BACKGROUND TO THE INVENTION Steel pipes have been lined with polymeric pipes by drawing down the polymeric pipe through a die before the polymeric pipe is inserted into the steel pipe. During the drawing down process, axial as well as radial stresses are imposed on the polymeric pipe. Excessive axial stresses pose a problem because they cause the polymeric pipe to contract over a period of time.

Contraction in an axial direction of the polymeric pipe over time can result in separation of the polymeric pipe from its flanges and consequent leakage from a pipe line made up of lined pipes.

Instead of using a die to draw down the polymeric pipe, successive sets of rolls have been used to reduce the diameter of the polymeric pipe. The process utilizing rolls imposes less axial stress on the polymeric pipe. However the sets of rolls are relatively large and expensive.

Hydrostatic reduction of the diameter of polymeric pipes prior to insertion into steel pipes is known. The advantage of hydrostatic reduction is that only radial stresses are imposed on the polymeric pipe. No axial stresses are imposed on the polymeric pipe. Consequently contraction of the polymeric pipe within a steel pipe in the axial direction over a period of time does not occur.

Hydrostatic reduction of the diameter of a polymeric pipe is described in South African patents 93/8890 and 94/3981. The apparatus described in these prior art patents which is used to achieve the hydrostatic reduction in diameter is relatively large and expensive.

SUMMARY OF THE INVENTION According to the invention a method of lining an outer pipe with an inner flexibly resilient pipe includes the steps of: introducing fluid into a pressure chamber to reduce the diameter of the inner pipe as the inner pipe is pulled into the outer pipe; increasing the length of the pressure chamber as the inner pipe is pulled into the outer pipe to maintain the diameter of the inner pipe pulled into the outer pipe in its reduced form; and reducing the pressure of the fluid in the pressure chamber once the inner pipe has been pulled into the outer pipe to discharge the fluid from the pressure chamber as the inner pipe expands within the outer pipe to line the outer pipe.

The method may include the step of pulling the inner pipe into the outer pipe with a winch. The method may include the step of attaching a towing head to the inner pipe. The towing head may be welded to the inner pipe.

The outer surface of the outer pipe may be reinforced against forces generated by the fluid under pressure in the pressure chamber. A mandrel may be inserted into the inner pipe to limit the reduction in diameter of the inner pipe and also to prevent the inner pipe from collapsing under pressure.

The leading end of the inner pipe may be pulled into a run-out pipe attached to the outer pipe before the pressure in the pressure chamber is reduced.

According to another aspect of the invention apparatus for lining an outer pipe with an inner flexibly resilient pipe includes a pressure housing for attachment to the outer pipe, the pressure housing having at least one fluid inlet and a static trailing seal for forming a seal between the inner pipe and the pressure housing, and a towing head for attachment to the inner pipe, a leading seal for forming a seal between the inner pipe or the towing head and the outer pipe as the inner pipe is pulled into the outer pipe, so that in use fluid can be introduced through the fluid inlet into a pressure chamber of increasing size, formed between the inner and outer pipes and sealed at opposite ends by the trailing static seal and the leading seal, to reduce and to maintain reduced the diameter of the inner pipe as the inner pipe is pulled into the outer pipe.

Reinforcing means may be provided for reinforcing the outer surface of the outer pipe against forces generated on the outer pipe by the pressure of the fluid in the pressure chamber. The reinforcing means may comprise a jacket located around the outer pipe. The jacket may have a slightly larger internal diameter than the external diameter of the outer pipe. In another form of the invention the reinforcing means may comprise a pressure jacket which forms an outer pressure chamber, between the outer pipe and the pressure jacket, into which fluid is introduced at the same pressure as the fluid which is introduced into the pressure chamber between the outer pipe and the inner pipe. The outer pressure chamber may contain a dynamic seal which can be moved at substantially the same rate as the leading seal so that the length of the outer pressure chamber increases at the same rate as the length of the pressure chamber between the inner and outer pipes.

