The Potential of DAS on Underwater Fiber Optic Cables for Deep-Sea Current Monitoring

<p><span>Distributed Acoustic Sensing (DAS) </span>enables<span> the </span>use of <span>existing </span>underwater <span>telecommunication </span>cables <span>as multi-sensor arrays</span>, allowing for detailed<span> study of the seismic wavefield. </span>Since <span>underwater </span>telecommunication <span>cables </span>were not deployed for seismological investigations, <span>the coupling between the </span>cable and the seafloor varies<span>, dramatically reducing the usefulness of </span>poorly coupled<span> cable segments</span> for<span> seismological research. In particular, underwater cables include segments that are suspended </span>in the water column across seafloor valleys or other bathymetry irregularities. Here, we propose that <span>ocean bottom currents may be studied by monitor</span>ing the vibrations of suspended cable <span>segments</span>. We analyze <span>DAS-strain recordings</span> on <span>three dark fibers deployed in the Mediterranean </span>S<span>ea. </span>Several cable segments, presumably suspended, feature h<span>igh-amplitude signals with harmonic spectra</span> as expected from<span> a </span>theoretical <span>model of in-plane vibration of hanging cables. The spatial shape of the vibration modes are determined by filtering and stacking. Their comparison to theory allows constraining the attenuation of longitudinal waves propagating along the cable in the non-suspended sections. The vibration frequencies change over time scales of tens of minutes. </span>Assuming that<span> oscillations of </span>suspended<span> sections are driven by deep sea currents, the temporal fluctuations of the vibration frequencies</span> <span>are related to changes of the cable</span>s<span> tension which, in turn, are related to the drag force induced on the suspended cable by </span><span>the shedding of </span><span>Karman </span><span>vortex</span>.<span> On this basis, we propose a method to infer changes of deep sea current speeds from the changes of fundamental frequency of cable vibrations</span>. Submarine optical reconnaissance campaigns and controlled smaller-scale experiments are planned to validate the approach. <span>The work aims at </span>demonstrating the potential of using suspended telecommunication <span>cable</span>s<span> to </span>monitor and investigate <span>marine currents in </span>deep <span>ocean environments. </span></p>

Optimal Design of Dampers for Multi-Mode Cable Vibration Control Based on Genetic Algorithm

Cables in cable-stayed bridges are subjected to the problem of multi-mode vibrations. Particularly, the first ten modes of long cables can have a frequency less than 3[Formula: see text]Hz and hence are vulnerable to wind-rain induced vibrations. In practice, mechanical dampers are widely used to mitigate such cable vibrations and thus they have to be designed to provide sufficient damping for all the concerned vibration modes. Meanwhile, the behaviors of practical dampers are complicated and better to be described by mechanical models with many parameters. Furthermore, additional mechanical components such as inerters and negative stiffness devices have been proposed to enhance the damper performance on cables. Therefore, it is increasingly difficult to optimize the damper parameters for suppressing multi-mode cable vibrations. To address this issue, this study proposes a novel damper design method based on the genetic algorithm (GA). The procedure of the method is first introduced where the damper performance optimization is formulated as a single-objective multi-parameter optimization problem. The effectiveness of the method is then verified by considering a viscous damper on a stay cable. Subsequently, the method is applied to optimize three typical dampers for cable vibration control, i.e. the positive stiffness damper, the negative stiffness damper, and the viscous inertial mass damper. The results show that the GA-based method is effective and efficient for cable damper design to achieve best multi-mode control effect and it is particularly useful for dampers with more parameters.

GALLOPING INSTABILITY ANALYSIS OF A CABLE-DAMPER SYSTEM

Viscous dampers are used widely to mitigate cable vibrations cable-stayed bridges. A damper attached to a stay cable leads to complex modes. The complexity can highly affect the aeroelastic stability of the cable. A galloping instability analysis of cable with an attached damper will be presented. A numerical example points out errors of conventional galloping analysis. The complexity of the mode shapes leads the cable being more unstable than ignoring it by treating the mode shapes as real.

Mitigating Large Vibrations of Stayed Cables in Wind and Rain Hazards

This paper presents an experimental investigation of stayed cable vibrations in dry-wind and rain-wind coupling hazards. To mitigate large vibrations of the cable, the use of spiral wires wrapped around the cable is proposed. By testing two cable models in a wind tunnel in dry and rain conditions for different yaw angles and wind speeds, the effectiveness of using the spiral wires to mitigate large vibrations is clarified. Finally, the paper provides a further understanding of the complex mechanism of wind-induced and rain-wind-induced vibrations. It is found that the low-frequency vortex flows in the wake play a significant role in the excitation of large responses of the cable in high wind speeds. The spiral wires dismiss these low-frequency flows and then reduce the large vibrations.

A Field Investigation on Vortex-Induced Vibrations of Stay Cables in a Cable-Stayed Bridge

A field study was conducted to identify the vortex-induced vibrations (VIVs) of stay cables in a cable-stayed bridge. A full-scale health-monitoring system was established to observe the wind effects of the selected cables. The vibration amplitudes in the twenty selected stay cables were first studied. The results indicate that only cable CAC20 has large amplitudes with a multimode and high-frequency vibration in the investigated period. The correlation between the wind and cable vibration was subsequently investigated. The large vibration amplitudes are primarily located in the mean speed scope of 4 to 6 m/s, simultaneously close to the reduced velocity of five when the wind was almost perpendicular to the bridge axis and had a smaller turbulence intensity. Moreover, the relationship between the maximum vibration amplitude with the mean wind speed was fitted by a function that was validated by the measured data. Finally, an estimation method was presented to predict the participative vibration modes that would happen in the VIVs of the stay cables, according to the known wind and cable parameters. The measured cable vibrations were employed to validate this estimation method. The results indicate the estimated vibration modes are close to the measured vibration modes.

Experimental Hydraulic Investigation of Angled Fish Protection Systems—Comparison of Circular Bars and Cables

The requirements for fish protection at hydro power plants have led to a significant decrease of the bar spacing at trash racks as well as the need of an inclined or angled design to improve the guidance effect (fish-friendly trash racks). The flexible fish fence (FFF) is a new developed fish protection and guidance system, created by horizontally arranged steel cables instead of bars. The presented study investigated experimentally the head loss coefficient of an angled horizontal trash rack with circular bars (CBTR) and the FFF with identical cross sections in a flume (scale 1:2). Nine configurations of different bar and cable spacing (blockage ratio) and rack angles were studied for CBTR and FFF considering six different stationary flow conditions. The results demonstrate that head loss coefficient is independent from the studied Bar–Reynolds number range and increases with increasing blockage ratio and angle. At an angle of 30 degrees, a direct comparison between the two different rack options was conducted to investigate the effect of cable vibrations. At the lowest blockage ratio, head loss for both options are in similar very low ranges, while the head loss coefficient of the FFF increases significantly compared to the CBTR with an increase of blockage. Further, the results indicate a moderate overestimation with the predicted head loss by common head loss equations developed for inclined vertical trash racks. Thus, an adaption of the design equation is proposed to improve the estimation of head loss on both rack options.