Developments in composite sleeves for permanent pipe and pipeline repair

With over 15 years in-service experience now “˜clocked up,’ the range of repairs completed is continuing to broaden. The technique has been adopted widely, and applications now include pipeline, offshore, refinery and chemical industries. Each has taken advantage of the ability to install repairs without disruption to operations.

What is a composite repair?

The interest in repairs such as Clock Spring was prompted by the potential they offered to combine simple installation with high performance, giving a highly reliable and cost-effective technique.

The original Clock Spring product is a continuous coil, proven for high pressure pipeline applications. Initial developments led to a similar product, the Snap Wrap, which is constructed from the same materials but is a split-sleeve system applied in multiple layers, and used for lower pressure applications up to 3,500 kPa such as piping in plants, refineries and offshore.

These “˜advanced composite’ materials derive their strength from E-glass fibres aligned in a polyester resin matrix. They are manufactured in bespoke facilities under a recognised quality assurance system. Both products come in a standard thickness, sized to satisfy requirements for repairs allowed by relevant construction standards (such as ASME B31 parts 3, 4 and 8). The mechanical performance of the system was extensively characterised before their introduction by the independent Gas Research Institute in the US, to give confidence in the long-term performance of the repairs installed. A 50-year life for the repairs was demonstrated to be realistic, and has been accepted by regulators for repairs on high pressure gas transmission lines.

The installation of the repairs is very straightforward. The surface of the pipe must first be cleaned, and once cleaned it is necessary to provide a smooth surface over which the Clock Spring can be applied. This is to ensure the Clock Spring is in intimate contact with the pipe around its whole circumference, whilst retaining a circular geometry. The required surface is achieved by means of filler built on the same chemistry as the adhesive used to bond the Clock Spring in place. These fast-curing, high strength materials were selected because of the flexibility they bring to site work. The chemistry of the Clock Spring adhesive enables the cure time to be tailored to a standard two hours irrespective of temperature.

The Clock Spring is wrapped around the pipe, with a layer of adhesive applied between each layer. The final step is to “˜cinch’ the product tight so that it forms a tight fitting sleeve around the pipe. This step extrudes any excess filler and adhesive, and ensures that when the filler cures there is intimate contact between the defect, filler and composite, guaranteeing support to the thinned area.

The Snap Wrap is applied in a similar manner – differing slightly in that the layers are applied individually – and then tightened down using band clamps, which should be removed once the adhesive has cured.

These techniques are simple to execute and makes Clock Spring ideally suited for site conditions. The need for welding, cutting the pipe and associated hot work can be eliminated, and the repairs can normally be made whilst the line is still in operation. No special handling equipment is needed, just a team of two trained technicians. Installation normally has a very minor (if any) impact on plant operations, meaning significant cost savings can be made. The overall result is a recognised repair under AS2885, reliable, economical, and safer to install for live lines than welded solutions and proven to be serviceable for 50 years.

Putting the “˜tee’ in integrity

One of the limitations of the Clock Spring is that it is difficult to repair geometries other than straight pipe. Theoretical and test work was completed to demonstrate reinforcement of elbows is possible provided that the gap between coils is maintained to below 12.7 mm on the outside of the elbow. However, more complicated, yet common, geometries could still not be repaired. This shortcoming has now been addressed by the Clock Spring Contour product. Contour is a combination of a high-strength fibreglass fabric and a chemically resistant epoxy resin. The two materials are combined on site and laid on the substrate before the resin has cured. This enables them to be applied to a wide range of different geometries.

Contour is often termed a “˜wet wrap’ – used in the sense that the wrap is applied before the resin matrix cures solid. Whilst the scope of repairs that can be considered is wider, there are a number of points regarding wet wraps which should be considered. In contrast to the Clock Spring, which is manufactured in a workshop, the material is actually made on site on top of the substrate to be repaired. Workmanship issues are therefore more demanding, with poor workmanship impacting on both the mechanical properties and quality of application. This needs to be addressed in the engineering design of a repair, and is generally reflected in thicker repairs being applied than when using Clock Spring or Snap Wrap. The time it takes to install wet wraps is longer, as is the time then required to allow them to cure to full strength. Clock Spring recommends use of the pre-cured products where suitable and the Contour where the pre-cured products cannot be applied.

The thickness of the Contour repair is determined by an engineering assessment in accordance with current ASME and ISO guidance. These documents require each application to be considered individually and the repair length and thickness calculated.

Examples of common repairs

Clock Spring has been used to solve a number of problems common across pipe applications in all industries. Repair of mechanical damage (third party interference) and external corrosion are the most common applications on pipelines, with tens of thousands of installations being completed each year. In the refinery industry general external corrosion, such as corrosion under insulation (CUI), is a relatively common problem both locally on a line and generally along longer lengths. Many of the lines affected would be difficult (and therefore expensive) to remove, and so the ability to repair in situ leads to a big saving to the operator. For straight lines the standard Clock Spring or Snap Wrap are normally the most cost-effective solutions for rehabilitation because they are quick to install. The repairs can normally be implemented whilst the system is on line.

For more demanding geometry it is necessary to use the Contour system. This can then address elbows, nozzles, reducers, tees, etc. An example showing the two systems working together is shown in Figures 1-3. These repairs are on an oil export header. Over 50 per cent wall loss had been recorded generally along the line, with some local points showing deeper pits. Hot work was only permitted on the export header for the short periods when the line was not in use. Surface preparation was therefore completed to ensure all corrosion product was removed (cleanliness demonstrated by ST3 finish) in these downtimes, and the surface preserved using an initial layer of repair material. Since installation is a cold work technique, the repair materials could be applied whilst the line was live. This split enabled the line to be worked on without interruption, enabling normal operation to continue. Significant interruption to operations would have led to delays in export with associated lost revenue in millions. The benefit of the repairs is that further external corrosion is prevented when the repairs are applied in line with prescribed procedures.

In contrast, progress of internal corrosion is not slowed by work on the outside of the pipe. However, successful repairs have also been made in these circumstances. A recent project within a US refinery led to a cost saving of nearly $US4 million. The line had suffered extensive internal corrosion, and doubler plates had already been welded in place but were now themselves corroding. A repair was developed which would accept total loss of the line. It spanned the corroding area totally, terminating either side on unaffected pipe. The installation was challenging for three reasons:

1. A group of nozzles was connected to the pipe at the corroding location.
2. The location of the damage was on an overhead pipe at a significant height above the ground.
3. The line was running at 130°C.

Whilst these issues made the repair difficult, it remained possible.

Although access to this line was difficult, other repairs have been installed where the access around the line is limited. Even though the line can be reached by hand it is not possible to get access between the line and cable tray above, which unfortunately coincided with the location of the damage. However, the simple application technique of the Clock Spring enabled this limitation to be overcome and a repair was installed.

Other areas where the benefits of these repair techniques have been employed include corrosion at ground to air interfaces, corrosion at pipe supports and extended rehabilitation of corroded lines.

As well as being used for remedial repairs, the materials have been applied as a preventative measure. Areas know to be susceptible to corrosion, such as pipe supports and ground to air interfaces, have been protected either at construction, or as part of an upgrade campaign, to ensure these known trouble spots remain corrosion free.

Summary

Repairs to corrosion damage to pipes can now be undertaken with confidence using Clock Spring and related products. Repair and protection of straights, elbows, tees, nozzles and other components has been completed and can give rise to significant cost savings by avoiding expensive shut downs. With industry experience now spanning over two decades the techniques are being increasingly used as standard.

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