A Fluid-Structure Interaction (FSI) Method for Predicting Cable Breakage in Downhole Rigs
Authors: Song Lijun , Ji Rigelesaiyin , Hofacker Mark , Zhang Haitao , Battentier Amandine 
Published: January 2025

Abstract
Armored wireline cables are frequently used in well logging and well construction services. The cables typically include multiple conductors encased in a protective armor of metal components. The cost of cable failure to the operation would be significant, underscoring the necessity for robust durability and reliability. Typically, the cables are engineered to withstand harsh downhole conditions such as corrosive fluid and gas, tool-string shock and vibrations, high pressures, temperatures and pull stress. One of the possible mechanisms leading to cable failure is flow-induced cable vibration, which causes the cable to repeatedly impact the collar housing, eventually leading to its breakage. This mechanism has been poorly understood in the past, presenting even greater challenges in terms of prediction.
In this study, we developed a novel fluid-structure interaction (FSI) method to model the cable to housing impact resulting from flow-induced vibration and predict the associated risk of cable breakage failure. The method employs one-way coupling between computational fluid dynamics (CFD) and finite element analysis (FEA). First, CFD simulations are carried out at specified flow rate conditions and fluid properties, where the flow typically exhibits transient and highly turbulent behavior. To accurately capture the transient velocity, pressure, and force oscillations on the cable surface, we utilize the stress-blended eddy simulation (SBES) model for turbulence. Our analysis demonstrates that the SBES model effectively captures vortex sizes comparable to the cable diameter, while vortex sizes within the wall boundary layers, much smaller than the cable diameter, are considered irrelevant. The transient forces extracted from the CFD simulations are then transferred to an FEA model spanning the length of the cable surface, for an explicit dynamic analysis. Specifically, we evaluate the recurring impact frequency and force magnitude between the cable and housing, crucial factors in assessing the risk of cable breakage failure. Through this integrated approach, we aim to enhance our understanding of flow-induced cable vibrations and improve the predictive capabilities regarding cable integrity under downhole environments.
The effectiveness of this method has been confirmed through validation against a real-world incident where a cable failed under relatively high flow rates and the cable broke into multiple pieces after several days of downhole operation. Our FSI simulations, integrating CFD and FEA, were conducted under identical flow rate conditions and fluid properties to those encountered in the field. Remarkably, simulated results pinpointed the highest flow-induced vibrations at locations characterized by step changes in housing diameter. Moreover, the analysis revealed that the point of most frequent impact of cable on housing, as predicted by the simulations, aligned closely with the observed point of cable breakage in the field. This alignment underscores the accuracy and utility of our FSI approach in predicting cable failure due to flow-induced vibration under real-world operating conditions.