A NEW WAY OF UNDERWATER 3D MONITORING
Kongsberg, Hamburg, Germany
The last decade has been witness to an expanding offshore industry, supported by the investment in oil and gas and offshore wind energy. The offshore industry is dependent upon the diversity of underwater structure (or structures and installations) installations, having a direct influence on the structural integrity and safety of markets that are important for operating companies.
It is well known that due to natural or artificial processes the seabed changes over time, causing scour development or sediment build up. The process of scour development is usually slow, but structures can ultimately become dangerous, reaching a point of collapse. The first effect is the degradation of foundation material, which exposes the footing and lowers its factor of safety regarding sliding or lateral deformation. The greatest loss of sediment to scour occurs at high water velocities, such as during flooding events. Secondly, pier movement may occur because of material loss beside and beneath the base of the footing, producing structural distress and ultimately resulting in structural collapse. In most cases, catastrophes occur due to overconfidence or insufficient risk mitigation. Therefore, real-time bathymetry information is extremely valuable to offshore operators. Traditionally, offshore surveys are conducted when weather conditions are good and there are nearly no waves or currents. A calm water state has been known to produce false scour, giving invalid survey results.
The sonar was deployed using a pole, which was fixed with straps around the Dolphin for a period of nine months. The deployment method, was simulating similar condition in case the system is installed on offshore windmills, where in most cases the welding is forbidden. Right after the mechanical installation of the system, the first baseline survey was performed. The system was configured to monitor the area with radius of around 10 m in front of the dolphin. The processing algorithm was configured to generate grid with cell size of 0.2 m, and the alarm threshold was set to 0.4 m calculated as vertical deviation on the seabed. The data from the baseline survey was used as a reference for calculating any seabed deviations.
The results showed that the system can improve the reliability of the recorded data by reducing the possibility of incorrect measurements due to a false scour occurrence. The monitored seabed area was not dinamic, so the data analysis for the period of six months showed only small changes on the seabed. The first deviations occurred after three months monitoring the seabed, on the far right edge of the dataset approximately 10 m away from the foundation of the dolphin. The analysis showed that the errosion occured due to the thrusters of a vessel moving in the area and not because of the currents.
It is important for the industry to represent accurate conditions of the foundation of the inspected structure. Therefore, the deployment of a 3D scour monitoring system over long periods and performing a scour inspection during bad weather conditions or other forces that are not taken into account is crucial. In addition to scour development information, underwater 3D inspections provide surveyors with important information about the condition of the structure. Even the smallest cracks or deviations detected could be important information in getting the full picture and deciding the next step in maintenance.
- Moving from traditional way of underwater inspection to permanent structural 3D monitoring.
- Adaptation to different monitoring tasks, ranging from small to complex 3D structural and scour monitoring systems around subsea structures and near to surface monitoring applications such as; sediment displacement, obstacle and debris detection and even environmental monitoring around the location of the structure.
- Real - time data available from the seabed to the office.
- Cost-efficient operation by providing intelligent system control and advanced data processing capabilities.