一种船体清刷ROV的设计

Design of a Remotely Operated Vehicle (ROV) for Underwater Ship Hull CleaningLee Min Wai Serene1, Koh Cheok Wei2AbstractA remotely operated vehicle (ROV) was designed with the purpose of ridding ship hulls of fouling organisms such as barnacles and zebra mussels. This initiative aims to resolve the problem brought about by the ban on current tin-based antifouling paints. In the design of the ROV, various considerations such as applicability, efficiency, safety, resources and cost were taken into account. A scaled architectural drawing was generated to depict the ROV structure and dimensions. Based on this design, a prototype can be constructed to determine its feasibility.Introductory BackgroundBarnacles pose a huge problem to ships by attaching onto the underwater hull structure through an adhesive protein polymer. This results in additional weight and an increase in frictional drag as the underwater hull, propellers and shafts become rougher [1]. This translates into slower speed, lower fuel efficiency and longer travelling time. At present, the most common solution is to coat the surfaces with antifouling paints. Antifouling paints contain biocides such as copper oxide and Tri(n-butyl)tin (TBT), which are released from the paint surface to inhibit the settlement of organisms. However, the International Maritime Organisation Antifouling Systems Convention (IMO-AFS) has specified a ban on the application of TBT antifouling paints to vessels from January 1,2003 and will phase out the presence of TBT antifoulings on ships’ hulls from January 1, 2008 [2]. Hence, TBT antifouling paints can no longer be used in future.Alternatively, silicone-based and fluoropolymer-based non-stick coatings are able to weaken the adhesive bond between fouling organisms and the hull. The organisms will slip off the coated surfaces as the vessel moves through the water. Although these coatings are non-toxic and in compliance with IMO standards, they are not very effective for slower ships as the force acting on the organisms is not sufficient to dislodge them [3]. However, propellers and shafts are usually left unpainted, due to their high-speed rotation, which prevents effective adhesion of the paint. As a result, the lack of protection by TBT paint in the surrounding water leads to more severe fouling on these areas, resulting in reduced speed and lower fuel efficiency.Possible Solutions and ApplicationsAt present, there are various antifouling applications, including the use of a sonic vibration system and the application of electric potential difference. The former involves resonators which produce a low frequency shield around the ship hull that eliminates possible attachment of marine growth onto the hull and running gear [4]. However, as the resonators can only be positioned inside the hull, the shield is less “concentrated” around the propeller-shaft region, rendering the antifouling mechanism less effective. The electrical method involves introducing an electric current through the structure, transforming chloride ions by electrolysis into hypochlorite, which is highly toxic to fouling organisms. Although hypochlorite decomposes rapidly in water, mildly toxic halogenated by-products are created in the process [5]. Paint trials for ship propellers and shafts with antifouling paints with low biocide content are currently underway, but its effectiveness is still unknown due to the rotation of the running gear which causes the paint to wear off rapidly._______________________________1 SRP student, Raffles Junior College, 53 Mount Sinai Road, Singapore 2768802 Naval Logistics Department, HQ RSN, MINDEF Building Gombak Drive, Singapore 669645Taking these factors into consideration, existing fouling prevention methods are not fully applicable. Hence, the only solution is to take the corrective approach through frequent maintenance in the form of underwater hull cleaning. Current practice involves upslipping the ship in a dry dock to have her underwater hull cleaned. This incurs high docking charges and maintenance downtime, which will affect the efficiency of the shipping schedules. As a result, a more effective method is to perform the cleaning in water with an ROV to facilitate the job, which is commonly carried out by divers. This would also reduce the manpower required to only one person for the control of the submersible from the wharf. Moreover, the elimination of the use of divers would mean that human safety is no longer a pressing concern.Principal Design ConsiderationsThe aim for this ROV is to carry out underwater hull cleaning in the most efficient and safe manner to reduce downtime and cost. The principal considerations are as follows:1.Manoeuvrability in confined space2.Efficient cleaning process for irregularly shaped structures (propellers and shafts)3.Watertight integrity of the system4.Visual inspection capability5.Cheap to constructROV DesignThe ROV is a small submersible connected via a main cable to a remote control unit on the wharf. The camera on the ROV will provide the input to the display for the single operator to inspect hull and shaft conditions, and observes the cleaning process while manoeuvring the submersible underwater.StructuralThe main body of the ROV consists of a cylinder (300mm in diameter; 1000mm in length) with a hemispherical front to reduce the presence of sharp corners for safety reasons, as well as to reduce drag. Its small size is for the ease of navigation in water, minimising possible collision with the propellers, shafts or any protruding appendages of the underwater hull during the cleaning process. Internally, there are web frames at 200mm interval to support the connection cables.There are two horizontal side wing plates (100mm x 650mm) designed to increase the stability of the submersible, as small ROVs are prone to “rolling