Assessing the Validity of Analytical Equations for Offshore Power Cable Bending with Fixed and Loose Tube Fiber Strain Sensors

Background

Subsea power cable failures in offshore wind farms result in significant financial losses. One common failure mode is submarine power cable bending.

Objective

The primary objective of this study is to validate two analytical models using strain readings obtained from a novel 3-point bending setup designed for power cable specimens. The setup incorporates two types of optical fiber sensors for simultaneous strain measurement.

Methods

A 3-point bending setup is constructed, integrating optical fiber sensors installed on the embedded fiber optic cable within the submarine power cable. One set of sensors is fixed to the fiber optic cable sheath, while a second set consists of loose tube fibers that are inside the fiber optic cable. The strain readings of the fixed sensors are compared to two analytical models. The first analytical model assumes a constant power cable curvature, while the second model considers variable curvature.

Results

The analytical models both predict nearly flat strain profiles and are in line with each other. The strain data, however, approaches zero strain away from the cable center. Model assumptions such as perfect sensor positioning and zero slip of the fiber optic cable cause this discrepancy. The results of the constant curvature model agree well with strain averages of the fixed sensors around the central region of the power cable, and both scale linearly with amplitude. Finally, the strain readings from the loose tube fibers demonstrate high reproducibility, facilitating the development of a calibration curve for estimating power cable curvature.

Conclusions

The analytical models surpass existing models by providing good agreement with the measured strain around the cable center. Moreover, the highly reproducible strain readings from the loose tube fibers allow estimating power cable curvature.