UMS camera and ultra bright red LEDs
disconnectable turret mooring systems and mid-water riser support arches, and the industry’s questions about their underwater behaviour, in 2004 MARIN decided to develop a 6 DoF, contactless Underwater Motion Measurement System (UMS), compa- rable to the above-water system. In close cooperation with the Krypton Corporation a modified version of its K600 optical meas- urement system was developed.
K600 adapted for underwater Several changes had to be made to the original K600 system to be able to use it underwater. First, the infrared Light Emitting Diodes (LEDs) used for structures above water had to be replaced by different colour LEDs because the absorption of infrared light underwater is very large. After some re- search ultra bright red LEDs were chosen for the new UMS. Secondly, a watertight housing for the camera beam, including a stiff connection frame to the basin carriage, had to be manufactured. Using MARIN`s diffraction
software, DIFFRAC, the wave loads on the connection frame and camera housing were determined and taken into account in the design. The frame has to be very stiff in order to minimise the motions of the camera beam because that has a big impact on measurement accuracy.
Improved measurement quality Fur- thermore, a system of tracks was developed to be able to mount the camera on the basin floor, allowing camera positions below the object to be tracked. This track-mounting option reduces the chance of reflections of the LEDs on the water surface, thus improv- ing measurement quality and accuracy. The engineering of the camera control soft- ware comprised the last stage of the devel- opment. An important requirement in the specifications was that the UMS should be able to measure in the same coordinate system as the above-water system. This is essential for disconnectable turret systems for instance, to be able to determine buoy positions with respect to the floater. A spe-
cial calibration device was developed to be able to convert the UMS coordinate system to the MARIN standard coordinate system.
This new measurement system is able to measure 3 DoF of up to 30 individual LEDs (e.g. for riser shape measurement), or 6 DoF of up to 3 bodies, (e.g. for underwater buoys). The system can measure in a large area (maximum of 3 * 3 metres at a distance of 6 metres from the camera), with a typical specified measurement resolution of 0.02 mm and a specified measurement uncer- tainty of 0.3mm (U95).
In recent years the UMS system has been applied for numerous complex measure- ments. It enabled the study of disconnectable turret systems in survival conditions, hybrid riser towers and other subsea systems in current and waves. The accurate meas- urements made it possible to study motion behaviour but also possible interactions between different structures that could otherwise not have been quantified.
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