SUBSEA CLEANING SYSTEM
Technical Field
The present invention relates to subsea cleaning of biofouling. More specifically, the invention relates to a system and a method for cleaning surfaces from biofouling, such as ship hulls and windmill structures, by rendering the biofouling harmless.
Background Art
On a clean ship hull floating on seawater, biofouling starts to form after few minutes, starting as proteins, diatoms and bacteria settling on the submerged part of the hull surface. After a few hours, a microbial biofilm starts to form.
Then, formation of microfouling starts, wherein the biofilm receives secondary colonizers. After days or weeks, the microfouling will allow attachment of invertebrate larvae. Later, invertebrate larvae and algae will grow, forming macrofouling, which can grow very thick with algae and calcareous species.
The effect on fuel consumption by biofouling is surprising. Increase of 25% of fuel consumption caused by only 0,5 mm thickness of soft biofouling with 50% hull coverage on a 175 m bulk carrier has been reported.55% increase in fuel consumption has been reported with 5 mm thickness of barnacles (small calcareous fouling or weed) and only 1% coverage on a 320 m tanker.
Reducing the overconsumption of fuel caused by biofouling on ships will contribute significantly to the reduction of greenhouse gas emissions and help accelerating the shift towards a more sustainable economy.
Another effect of biofouling is spreading of invasive species, resulting in damage to existing ecosystems. On the website of IMO, the International Maritime Organization, the United Nations specialized agency for the safety and security of shipping and the prevention of marine and atmospheric pollution by ships, a non-exhaustive list of invasive species and effects caused by the invasive species can be found, at: https://www.imo.org/en/OurWork/Environment/Pages/Common-Hull-Fouling-Invasive-Species.aspx
A further serious problem, not well known yet, is spreading of diseases, such as ILA (infectious salmon anaemia), between salmon farming plants and further to other aquaculture facilities. For example, service vessels can transport disease from one plant to another.
A further problem is damage of antifouling coating when removing biofouling, since friction, chemicals and high-pressure flows may leach the active antifouling substances, such as copper, and wear the antifouling and any other coating. Spreading of biocides and microplastics are the results.
An additional problem is removal of biofouling inside pipes, tanks, on intricate areas on ships, on windmill structures, wawe power plant structures, cooling structures, anchoring systems, oil and gas installations, pipes, cables; on ships in dry docks or wet docks, and on areas subject to water spray, such as on windmill structures and support structures. Biofouling can be a step towards structural problems and operational problems, and can initiate corrosion on the structure. Removal of biofouling in said contexts is a current industrial problem without good solutions yet.
The objective of the present invention is to provide a system and a method contributing to reducing or eliminating some or all of the problems described above.
Summary of invention
The invention provides a system for subsea cleaning, distinctive in that the system comprises a two-dimensional array of laser units, or one row of laser units, providing full coverage of laser irradiation when moving the array in at least one direction along a surface to be cleaned for biofouling.
The system of the invention, comprising two or more lasers irradiating the surface to be cleaned simultaneously, apparently is novel and unique. The system of the invention is designed to be capable of irradiating at an intensity and coverage destroying the biofouling, rendering the biofouling harmless and dead, which in this context is referred to as cleaning the surface for biofouling. The detailed parameters and functionality required, as determined by extensive research and testing, is described in detail below. Research and testing indicate that the dead biofouling on most coatings, such as coatings based on silicones, will fall away quickly when the ship starts sailing, thereby the surface will be literally cleaned. However, biofouling rendered harmless by the system of the invention on coating based on metal-containing antifouling, such as copper, may require active steps for removal and collection.
The system and method of the invention can render harmless soft biofouling at thickness from start of formation up to at least 3, 4 or 5 mm thickness, research and testing have revealed. Thick biofouling, such as macrofouling containing calcareous species and algae seaweed, is not effectively made harmless by the system and method of the invention.
The system of the invention preferably is coupled to or integrated into an ROV (Remotely Operated Vehicle), AUV (Autonomous Underwater Vehicle), robot or craft/vehicle, capable of moving the array along a submerged surface. In other preferable embodiments, the system can be arranged on a manipulator arm, be operated with a quay crane, or is configured for operation by a diver, by having buoyancy elements integrated. For many preferable embodiments, the system of the invention is piggybacked on existing robot (ROV, AUV) systems or cleaning and/or collecting systems.
The array or row of laser units of the system of the invention has three main embodiments:
Firstly, comprising a laser array of at least 2 x 2 laser units, closely packed with each laser displaced from the neighbouring lasers by a suitable distance ensuring full coverage of the surface to be cleaned when moving the array along the surface to be cleaned. Preferably, every second row is displaced half the distance between laser units compared to the row above and below.
Alternatively, the displacement of laser units is slightly displaced for each row and/or column, for the full row in one direction or symmetrically about a midcolumn- or mid-row position in opposite directions.
Secondly, comprising a laser array of at least 2 x 2 laser units or a row of at least 2 laser units, closely packed and arranged resulting in full coverage of the surface to be cleaned in a single pass. For these embodiments, quadratic or hexagonal or otherwise shaped laser units are packed closely, adjacent element to element. This represents the closest type of arranging the laser units.
