Intro to Naval Architecture 3E Episode 5 docx

110 SEAKEEPING Such values as the significant motion amplitude in the given sea can be used to compare the performance of different designs in that sea. There remains the need to consider more than one sea, depending upon the areas of the world in which the design is to operate, and to take into account their probability of occurrence. LIMITING FACTORS IN SEAKEEPING A number of factors, apart from its general strength and stability, may limit a ship's ability to carry out its intended function 3 . Ideally these would be definable and quantifiable but generally this is not possible except in fairly subjective terms. The limits may be imposed by the ship itself, its equipment or the people on board. The seakeeping criteria most frequently used as potentially limiting a ship's abilities are speed in waves, slamming, wetness and human reactions. Speed in waves As the waves become more severe die power needed to propel the ship at a given speed increases. This is because of increased water and air resistance and the fact that die propulsors are working under adverse conditions. At some point the main machinery will not be able to provide the power needed and a speed reduction will be forced upon the master. The master may choose, additionally, to reduce speed to protect the ship against the harmful effects of slamming or wetness. Slamming Slamming is a high frequency transient vibration in response to the impact of waves on the hull, occurring at irregular intervals. The most vulnerable area is the ship's outer bottom between about 10 and 25 per cent of the length from the bow. The impact may cause physical damage and can accelerate fatigue failure in this area. For this reason this area of the outer bottom should be given special attention during survey. Slamming is relatively local and often in a big ship, those on a bridge well aft may not be aware of its severity. Because the duration of the slam is only of the order of $5 of a second, it does not perceptibly modify the bodily motion of the ship but the ensuing vibration can last for 30 seconds. A prudent master will reduce speed when slamming badly. This speed reduction leads to less severe slamming or avoids it altogether. Often a change of direction helps. Lightly loaded cargo ships are particularly liable to slam with their relatively full form and shallow draught forward, and enforced speed reductions may be as high as 40 per cent. Slamming is less likely in high speed ships because of their finer form. SEAKEEPING 111 Slamming is likely when the relative velocity between the hull and water surface is large and when the bow is re-entering the water with a significant length of bottom roughly parallel to the sea surface. It is amplified if the bottom has a low rise of floor. The pressure acting in a slam can be shown to be proportional to the square of the velocity of impact and inversely proportional to the square of the tangent of the deadrise angle. Wetness By wetness is meant the shipping of heavy spray or green seas over the ship. The bow area is the region most likely to be affected and is assumed in what follows. It may limit a ship's speed and the designer needs some way of assessing the conditions under which it will occur and how severe it will be. To some degree wetness is subjective and it certainly depends upon the wind speed and direction as well as the wave system. In the past it was often studied by running models in waves but it is now usually assessed by calculating the relative motion of the bow and the local sea surface 3 . The assumption made is that the probability of deck wetness is the same as that of the relative motion exceeding the local freeboard. The greater the difference, the wetter the ship is likely to be. Increased freeboard, say by increasing sheer forward is one means of reducing wetness. At sea the master can reduce wetness by reducing speed and, usually, changing the ship's heading relative to the predominant waves. Good round down on the deck will help clear water quickly. A bulwark can be used to increase the effective freeboard but in that case adequate freeing ports are needed to prevent water becoming trapped on the deck. The size of freeing ports to be fitted is laid down in international regulations. The designer would avoid siting other than very robust equipment in the area where green seas are likely. Any vents would face aft and water traps provided. Propeller emergence The probability of the propeller emerging from the water, as the result of ship motions, can be assessed in a similar way to wetness. That is, by calculating the motion of the ship aft relative to the local sea surface. If the propeller does emerge, even partially, it will be less effective in driving the ship. It will tend to race arid cause more vibration. Human performance It is a common experience that ship motions can cause nausea and then sickness 4 - 5 . This discomfort can itself make people less efficient and make them less willing to work. Motions can make tasks physically more difficult to accomplish. Thus the movement of weights around the ship, say when replenishing a warship at sea, is made more difficult. Also tasks 112 