The relationship between the effective service-life and the stress to which it is exposed of a damaged pipeline
In accordance with the relevant codes, a pipe with an additional protective outer sheath must be used if damage of greater than 10 % can be anticipated during the planned installation operation. This requirement is all the more important in view of the fact that the pipes effective service-life depends on the stress to which it is exposed. This relationship is illustrated below, using the example of a PE 100 material.
Exposure to internal pressure of 16 bar generates in a PE 100 pipe of dimensions OD 110 mm x 10 mm a stress of 8 N/mm2 in the pipe wall. Decreasing the pipe wall thickness increases stress to 9 N/mm for the same internal pressure. Further reduction of wall thickness to 8 mm (simplified assumption in case of 20 % damage) will result in stress exceeding the minimum strength of 10 N/mm2 guaranteed by the material supplier for a PE 100 material. Achievement of a service-life of at least 50 years will then not be likely.
In the case of the SLM® RCplus pipe system, the damage occurring during trenchless installation is absorbed by the protective sheath. The undamaged pipe under the sheath is then not subject to any restrictions on pressure resistance and effective service-life.
This concludes our outline of the essential requirements which pipes for trenchless installation must fulfill.
During operation of a pipe, the loadings generated by internal pressure are borne by the pipe wall. In the case of pipes not installed conventionally, i.e., in a sand bed, the pipe is also exposed to additional loads caused by stones or fragments of cast-iron material (from the old pipe). The results are local stress peaks and the component will fail prior to reaching the planned service period if the pipe material used does not have special resistance to stress-induced cracking. The PE 100 RCplus is suitable for non-conventional installation.
Compressive stresses caused by punctiform loads generate tensile stresses on the inner side of the pipe which must also be borne by the material in addition to internal pressure. If the material used does not have special resistance to stress-induced cracking, the additional stresses in the material will be dissipated by cracking before achievement of the planned service-life. Demonstration of resistance to stress-induced cracking is performed by independent inspectors using the Full Notch Creep Test and point-load tests.