The salvage operations for the Costa Concordia, which went aground on the island Giglio on January 13th, proceed steadily. The Costa Concordia’s potential slipping off of the rock plateau, on which the ship has been gone aground, presents a big problem. The position of the ship must be monitored to do not endanger the salvage crew. In technical terminology, it is termed monitoring.
Modern total stations and laser scanners used for monitoring
For the documentation of the Costa Concordia’s change in position, modern surveying instruments are employed. These include total stations (also called tachymeter) as well as laser scanners.
Der RIEGL LMS-Z420i Laserscanner der Abteilung für Geowissenschaften der Universität Florenz vermisst das Schiffswrack millimetergenau
Source of image: RIEGL Laser Measurement Systems
The total stations take single-point measurements. These can be made without targets (without reflector) or with targets (mirror or adhesive labels). Laserscanning Europe has talked to the surveying specialists on site. In this case, several reflectors (so-called surveying prisms) have been fixed on the ship by specialists.
Then, these reflectors can be surveyed by means of total stations and the positions are saved. Afterwards, a regular automatic survey of the surveying positions can be made. The intervals can be defined autonomously by the user.
For this surveying process, a method called ATR is used. This is automatic target detection. The total stations roughly approach the different positions by using the Robotic mode and the ATR takes the vernier adjustment on the surveying prism. For each surveying position, repeatability accurate to the millimeter is, thus, obtained.
Finally, the surveying results for all targets are automatically saved in a software. This software compares the new surveying results with the results achieved before and records them. If limits for the change in position set by the user are crossed, alarm would automatically be set off.
This method is not new but is used in technical engineering constructions or monitoring surveys and surveys of acclivity in the mountain for a long time.
In the case of the Costa Concordia, measuring instruments and software solutions of the companies Leica Geosystems and Topcon are employed.
For the monitoring survey, it is also possible to use highly accurate GPS antennas on the shipwreck. This expensive measuring technology would be lost if the ship sinks. As a result, this technology is not used in this case.
Laser scanners for monitoring survey
Laser scanners are employed additionally. The laser scanners take surveys from the shore. Here, pulsed laser scanners are advantageous since their range is clearly wider than the range of phase laser scanners. In the image, a laser scanner of the Canadian company Optech is employed. This scanner belongs to the laser scanners on the market which can realize measuring ranges up to 2km. Large-volume recordings (so-called laser scans) are regularly taken. Thereby, laser scanners scan the shipwreck with millions of points and create realistic 3D copy of the visible ship surface.
Laser scanner of the company Optech employed
Source: http://www.wday.com/event/image/id/8506/headline/Costa%20Concordia/
In contrast to the surveys with the total station, changes of the hull can additionally be detected by laser scanning. These changes include changes of the ship’s geometry. They could point to fractures or deflections. But the huge amount of data cannot yet be interpreted automatically. The specialists use the scans as a further monitoring source in order to draw conclusions. The surveying accuracy of the total stations is higher since they determine the measured section from the survey to the surveying prisms. Laser scanners measure without reflectors. Indeed, this allows a fast registration of many points, but the accuracy of the single surveys is lower than the accuracy of the survey with the total station for this wide scanning range.
A great advantage of the laser scanner is that there are no workings at the wreck necessary for surveying the wreck. If entering the wreck is not possible anymore and all fixed reference marks are under water, reliable and accurate scanning results could still be determined due to laser scanning.
The surveying methods employed for the salvage of the wrecked Costa Concordia show the useful combination of modern surveying instruments such as 3D laser scanners and total stations.