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Dredge Monitoring Gives Shipwreck the Deep Six
01 Nov 2004
An innovative solution to the problem has taken the form of dredging seabed sand from around the wreck, thus creating an adjoining cavity on the bottom into which the vessel would sink deeper until it was no longer a threat to safe passage.
Royal Boskalis Westminster's newbuild 15,850m3 capacity trailing suction hopper dredger Prins der Nederlanden wasmobilised to undertake the work.
A safe and accurate dredging operation required sophisticated underwater monitoring equipment which could track the precise location of the wreck throughout operations and function as a warning system in case it should move suddenly. Noordhoek Offshore Survey was contracted to provide these services.
Noordhoek mobilised a USBL system, consisting of a combination of omni-directional beacons (Compatts) and directional beacons mounted with an acoustic release for retrieval at project completion.
Dredging operations do not provide an ideal working environment for underwater acoustics. Noise levels from the suction pumps are high pitched and the acoustic signal from subsea beacons are not likely to register with the USBL system on board the dredger.
With these conditions in mind, a complete wreck motion monitoring system was designed, manufactured and installed by Noordhoek's wholly owned subsidiary, Seatec Underwater Systems. Seatec designed the motion sensors using the measurement of gravitational vectors and angular velocities for all three main axes. A software module used these measurements to calculate true roll, pitch and movement of the sensor.
Seatec delivered complete motion sensor sets for the wreck's bow and stern sections.
A single sensor system consisted of the submersible motor sensor, subsea cabling, buoy mounted telemetry and a monitoring system on board the dredger.
Dedicated software warning thresholds would give immediate notice should either of the wreck sections start to move. The system was so sensitive that even tension spikes in the wreck's hull could be measured. This gave the dredger timely notice that movement was eminent and the suction pipe should be removed to prevent damage.
All acquired data was logged and exported to the client's dredge monitoring system, enhancing the confidence of the project team and increasing the safety of dredging operations.
MJ Information No: 20026
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