SEAKEEPING requiring careful alignment of two elements may become impossible without some mechanical aid. Over and above this the motions, and the drugs taken to alleviate the symptoms of motion sickness, may adversely affect a person's mental dexterity. In broad terms the effects of motion on human behaviour depend upon the acceleration experienced and its period. The effect is most marked at frequencies between about 0.15 to 0.2 Hz. The designer can help by locating important activities in areas of lesser motion, by aligning the operator position with the ship's principal axes, providing an external visual frame of reference and providing good air quality free of odours. OVERALL SEAKEEPING PERFORMANCE An overall assessment of seakeeping performance is difficult because of the many different sea conditions a ship may meet and the different responses that may limit the ship's ability to carry out its function. A number of authorities have tried to obtain a single 'figure of merit* but this is difficult. The approach is to take the ship's typical operating pattern over a period long enough to cover all significant activities. From this is deduced: (1) the probability of meeting various sea conditions, using statistics on wave conditions in various areas of the world 6 ; (2) the ship speed and direction in these seas; (3) the probability of the ship being in various conditions, deep or light load; (4) the ship responses that are likely to be critical for the ship's operations. From such considerations the probability of a ship being limited from any cause can be deduced for each set of sea conditions. These combined with the probability of each sea condition being encountered can lead to an overall probability of limitation.The relative merits of different designs can be 'scored' in a number of ways. Amongst those that have been suggested are: (1) the percentage of its time a ship, in a given loading condition, can perform its intended function, in a given season at a specified speed; (2) a generalization of (1) to cover all seasons and/or all speeds; (3) the time a ship needs to make a given passage in calm water compared with that expected under typical weather conditions. It is really a matter for the designer to establish what is important to an owner and then assess how this might be affected by wind and waves. SEAKEEPING 113 Acquiring seakeeping data Computations of performance criteria require good data input, including that for waves, response operators and limitations experi- enced in ship operations. Wave data The sources of wave data were discussed in Chapter 5. The designer must select that data which is applicable to the design under review. The data can then be aggregated depending upon where in the world the ship is to operate and in which seasons of the year. Response amplitude operators The designer can call upon theory, model testing and full scale trials. Fortunately modern ship motion dieories can give good values of responses for most motions. The most difficult are the prediction of large angle rolling, due to the important non-linear damping which acts, and motions in quartering seas. The equations of motion can be written down fairly easily but the problem is in evaluating the various coefficients in the equations. Most modern approaches are based on a method known as strip theory or slender body theory. The basic assumptions are those of a slender body, linear motion, a rigid and wall-sided hull, negligible viscous effects apart from roll damping and that the presence of the hull has no effect upon the waves. The hull is considered as composed of a number of thin transverse slices or strips. The flow about each element is assumed to be two-dimensional and the same as would apply if the body were an infinitely long oscillating cylinder of that cross section. In spite of what might appear fairly gross simplification, the theory gives good results in pitch and heave and with adjustment is giving improved predictions of roll. The same principles apply to calculating vibration frequencies as discussed in Chapter 11. To validate new theories or where theory is judged to be not accurate enough, and for ships of unconventional form, model tests are still required. For many years long narrow ship tanks were used to measure motions in head and following regular waves. Subsequently the wavemakers were modified to create long crested irregular waves. In the 1950s, as the analytical tools improved, a number of special seakeeping basins were built. In these free models could be manoeuvred in short and long crested wave systems. For motions, the response operators can be measured directly by tests in regular seas but this involves running a large number of tests at different speeds in various wavelengths. Using irregular waves the irregular motions can be analysed to give the regular components to be compared with the component waves. 