Flexible Riser Dropped-Object Impact Resistance

Abstract The ability to accurately predict the resistance of flexible risers to impact from dropped objects is critical to defining field layouts (e.g., selecting hang-off locations or keelhauling flexible risers beneath FPSOs), restricting FPSO crane lifting operations where possible, or determining flexible riser capacity for continued operation post dropped-object impact or time to replacement. Developing such predictive capabilities also enables the transfer of insights gained from dropped object tests performed on a pipe structure used in one Project, to predict the dropped-object impact resistance of a different pipe structure that is to be used by another Project. To achieve these objectives full-scale dropped object tests have been performed on a novel four-tensile-layer riser-pipe structure. Five different tests were performed with impact energy levels ranging between 7kJ and 55kJ. For each dropped object test high-speed camera measurements of the instantaneous pipe response, and carcass plastic deformations have been measured. The most informative test results have been used and modeled using three different analytical and numerical methods (each trading speed with accuracy of prediction at a different level) and each method used to approximate the impact resistance of a different four-tensile-layer riser-pipe structure. The five different impact energy test levels were determined from dynamic simulations of 15 tons and 35 tons containers dropped from an FPSO after discounting the impact energy absorbed by lateral displacement of a selected lazy wave riser configuration. Different ABAQUS models were used to model eighteen-layer pipe structure starting with equivalent layer model and ending up using a detailed model of each structural layer of eight million degrees of freedom and explicit integration scheme. Comparison is drawn with DNV RP F107 stated limit of rupture at impact energy exceeding 20kJ expected for 8–10 inch (203–254 mm) ID (Inner Diameter) flexibles, and the test result to 25kJ impact on 9.5 inch ID flexible demonstrating pipe residual capacity to resist loss of containment. FEA tool predictions are compared with full scale test measurements, and recommendations are made for engineering assessment and the integrity management of flexible risers impacted by dropped objects.