Design optimisation of submerged floating tunnel high-potential energy system

Although proposals to build submerged floating tunnels (SFTs) have existed for over a century, their history is littered with well-meaning suggestions of form and type, but very few have been built. More recently, however, their potential as a highly cost-effective and sustainable crossing has rejuvenated industry interest. The main types of SFTs are pier-, tether-, pontoon-supported and free-spanned. With the introduction of a novel high-potential energy (HPE) system, the benefit of providing axial tension along a tether-supported SFT is studied. The system imposes permanent axial tension in the tube by pre-tensioning the tube from the shore during construction. The system enables a positive adjustment of the stiffness and natural vibration frequency. This novel energy system has three main advantages: (i) expensive and challenging construction in the offshore and submerged environment can be dramatically reduced; (ii) introducing design flexibility can ensure a globally optimised design; and (iii) the risks of system failure due to changing environments or uncertain system behaviours can be eliminated. Design comparisons are made of the SFT’s effectiveness against tethers layout, wave period, and crossing distance, with and without the high potential energy system. An optimisation approach allows for a desktop study that minimises system cost while satisfying multiple design criteria, such as tube deflection and acceleration, tether breaking strength, no slack in the tethers, supported by evicence from wave-basin and free-decay vibration tests.