Addressing Horizontal Movements and Trenching at the Seabed of A Flexible Riser Through Advanced Sensor Integration and Dynamic Modeling

 Abstract

A flexible riser in the North Sea experiences unexpected lateral motions and self-trenching. This introduces risk of contact with subsea structures, production shutdown, and potential for leakage or burst. This paper aims to understand the cause of these motions by using sensor data analysis and numerical model simulations, enabling integrity validation of the existing, a modified, or a new riser configuration.

The production riser was monitored by one autonomous IMU, attached near the seabed and one cabled IMU attached at the hold-down clamp. Additionally, a current profiler was installed at the seabed, which in combination with hindcast weather data and platform motions, is used to capture external forces. Sensor data analysis is performed to establish correlations and relationships, and the numerical model parameters are tuned to capture the behavior measured by the IMUs, ensuring a more accurate representation of the riser’s dynamics. Hindcast weather data and measured currents are applied to impose riser motions in the numerical model, which are then compared to the actual motions recorded by the IMUs.

The sensor data analysis revealed that the riser exhibits lateral movements continuously, even when external forces are low. The sideways motions exhibit a frequency range around 0.13 Hz for the largest motions, a frequency range which is present almost regardless of external excitation, and that aligns well with platform motion frequencies. This indicates that the lateral motions at the seabed are due to a triggered system eigenmode. The same system eigenmode is identified in the OrcaFlex eigenfrequency analysis.

For the largest sea-states, which coincided with the largest currents observed, the response near the seabed was significantly reduced, but with a higher oscillation frequency than usual. The analysis suggests that current has a dampening effect on the motions, but it cannot be ruled out that some current conditions excite the riser. The analysis shows that the motions induced during the largest sea states are caused primarily by non-linear effects.

The paper presents a data-driven approach that, to the authors’ knowledge, has not been performed before to such an extent. The comprehensive use of diverse data streams has significantly reduced the uncertainties typically encountered in global dynamic analysis of flexible risers, providing a more accurate representation of the riser’s behavior under real-world conditions.

Authors

Einar Steen-Johnsen
Engineer at 4Subsea

Stian Brurås
Engineer at 4Subsea

Aurora Åsgård Pedersen
Engineer at Subsea7