Paul Haywood looks at techniques available for pipeline renovation
The terminology used to describe pipeline renovation is often confused. Rehabilitation means the improvement or restoration of a pipeline by any means (trenchless or otherwise) which incorporates the fabric of the existing pipeline and is aimed at enhancing its performance or extending its life.
Renovation is the rehabilitation of a pipeline by trenchless methods. In practice, trenchless can mean minimum excavation and the term is generally applied to techniques that avoid continuous open-trench construction.
Pipeline renovation is no longer new technology. There are some new systems but the science has a track record going back decades. With increasing traffic volumes and penalties on utility companies for causing disruption, the popularity of pipeline renovation continues to increase. Not only is it generally more cost-effective than traditional methods, it is also quicker and causes less aggravation to residents, businesses and motorists. In terms of lifespan, most structural renovation systems can be expected to extend the pipeline’s life by more than 50 years – somewhat longer than many of the clayware and concrete pipes that have failed.
The simplest and oldest renovation technique is sliplining – meaning the threading of a new pipe up an old one, and then usually grouting the annular space between the two. The technique has lost popularity in many countries, mainly because of the reduction in bore and hence capacity, and the problems associated with annulus grouting which is generally the most difficult and risky part of the operation. It does have the advantage it can be carried out in live sewers without flow diversion, provided the flows are not too great. Sliplining is also popular for pressure pipelines, although modified sliplining has become more popular in recent years.
Pipeline renovation has its modern roots in the late1970s and early 1980s when advances in CCTV technology allowed for the first time the condition of sewers to be determined before something serious occurred. Before then, preventative maintenance was almost impossible in non-man-entry pipelines. Market forces have whittled down the number of renovation methods to a handful of tried-and-tested techniques. Proprietary variations on a theme are launched from time to time, but there have been few radically new ideas in the last decade or so.
Sliplining has already been mentioned. Modified sliplining – involving the temporary reduction of liner diameter by swaging or folding the liner – is common for renovating pressure pipes, and there are also techniques such as Nu-Pipe and AM-Liner aimed at gravity pipes. After insertion into the host pipe, the liner is reverted to a circular shape by steam and pressure. Swaged or fold-and-form liners are quicker to install than cured-in-place-pipe liners (CIPP) but are not available in as wide a range of sizes.
Many people will be familiar with the CIPP process which is now the most popular method of renovating gravity pipelines. A felt or glassfibre/felt tube is impregnated with a resin (usually polyester or epoxy), inserted by some means into the host pipe, inflated and cured to create a close-fit pipe within a pipe. The original CIPP system was introduced by Insituform in the 1970s, and since the expiry of the principal patents there have been a host of similar proprietary systems. CIPP is versatile – it can accommodate slight bends and variations in shape, it is suitable for most sizes of pipeline from less than 100mm diameter to man-entry size, and it is generally economical now there is plenty of competition.
There are alternatives to curing CIPP liner by hot water circulation – ultra-violet is one of the most common – and various liner fabrics are available including felt/glassfibre composites and woven materials which can be used to line around tight bends without wrinkling. Alongside manhole-to-manhole systems are patch repair techniques for localised defects, which involve installing and curing a short length of resin-impregnated tube. Most system developers and contractors offering the full-length CIPP technique also have a localised repair method based on similar materials. Beware the possibility of shrinkage in patch repair systems – inappropriate materials or installation techniques can lead to a patch coming loose and ending up in a manhole.
So-called non-structural systems (which means the structural enhancement cannot be quantified) include resin injection techniques for leak sealing individual joint and circumferential cracks, and fill-and-drain systems such as Sanipor, which entail flooding the sewer (and connections) with two chemicals sequentially to create an impermeable mass around areas of leakage. The injection method is generally better for isolated problems or small-scale treatment, whilst the fill-and-drain technique is aimed at larger projects where the entire sewer, laterals and manholes can be treated in one operation.
