Aims and methodology
The objectives were to carry out resonance fatigue testing on six MLPs, which were previously subjected to reeling at atmospheric pressure, in order to qualify the fatigue performance to DNV fatigue curves (DNV, 2012). Additional tests were carried out using DPI and ECT at the interface between the clad overlay weld and the liner, 2.5m from the pipe end, to detect cracks.
The pipes were fatigue tested at ambient temperature, at a frequency of approximately 30Hz, in TWI’s fatigue testing machine. The specimens were pressurised to achieve an axial mean stress of 90MPa and two specimens were tested at 80MPa, 120MPa and 175MPa stress ranges. DPI and ECT were developed and a dedicated inspection tool was built to allow the inspections (Figure 1). DPI was undertaken prior to fatigue testing, after reaching the DNV class D target curve and on test completion. ECT was carried out after reaching the DNV class D target curve when suspicious indications were found with DPI. Only one indication, of 2mm, was detected in one pipe and all fatigue test results reached the DNV class D target curve (Figure 2).