Measuring Hull Resistance To Optimize Cleaning Intervals

Transcription

1 Measuring Hull Resistance To Optimize Cleaning Intervals Torben Munk, Propulsion Dynamics Inc. (PDI) Marine fouling on the hulls and propellers of ships increases resistance (drag), resulting in higher fuel consumption to maintain speed. Ships are responsible fo r approximately 5% of the global oil consumption, and a considerable amount could be saved by cleaning the ship s underwater hull and propeller at optimum intervals. Ship owners are conscious of the costs incurred by marine fouling on vessel hulls and of the costs of drydocking to apply antifouling coatings. They are also aware of the wide range of antifo u l- ing coating technologies that will give up to 5 years protection depending on the technology and vessel s pattern of operation. However, owners are not coating specialists, so they must rely to a certain extent on the coating suppliers to advise 46 them of the best antifouling for a given vessel and how to apply the coating. S i m i l a r l y, many ship owners are not aware of the precise impact that fo u l i n g has on vessel performance, owing to the inherent limitations of perfo r m a n c e monitoring systems currently used. This article describes a novel method of monitoring the performance of ships, based on the standard measuring equipment onboard (in conjunction with vessel design and sea trial data). The article will also give examples of the results achievable with the novel method as well as measures a ship owner can take to improve fuel conservation, mitigate performance losses, and benchmark the performance of hull coating systems. Background on Ship Performance As background to the monitoring technique, it is necessary to have a baseline from which to work. This baseline can JPCL / July 2006 / PCE conveniently be the power versus speed curve provided fo r each vessel on delivery from the shipyard. For most ships delivered, there is a diagram showing the relation between speed and required power for one or more loading conditions (Fig. 1). The diagram has been prepared based on theoretical calculations and in most cases has been confirmed by model tests and by a speed trial immediately before delivery. This speed trial is complicated and time consuming. The ship must be loaded correctly; the weather needs to be reasonably good; and the trial has to take place in a test area with deep water at a time when there is no other immediate traffic. Time must be given to accelerate the ship up to a constant speed and, because a sea current may be present,

2 Photos courtesy of Muldoon Marine, Inc., Long Beach, CA Speed Knots Fig. 1: Power versus speed Speed Knots Fig. 3: Power versus speed, trials The actual marine fouling for any particular ship may be worse. The condition will be discovered only if the fouling is significant, because it is very difficult in practice to get a reliable and reasonably accurate picture of the speed/power performance of a ship in service. Speed Knots Fig. 2: Power versus speed, service each speed run has to be made twice, in opposite directions, to compensate for the current. Consequently, only a limited number of draft/speed combinations are tested, so the achieved speed/power results properly adjusted for temperature, s a l i n i t y, weather, and draft differences are used only to confirm or adjust the already existing diagram. If the engine s maximum continuous rating (MCR) is plotted in the diagram of power vs. speed, the maximum speed for the ship may be found (Fig. 2). Ship owners know that they cannot expect the speed in Fig. 2 in daily operation. For commercial considerations, ship o w n e r s define a service speed. This service speed is traditionally found by adding 15% to the power curve and subtracting 15% from the engine power line as shown in Fig. 3. The 15% added power is expected to consist of 5% for speed losses caused by weather and 10% for speed losses caused by surface roughness resulting from marine growth and corrosion on the hull and propeller. Degradation of the Performance The main reason for performance degradation is marine growth on the ship s hull. (This subject has been treated thoroughly in the technical literature, for instance in Ref. 1.) Here it shall only be mentioned that ship owners are allocating a lot of time and money to prevent or mitigate the degradation. The main remedies are various types of coatings applied in drydock to the underwater part of the hull at regular intervals, and, in some cases, in-water brushing of the hull and polishing of the propeller. The total costs of all ship owners measures to prevent fo u l- ing are approximately 1.5 billion USD per year, or approximately 5% of the total spent on marine fuel oil. Unfo r t u n a t e l y, it is difficult to determine if this money is invested optimally. There are many different types of hull treatments, and the price for the coatings varies greatly. In addition, each ship owner has his own way of handling coating selection and maintenance. Furthermore, it is difficult to evaluate and compare the effect of the different hull treatments, unless reliable methods of performance analysis are available. Monitoring of Ship Performance Most ship operators have established a procedure for monitoring speed vs. power, for instance, by measuring the daily fuel consumption and the daily distance covered. In this way, the JPCL / July 2006 / PCE 47

