Cable Dynamics 1
Taught cables have dynamic characteristics that can often be efficiently analyzed with closed-form analytical solutions and bespoke Excel applications, typically in the frequency domain, developed by STA. OrcaFlex adds further sophisticated numerical frequency and time domain solution capabilities.
The above figure shows one stage of the lifts involved with Marine Heavy Lift Dynamics of the Sewol Salvage operation, as planned and analyzed by STA in 2016, with OrcaFlex as the primary tool for the complex dynamic analyses.
Each of the upper HMPE slings, 16 each side, 32 total, is a continuous sling, doubled. Hence each sling has four parts. The mean tension in a 4-part sling under the largest static load is around 260 metric tons. Each part carries one quarter of this value. Owing to various issues, there may be significant variations from the mean tension. Sling angles range from 57º to 72º to horizontal. Lengths from 62m to 82m.
The slings may experience vortex induced vibrations (VIV) and galloping vibrations from wind loads during the lifts.
Using analysis methods laid out in FHWA-RD-05-083, 2007, for cable-stayed bridges, an Excel tool for calculating the VIV and Galloping of Dry Inclined Cables was developed by Bil Stewart. Key inputs and initial VIV calculations are shown in the table above. Note the sling mass density, m = 5.77 kg/m. The stability diagram is shown below.
The diagram indicates that cable dynamics are likely to be significant. A comparison can quickly be made with steel cables, as they have the same geometry and approximately the same elastic axial stiffness (AE value) but seven times greater mass density. The chart below shows the comparison.
With steel (mass density of slings = 40.39 kg/m) the cable dynamics is small amplitude and no galloping occurs.
High quality marine and offshore technology consultancy can be expected when working with STA, and this is especially the case when it comes to cable dynamics. Another example cable dynamics example can be seen at CRANE BOOM RECOIL. Stewart Technology Associates has yielded impressive results in this area with its clients, and this is due not only to decades of experience, skill, and dedication, but also to the fact that we utilize only the most robust software in Orcaflex by Orcina. STA possesses the experience, knowledge, and resources your organization needs for success.
See also the STA CHAINLENGTH Catenary Calculator.