Vortex-induced vibrations of a free spanning submarine power cable

Abstract

Vortex-induced vibrations (VIV) of a flexible structure occur when the vortex shedding frequency is close to the natural frequency of the structure. Submarine power cables exhibit more complex vortex-induced dynamics than the top-tensioned risers and submarine pipelines, because their modal properties such as the frequencies and modal shapes are affected by the structural sags. In order to better understand this problem and improve the frequency-domain analyses of vortex-induced cable fatigue damage risks, a laboratory study on VIV of a free spanning submarine power cable in steady currents with four sags is conducted. The scaled laboratory model is designed based on the gravity similarity and elastic similarity. With the geometric similarity ratio of 1:7, it represents in prototype a 500 kV self-contained oil-filling submarine power cable. The measured data indicates that the lock-in of the first antisymmetric, first symmetric and second symmetric modes all take place within the flow ranges in the experiments. The analysis further shows that the VIV features of the submarine power cable are significantly affected by the sags. At sag d = 0.23m, the cable mainly exhibits a single-mode VIV response. As the sag decreases from 0.23m to 0.09m, the natural frequencies of the cable are more closely spaced and its VIV tends towards a multi-mode response. During the multi-mode response, the first antisymmetric, first symmetric and second symmetric modes are observed to coexist. Among the excited modes, because the natural frequency of the first symmetric mode is the intermediate one, its vibration is apparently suppressed. The maximum root-mean-square value of the modal weight of the first symmetric mode decreases from 0.26 to 0.086 as the sag decreases from 0.23m to 0.09m.