3 Fig. 4: Plot for a well-maintained container ship daily mean power and mean speed may be calculated, and the result may be plotted in the speed/power diagram for comparison with the trial trip results. Unfo r t u n a t e l y, results achieved in this way usually scatter so much that it is impossible to conclude anything directly from such a diagram, as may be seen from Fig. 4, a plot for a well-maintained container ship. Procedures may also have been established for more precise measurements with longer intervals, for instance once a month. A day with nice weather may then be chosen. In such cases, and where the prime mover is a slow running diesel engine, the power may be measured more accurately by cylinder pressure, and speed may be measured over a set period such as two hours at constant power on a constant course. The result of such an exercise will be more accurate than one based on noon data (obtained through daily observance at mid-day under nice weather conditions). However, even such monthly results may scatter to an extent that an accurate service speed prediction may be difficult or impossible. (Although underwater inspections of the hull are useful supplements to speed and power measurements, the inspections do not provide a meaningful relationship between surfa c e roughness and impact on vessel performance. For more info r- mation on the biological aspects of hull coatings, see Ref. 2.) Days for development of added resistance Fig. 5: Added resistance developed slowly at <0.5% month Influences on the Monitoring of Speed and Power There are many reasons why the values directly obtained from speed/power are scattered in Fig. 4. The main factors are described below. 1. Drafts. Mean draft and trim (the depth of the vessel in the water and how level the ship is sailing, respectively) have a great influence on a ship s resistance. It is reasonably easy to adjust the results for differences in mean draft, but differences in trim are more difficult to address, especially when most ships today are equipped with a bulbous bow. 2. We a t h e r. Wind and waves can seldom be totally ignored; t h e r e fore, the results of monitoring speed and power will need to be corrected accordingly. It is not that difficult to measure and make corrections for the wind, but waves can neither be measured nor easily corrected fo r. 3. Sea current. Today the speed over ground may be measured with great accuracy by means of a global positioning satellite; however, the true speed of a ship, the speed through the water, is more difficult to measure because of the sea current. The speed log not only measures the ships speed underwater at the hull but also measures other water movements, which can result in an inaccurate measurement. Normally, it is not possible to correct the speed for sea current without performing a reciprocal run, which is usually considered too timeconsuming during commercial operation. 4. Temperature and salinity. These two factors do have some i n fluence on the result, but they are seldom taken into account. 5. The lack of method for interpreting the results. Even if reliable speed/power values, corrected for all the above-mentioned factors, are obtained and plotted in a diagram of power vs. speed, it may be difficult to accurately describe the degradation of the performance. The reason is that the ship s resistance may be roughly divided into frictional resistance and wave-making resistance. The fouling influences only the frictional resistance, and the portion of total resistance that makes up the frictional resistance depends on the speed and the draft. Proposed Measure for Performance Degradation The effect of hull resistance on propulsion performance is difficult to describe. The primary effect is that more water is dragged forward along with the ship, which will increase the resistance of the ship. The increased forward velocity of the water in the ship s boundary layer will also cause the infl o w velocity to the propeller to be reduced. This reduction has several effects. The efficiency of the propeller will decrease, but some of the power lost in the boundary layer will be re-gained. The total required power will increase, but not by as much as the resistance. Since it is not possible to state a fixed relation between added resistance and added power, for simplicity it is proposed to use the added resistance as a measure for degradation and not the added power. 48 JPCL / July 2006 / PCE

