Developing laser systems for cleaning ship hulls underwater
Researchers are developing laser systems capable of cleaning ship hulls underwater
Throughout their lifetime the undersides of ships can accumulate microorganisms, vegetation and algae – a process known as biofouling. If left unchecked, biofouling will build up overtime, which can have a dramatic impact on the weight and streamlining of a ship.
A reduction in streamlining lowers the efficiency of a ship’s movement, leading to an increase in operating costs due to the additional engine power and fuel required to propel it. With fuel consumption making up around 80 per cent of the operating costs of any ship1, controlling biofouling is important, not only for improving the economic effectiveness of their operation, but also for ensuring the minimal output of CO2 emissions from maritime transport as a whole.
But just how bad can biofouling get? According to a group of Russian researchers at the Institute for Marine Technology Problems, in Vladivostok, the weight of small shells – a type of biofouling – on a ship’s hull can be as much as 50kg/m2. Although the layer of fouling is only 2 to 3cm thick, multiplying this over the approximately three thousand square metres of a ship’s hull can result in more than 150 tonnes of biomass weight.
The inspection and cleaning of ship hulls is carried out periodically to prevent the continual build-up of biofouling, and the resultant degradation in efficiency. Usually this process is carried out in a dry dock2, but can also be done underwater by divers with video systems, devices for the non-destructive testing of hull structures, and tools for removing biofouling. Such tools include those used for mechanical cleaning, such as brushes with a hydraulic drive, cutters or vibrating knives; or hydrodynamic cleaning, which involves high pressure water jets containing cavitation voids of water vapour.
Both mechanical cleaning and hydrodynamic cleaning offer their own advantages and disadvantages. For example, while mechanical cleaning enables the removal of large outgrowths of biofouling over a large area, there is a risk the ship’s coating can be damaged in the process. In addition, mechanical tools can often be fragile and require large inputs of energy to operate, leading to high maintenance and running costs. Hydrodynamic cleaning is effective at removing mild outgrowths of biofouling without damaging a ship’s hull. However, it does require the constant intake and pumping of water, and is generally more effective above water than underwater.
The developed ROV device for the remote inspection and laser cleaning of ship hulls.
The researchers from the Institute for Marine Technology Problems are exploring laser cleaning as an alternative method for removing biofouling. Laser cleaning involves clearing away undesired material from a solid surface by irradiating it with a laser beam. As the targeted material absorbs the energy from the laser, it is heated very quickly, causing it to vapourise.
In two papers published both this and last year in IOP Conferences Series: Earth and Environmental Science, the researchers describe how the technique offers several advantages over mechanical and hydrodynamic surface cleaning. Laser cleaning is not only a high-throughput, non-contact process that enables the selective treatment of a hull surface without causing damage, but it also uses zero consumables and its parameters can be controlled remotely with ease using software.
Robotic underwater laser cleaning
The papers cover the researchers’ development of a prototype device capable of performing remote inspection and cleaning of afloat vessels, based on a remotely operated uninhabited underwater vehicle (ROV) equipped with a built-in laser cleaning system.
When developing the ROV, the researchers knew that their cleaning system design would have to offer a certain range of functionality. The propulsion system would need to enable movement along a steel hull in both water and air – guided by video cameras – in a wide range of longitudinal and transverse velocities with depth roll and pitch control. The ROV would also have to be held to the hull of the ship, which would require integrating magnets into its wheels. The system would also have to include vertical thrusters to compensate for the reaction of the air flow from the nozzle of the laser cleaning module, while the module itself would have to be sealed to make it waterproof.
The resultant ROV, pictured in figure 1, has dimensions of 0.73 x 0.72 x 0.45m, weighs 78kg, can operate at a depth of up to 10m, and can move forward at a speed of 0.6m/s underwater.
The laser cleaning module comprises a 2D Mid-Power Scanner from IPG – designed to work with fibre lasers of 1,070 to 1,080nm wavelength – AC-DC modules for onboard power conversion, a durable container with a porthole for laser radiation, a fibre optic connector and a slotted nozzle with fittings for an airline to drain the surface to be cleaned. The module’s functional diagram is shown in figure 2.
