing unit may be incorporated to ensure better cleaning.
Number | Date | Country | Kind |
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0206246.1 | Mar 2002 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP03/50065 | 3/17/2003 | WO | 9/15/2004 |