The apparatus may include a mandrel for location within the inner pipe to limit the reduction in diameter of the inner pipe. The mandrel may be a pipe. The mandrel may be lined on its outer surface with a flexibly resilient material to cater for variations in the wall thickness of the inner pipe. The flexibly resilient material may be rubber or polyurethane.

The apparatus may include a run-out pipe for attachment to the end of the outer pipe remote from the end to which the pressure housing is attached.

The run-out pipe and the pressure housing may consist of two sections which can be split from one another along their longitudinal axes to facilitate their attachment and removal.

The apparatus may further include pulling means for pulling the towing head through the outer pipe. The pulling means may be a winch with a cable which can be connected to the towing head.

The outer pipe may be a steel pipe and the inner pipe may be a polymeric pipe.

The outer pipe may be curved and may be a pipe bend.

The scope of the invention extends separately to the pressure housing and to the towing head.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic cross-sectional side view of the apparatus according to the invention; Figure 2 is a cross-sectional side view of the apparatus showing the inner pipe pulled part way into the outer pipe; Figure 3 is a cross-sectional side view of the pressure housing according to the invention; Figure 4 is a cross-sectional side view of the towing head according to the invention; Figure 5 is a cross-sectional side view of the mandrel ; Figure 6 is a cross-sectional side view of part of the apparatus showing a reinforcing jacket on the outer pipe; and Figure 7 is a cross-sectional side view of apparatus according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to figure 1, apparatus 10 for lining an outer steel pipe 12 with an inner high density polyethylene pipe 14 includes a pressure housing 16 and a towing head 18.

The pressure housing 16, as can be seen in more detail in figure 3, has a static trailing seal 20, water inlets 22 and a flange 24. The outer steel pipe 12 has two flanges 26 and 28. Flange 26 is attached to flange 24 of the pressure housing 16 to secure the pressure housing 16 to the outer steel pipe 12 to form a seal between the two flanges.

The towing head 18, as can be seen in more detail in figure 4, has a leading dynamic seal 30 located in a groove 32 in a high density polyethylene body 34 from the rear of which a hollow cylindrical section 36 extends. An eye bolt 38 is fixed to the body 34. The body 34 and part of the section 36 are tapered.

A mandrel 40 is inserted into the inner pipe 14. As can be seen from figure 5, the mandrel 40 consists of a steel tube 42 with a flexibly resilient rubber coating 44 on its outer surface.

A run-out pipe 46 has a flange 48 which is attached to the flange 28 of the outer pipe 12 to secure the run-out pipe 46 to the outer pipe 12 and also to provide a seal between the two flanges.

A winch 50 is connected to the eyebolt 38 of the towing head 18 by a cable 52 which passes through the run-out pipe 46 and the outer pipe 12.

The towing head 18 is fusion welded to the inner pipe 14 and then winched into the outer pipe 12 by the winch 50. As can be seen from figure 2, a pressure chamber 60 is initially formed between the inner pipe 14 and the pressure housing 16, and that as the inner pipe 14 is pulled into the outer pipe 12, the pressure chamber increases in length and is thus also formed between the inner pipe 14 and the outer pipe 12. The pressure chamber 60 is sealed at its opposite ends by the static trailing seal 20 and the leading seal 30.

The pressure housing 16 has a section 62 which has the same internal diameter as the outer pipe 12. This internal diameter may be less than that of the outer pipe 12 to introduce a degree of axial stress into the inner pipe 14 should this be required. When the leading seal 30 has passed the water inlets 22 and abuts the section 62, water is introduced under pressure into the pressure chamber 60. The pressure of the water reduces the diameter of the inner pipe 14 and forces it onto the mandrel 40. As the towing head 18 is pulled through the outer pipe 12, so the size or length of the pressure chamber 60 increases. As the length of the pressure chamber 60 increases, so further water is pumped into it to reduce, and to maintain reduced, the diameter of the inner pipe 14. The mandrel 40 prevents the inner pipe 14 from collapsing under the influence of the pressure of the water in the pressure chamber 60.

