In this publication, a novel mechanical test rig is described, which is built to determine the structural and material damping in axially tensioned subsea power cables and umbilicals, during both high frequency oscillations and low frequency wave frequencies. The test set-up includes measurements from load cell sensors, strain gauges, position lasers and fibre bragg grating nodes that are processed further into the damping of a full scale test sample.
Results from a pilot test are presented, which are used to evaluate the performance of the test rig and sensor measurements.
During installation of subsea cables and umbilicals the target is to achieve a low bottom tension, which in turn returns a product that follows the seabed topography and reduces the number of free spans and their lengths.
In areas where the product is installed with free spans, vortex induced vibration (VIV) can occur when exposed to ocean currents. Such vibration of the product entails undesirable fatigue damage in the product, that can lead to a shorter life expectancy. Thus, the lay-route of a subsea cable is carefully planned with focus on minimizing the possibilities of free spans versus cost. However, in some conditions free span occurs as a result of too high bottom tension during installation, as shown in Fig. 1, while in other scenarios it is not viable to avoid free spans, as shown Fig. 2, where deep seabed depressions returns a free-span.
One parameter that can reduce the calculated fatigue damage in products, is the structural and material damping, which is often an unknown parameter for the various products.
The target for this publication is to present the capability of a novel test rig that is able to measure the damping in tensioned free spans. The same test rig, can be reconfigured to enable a low frequency test, where the test sample will be forced to move with a specific amplitude and cycle period. This set-up is somewhat similar to a bending stiffness test rig, see (Jordal, et al., 2017; Jordal & Vermeer, 2021), but with a controlled axial tension applied.
