Currently, there are a number of very different butt fusion welding procedures used around the world, for welding pipes made of the same polyethylene (PE) material. For example, even in the UK, there is a difference between the welding parameters used for gas pipes and water pipes made from PE, even though the only difference between the two is the colour. In the most extreme case, the welding pressures specified in the US are 300% higher than those specified in Europe. This state of affairs has been accepted by the gas and water industries around the world until recently. However, there is now a major effort in the nuclear industry to use PE pipes in safety-critical applications and, in order to get approval from the national regulatory bodies (in particular NRC in the US, ONR in the UK and ASN in France) the industry must provide proof that the welded pipe systems will last for the design life of the system.
The reason for such major differences in the specified welding procedures around the world is that different mechanical test methods were used to optimise the welding parameters. In the UK water industry, the welds were assessed using a short-term tensile test on specimens cut from the joint, whereas in Germany they were assessed using a specimen bend test and specimen creep rupture test. In the US, the welds were assessed using a tensile impact test on specimens and hydrostatic pressure testing of the welded pipe. Previous work has shown that a number of standard tests, including the bend test and elevated temperature hydrostatic pressure test were insensitive to differences in welding parameters and also that there is no correlation between the results from short-term and long-term tests, or between the results from specimen and whole pipe long-term tests. It is therefore not surprising that such differences in specified welding procedures exist around the world.
The proposed project supports the development of one of POL’s key technology strands, testing of polymer pipe welds, with the aim of making TWI the foremost research organisation in the world in this activity. This is to be achieved through the generation of test data from our unique whole pipe tensile creep rupture (WPTCR) test equipment. This is the only whole pipe test that can consistently generate slow crack growth (SCG) failures in butt fusion welds in PE pipes; SCG is the recognised long-term failure mechanism for PE pipes.