GB2324579A - A liner for a well shaft - Google Patents

Abstract

The liner (1) has sections insertable one (21) inside another (31) with coupling by screw threads (22, 32) for use during installation, one of the threads (32) being formed in a boss (33) that is shearable from one section, e.g. the outer (31), in response to significant longitudinal settlement loads applied to the liner and thus allowing telescopic movement of the liner to avoid buckling. The shearing can be provided by brittle shear pins mounted on the section (31) and in apertures of the boss (3); alternatively, the boss (33) is welded to the section by welding of a known strength. Other section jointing is disclosed, i.e. L-slot bayonet, sprung pins, or push-fit.

Description

A LINER FOR A WELL The present invention relates to a liner used to keep open a well by inhibiting the collapse of surrounding material.

As an example, on a landfill or land raising site it is desired to monitor levels of methane and leachate (dirty water). This is achieved by providing one or more wells in the landfill material. Commonly, landfill sites are filled with a mixture of dry waste materials and are covered or interspersed with layers of soil. A tubular liner is inserted into a well shaft or built up in sections with the layers of landfill material. The liner may be surrounded by an annulus of gravel or the like, for stability and to inhibit the landfill material from making direct contact with the liner.

A typical prior art liner comprises a plurality of plastics material tubular sections. The sections are joined by means of male and female in-wall threads at either end of each section. The threads couple to give a flush butt joint. The liner is not usually desired to be liquid or gas tight, and some leakage or permeation at the joints is permissible.

Some sections, known as casing sections, have a continuous wall surface. Others, known as screen sections, have a plurality of apertures such as horizontal slots. Gas or liquid from the landfill material permeates the gravel annulus and enters the screen sections. The gas or liquid can be monitored, or extracted by pumping, with the uppermost end of the liner coupled to a collection system or wellhead.

A problem arises in that, depending upon the type of material, landfill and land raising sites can settle by up to around 30 per cent. During settlement, skin friction between the landfill material and the well liner exerts a considerable vertical load on the liner. Typical prior art liners cannot accommodate vertical settlement loads and are vulnerable to buckling; thus rendering the well unusable. Although a typical landfill site is provided with a number of wells, it is sometimes necessary to drill new wells to replace wells lost through buckling, involving significant extra costs.

An aim of the present invention is to provide a liner which does not buckle in use during settlement of the surrounding material.

According to the present invention there is provided a liner for a well, comprising: a plurality of tubular lining sections, a first of said sections being insertable into a second of said sections; and coupling means for coupling said first section to said second section, said coupling means comprising an external first screw thread portion provided on said first section being operatively engageable with an internal second screw thread portion provided on said second section, one of said first or said second screw thread portions being shearable from said first or second section respectively in response to longitudinal settlement loads applied to said liner.

Conveniently, the first screw thread portion is provided integral with the first section, such as by being machined into a wall thereof. The second screw thread portion is preferably formed in a shearable boss locatable concentrically inside said second section. A retaining means for retaining the boss initially in place against the second section may take any suitable form, including shear off pins or a weld using material of known shear strength.

During installation, the first section is inserted into a second section and respectively rotated to couple the screw threads. In this position, the sections are nonretractably coupled and an upper section can support a lower section hanging therefrom such as when a liner string is lowered down a well.

During settlement, skin friction between the liner and the surrounding material causes a settlement load to be applied generally longitudinally along the liner.

Settlement loads are typically much greater than the weight of even a relatively long liner string, and are sufficient to shear the second screw thread portion from the interior wall of the second section. The first section may now telescope freely inside the second section, or until a stop arrangement is reached in either direction. In use, an upper section moves to increase overlap with a lower section (or vice versa). Buckling is avoided because the well liner can move with the surrounding material during settlement and loads on the liner are reduced.

A preferred embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a side view of a liner; Fig. 2 is a sectional side view of coupled sections, showing a preferred coupling arrangement.

