Boskalis is a global market leader in the field of dredging and inland infrastructure, offshore energy and salvage, with a track record of success spanning over 100 years. Boskalis contacted 2G Robotics for assistance in conducting subsea 3D laser scans of the internal surfaces of 2 offshore wind monopiles.
In some cases, the monopiles have encountered buckling and other deformation. These needed to be quantified, analyzed and recorded.
The equipment used in these scans included:
- 2G Robotics ULS-200 Laser Scanner
- iXSea Rovins Inertial Navigation System
- Paroscientific Digiquartz Depth Sensor
- Valeport Altimeter
- Aqua Vision Surveyor SD Camera
The 2G Robotics ULS-200 was attached to a steel bracket, mounted under a cutting tool. The tool was lowered into place, where it would scan the interior of the pile. The ULS-200 takes a single profile every 0.018 degrees for a full 360 degrees, before being shifted up or down to the next position.
The duration of each scan was dependent on the desired amount of filtering, with most scans requiring less than 15 minutes.
The ULS-200 operates by emitting a 50 degree laser swath, calculating the position of 480 points along a line projected on the target before incrementing to a new angle. This process is repeated until a full 3D point cloud representation of the area surrounding the scanner is generated.
A common use case is to perform a series of overlapping 3D scans, and then register them in post-processing.
To ensure faster processing and more accurate alignment, the previously mentioned tools were integrated with the ULS-200.
A sound velocity profile through the water column was recorded prior to scanning, in order for the online acquisition software to convert raw pressure to depth. The depth readings were reduced using real time tide data to establish Mean Sea Level heights.
The first scan of each pile was carried out at the highest possible section allowed by the cutting-tool’s hoisting-wire setup, before lowering the cutting tool down to a meter above the drilled seabed depth.
The camera and altimeter were used to ascertain when the laser was close to the seabed. Static scans were carried out every 0.5m where the pile showed significant deformation and at 0.75m steps where the pile returned to the normal circular shape. Each scan covers a 1.27m vertical section, thus giving 100% coverage of the pile in between top and bottom values.
During scanning, the survey-bracket was immobilised by forcing the bracket-cylinders against the pile-surface. To check stability the MRU pitch / roll / heading was logged and checked continuously during the measurements.
The data was fed up to the vessel through a cable, and acquired using 2G Robotics’ proprietary ScanSoft program.
A control survey was carried out above water prior to the measurement, using a section of spare pile . This control survey provided a baseline for what a pipe would look like without deformation, in order to establish the practical error.
This image shows the benchmark scan, and the diameters across various points. The theoretical inner diameter of the pile is 2.598m.
The following image shows how the ovality observed in Pile 2, compared to the benchmark scan.
The following image shows the ovality observed by measuring the diameter across various points on Pile 2, at the site of the observed deformation. They range from 2.43 to 2.61 meters, for a difference of 18cm between the widest and narrowest axes. Compare this to the benchmark pile, where the widest and narrowest axes differ by less than 1cm.
The following image shows a 3D point cloud of the pile, which can be rotated for qualitative analysis and exploration. In this image, plastic deformation can be seen on the pile.
As this example demonstrates, the USL-200 – when combined with other measurement and navigation tools – provides a fast and cost-effective means for collecting large amounts of high-resolution quantitative and qualitative metrology data for wind monopile inspection.