4 Even a description of the hull degradation in the form of the added resistance as a percentage of the total resistance is ambiguous, unless one designates the speed and loading condition at which the percentage is valid. It is therefore further proposed to refer the added resistance to the design speed and the design draft instead of to the specific operating speed and draft. This reference is not precise, but it works in practice and is quite useful, not only for evaluation of the condition of a single ship, but also for comparison of several ships, which not need to be of the same shape and size. The implication here is that different coating systems may be compared, even if they are applied to ships of different size or hull fo r m. It should, however, always be kept in mind that the added resistance as defined here is not equal to the actual increase of p o w e r. Even at design speed and draft, the increase of power will normally be a few percentage points lower than the added resistance. At deep draft and low speed, the power increase will be more than the added resistance, and in ballast condition at full speed, it may be less than half of the added resistance. However, it is always possible to calculate the actual power increase for any draft/speed from the fo u n d added resistance. Collection of Performance Data As mentioned above, performance data may be collected daily o r, in a more detailed form, within an interval of a month or so. Some ships have an automatic data logging system, which files observations continuously. In principle, any of these methods may be relevant and useful, as long as the observations are made carefully. These different methods do have their advantages and disadvantages. 1. Continuous data logging excludes all human errors, but some data, for instance wave data, are normally not available in this way. Further, this method produces a lot of data, which means that some kind of data reduction and data selection must be introduced with the system. Still, it is difficult to assure that only data for valid navigation conditions are further processed. 2. Daily observations, the noon-data defined earlier, are useful for some purposes if dealt with carefully. But daily reports can be used for performance analysis only if all conditions have remained unchanged during the 24-hour, noon-to-noon period, and this is seldom the case. 3. Monthly, detailed observations over a time interval of a couple of hours are normally as reliable as observations can be and are, therefore, quite useful. However, we will see that these observations cannot stand alone, but have to be treated t o g e t h e r. Twelve sets of observations a year are, therefore, too few to establish a reliable time history for the development of the added resistance for a ship. A reasonable solution seems to be a procedure involving weekly observations. This interval is short enough that the routines for collecting the data are not forgotten, and are long enough to avoid the temptation of repeating the last set of data instead of recording a new set of data. In addition, it is usually possible to find a two-hour period with constant navigation conditions within a time interval of a week, and approximately 50 observations per year is still enough for a detailed time history. Processing of Performance Data One way of processing the performance data is to compare the observed power and RPM values to those found for similar weather and loading conditions using a mathematical model of the ship s propulsion performance. Then you can determine the speed through the water and the added resistance at which the calculated values match the measured values. Both speed through water and added resistance are then determined. This method requires that such a mathematical model be available or be established. There are complicated, theoretical methods for the calculation of resistance, propulsion system p e r formance, weather resistance, and influence of hull roughness for a specific ship. But in practice, a simple and robust general mathematical model, which can be adapted easily to any ship, is needed. Such a model may be established by combining theoretical considerations and approximation fo r m u l a s with empirical constants. The number of empirical constants in a model, which is developed in this way, is quite high, but fo r t u n a t e l y, some of these values are valid for all ships or for large groups of similar ships. Other constants are specific for the individual ships. The value of some of these latter constants may be found by careful analysis of the tank test and/or trial trip results, whereas other constants can be found only by statistical analysis of a large number of performance observations fo r the ship in service. An example of a solution for processing performance data is C A S P E R (Computer Analysis of Ship PERformance). This