Qualification of Polymer Materials for Dynamic Riser Service

Abstract

Polymer liners have been used extensively in water injection flowlines for several years, however, have only recently gained traction as a corrosion protection solution for Steel Catenary Risers and other similar rigid pipe risers as the industry moves to more novel systems. In order to qualify a system for a dynamic service environment such as a riser, it is important to understand the system's fatigue response and characteristics to ensure that all potential failure modes are addressed. This is typically accomplished for rigid pipes via full scale resonance fatigue testing whereby a test specimen is subjected to a representative fatigue environment and point of failure determined. Rigid pipe specimens with polymer liner installed have been trialed and reported previously. In this programme, the full-scale test strings demonstrated that all the lined system components can withstand the standard fatigue performance test curves. However, it did not confirm the boundary conditions for failure of the polymer liner, as failure of the metallic host pipe always occurred first. A similar method can be used to test the polymer material in isolation, however given the strain levels involved and the material's inherent fatigue resistance, it was expected that the duration of testing would be impractical. It was therefore necessary to implement a test method that allowed for identification of the polymer boundary conditions within a reasonable time frame, whilst also allowing for comparison with existing fatigue testing. In order to achieve this, a small-scale fatigue testing programme was setup in line with ISO 18489, whereby pre-notched dumbbell samples would be prepared and tested, firstly at 23°C but also at 0°C and 60°C to not only allow comparison with existing full-scale data, but also allow determination of suitability across the full temperature range expected in service. Testing results have demonstrated that the polymer's fatigue resistance far exceeds that of the steel pipe, even with the inclusion of pre-initiated cracking in the samples. The testing was able to provide key data on parameters and their influence on the material's fatigue life such as temperature, stress and strain. Further to this, an additional test programme was setup to evaluate the influence that the vertical orientation of the riser has on the polymer liner system. In this test programme, the interaction force between the steel pipe and polymer liner was assessed to establish the necessary design criteria to ensure that the interaction force always exceeds the vertical self-weight of the liner. Testing results demonstrated that the steel pipe/liner interaction force exceeded the equivalent self-weight of the liner eliminating potential failure modes associated with creep. As a result, the vertical orientation of the riser does not present a risk to the integrity of the liner system.