CDQM, Volume 11, Number 3, 2008, pp. 61-66 COMMUNICATIONS IN DEPENDABILITY AND QUALITY MANAGEMENT An International Journal UDC 629.5.024:620.191.35 Exploitation roughness of the hull and its influence on the ship s resistance Lazo Vujovic 1 1 Faculty of Maritime, Dobrota 36, 81330 Kotor, Montenegro E-mail: vlazo@cg.ac.yu accepted February 11, 2008 Summary Exploitation roughness is by its origin divided into one that is a result of degradation of the underwater coating, of a plating corrosion and mechanical damages of the plating, and the one that is a result of the plating fouling. The first one is difficult to control so it continues in the course of time and remains an irreversible process. As far as fouling of plating is concerned, this process is being tried to slow down by means of biocide coatings which emit poison as well as other ways. However, the effect of such a coating is short termed while all other techniques are of a limited range. For this, the ship must be docked and her underwater part is to be mechanically cleaned (scraped off). The paper deals with the process of overgrowth of the underwater part of the hull and the procedures aimed at assessing the range of overgrowth which in this respect affects exploitation roughness and ship s resistance as well. To maintain her speed, the ship must increase power of her driving engine, the result of which is an increase in fuel expense. These negative consequences of overgrowth can be removed by maintaining underwater part of the hull, i.e., by removing overgrown layers, as elaborated in this paper. Key words: Exploitation roughness, anti-corrosive coating, shells, fuel saving, preventive maintenance, hull cleaning. 1. INTRODUCTION Underwater part of the hull is exposed to overgrowth. i.e., live sea organisms settle on its surface. Hull overgrowth results in different types of roughness. The type of roughness depends on the character of overgrowth and the way the sea organisms are allocated on ship s hull. The term type of roughness refers to characteristics of overgrowth contents, density of overgrowth and distribution of the organisms over the ship s hull surface. Character of overgrowth refers to features of mutual distribution of the organisms on the surface. Although the progress of roughness 61
is not only the result of overgrowth, it its major cause. Besides, in the steps taken to maintain underwater part of the ship, roughness can be in major part tackled. 2. FOULING OF THE SHIP S HULL Major problem which occurs in the ship s exploitation is fouling of her underwater part by sea organisms. Despite the fact that every ship is vulnerable to fouling, the influence of fouling is different for each ship. It varies according to the type and size of the ship and can be illustrated as ratio between Froude s number and carrying capacity for different conditions of underwater part of the ship Figure1 [4]. Figure 1. Statistic approach to the relation between Froudo s number and deadweight capacity for different conditions of ship s underwater part overgrowth The type of ship s trading, route, operating ports and spent time will affect the ability of avoiding fouling process. The average time of fouling formation is 9 months and depends on the alkalinity of the toxic paint. Figure 2 shows the process of initial appearance and penetration of shells under the conditions of weakened protective film. The ship leaves the shipyard with the hull cleaned from overgrowth and protected with anti fouling coating, as illustrated on Figure 2.A. After some time, shells start to dwell on the surface of anti-fouling coating, as illustrated on Figure 2.B. In this phase of hull overgrowth, cleaning should be carried out. However, it is not always feasible. If cleaning fails to be undertaken in this phase, the shells overgrow and eat anti-fouling coating, as illustrated on Figure 2.C. Ingress of water in this zone causes lifting of the protective film of the anti-corrosive painting and bubble occurs. Certain shells die at this stage while the others continue to penetrate and end as given in Figure 2.D, where the shell passes the anti-corrosive coating and came in contact with the steel. Marine fouling progress differs with small, i.e. large ships. Sun exposure encourages fouling process. The higher the sea temperature, the quicker the process of fouling is. Flat bottoms of the ships with big draft lie in colder and darker water, therefore, the process of fouling of their surfaces is slowed down. Scope of cleaning in this case is based on cleaning side plating to the bilge keel [2]. 62
Figure 2. Formation and penetration of the shells There are different ways to assess the level of ship s fouling. Russian authors Domski and Dehtjarev assess ship s fouling as follows [3]: Minor fouling live organisms cover up to 20% of underwater surface, layer thickness up to do 2 mm, Medium fouling between 20 an 70 % of underwater surface, layer thickness more than 2 mm, Large fouling live organisms cover more than 70 % of underwater surface, layer thickness exceeds 10 mm. Other russian authors Baljakin and Revin divide the level of ship s fouling in three categories: I fouling in the form of bacterial mucous film and water overgrowth up to 100 mm long, II fouling with shells 10 mm in height and water overgrowth up to 200mm long, III fouling with shells from 10 do 30 mm in height and water more than 200mm long. It is obvious that, from the terotechnological aspect, pure leveling of fouling is not enough. In this view, distribution on the plating must be differentiated as well as influence of the various types of organisms on ship s resistance, that is, decrease of speed. For example, the layer of mucus, consisting of water bacteria, detritus and mud, hardly affects the