Laser cleaning proves successful for cleaning ship hulls
Diode laser manufacturer Laserline, together with research institutes Laser Zentrum Hannover (LZH) and Fraunhofer IFAM, have successfully developed an underwater laser cleaning process capable of removing biofouling from ship hulls and other maritime structures.
The new process could prove a faster, more economical alternative to mechanical cleaning methods, while dramatically reducing the fuel consumption of ships and the risk of species migration – which recently made headlines when a cruise ship was denied entry into an Australian port due to biofouling present on its hull.
What is biofouling?
Biofouling refers to the growth of algae, mussels and other marine organisms on ship hulls, harbour sheet pile walls or steel girders anchored underwater. Thanks to millions of years of evolution, their attachment can be as robust as industrial adhesives.
These often 2-3cm thick and highly compacted fouling layers are critical, especially in shipping traffic. This is because across the whole hull of a ship, the weight of biofouling can add up dramatically. For example, small shells – a type of biofouling – can weigh as much as 50kg/m2 . Multiplying this over the approximately three thousand square metres of a ship’s hull can result in more than 150 tonnes of biomass weight.
Algae, mussels, shells and other marine organisms make up biofouling, which adheres strongly to the hulls of ships, adding extra weight and reducing their streamlining
Biofouling therefore increases the weight and flow resistance of ships considerably, leading them to consuming up to 30% more fuel and releasing more CO2 emissions. It can also prevent ships from being allowed to berth at certain ports, for fear that foreign species may destabilise the local ecosystems. This happened only recently in December, when the 14-deck, 930-person Viking Orion cruise ship was forced to spend approximately one week anchored 17 miles off the coast of Adelaide, Australia while professional divers cleaned its hull of potentially harmful biofouling.
It’s not just the shipping industry that biofouling can affect though. For example, it can functionally impair tidal power plants, aquaculture cages or other movable constructions. In addition, marine organisms also attack anti-corrosion coatings and material structures of individual surfaces, thereby endangering even the long-term stability of purely stationary structures such as the foundations of oil rigs or wind turbines.
What methods are currently used to tackle biofouling?
As a preventive measure, biocidal antifouling coatings have often been applied by maritime firms in the past, either to prevent the growth of biofouling from the ground up, or to destroy the cell structures of adherent organisms. However, due to their toxic effect on other aquatic organisms, only a few biocides have been approved, and even these will completely be banned in the foreseeable future. Biocide-free alternatives have also been explored in the form of silicone-based antifouling coatings, which offer a particularly smooth surface structure to help prevent biofoul adhesion.
Cleaning ship hulls of biofouling using conventional methods is a time- and cost-intensive process
These coatings have only had limited success however, as they only tend to delay biofouling. The organisms colonise quickly and in large quantities, so even with the coatings, a considerable amount of new biofouling growth can still be observed within a very short time.
Such as was seen in December, mechanically cleaning is often deployed by diving teams and in dry docks to counteract biofouling. This involves, for example, using brushes with a hydraulic drive, cutters or vibrating knives, as well as high-pressure water jets containing cavitation voids of water vapour. Not only do such methods come at great expense and require a lot of time, but they can also release numerous marine organisms, which then migrate unwantedly into the local ecosystems; this is why mechanical ship cleaning is forbidden in many ports around the world. Lastly, it has proven near impossible to clean biofouled surfaces mechanically without damaging existing antifouling and anti-corrosion layers. Flaked areas or cracks in varnishes and silicone layers are not uncommon. This not only promotes the formation of rust spots, but actually increases the occurrence of biofouling, as organisms can more easily cling to cracks and scratches in surface coatings.
Could laser cleaning be the solution?
The partners of the FoulLas (Fouling removal of maritime surfaces using laser radiation underwater), which began in August 2019 and concluded in December, sought to address the challenges of biofouling using underwater laser cleaning.
Laserline, LZH and Fraunhofer IFAM have developed a process in which blue laser radiation is used to lethally damage biofouling underwater without damaging the underlying coating of a ship's hull. The cells of the biofouling are damaged in such a way that the fouling layers die and are simply washed away by the water after some time.
The new process could prove a faster, more economical and environmentally friendly alternative to current conventional mechanical cleaning methods, while reducing the need for repair coatings.
An underwater laser process has been developed to deliver a lethal does of energy to biofouling. (Image: LZH).
Experiments were conducted at a testing facility run by Fraunhofer IFAM and LZH in Helgoland, a small archipelago in the North Sea. Here, samples were exposed to biofouling under real conditions in seawater.
A laser system was developed using a 1.5kW LDMblue from Laserline, as well as beam delivery optics to produce line-shaped focus.
Biofouled samples were imaged, weighed, immersed in a seawater-filled tank using a handling system, and irradiated using the laser. Following the irradiation, the samples were imaged, weighed again and returned to the seawater.
Fouled samples were stored underwater at a research facility in Helgoland. (Image: LZH)
“After treatment with blue laser radiation, the biofouling showed a clear change in colour from a dark green/brownish appearance to a light green colour,” Dr Markus Baumann, Optics Engineer at Laserline and manager of the FoulLas project, told Laser Systems Europe. “The irradiated sample showed a clear reduction in biofouling after a 2-4 week observational period, and investigations showed lethal damage to the organisms. With simulated currents, as would be added in real life with a moving ship, the cleaning effect is further enhanced, with the removed, lethally-damaged biomass no longer being dangerous to foreign ecosystems.
The project ran for several years in order to monitor the seasonal influences of biofouling growth, and develop a range of process strategies accordingly.
Results of the samples irradiated with the laser and those not irradiated. (Image: LZH)
“We irradiated the samples with a broad range of parameters (laser power, power density, duration) and gained a good insight into which sets of parameters generate effective fouling removal,” said Baumann.
Making it industry-ready
The FouLas project was intended as a feasibility study to develop and prove the effectiveness of laser technology to counteract biofouling. As such, the technology has only currently been tested in laboratory settings, and is yet to be trialled on a ship hull underwater
“However, our results are very promising and we want to go a step further and develop a technique to implement this on a real ship hull,” said Baumann. “The goal will be to have a remotely operated underwater vehicle that can clean the ship during loading or unloading, equipped with cameras and sensors to ensure a specific grade of cleaning.”
The envisaged system would make it possible to achieve shorter cleaning intervals, which adds further benefit to maritime firms – in addition reducing fuel consumption costs and the risk of species migration.
 V V Kostenko et al 2019 IOP Conf. Ser.: Earth Environ. Sci. 272 022103