A monitoring strategy for cross-sectional stress distribution of marine unbonded dynamic cables

Abstract

Dynamic cables in offshore structures are susceptible to failure under the influence of alternating ocean environmental loads. Accurate assessment of cross-sectional stress distribution in marine unbonded dynamic cables brings a significant challenge. This study proposes a comprehensive monitoring strategy that integrates global (cable configuration) and local (cross-section) monitoring to obtain the cross-section stress distributions of unbonded cables. Our method stands out for its originality in integrating both global (cable configuration) and local (cross-section) monitoring, thereby overcoming the inherent difficulties in stress measurement in such cables. An innovative aspect of our work is the introduction of an integrated global and local algorithm. Through a meticulous analysis of the mechanical behavior of flexible cables in service, we devised a method to simplify the cable into a curved beam model subject to tension and bending loads. Simultaneously, the cable's cross-section structures were reduced to helical beam, cylindrical beam, and hollow cylinder models. The governing stress equations are utilized to establish the relationship between stress distribution in critical cable layers and the local cable configuration. Additionally, the curved beam governing equation is employed to determine the connection between the local and global cable shape, enabling a theoretical correlation between cross-sectional stress and global cable configuration. A sensor layout scheme is provided for the floating structure, along with specific locations on the cable, to obtain data on hull motions and cable inclinations. The monitoring data are then used to determine the global cable configuration, which serves as a boundary condition for the cross-sectional stress equations. This holistic approach enables real-time monitoring of the cross-sectional stress within the cable at a full scale. To validate the feasibility and accuracy of this proposed monitoring system, a model experiment and numerical simulations are meticulously executed. Comparative analysis between the numerical simulation model and monitoring results is conducted, encompassing the overall cable configuration, cross-sectional stress distribution, and the identification of maximum stress locations during cable operation. The results demonstrate that the present monitoring strategy accurately captures variations in cross-sectional stress and provides valuable technical support for the safe maintenance and operation of marine cables.