Referring to Fig. 1, liner 1 comprises a string of casing or screen sections 2 interspersed by coupling sleeves 3. In an alternate embodiment (not shown), each telescopic section is provide with one male" end 2 and one "female" end 3, such as by welding a casing section to a coupling sleeve, and thus avoiding the need for separate coupling sleeves on site. The liner is suitably made from plastics material, and preferably a thermoplastic material such as High Density Polyethylene (HPDE), Medium Density Polyethylene (MDPE), Polypropylene, or other polyolefin polymer.

The diameter and the wall thickness of the liner sections are selected according to site requirements as will be familiar to the skilled person. Typical dimensions are in the range of 90mm to 1600mm outside diameter. Each casing or screen section is typically 2.3m long, and each coupling sleeve typically lm long. Allowing for an initial assembly overlap of 150mm into each end of the coupling sleeve, the overall length of each telescopic section is around 3m. These lengths can be altered, in particular according to the expected amount of settlement.

As shown in Fig. 2, a male end 21 of each section is provided with an external thread 22, such as by machining.

A female end 31 of each section is provided with a shearoff portion, such as boss 33, having an interior thread 32.

Alternatively, the shear-off portion may be provided externally at or near the end of the male section 21.

During initial assembly and installation of the liner, the threads engage to couple adjacent sections.

The boss 33 is held in place by shear-off elements such as shear pins of known shear strength, conveniently located through the wall of the section and into apertures in the boss 33. Ideally, the pin members are of a brittle material such as nylon and shear off under the vertical load imposed on the well liner by the surrounding material.

The number and strength of pin members can be selected according to expected loading requirements.

Additionally or alternatively the boss 33 is welded to the section 21 or 31 with material of known weld strength, selected so as to be overcome by expected settlement loads but not by expected installation loads. In a typical example, the coupling arrangement is able to support the weight of a liner string of around 30m to 50m in length during installation.

The boss is shown as a complete ring, but a partial ring or ring sections can be used. Further, alternate coupling arrangements may be used in place of the internal and external screw threads. Examples include a twist-lock joint having pins locating in L-shaped receiving slots, sprung pins engaging receiving apertures, or a push-fit joint.

The coupling arrangement described herein is straightforward and efficient to manufacture and requires a minimum of labour and components. The coupling allows for a simple join operation, leading to easy and reliable assembly of the liner during installation. Also, the liner is self supporting during installation, i.e. whilst a gravel annulus is added. The liner is able to withstand loads imposed radially from the surrounding material. Since the join is covered by the outer section, there is minimal interference from the annulus or surrounding material.

Chamfered internal edges assist the coupling together of sections by guiding ends 21 and 31 into position. The chamfered edges also aid movement of equipment along the inside of the liner, such as when lowering a pump down the well, since the risk of equipment catching is reduced.

Whilst the coupling arrangement has been described with particular reference to a liner for a well, the skilled person will appreciate that the coupling arrangement is not limited to this application and may be employed in coupling any other tubular sections, including pipework.

Claims (5)

1. A liner for a well, comprising: a plurality of tubular lining sections, a first of said sections being insertable into a second of said sections; and coupling means for coupling said first section to said second section, said coupling means comprising an external first screw thread portion provided on said first section being operatively engageable with an internal second screw thread portion provided on said second section, one of said first or said second screw thread portions being shearable from said first or second section respectively in response to longitudinal settlement loads applied to said liner.

2. A liner as claimed in claim 1, wherein: said first screw thread portion is provided integral with the first section; and said second screw thread portion is formed in a boss locatable concentrically inside said second section and having shearable retaining means for retaining said boss with respect to said second section.

3. A liner as claimed in claim 2, wherein said retaining means comprises shear-off pins of a known shear strength.

4. A liner as claimed in claim 2, wherein said retaining means comprises a weld using material of known shear strength.

5. A liner substantially as hereinbefore described, with reference to the accompanying drawings.