Underwater infrastructure

LiDAR measurement to replace dives: optical sensors for more efficient condition monitoring

By their very nature, damage to critical infrastructure below the surface of bodies of water is not visible at first glance. This makes regular inspection all the more important. Today, condition monitoring of underwater infrastructure such as offshore wind turbines, pipelines or submarine cables, tidal lift power plants, drilling platforms, or port infrastructe is carried out visually by divers, exposing them to high risks. In view of the growing number of offshore wind turbines and underwater pipelines worldwide, it comes evident that purely visual inspections by humans can only be accomplished with an enormous amount of time and consequently high costs.

In addition to condition monitoring, the detection of moving objects under water is also becoming increasingly important: there are large quantities of unexploded ordnance on the seabed that must be recorded in order to ensure safe munitions clearance. The outer hull of ships must also be surveyed more systematically in future: Ship hulls are increasingly being used as carriers for smuggled goods. In addition, centimeter-thick deposits on ship hulls, known as biofouling, sometimes drive up the fuel consumption of ships. Against this backdrop, more efficient methods for measuring underwater objects are required. Measuring systems that perform inspection tasks automatically are in urgent demand. LiDAR scanners measure very precisely under water and thus enable real 3D recording of underwater infrastructure. In recent years, Fraunhofer IPM has succeeded in overcoming the particular challenges of using LiDAR technology underwater, including the strong attenuation of light and the turbidity of the water.

Capturing structures with millimeter precision at great depths

The Underwater LiDAR system ULi by Fraunhofer IPM is designed for the 3D inspection of large underwater structures. The measuring system works based on the pulsed time-of-flight method (TOF), having been fundamentally enhanced for use in the turbid medium of water. In addition to the hardware, the scanning system includes algorithms for fast and reliable data evaluation and interpretation. By analyzing several reflected pulses, the system ignores both results that do not originate from scattered light and interfering particles in the water. The integration of the complete system into an appropriate pressure housing and the use of special water-suitable components enables the operation on remote-controlled or autonomous underwater vehicles. For the first time, this will allow for efficiently obtaining very accurate 3D data for a wide range of underwater inspection tasks.

The ULi sensor inspects 3D structures under water and can be operated statically or on a ROVs (remotely operated vehicles) or ship. ULi dives hundreds of meters deep and detects objects with millimeter accuracy over distances of several tens of meters. The clearer the water, the better the measurement result. The measuring distance is around twice as far as the visibility, with structures being resolved to well under one centimeter. This means that the system measures up to ten times more accurately than sonar systems, for example, and provides a precise 3D model of the object. ULi records underwater infrastructure based on the pulsed time-of-flight method using laser light with a wavelength of 532 nm and records up to 100,000 measuring points per second. The measuring beam is deflected by two rotating wedge prisms. This allows the scanner to capture the full field of view without having to move the sensor.