During sinking and submerging operations, the traditional way of positioning tunnel elements is indeed to use high precision total stations and large towers fitted with reflectors on the elements. The reflectors positions are updated every 2-4 seconds. Using this technique, the position updates are infrequent and the dynamic motion of the element is unavailable.
Deep bv was commissioned to develop a system to position elements using the newest techniques available to the market to monitor the 3-D dynamic position of the tunnel elements during installation. The all-time accuracy should be better than 1 cm in X, Y and Z.
In close cooperation between Delta Surveys, experts in 3-D measurements, and IXSEA, manufacturer of innovative navigation, positioning and imagery systems, the definitions were made while available techniques inventoried. This resulted in the following set-up:
The element’s primary position was measured by a Leica TCA2003 Robotic Total station, which was set up near the construction site and was directly interfaced with the Seabed Signal Box.
An IXSEA PHINS Inertial Navigation System was installed on the element and calibrated prior to the positioning works. PHINS was powered using large capacity batteries and data was transmitted using telemetry. PHINS can output positioning data as well as heading, roll and pitch at high speed (100hz) with an accuracy of 0.01 degrees for heading, roll and pitch. While a Seabed Signal Box was used to route and format all data strings for direct input into the positioning computer.
Using the IXSEA PHINS and primary positioning data, all relevant offset positions of the element could be calculated and displayed in real-time by the dedicated software. A large LCD display provided numerical and graphical data during the positioning and ballasting of the element.
An additional Robotic Total station measured a second target position on the element providing the software with redundant data. This way, the quality of the independent measurements was checked online.
The system proved to give online position information of every part of the element with an accuracy of better than 1 cm in X, Y and Z. The update rate was much higher than using the conventional techniques and the reliability for the complete installation, during almost all types of weather and even when the operators were tired, was 100%. Galamadammen was a relatively small project, but the system proved to be ready for large tunnel installation projects.