Thirdly, comprising a laser array of at least 2 x 2 laser units or a row of at least 2 laser units, packed and arranged, wherein some or all of the laser units comprise a spreading lens, resulting in that a full surface passed by moving the laser array over the surface to be cleaned is covered. For this embodiment, the effect or intensity is spread over a larger area for each laser unit, and the laser units are not necessarily closely packed or displaced for full coverage, since the spreading provides full coverage of the area to be cleaned.
The cleaning system of the invention also comprises any combination of two or three of the array general embodiments defined above.
A preferable embodiment of a system of the invention comprises a laser array with for example 16 x 9 rows, with displaced position between every second row in one direction, positive or negative half the distance between laser units in a row, providing laser effect controllable by moving the array along a surface to be cleaned at a specific speed, and/or controlling the laser intensity.
The number of laser units can be for example 9 in row 1 and 3 and 8 in row 2, wherein row 2 is displaced half the distance between laser units. The number in even and odd rows and/or columns must not be equal, ± 1 is feasible.
An array of lasers means at least 2 x 2 lasers or laser units arranged side by side, as at least 2 rows and 2 columns. The number of rows and columns are preferably larger, such as 2x2, 3x3, 4x4, 5x9, 6x12, 16 x 9 and any other combination not resulting in being too heavy and/or large to handle by an robot (ROV, AUV) or otherwise. As integrated with a robot (ROV, AUV), the width of the array preferably is equal to the width of the robot (ROV, AUV), for example the array is 0,5 m wide and 0,3 m long. The number of column x number of rows is determined by the desired efficiency based on the case.
Full coverage of the laser array means that any point under which the laser array is positioned or moved over, in at least one positive or negative direction, is subject to laser radiation at an accumulated intensity sufficient for the purpose of killing the biofouling by the laser radiation. What this means will be clear from the detailed description.
Moving the laser array means moving along the surface to be cleaned.
However, for the second and third main embodiment, moving also includes positioning the array successively over areas to be cleaned and irradiating for a prescribed period, for providing sufficient irradiation for the purpose.
Embodiments with only a row of laser units arranged closely with coverage between laser units, possible for the second and third main embodiment, must be moved transverse to the row.
The system may include removal and collecting of the biofouling. For ships travelling in local waters, the irradiated and thereby dead biofouling can probably be allowed to fall from the ship hull when the ship is sailing, since there will be no risk of spreading of invasive species or diseases.
In some embodiments, the dead biofouling is removed and collected, for example by high pressure nozzles and/or brushes and/or vacuum suction, preferably including sucking in the killed, loosened biofouling for collecting and possible later use as fertilizer or disposal.
Power is electric power via an umbilical, preferably a combined handling and power umbilical. Alternatively, power is by batteries, such as Li batteries operatively coupled to or integrated in the system/ROV/AUV.
The operation is with an ROV, an AUV, a manipulator that can be on quay or on a vessel, by a quay crane, by divers, by operators in a dry dock or wet dock, or by other means, including fixed installations irradiating for example critical parts of pipes or equipment, for example irradiating subsea coolers for preventing biofouling and reduced efficiency. A typical robot (ROV, AUV) is about 0,5 x 0,7 m and weighs about 30 kg and is an observation robot (ROV, AUV) or a light work class robot (ROV, AUV), by robot (ROV, AUV) terminology.
The system, in a typical embodiment, operatively arranged to the robot (ROV, AUV), weighs about 20 kg, resulting in about 50 kg weight in air but far lower weight submerged. Larger systems for large ships can be much larger and heavier, while smaller systems for coastal use, use from quays, and for covering intricate geometries can be much smaller and lighter for increased access and dexterity.
The laser used for the system and method of the invention can in principle be almost any laser small enough and with sufficient intensity, and for marinizing into laser units in an array of lasers, as described in the detailed description. The lasers of lowest cost and best availability are blue laser diodes (LD), which therefore are preferable. A supplier is for example Nichia, of Tokushima, Japan.
The laser units are marinized for withstanding at least 3, 5 or 8 bar pressure, or higher pressure, as small gas filled, liquid filled or vacuumed containers with cooling as required and with common or individual intensity control. For large depth operation, the marinization can include liquid filling and pressure compensator, enabling operation down to hundreds of meters of depth.
Research has revealed that the irradiation by the system and method of the invention destroy the photo destructible pigments within the cells. More specifically, the laser attacks the phycoerythrin pigment. The effect can be observed visually since the biofouling changes colour when rendered harmless.
The invention also provides a method for cleaning surfaces immersed in water, distinguished in that the system of the invention is operated for rendering the biofouling harmless by laser radiation, and then, optionally removing and/or collecting the dead biofouling.
Brief description of drawings
Figure 1 illustrates a typical laser array and laser unit of a typical embodiment of the invention,
Figure 2 illustrates a typical laser unit and a longitudinal section thereof, of a typical embodiment of the invention, and
Figure 3 is a flow chart illustrating the laser array of a system of the invention.
