TYPE A SLEEVE

The mechanism by which composite materials reinforce areas of damage on operating pipelines is much more complex. While the strength of composite materials is similar to that of line pipe steel, the elastic modulus is significantly lower. For a composite material to prevent a defect from rupturing, the defect must typically plastically deform in the process of the load being transferred to the composite. Brittle pipe material or seams may only be able to tolerate a very small amount of plastic strain before a defect in the pipe or seam grows and fails.

For both steel sleeves and composite repairs, a high-compressive strength filler material is used to fill defect areas so that load is effectively transferred to the repair material.

With very few exceptions, the applicability of Type A sleeves and composite repairs are similar. The two exceptions are repair of arc burns, where Type A sleeves can be used without complete removal of metallurgically altered material whereas composite repairs cannot, and repair of hydrogen-induced cracking (HIC) and blistering, where Type A sleeves can be used and composite repairs cannot. Both of these exceptions are the result of composite materials having an elastic modulus that is significantly lower than that of steel. As a result of the lower elastic modulus and the susceptibility of these defects to grow upon the application of comparatively small amounts of strain, the composite repairs allow these defects to strain sufficiently to cause them to fail or grow by fatigue.

Type A sleeves and composite repairs are all acceptable for relatively shallow to moderately deep external corrosion (less than 80 per cent deep). Very deep external corrosion (80 per cent deep or greater) should be repaired only using methods that are appropriate for leaks.

Another claimed advantage of composite repairs is that their installation is simpler than steel sleeves. While this may be the case for Type B sleeves where welding to an in-service pipeline using specially qualified welding procedures and specially qualified welders is required, this is certainly not the case for Type A sleeves. Type A sleeves require no in-service welding, can have fillet-welded overlapping side strips (see Figure 4), and are very simple to fabricate and install. They can be supplied with the side strips fillet welded to the bottom half of the sleeve so that the only welds required in the field are the fillet welds on the top of the side strips. These welds can be readily made by welders without special training or qualification. The raw materials required to make Type A sleeves are significantly less expensive than composite materials. Type A sleeves can be fabricated in the field by simply splitting a length of pipe of equal, diameter, wall thickness and grade as the line-pipe material. Unlike composite materials, steel sleeves have no finite “÷shelf life’. Also, the stiffness and long-term performance of Type A sleeves are equivalent to that of line-pipe steel.

While the long-term performance of steel is well established, the long term performance of composite materials on buried pipelines beyond about 20 to 25years has not yet been demonstrated. The significantly reduced stiffness of composite materials compared to steel makes the use of composite repairs questionable for application to pipelines that experience cyclic pressure fluctuations. Cyclic strains in the defect area may cause the defect to grow and eventually fail in subsequent service.

Both Type A sleeves and composite sleeves rely upon the use of an effective sealer to keep potentially corrosive fluids (e.g. groundwater) from entering the crevice area between the carrier pipe and the repair. Corrosion under Type A sleeves can be prevented by using either an elastomeric sealant or a hardenable sealant.

While Type B sleeves do have to be fillet welded to the pipeline (see Figure5), they can be used where composite repairs cannot, such as for repair of defects that are 80 per cent deep or greater, circumferentially-oriented defects, leaking defects or for defects that will eventually leak (e.g. internal corrosion), and cracks. The raw materials required to make Type B sleeves are significantly less expensive than composite materials and the stiffness and long-term performance of Type B sleeves are equivalent to that of line-pipe steel.

When considering the cost of various repair options, both material cost and cost of installation need to be considered. Mobilisation cost and the application cost of steel sleeves may be highly variable, depending upon availability and labour rates for welding personnel. While the mobilisation cost for composite repair installation may be less in some cases, the materials are more expensive. Composite sleeves or wet layup composite repair kits are typically designed for a standard, fixed length. If the length of the area requiring repair is longer than that which can be repaired by a single composite sleeve, the use of steel sleeves may be significantly less expensive than composite repairs.