Polymer layers have been used in the offshore Oil & Gas industry for many years in unbonded flexible pipes for static and dynamic applications. The evaluation of fatigue performance for polymer grades in such applications is complex and no qualification method is well defined nor standardized.
Though historically no reported polymer fatigue field failures, the complexity of unbonded flexible pipes and the conservatism of this industry make the introduction of new polymer grades in risers for dynamic applications challenging. The development of a polymer fatigue methodology will allow the industry to quantitatively evaluate risks of fatigue failure of polymer sheaths and thereby be important in introducing new material grades.
In this study, a design fatigue methodology is built based on inputs from a specific project and results from small- and full-scale tests. The pipe curvature ranges from the project are used to obtain the accumulated tensile strain that is then converted into an accumulated tensile stress. The fatigue damage can then be determined using a design curve established from bending or tension-tension fatigue testing performed on dumbbells and pipe samples. Parameters are included to reflect geometrical effects from manufacturing process but also service life conditions with several temperature profiles.
This method can be applied for pressure sheaths as well as sealed outer sheaths with relevant inputs related to their operating conditions. The obtained fatigue damage can then be used to validate the use of polymer grades for specific unbonded pipe designs under their service conditions.