Analysis of the tunnel immersion for the Busan-Geoje fixed link project through scale model tests and computer simulations

In Korea a four lane motorway is constructed between the city of Busan and the island Geoje, reducing traveling times from 1 hour by ferry to just 10 minutes by car. The so-called Busan-Geoje Fixed Link consists of 2 cable-stayed girder bridges and a tunnel, crossing the bay of Jinhae. The submerged tunnel is built by transporting each of its 18 elements below 2 pontoons from a construction dock to their final positions and lowering them on the sea bed. The project is unique, because the tunnel elements are installed in a bay with direct access towards open sea. For this reason, the effects of incoming swells and wind seas were investigated in detail, so that the operational limits of the tunnel element immersion could be accurately determined. This was achieved by using an approach of combined hydrodynamic scale model tests and time-domain computer simulations.
First, scale model tests were carried out in MARIN's Shallow Water Basin. A detailed test set-up was constructed, including the trench in which the tunnel elements are placed, as is shown in the photograph. Models of a tunnel element, two pontoons, the mooring system, contraction lines and suspension wires were constructed at a scale of 1:50. The motions of the pontoons and the submerged tunnel element, as well as the tensions in the lines, were measured in a range of different wave conditions. Different stages of the tunnel immersion were investigated. Second, a simulation model of the pontoons and tunnel element was constructed in MARIN's time-domain simulation tool aNySIM. The large number of mooring lines, contraction lines and suspension wires resulted in a relatively complex numerical model. The simulation model was calibrated such that the results from the model tests could be accurately reproduced. Subsequently, a sensitivity study was carried out, investigating the parameters most critical to the operation and the mooring system of the pontoons was further optimized.
Finally, the operational limits of the tunnel immersion were evaluated by carrying out more than 6,500 time-domain simulations, investigating a large number of different combinations of wind sea and swell. The simulation results included motions, velocities and accelerations, as well as line tensions. The extreme values were used to perform a combined evaluation of more than 10 structural and operational criteria.