Detailed description of the invention
Figure 1 illustrates the laser array 2 of a typical embodiment of the system 1 of the invention, as seen from underneath, and with one laser unit 3 highlighted for illustrating more details. When moving the laser unit array along positive or negative y direction, full coverage is ensured. A spreading lens 4 (not visible on Figure 1) on each laser unit 3 ensures that full coverage is achieved also in the transverse x-direction. The laser diode 7 is in this embodiment an array of laser diodes but can be a single laser. The result is a laser array according to a combination of the first and third embodiment as described above. This represents a preferable embodiment of the laser arrays, for many systems of the invention, since the investment cost is relatively low and the cooling requirement is reduced compared to closer arranging of the laser units. But more time for irradiation/lower speed of the robot/higher intensity of lasers/shorter distance between laser and biofouling can be required, compared to closer arranging or packing of laser units, but all parameters are controllable.
Full coverage means that no area irradiated has received 0% of the intended irradiation dose, while overlapping radiation by neighbour laser units is less than 5%, 3% or 1%, of the intended accumulated dose of irradiation.
Figure 2 illustrates a typical laser unit 3 as viewed from the side and a longitudinal centre section thereof, of a typical system embodiment of the invention. The piping 5 of the cooling system is visible, running through the mounting of the laser unit. Visible are also the laser array, lens 4 and mirror 6 on top. In the longitudinal section, the spreading lens 4, typically a quartz spreading lens designed for the purpose, is easily recognizable. The laser diodes 7 themselves are in this embodiment also an array, but an array of laser diodes, closely arranged. Laser array diodes, laser diodes or other lasers feasible for the purpose are available commercially. Also, spreader lenses and feasible housings can be custom designed and built for the purpose, or can be found as commercially available components that can be arranged together by a person skilled in the art.
In most embodiments, the array is coupled to or integrated in a typical robot (ROV, AUV) system for the purpose. The speed of the robot (ROV, AUV)/system of the invention on the surface to be cleaned, and the intensity, is controlled to ensure that the accumulated irradiation will be within the prescribed range for rendering the biofouling harmless while at the same time not damaging the antifouling coating. The distance between the laser array and the biofouling on the surface to be cleaned is set to be constant by distance wheels on the robot (ROV, AUV), or by other means. Each laser unit is controllable within 1 – 200 w, preferably.
Figure 3 is a flow chart illustrating the laser array of the system of the invention. Positioning data is gathered from the robot (ROV, AUV). This information is the positioning and the speed of the robot. Biofouling detection is information on what kind and how much biofouling that is in front of the laser system, as visually observable. Norsjór effective cleaning matrix is a matrix that gives out the most effective cleaning parameters without damaging the antifouling. The parameters are radiation time (robot speed) and laser intensity. The Controller is the brain of the system. It uses information from the biofouling detection, positioning data and Norsjór effective cleaning matrix to control laser intensity (voltage) and robot speed. Power supply delivers electricity to the laser arrays. The power supply has a voltage regulator to control the intensity. The water cooling system is in the typical embodiments a closed looped water cooling system that cool down the lens system and blue laser diode arrays. Blue laser diode array comprises a number of blue diode laser units arranged as an array and connected in a parallel. The number of diode units of the arrays is not necessarily fixed but is found for each case. The number of diodes corresponds to the efficiency of the system. Lens system are preferably included and are lenses that collimate the laser beam.
Laser intensity, accumulated when the array of the system is moved over the surface area to be cleaned, is sufficient to kill/damage the biofouling, but below a limit causing damage/leaching to the antifouling/coating system.
The laser units preferably have wavelength 420-499 nm, more preferably 430-470 nm, most preferably 450 nm, since 450nm window is a window that lets laser light through with minimum optical power loss.
The laser units preferably have intensity 1 – 200 w, preferably adjustable. This will enable killing/destroying/making harmless soft biofouling in thickness 0,0 – 8 or 5 mm, and the intensity, together with the specific coverage and speed of moving for the specific system, will enable the intended effect, as found by both research and testing.
The observed actual biofouling thickness estimate, based on visual inspection by the ROV/AUV or otherwise, and time since last cleaning, preferably is used as input for determining the required combination of intensity and speed for the radiation by the system of the invention.
Preferably, the system and method of the invention are used proactively, meaning frequent cleaning of biofouling with small thickness of biofouling, in the range 0-3 mm. Preferably, the intensity and speed is set so as to ensure that in any areas of 0 mm biofouling, the antifouling coating is not damaged.
It is possible to find feasible operation parameters analytically, but usually after much research and testing. Antifouling coatings vary, type and thickness of biofouling vary, and it is in practice easier to build up/calculate an operation method matrix as part of the Controller. To calculate intensity of the laser and radiation time multiple research attempts are applied in a matrix. Maximum intensity and radiation time is found through testing different antifouling coatings to find the destructive threshold. Each coating will have its own individual maximum destructive threshold to not harm the coating. Efficiency results (shortest time to lethally harm the biofouling) from various biofouling tests with variable parameters such as laser intensity, radiation time and biofouling thickness will create the matrix for most effective cleaning.