Figure 2: Functional diagram of the laser cleaning module
Tests of the ROV were carried out to determine the optimal parameters of the underwater laser cleaning process, such as laser power, scanning speed, the air pressure required at the entrance to the slotted nozzle, and the speed of movement relative to the surface being cleaned.
Continuous laser radiation of 800 ± 200W of power was determined to be sufficient for removing biofouling at a linear scanning speed of 40mm/s. Effective drainage of the chamber volume was ensured by using a compressed air pressure of 0.4 ± 0.2MPa. An example of the underwater laser cleaning of a test plate can be seen in figure 3.
Figure 3: The underwater laser cleaning of a test plate using a continuous mode laser.
In summary, the researchers succeeded in designing and developing a prototype remote underwater robotic inspection device that is capable of laser cleaning the hulls of ships. Their experiments demonstrated that the device could move in both air and underwater, and the ability of laser cleaning equipment to remove biofouling was confirmed. The developed device will enable the inspection and cleaning of hulls efficiently and safely, without requiring the vessel to be docked.
The Russian researchers are not the only scientists looking into laser cleaning for biofouling removal. In the multi-year research project ‘FoulLas’, scientists from the Laser Zentrum Hannover, Laserline and Fraunhofer IFAM are currently looking to develop their own underwater process capable of removing biofouling without damaging underlying paint-based antifouling and corrosion protection coatings.
A blue diode laser from Laserline is being tested for the removal of biofouling in the FoulLas project.
The process will damage the vegetation cells in such a way that, ideally, the water flow washes away the remaining material.
The project partners are working with diode lasers provided by Laserline in both blue and infrared wavelengths. Whether or not the laser will be equipped to an ROV has yet to be revealed by the project partners.
Lasers create anti-fouling surfaces for ship hulls
An ongoing European research project is using laser technology to structure the surfaces of ships in order to improve their fuel efficiency.
In the three-year ‘MultiFlex’ project, scientists are using the world’s first ‘dot matrix’ laser to create hydrophobic, anti-fouling metal and plastic surfaces. These could soon replace toxic ship paints and varnishes that are used to stop algae or barnacles sticking to hulls, reducing maintenance costs, fuel consumption and CO2 emissions.
Through harnessing a 1kW ultrafast laser and beam-splitting optics, the researchers can split a single high-energy pulse into a grid of 64 beamlets, each of which can be turned on, off, positioned and tuned individually. The grid can be used to etch microscopic ‘spike’ structures onto sheet metal or plastic at a fast rate, creating a rough surface that reduces drag to inhibit the growth of bacteria, algae or even barnacles. The structures imitate the incredibly efficient skin of sharks, which are covered in millions of tiny protruding scales that reduce drag, making them highly efficient swimmers.
‘Existing ultrafast lasers are known for their precise ablation and cutting results. Unfortunately, processing large parts with such lasers can take weeks,’ said Dr Johannes Finger, project co-ordinator at MultiFlex. ‘Our system will ablate more than 150mm³ in one minute, therefore making it hundreds of times faster than existing technologies.’
The MultiFlex partners consist of Fraunhofer ILT, RWTH Aachen University, Amplitude Systèmes, Lasea, and AA OptoElectronic.
References
- Development of the Underwater Robotics Complex for Laser Cleaning of Ships from Biofouling: Experimental Results - A Yu Bykanova, V V Kostenko, A Yu Tolstonogov. IOP Conferences Series: Earth and Environmental Science. doi:10.1088/1755-1315/459/3/032061
- Underwater Robotics Complex for Inspection and Laser Cleaning of Ships from Biofouling - V V Kostenko, A Yu Bykanova, A Yu Tolstonogov. IOP Conferences Series: Earth and Environmental Science. doi:10.1088/1755-1315/272/2/022103
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