is based on a general mathematic model: a build-up of wellknown, state-of-the-art elements for the calculation of ship resistance, propeller performance, weather resistance, etc. The general model, based on the type and main dimensions of ship and propeller, may stand alone and may be used directly fo r comparison to actual performance data, but a more reliable model can be established easily by adjusting the general model based on tank test/trial data. Even this model will not normally reflect all changes in the operational conditions, and the model is therefore not used fo r p e r formance evaluation until it has been adjusted further by means of a statistical analysis of a number of perfo r m a n c e observations. In general, sets of observations are JPCL / July 2006 / PCE 49

5 Days for development of added resistance Fig. 6: Plot showing ship with initial high resistance before sides and bottom of hull were brushed required for statistical analysis, and the model can then be used for performance analysis and predictions. The adjustment of the model continues as more observation data are received. N o r m a l l y, the basic constants of the model will remain unchanged after sets of observations, but the constants describing the condition of the hull and propeller resistance will be updated in real time as data from the ship is acquired. Accuracy of the Analysis In practice, the accuracy of the analysis depends more on the accuracy of the observation data than on the accuracy of the mathematical model itself. Experience shows that the actual added resistance as earlier described may be found with an accuracy of approximately ±1%, and that the result from a single set of observations will not normally deviate more than 3% from the mean value. The actual speed/power diagrams, which may be produced from the adjusted mathematical model, are t h e r e fore fully valid for all practical purposes (such as transport cost calculations, cost-benefit decisions for drydock treatment, coating selection, and maintenance intervals). Examples of Added Resistance Diagrams Several diagrams are used here to illustrate the method described above. The individual analysis results are shown, and a first order curve (a straight line) is plotted through the points to show the development. Figure 5 is a typical example of the development of added resistance. It is seen that the added resistance (after docking out) in this case develops very slowly, less than 0.5 % per m o n t h. The information obtained from these types of diagrams is discussed below with reference to Figs The ship described in Fig. 6 initially had a high added resistance, approximately 50%. When added resistance was discovered, the propeller was polished and the ship s sides were brushed, with marginal effects. The operator was advised to have the ship dry-docked, but because this was inconvenient at the time, he decided to brush the sides and bottom of the hull thoroughly a few weeks later. The result of this brushing was remarkable, but the antifo u l- ing was apparently depleted, so the result did not last long. The ship was dry-docked on schedule. After the dry-docking, the added resistance fell, then slowly increased until it exceeded 20%, at which time the hull was brushed. The ship described in Fig. 7 came out from the dry-dock with a remarkably high added resistance of 40%, which was constant for a period, then suddenly dropped. After dropping, the added resistance developed very fast (6% per month). A hull brushing removed approximately half of the added resistance, but the resistance developed fast again after the brushing. The ship gives a clear example of a poor treatment in the dry-dock. The explanation for the high, constant, added resistance (in Fig. 7) after dry-docking can only be that something adhered to the hull. It could be keel blocks, plastic sheets, or other objects, which may have been under the bottom of the ship b e fore docking out. Whatever adhered, it disappeared suddenl y, and the resistance dropped down to a usual development line. Assuming that something adhered to hull was the case, the added resistance after dry-docking was still high at ~16%, which indicates that not much treatment had been done (in dry-dock) to make the hull smooth. Further, the fast develop- Days for development of added resistance Fig. 7: Plot showing ship with high, constant, added resistance after dry-docking ment of the added resistance indicates a very inefficient a n t i fouling paint had been applied. For this ship, a hull cleaning at least every half year will be advisable to mitigate what would otherwise be even higher fuel penalties. The seven ships of similar hull form described in Fig. 8 came out of dry-dock between 100 and 500 days ago. The effect of the drydock treatment and coating performance is clearly shown for all seven ships. (Four have one type of antifo u l i n g coating applied, and the other three have a different type. Individual data points have been removed for clarity.) C o n c l u s i o n s Because this technology has been used for more than 10 years and on more than 100 ships before commercialization, we can draw some general conclusions from the results. 1. The added resistance (due to fouling of the hull and propeller) varies from around 6% to 80% in the worst cases. On 50 JPCL / July 2006 / PCE

