WO2000015993A1

WO2000015993A1 - A method of installing crosslinked thermoplastics pipelines

Info

Publication number: WO2000015993A1
Application number: PCT/GB1999/003027
Authority: WO
Inventors: Colin Burley; Nigel Wright
Original assignee: Advanced Engineering Solutions Limited
Priority date: 1998-09-11
Filing date: 1999-09-13
Publication date: 2000-03-23
Also published as: GB9819718D0; AU5873999A

Abstract

A method of installing crosslinked thermoplastic pipe comprises laying a pipeline of crosslinkable plastics material and subsequently causing or allowing the plastics material to crosslink. The crosslinkable plastics material is preferably a polyethylene or the like to which vinyl silane groups are grafted, crosslinking being brought about by exposure to moisture.

Description

Title: A Method of Installing Crosslinked Thermoplastics Pipelines

This invention relates to a method of installing crosslinked thermoplastics pipelines.

Thermoplastics pipes are now a common material for installing new pipelines, for example gas and water distribution, sewerage and process plant pipework. New pipeline installations typically use thermoplastics materials, such as polyethylene (PE), polypropylene (PP) or other polyolefms. These pipelines have the advantage of being lightweight, corrosion resistant and easily joined and installed. Jointing methods include heat fusion joints, for example butt fusion or electro fusion. Installation techniques include the jointing of individual lengths or coils of pipes, to form long strings of pipelines, and then installing into the trench. The disadvantage of these types of pipelines is that in they can fail by fracture propagation, either in the long term from slow crack growth from defects or stone impingement during installation or in the short term from fast fracture propagation during pressure testing or cold weather operation.

Crosslinked polyolefms, in particular crosslinked polyethylene (PEX), has superior properties to conventional thermoplastics materials including strength, toughness and corrosion resistance, and is able to resist failure from crack propagation. PEX pipes have been used successfully for buried pipelines in particular in countries having cold climates where the lower temperatures significantly reduce the toughness of conventional thermoplastics materials. The major disadvantage of PEX pipes is the inability to fusion weld PEX to PEX pipes, resulting in the use of relatively expensive mechanical fittings to join the individual lengths.

There has now been devised a method of installing crosslinked thermoplastics pipelines which overcomes or substantially mitigates these and/or other disadvantages of the prior art.

According to the present invention, there is provided a method of installing crosslinked thermoplastic pipe, which method comprises the steps of a) laying a pipeline of a crosslinkable plastics material, and b) subsequently causing or allowing the plastics material to cross-link.

The method according to the invention is advantageous primarily in that the lengths of pipes making up the pipeline may be joined in a conventional manner, eg using fusion welding procedures to create long lengths of pipes. The subsequently crosslinked pipeline nonetheless has the desirable mechanical properties of pre-crosslinked material.

Polyethylene pipes for the transportation of gas and water may be characterized in terms of a bursting pressure. Such a pressure typically falls over the course of time, the pressure rating being calculated from the burst pressure at 50 years, which is significantly less than the short term burst pressure, with a safety margin also being taken into account.

During the early life of the pipeline formed according to the invention, the pipeline may be operated using the relatively high 1 year pressure rating of non-crosslinked PE. Once fully crosslinked, which would generally occur before the end of the first year of operation, the burst pressure of the pipe would decay in a manner characteristic of PEX. The PEX burst pressure curve is significantly shallower than the curve for non-crosslinked PE. This allows the pipeline to operate at higher 50 year pressures.

The pipeline formed in accordance with the invention also has good toughness, excellent chemical resistance and a broader operating temperature range than PE.

As explained above, existing PEX pipelines are difficult to construct since the butt welding process tends to produce brittle welds because the pipes are fully crosslinked before welding. In this pipeline formed in accordance with the invention the welds are ductile since the welding is performed when the pipe is in its (non-crosslinked) PE state. Any pipeline repair or modification done on the pipe after it is crosslinked may use uncrosslinked crosslinkable fittings which again would crosslink up after a period of time. A further advantage of the method of the invention is that the cost of such the pipeline could be reduced by using lower cost low density PE. Normally such material would not have the strength or the toughness for pipeline service. However, once crosslinked, the strength conforms to the pressure curve for PEX and the toughness dramatically improves.

The pipe material may be crosslinked in situ either by the active application of a crosslinking agent or catalyst, eg by passing such an agent through the pipeline. Alternatively, the pipe material may be passively allowed to crosslink by exposure to a crosslinking agent or catalyst in the surrounding environment. For example, the crosslinking catalyst can be moisture or water in the surrounding soil for buried pipelines, or water vapour in the air for above ground installations.

Where the pipeline is crosslinked by active application of crosslinking agent or catalyst material, that material may simply be pumped down the pipeline. However, in view of the length of pipeline which may be involved in practice (commonly 1km or more), the handling and disposal of the required volume of material may be a problem. In such circumstances, it is preferred for a smaller volume to be held between a pair of spaced apart barriers which are progressively traversed along the pipeline. The barriers may be seals provided on one or more pipe pigs. A single pig may have two such seals, the space between the seals forming a chamber. Alternatively, two pigs may be moved in unison along the pipeline, the space between the pigs containing the material.

