Using Field-Measured TLP Vortex-Induced Motion Data to Extend Service Lives of Aging SCRs

Abstract Early steel catenary riser (SCR) designs often did not adequately address vortex-induced motion (VIM) fatigue. Many such designs are now approaching the end of their original design life. During life extension assessment of a 6-inch production SCR tied back to a tension-leg platform (TLP), it became apparent that achieving a 5-year life extension would be challenging due to unexpected fairing loss and the lack of a VIM assessment during the original design. To address this challenge, the VIM response of the TLP was estimated from 10 years of field-measured data. This paper aims to demonstrate how these field measurements are used to validate the amplitude-to-diameter (A/D) curve applied in the VIM assessment, and how quantifying the conservatism inherent in this curve helps achieve the SCR life extension target. The study utilizes current speed data from an Acoustic Doppler Current Profiler (ADCP) and vessel position data from the TLP monitoring system to identify VIM events. Screening criteria including wind speed thresholds and vessel offset relative to current direction are applied. Forty days of data are analyzed and categorized into reduced velocity regions. Spectral analysis is then conducted to filter and identify crossflow and in-line motions. Global riser analysis is conducted in Orcaflex to identify SCR stress response in the touchdown zone. Rainflow cycle counting is performed to compare fatigue damage rates and quantify the conservatism of the A/D curves against field measurement derived data. Significant conservatism is shown to be present in using the A/D curve to predict VIM damage rates in the SCR. The conservatism is a combination of two effects: Overestimation of motion amplitudes during peak response.The design assumption that every current event with speeds above the lock-on reduced velocity induces hull VIM. The findings show that the actual VIM response of the TLP is characterized by lower peak amplitudes and occurs less frequently than the A/D curve predicts. The fatigue damage rates in the touchdown zone of the SCR are overestimated by a factor of approximately 4, even when the A/D curve is calibrated to envelop field measurements of TLP motion. Application of this factor reduces combined fatigue damage rates allowing the service life of the SCR to be extended beyond the end of its original design life. This paper demonstrates that field data can be used to refine VIM fatigue damage predictions of other aging risers to better reflect actual operating conditions. Refining the A/D curve with field-measured data enhances accuracy while maintaining a conservative approach for VIM analysis. This approach ensures that the VIM curve better reflects actual conditions, enhancing reliability without compromising safety. The method can be applied in life extension assessments of dynamic risers nearing the end of their design life and uses minimal data requirements like near-surface current speed and vessel position, which are commonly available on most floating facilities.