motions” due to underwater currents. This improved stability will enhance the vehicle-control in its neutral buoyancy state, thus facilitating the cleaning process. In addition, the wing plates provide sufficient moment arm for effective manoeuvre, which will be discussed in the mechanical aspect.There are also two supporting plates at the bottom of the submersible inclined at 45˚ to serve as supports for the submersible when recovered on land. They also act as additional stabilisers to prevent unnecessary roll caused by currents.MechanicalThere are altogether 4 sets of propulsion systems with2 positioned on the end of the side wings and the other 2 onthe main body. Their positions are determined based on the 6Degrees of Freedom (DOF) of movement required for itsmanoeuvres in water. (Refer to Table 1 for the 6 DOF motionsand the respective controls) Each set of propulsion systemconsists of an electric motor, shaft and propeller. The electricmotor drives a short length of shaft maintaining sufficientroom for the development of flow generated by the propellersin the reverse direction. This would prevent stalling due to turbulent flow in the region. However, if the shaft is too long, it will result in off-axial movement of the propeller causing irregular movement of the ROV.To maintain watertight integrity, a combination of o-rings and grease can be used to seal the gap between the ROV body and the rotating shaft. Silicone is also used to seal any gaps between joining sections during construction. Watertight integrity is important as it maintains the buoyancy and stability of the ROV, and prevents water ingression which will damage the internal electrical circuitry.Table 1: 6 DOF motion and propeller controlsThere are 2 sets of brushes catered for effective cleaning of the hull, propellers and shafts. The forward brush in the shape of a hemisphere that protrudes from the base of the submersible is for cleaning the propeller blades and curved hull surfaces. It is shaped in this manner to maximise contact surfaces and to reach awkward areas such as the curvature of the skewed propeller blades. The other pair of brushes on the topside is for cleaning the shaft, which is inaccessible to the forward brush. As viewed from the aft (Fig. 1), there are only bristles on one side (sides D &E) of the brushes. The reason for such design is to maintain a channel for aligning the shaft between the brushes. The opposite side (sides F & G) of the brush bristles is trimmed to the shape of the shaft, such that the entire circumference is covered when the arched bristles are face to face with each other (Fig.2). This will ensure every part is cleaned, as the ROV glides along the length of shaft. For the actual model, the brushes are interchangeable with the appropriate sizes to fit shafts of different diameters.Fig. 3ElectricalA camera is situated at the forward part of the ROV for the purpose of manoeuvres and inspection. It is equipped with a panning angle of 120º to allow a sufficiently wide field of vision to observe the cleaning by both sets of brushes. 2 underwater lights are situated on both sides of the camera to provide illumination for the navigation of ROV as well as the positioning of the brushes and inspection of surfaces. The images from the camera are piped to the display through the connection cables to the surface.In the remote control unit, a total of 4 double-throw switches areused. An example is illustrated in Fig. 3. The switch is made to pivot at points C and D, which allows a change in direction of current applied across the motor. This causes the motor to rotate either in the clockwise or parallel for the 4 propellers to achieve the desired manoeuvrability. Thereare also 2 switches for activating the brushes in the control unit.Electrical Block DiagramMaterial Requirements(For scaled model)S$)(in Materials/Parts Purpose Cost Camera With panning ability to enable operator to guide$80submersible2 lights For navigation in water $72 x 2 = $1443 brushes For cleaning of the hull, propeller and shaft$7 x 3 = $21surfaces$20 x 4 = $804 propellers with shafts For propulsion and descending/surfacing ofsubmersible7 motors 4 to power propellers, 3 to power rotating$ 40 x 7 = $280brushes for cleaning$20 x 2 = $402 remote controls Each to control 4 propellers in clockwise andanti-clockwise directions2 switches 1 for forward brushes, 1 for topside brushes $1.50 x 2 = $3Wires For the electrical circuitry $20Perspex For main body and appendages $200To maintain watertight integrity of the structure $20O-rings, grease andsiliconeTotal: $888ConclusionIn conclusion, the above ROV design integrates the basic considerations of submersibledesign with its main purpose of facilitating underwater hull cleaning. This design paves the way forthe construction of a prototype in future, which will be tested in water to evaluate its effectiveness andfeasibility. It is hoped that such an ROV can be developed as a cost-effective interim solution toalleviate problems brought about by marine fouling.AcknowledgementsI would like to express my heartfelt gratitude to my mentor, CPT Koh Cheok Wei, for hisguidance throughout the course of this project; the Naval Logistics Department for providing thenecessary facilities to generate my project design, and last but not least, everyone else who has helpedme in one way or another.References[1]Singapore Zoological Gardens. Coral Reef Creatures – Barnacles. /ff/f-reef7e.htm. (May 1, 2003).[2]Brown, Jim. 2002. Propeller International. Issue 14, August 2002. International Coatings Ltd,U.K.[3]Intertidal Environments. .au/BoZo/backwell5/intertidalp.htm. (April 18, 2003).[4]Barnaclean Seas Company. Barnaclean Sonic Anti-Fouling System./barnaclean.html. (May 1, 2003).[5] Watermann, Burkard. 1999. Alternative Antifouling Techniques (Present and Future).LimnoMar©, Hamburg.。