The towing head 18 is pulled into the run-out pipe 46 by the winch 50. At this stage the pressure chamber 60 has reached its maximum length. The pressure in the pressure chamber 60 is then reduced to allow the inner pipe 14 to expand and in so doing to discharge water from the pressure chamber 60 through the water inlets 22. As the inner pipe 14 expands it abuts the inner surface of the outer pipe 12 and an interference fit is formed between the inner and outer pipes.

The run-out pipe 46 is disconnected from the outer pipe 12 and removed from that part of the inner pipe 14 protruding from the outer pipe 12. The towing head 18 is then severed from the inner pipe 14. The pressure housing 16 is also disconnected from the outer pipe 12 and removed from that part of the inner pipe 12 protruding from the outer pipe 12. The parts of the inner pipe 14 protruding from the opposite ends of the outer pipe 12 can be used to form flanges (not shown) which abut the flanges 26 and 28.

A reinforcing jacket in the form of a steel pipe 62, see figure 6, can be located over the outer pipe 12 to prevent the outer pipe 12 from undergoing any significant radial expansion caused by the pressure of the water in the pressure chamber 60. Thus a relatively thin walled outer pipe 12 can be lined with an inner pipe 14, without the outer pipe 12 being damaged during the lining process.

Referring now to figure 7, a pressure housing 16.1 has a static trailing seal 20.1, water inlets 22.2, a tapered die section 64 and an oil cavity 66.

The oil cavity 66 is formed between a radially inwardly extending lip 68 and the static trailing seal 20.1.

The tapered die section 64 has bolt holes 68 to which a flange 26.1 of an outer steel pipe 12.1 can be fastened by bolts (not shown).

A free leading dynamic seal 30.1 is provided for sealing the leading end of the pressure chamber 60.1. The pressure chamber 60.1 is sealed at opposite ends by the static seal 20.1 and the leading seal 30.1, and is formed between the inner pipe 14.1 on the one hand and the outer pipe 12.1 and the pressure housing 16.1 on the other hand.

The leading seal 30.1 has an outer seal 72 which can seal against the outer tube 12.1, and an inner seal in the form of an O-ring 74 which can seal against the inner tube 14.1. The leading seal 30.1 has a circumferentially extending groove 75 which is in use filled with water under pressure.

It will be appreciated that a plurality of seals may be used instead of any of the single seals illustrated in figure 7.

In use, as the inner pipe 14.1 is pulled into the outer pipe 12.1 oil is introduced into the oil cavity 66 and water is introduced under pressure into the pressure chamber 60.1. The water urges the static seal 20.1 radially inwardly to seal against the inner pipe 14.1. The water also enters the groove 75 and urges the O-ring 74 radially inwardly to seal against the inner pipe 14.1, and the seal 72 radially outwardly to seal against the outer pipe 12.1.

A spacer (not shown) may be interposed between the leading seal 30.1 and a sleeve 78 of the towing head 18.1 to prevent the leading seal 30.1 from moving forwardly along the inner pipe 14.1. However, the bore of the leading seal 30.1 may have a plurality of spaced circumferentially extending grooves for engaging the inner pipe 14.1 to prevent the leading seal 30.1 from moving along the inner pipe 14.1.

The inner pipe 14.1 is pulled into the outer pipe 12.1 and the length of the pressure chamber 60.1 increases as the dynamic seal 30.1 moves away from the static seal 20.1.

Instead of using water, a mixture of water and an anti-corrosion agent such as glycol may be used in the pressure chamber.

It will be appreciated that the outer pipe need not be straight and that the method described could be used to line a pipe bend with an inner pipe.

The tapered die section 64 can be used for full or partial reduction of the diameter of the inner pipe 14.1 to introduce axial stresses into the inner pipe 14.1 should this be required.

It will be appreciated that many modifications or variations of the invention are possible without departing from the spirit or scope of the invention.