6 Fig. 8: Diagram of coating perf o rmance for seven ships of similar hull design average, the added resistance for a ship is approximately 30%, if no special attention has been paid to the ship. a.) Roughly one-third of all ships are in good condition with added resistance less than 20%. b.) Half of all ships are in a reasonable condition, but could be improved, with an added resistance between 20% and 40%, and exhibit no unusual fouling pattern. For these ships, improvement in performance can be achieved by some standard maintenance procedures without interfering with the normal course of operations. c.) The remainder of the world fleet (over 10,000 dwt) is in poor condition, with the added resistance over 50%. 2. The basic hull treatment in the dry dock has a pronounced influence on the added resistance after the drydocking. (The lower the added resistance is, the better the treatment is carried out.) In the best cases, the baseline added resistance will only be 0% to 4%. A partial treatment in dry dock has been seen to result in an added resistance of 5% 20%, while in the worst cases there may be no effect at all from the dry-docking. 3. The type of coating has a pronounced influence on the development of the added resistance. It is also important that the coating is applied in correct thickness and that the dissolution speed (or, for self-polishing paint, the polishing speed) is carefully adjusted to the service speed and operational patterns of the ship. (For many ships under analysis, it has been found that the coating thickness and/or dissolution rate and polishing speeds were not fine-tuned.) With silicone coatings, the treatment in dry-dock is even more critical than with paint systems to ensure good adhesion. 4. Hull brushing between dry-dockings may have a remarkable effect, especially if one of the less active types of a n t i foulants has been used. Hull brushing may to a certain degree compensate for low efficiency of the antifoulant. It is advisable to clean the hull before the slimy layer of bacteria and algae has turned into a layer of seaweed. In that case, very soft brushes (for example softer than the bristles of a toothbrush) can be used, and the antifouling paint will not be damaged. This stage corresponds to approximately 12% of resistance added to the resistance value after dry-docking. At a later stage, harder brushes are required, and although they easily can remove the seaweed, they will most likely remove some of the antifouling paint, which may increase the added resistance after the brushing. With rigorous, established methods of analysis, the fo l l o w i n g will be possible. Evaluation of the efficiency of dry-docking treatment (gritblasting, water-blasting, robotic systems) and other emerging t e c h n o l o g i e s Tracking of the development of hull and propeller resistance for individual ships, and taking action when economically justified on a ship-to-ship basis. Evaluation includes the befo r e - a n d - a fter effects of hull cleanings, water-pressure cleanings, propeller polishing, and other treatments. Benchmarking the efficiency and true life cycle cost (total cost of ownership) of any coating system by comparing ships with different coating systems applied to the hull and/or prop e l l e r. Ships need not be identical in hull form. Experience has shown that at least 10% may be saved, on average, in fuel costs. For a ship that burns 100 tons of fuel per day, at least 10 tons per day may be saved. This represents a value of approximately $3,000 USD per day or approximately $800,000 USD per year. Looming on the horizon are higher bunker fuel prices (as of this writing, bunker fuel is $315 USD per ton), greater demand to reduce pollution, and the need for defining true life cycles of new coatings. It is in all parties interest that ship operators do their utmost to establish accurate and reliable methods of analysis for maximizing fuel conservation and improving vessel perfo r m a n c e. R e f e r e n c e s 1. M. Candries, Paint Systems for the Marine Industry, Notes to Complement the External Seminar on Antifoulings. Department of Marine Te c h n o l o g y, University of N e w c a s t l e - u p o n - Tyne, Dec h t t p :// w w w.geocities. com/maxim_candries/paintr e v i e w. h t m l. 2. Dr. Julian Hunter, CEPE Prospects for Nontoxic Fouling Control Coatings, h t t p :// w w w. d b u. d e / c a l e n d e r / fo u l i n g r e l e a s e / d o w s / 1 8 % 2 0 H u n t e r. p d f. JPCL / July 2006 / PCE 51