Comparative Study for a Dropped Riser Simulation Using Two Commercial Finite Element Analysis Software

Abstract Although the possibility of a riser dropping off from its top hang-off point during installation or in service is minimal, such incident can pose significant risks to surrounding structures once it occurs. To mitigate this risk, emergency stopper systems are employed during installation to prevent this. Despite of these precautions, the process of a riser falling could still cause damage that might affect its compliance with operational and serviceable requirements. This study aims to analyze the potential damage from a dropped riser using commercial finite element analysis (FEA) software. The paper provides a comparative analysis of two commercial FEA software for global riser analyses, OrcaFlex and Flexcom, focusing on their effectiveness in modelling the process of a dropped steel catenary riser (SCR). Simulation is conducted to demonstrate the consequences of riser dropping off from its hang-off point. The analysis uses hypothetical parameters for the riser, load conditions, and drop distances. The distance of the stopper from the hang-off point is determined for illustration purpose in this study. Each software package offers distinct simulation tools, such as winches and links, tailored for simulating the dropped riser scenario. The study investigates three scenarios:1) a freely dropped riser descending 10 meters, 2) a riser with partial top support descending 10 meters, and 3) a SCR freely dropped downward to the seabed. The SCR dropping-off simulation study provides insights into the performance of each commercial FEA software in modelling a dropped riser event. This study uses tension and displacement as calibration parameters to determine how well each software models the dropped riser behavior under different conditions and how accurately it predicts the dynamic responses during the drop event. The maximum stress experienced by the riser during the drop are identified as pivotal results. By analyzing the stress distributions, the study assesses whether the riser remains serviceable or if specific sections have sustained damage that necessitates repair or replacement. While the study provides a thorough comparison of the software tools, it does not endorse any one for dropped riser analyses. By systematically comparing the two software, the study provides valuable insights into the capabilities of each software in simulating the complex dynamics of riser drop events. The results from the study emphasize that partial top support during a dropped riser scenario significantly mitigates the risks of structural damage, reducing the dynamic forces and stresses experienced by the riser, particularly in the critical TDZ and top section. These findings are crucial for understanding the potential damage a riser may sustain and assessing its continued serviceability.