The speed of the pigs will be governed by the rate of the crosslinking process, which in turn may be dependent on the temperature and/or pressure of the material. The material, eg water, between the seals may be heated by an electric heating element carried between the seals. Alternatively, water or saturated steam may be heated externally of the pipeline and pumped to the space between the seals.

Where the pipeline is crosslinked by exposure to water, and where the pipe is extruded prior to use, it may be necessary to wrap the pipe to prevent premature crosslinking due to exposure to rain or atmospheric moisture. However, where the pipe is to be installed within a relatively short period of manufacture such measures may not be necessary, a certain degree of cross-linking prior to use being acceptable.

The preferred material used for the pipeline is a crosslinkable polyolefm, most preferably crosslinkable polyethylene.

One preferred form of crosslinkable material comprises a polyolefm, eg a polyethylene or the like, to which vinyl silane groups are grafted. A process for producing such materials is described in detail in GB 1286460 ("the Silane Process"), the disclosure of which is incorporated herein by reference. Essentially the process involves reacting a polyolefm (e.g. polyethylene) with a silane containing at least one vinyl group and at least one hydrolysable organic radical.

Materials of the preferred kind can be crosslinked by exposure to moisture. Thus, following jointing and installation of the crosslinkable pipe, it can be crosslinked by passing water, water vapour down the pipeline, or dosing fluid within the pipeline with water, crosslinking from the inside to the outside of the pipe. Alternatively the crosslinking can take place from the humidity in the air, for above ground installation, or from the water in the ground, for buried pipeline installation, crosslinking from the outside to the inside of the pipe.

The speed of crosslinking will be determined by the effectiveness of the catalyst used in the silane process and the operating temperature of the pipeline.

Prior to the action of the catalyst, where the degree of crosslinking of the pipe is less than about 50%, the material behaves in a similar manner to uncrosslinked polyolefm. This allows fusion welding, e.g. butt welding, electro fusion welding or other known welding processes, of the crosslinkable pipe lengths to form long lengths of pipeline in the conventional manner. Additionally it has been demonstrated that it is possible to fusion weld crosslinkable pipe to crosslinked pipe manufactured from similar materials, which allows for subsequent repair or extensions to the pipeline by fusion welding, e.g. butt fusion, the new section of crosslinkable pipe to the existing crosslinked pipe and allowing the crosslinking to take place in situ. Again the crosslinking can take place by passing water or water vapour down the pipe, or dosing fluid within the pipe with water, crosslinking from the inside to the outside of the pipe. Alternatively the crosslinking can take place from the humidity in the air, for above ground installation, or from the water in the ground, for buried pipeline installation, crosslinking from the outside to the inside of the pipe.

The invention will now be described in greater detail, by way of illustration only, with reference to the following Example.

Example

Installation of crosslinked underground pipeline

Straight lengths of crosslinkable polyethylene pipe prepared in accordance with the silane process are extruded to lengths of approximately 12m. As an alternative, eg where the diameter of the pipe is 180mm or less, the pipe may be manufactured in 50m coils.

The lengths of pipe are positioned adjacent a previously excavated trench and butt jointed by conventional methods.

The jointed pipe is then installed in the trench and back-filled.

Cross-linking of the installed pipe then occurs over time, due to the action of moisture in the surrounding soil.

Claims

1. A method of installing crosslinked thermoplastic pipe, which method comprises the steps of a) laying a pipeline of a crosslinkable plastics material, and b) subsequently causing or allowing the plastics material to cross-link

2. A method as claimed in Claim 1, wherein the pipe material is crosslinked by the active application of a crosslinking agent or catalyst.

3. A method as claimed in Claim 2, wherein active application of the crosslinking agent is achieved by passing such an agent through the pipeline.

4. A method as claimed in Claim 1 , wherein the pipe material is passively allowed to crosslink by exposure to a crosslinking agent or catalyst in the surrounding environment.

5. A method as claimed in Claim 4, wherein the crosslinking catalyst is moisture or water in the surrounding soil for buried pipelines, or water vapour in the air for above ground installations.

6. A method as claimed in Claim 3, wherein the crosslinking agent is held between a pair of spaced apart barriers which are progressively traversed along the pipeline.

7. A method as claimed in Claim 6, wherein the barriers are seals provided on one or more pipe pigs.

8. A method as claimed in any preceding claim, wherein the material used for the pipeline is a crosslinkable polyolefm.

9. A method as claimed in Claim 8, wherein the material is crosslinkable polyethylene.

10. A method as claimed in Claim 8 or Claim 9, wherein the crosslinkable material comprises a polyolefin to which vinyl silane groups are grafted.