increase of resistance so this kind of fouling in technological sense can be neglected. In this sense, we should consider the characteristics of prevalence of microorganisms over the hull, i.e., organisms with dimensions approximating 1 mm or more. The knowledge of roughness due to fouling and its typology imply that it, as a final occurrence, belongs to accidental (stochastic) processes, very complex ones. The shape of roughness, their length and mutual distance are entirely independent. Besides, the distribution of roughness is subjected to the laws of normal (Gauss) function. In addition, the probability of roughness distribution over the ship s hull surface corresponds to Poison s process. Therefore, it is evident that dealing with the problem of fouling in the present phase hasn t reached necessary simplicity for practical anticipations and so to become useful support in terotechnological approach. 3. CHANGE OF DEPENDENCE OF POWER ON SHIP S SPEED IN FUNCTION OF HULL S FOULING Fouling of the underwater part of ship s hull, imposes increased resistance. Under conditions of increased ship s resistance, the power of driving engine must be increased as well, so that the ship 63
could keep the intended speed. Otherwise, the ship will lose her speed and have delays in performing shipping operations. This has negative effects on economic, performance of the ship. Increase of power in function of ship's speed is presented on Figure 3. Increase of power, on the other hand, imposes increased fuel consumption, and this, again, economic performance of the ship [1]. Figure 3. Dependence of power on the speed in function of hull s fouling under the same capacity of ship for tanker of 35.000 TDW A trial drive; maximal speed up to 16 knots, B drive after 6 months, before annual docking, C upon driving of 9 days, with performed sanding up to the very metal, and applied coatings in accordance with modern systems of protection and painting Exploitation by curve B will impose drastic increase of fuel consumption. If main propulsive 3 engine is of a diesel type and navigation is performed according to the propeller law P 1 1 then the load is in the zone of propeller, or in the forbidden area. n = P2 n 2 Consequence of such a work is constant overload of engine above the limit of nominal medium effective pressure and permitted load of bearings. This results in frequent averages, increased maintenance, high cost of spares, materials, increased work of crew, delays and stoppages of ship s performance and increase in ship s exploitation expenses. Paintings manufacturers tend to improve coating efficiency. Materials used in composing such protective coatings are very expensive and applied exclusively while the ship is in dry dock. Because of this, application of underwater cleaning is favorable also in terms of prolonging the time between two dockings. Periodical maintenance will decrease exploitation costs and reduce potential possibility of expensive damage to the steel hull. Such periodic inspections will also allow ship-owners to estimate the condition of hull and plan their stay in dry dock. 64
4. FUEL SAVING Fuel saving is the most important factor for preventive hull maintenance. Fuel expenses make 60% of the ship s exploitation costs, while this percentage is larger in container ships. Research covered 400 ships (tankers) of different carrying capacity and different driving engines. The results of research have been classified and presented on illustrations 4 and 5. Figure 4 shows variation of the speed at constant fuel consumption, with alternative cleaning of the hull. Figure 5 shows increased fuel consumption so as to keep constant speed of 15 knots, with and without hull cleaning [2]. Figure 4. Variation of the speed at constant fuel consumption, with alternative cleaning of the hull; x 1 speed without hull cleaning, x 2 speed with hull cleaning Figure 5. Increased fuel consumption so as to keep constant speed of 15 knots, with and without hull cleaning; y 1 fuel consumption without hull cleaning, y 2 fuel consumption with hull cleaning Within 24 months, there is tremendously high saving of fuel comparing ships with and without hull cleaning at a constant speed of 15 knots. Example of saving fuel for two years period: 65
- Tanker 50.000TDW (diesel engine) saving 1.900 t, - Tanker 50.000TDW (steam turbine ) saving 2.500 t, - Tanker 200.000TDW (steam turbine) saving 3.200 t. 5. CONCLUSION Underwater part of the ship should be constantly maintained in terms of removing overgrown parts whether in water or dock (more efficient but also more expensive method), so as to reach the following positive results: - Reducing fuel expenses cca 500.000 $USA during two years period of ship s exploitation (savings depends on ship s size and type of main propulsive engine), - High-quality maintenance of the hull, underwater sea intakes, propeller and rudder, - Protecting steel plating of the ship of corrosion, - Reduced overcapacity of diesel engine as main propulsive engine, - Lessened need for main ship s engine maintenance, reducing costs incurred by spare parts, material and manpower, - Maintaining constant ship s speed at a same guaranteed rate of fuel consumption. REFFERENCES [1] Vujović, L. : Maintenance of the ship s underwater part, XXIV Montenegro s Conference of Maintenance Working Resources, Budva, 2001 (in Serbian). [2] Antunović, I.: Review influence fouling of the ship s hull and propeller on increase power source necessarly for constant speed, Brodarski institut, Zagreb 1980 (in Serbian). [3] Tokidzo, M.: Vlianie obrastanija na skorost sudna, CNIIMF, Dalnevostočnii filiial, Vladivostok 1975. (in Russian), [4] Lovrić, J.: Elements of the ship s terotechnology, Maritime Faculty - Dubrovnik, 1989 (in Serbian). 66