Responses of Submarine Telecommunication Cables to Earthquakes
Student thesis: Doctoral Thesis
Related Research Unit(s)
Submarine telecommunication cables carry about 99% of Internet-based international telecommunications and are the physical backbone of the Internet. They are usually buried shallowly beneath the seabed near shore for protection or placed directly on the ocean floor in the deep sea. However, in recent years, it has been observed that many submarine cables are damaged due to earthquakes, which not only causes enormous financial losses, but also severely impacts normal communications through the Internet. Although some studies have been performed to explore the mechanism of cable damage, it is still an unknown issue that how submarine telecommunication cables behave and how they are damaged under seismic effects. This thesis aims at developing numerical and physical models for studying the response of submarine cables to seismic waves and seabed movements.
To address this problem, a finite element (FE) model is first developed. The submarine cable is discretized using continuous beam elements. The cable-soil interaction is modeled using the axial, transverse horizontal, and transverse vertical soil springs with different force-displacement relationships. The effect of seismic waves on submarine cables is first investigated. An analytical solution is presented to validate the FE model. Through a comprehensive parametric study covering various characteristics of common seismic waves and submarine cables, the FE results show that the influence of seismic waves is limited and incapable of damaging submarine cables. Then the effects of earthquake-induced seabed movement are also studied using the FE model. The FE model is validated by a large-scale test on submarine cables and other comparable physical tests on buried pipeline. The FE results show that the lateral seabed movement has a significant influence on submarine cables.
Therefore, a series of parametric studies is performed using both the developed FE model and large-scale physical model. Considering the typical ranges of soil properties, cable burial depths, cable characteristics and seabed movement characteristics, the FE parametric study results are summarized in a dimensionless plot followed by a regression analysis. Regression equations are developed to directly predict the maximum cable strains induced by lateral seabed movements. To reveal the mechanism of cable damage due to ground movements and validate the FE model, a series of large-scale physical tests are conducted. A large-scale split-box test apparatus is designed and fabricated. The influence of two key factors (i.e., the unit weight of backfill soil and the intersection angle between the ground movement direction and cable axial direction) is explored. The results show that the cable strain is mainly controlled by the intersection angle, while the influence of the soil unit weight is minor. The cable damage is attributed to the excessive axial strain while the contribution of bending strain is negligible. Safety criteria for submarine telecommunication cables are proposed for evaluating the cable safety when subjected to lateral seabed movements for engineering practice. In addition, the influence of the intersection location between the submarine cable and seabed movement is also explored. The results show that its influence on the maximum cable strain is insignificant.
Considering the uncertainty associated with cable-seabed interaction, the Monte Carlo simulation integrated with FE analysis is performed. The uncertain parameters are modeled using random variables and random fields, respectively. The probabilistic analysis of cable damage is performed to assess the effects of parameter uncertainties on the cable damage probability. In addition, the influence of correlation lengths of random fields on the maximum cable strain is also investigated.