Robotic systems such as KA-TE are popular in some countries and hardly ever seen in others. The idea is to grind out cracks and fractures using a powerful milling robot, and inject epoxy mortar into the slots with a second filler robot. As with resin injection it can be difficult to assess the strength of the finished repair, so it is a matter of opinion whether robotic repairs are structural or not.
The renovation of man-entry sewers is often carried out using pre-cast GRP or GRC elements which are back-grouted after installation. The technique is long-established, and perhaps the most important consideration is the competence of the operatives and supervisors, not just in carrying out the work but also in their attention to health and safety in a potentially hazardous environment.
In-situ repairs to man-entry pipes and chambers can be undertaken using sprayed concrete, gunite or ferrocement. A pre-cast channel block is often used in conjunction with in-situ lining. Manhole renovation can involve the same techniques used in large diameter pipes. There are also spray-lining systems such as Permacast which do not require entry to the chamber.
One of the principal considerations for potable water renovation systems is that materials must be approved by the relevant authorities for use in contact with drinking water. For corrosion protection, either cement mortar lining or epoxy spray-lining can be used to extend the life of an elderly pipe. There is controversy, sometimes fierce, about the relative merits of cement mortar and epoxy lining. Many proprietary systems and materials are available, and most water companies have their favoured method.
Modified sliplining such as U-Liner or Subterra’s Rolldown and Subline allows a reduced-diameter liner to be reverted after installation so it is in close contact with the wall of the host pipe. With pressure pipes a thin-walled liner is often adequate and the folded polyethylene systems (fold-and-form) are quite popular. They work in a similar way to the PVC fold-and-form systems for gravity pipes. Swaged liners have their diameter reduced by passing through a system of rollers, either on-site or beforehand, and are reverted to their original diameter after installation.
Structural CIPP systems which are common for gravity pipelines are fairly rare in water mains, although resin systems have been developed which pass the required standards. Hose liners such as Insituform’s Thermopipe are somewhere in between spray-lining and full structural pipe-within-a-pipe systems. Thermopipe uses a pre-formed polyester-reinforced polythene hose which is flexible prior to cure. The liner is folded into a C-shape and winched into the host pipe. It is then inflated by air and steam pressure which expands it to the pipe wall and cures the liner.
Individual leaking joints in man-entry pressure pipes can be repaired by internal seals such as Amex-10 or Weco, which are suitable for internal pressures of up to 20bar. This can be a cost-effective alternative to relining where the basic by-structure is satisfactory.
Work is being carried out on the lining or trenchless replacement of small-bore service pipes. Interest arises partly from the incentive to reduce overall leakage and partly from concerns over the health implications of old lead pipes. EU directives setting out a timetable for the elimination of lead pipes have increased activity in this area and several systems are under development or at prototype stage.
It is worth mentioning certain forms of trenchless replacement are challenging pipe lining on price. In particular, static bursting techniques are between half and two-thirds the cost of CIPP lining in some countries, particularly the USA.
These systems use a passive bursting or splitting device attached to rods which are pulled through the old pipeline by a hydraulic rod-pulling machine located at a reception pit or chamber. The usual procedure is to first push the rods through the existing pipeline and then fit the breaker/splitter, followed by an expander cone to which the new pipe string, usually continuous PE, is attached. As the rods and bursting head are pulled back, the old pipe is broken out and the new one pulled in simultaneously. Some bursters are designed for use in ductile materials such as steel, iron or plastics, whilst others are aimed more at brittle pipelines. The main drawback is the need to disconnect laterals or service connections prior to bursting and to reconnect externally afterwards. This limits the technique’s viability in urban sewers or water mains with a large number of connections.
As environmental considerations and costs become more important, it is inevitable the social and financial incentives to repair and replace old pipelines by trenchless methods will increase. The present rate of expenditure on sewer repair and replacement in the UK is approximately 0.2% of asset value per annum, which implies sewers are supposed to last about 500 years. Once water companies and Ofwat realise the absurdity of this there is every prospect the market will increase still further