114 SEAKEEP1NG Because the irregular surface does not repeat itself, or only over a very long period, a number of test runs are needed to give statistical accuracy. The number of runs, however, is less than for testing in regular waves. A third type of model test uses the transient wave approach. The wavemaker is programmed to generate a sequence of wave lengths which merge at a certain point along the length of the tank to provide the wave profile intended. The model is started so as to meet the wave train at the chosen point at the correct time. The model then experiences the correct wave spectrum and the resulting motion can be analysed to give the response operators. This method can be regarded as a special case of the testing in irregular waves. Whilst in theory one run would be adequate several runs are usually made to check repeatability. The model can be viewed as an analogue computer in which the functions are determined by the physical characteristics of the model To give an accurate reproduction of the ship's motion the model must be ballasted to give the correct displacement, draughts and moments of inertia. It must be run at the correct representative speed. To do all this in a relatively small model is difficult particularly when it has to be self- propelled and carry all the recording equipment. The model cannot be made too large otherwise a long enough run is not achievable in the confines of the tank, Telemetering of data ashore can help. Another approach has been to use a large model in the open sea in an area where reasonably representative conditions pertain. Wetness and slamming depend upon the actual time history of wave height in relation to the ship. Direct model study of such phenomena can only be made by running the model in a representative wave train over a longish period. However, tests in regular waves can assist in slamming investigations by enabling two designs to be compared or by providing a check on theoretical analyses. Then there are full scale ship trials. Some full scale data has been obtained for correlation with theory and model results. Direct correlation is difficult because of the need to find sea conditions approximating a long crested sea state during the trial period when the ship is rigged with all the measuring gear. A lot of useful statistical data, however, on the long term performance can be obtained from statistical recorders of motions and strains during the normal service routine. Such recorders are now fitted in many warships and merchant ships. Deducing criteria It is not always easy to establish exactly what are limiting criteria for various shipboard operations. They will depend to some extent upon the ability of the people involved. Thus an experienced helicopter pilot SEAKEEPING 115 will be able to operate from a frigate in conditions which might prove dangerous for a lesser pilot. The criteria are usually obtained from careful questioning and observation of the crew. Large motion simulators can be used for scientific study of human performance under controlled conditions. These can throw light upon how people learn to cope with difficult situations. The nature of the usual criteria has already been discussed. SHIP FORM AND SEAKEEPING PERFORMANCE It is difficult to generalize on the effect of ship form changes on seakeeping because changing one parameter, for instance moving the centre of buoyancy, usually changes others. Methodical series data should be consulted where possible but in very general terms, for a given sea state: (1) increasing size will reduce motions; (2) increasing length will reduce the likelihood of meeting waves long enough to cause resonance; (3) higher freeboard leads to a drier ship; (4) flare forward can reduce wetness but may increase slamming; (5) a high length/draught ratio will lead to less pitch and heave in long waves but increase the chances of slamming; (6) a bulbous bow can reduce motions in short waves but increase them in long waves. Because form changes can have opposite effects in different wave conditions, and a typical sea is made up of many waves, the net result is often little change. For conventional forms it has been found 7 that overall performance in waves is little affected by variations in the main hull parameters. Local changes can be beneficial. For instance fine form forward with good rise of floor can reduce slamming pressures. A ship's rolling motions can be reduced by fitting a stabilization system. In principle pitch motions can be improved in the same way but in practice this is very difficult. An exception is the fitting of some form of pitch stabilizer between the two hulls of a catamaran which is relatively shorter than a conventional displacement ship. In this section attention is focused on roll stabilization. The systems may be passive or active. 116 SEAKEEPING Bilge keels Of the passive systems, bilge keels are the most popular and are fitted to the great majority of ships. They are effectively plates projecting from the turn of bilge and extending over the middle half to two-thirds of the ship's length. To avoid damage they do not normally protrude beyond the ship's side or keel lines, but they need to penetrate the boundary layer around the hull. They cause a body of water to move with the ship and create turbulence thus dampening the motion and causing an increase in period and reduction in amplitude. Although relatively small in dimension the bilge keels have large levers about the rolling axis and the forces on them produce a large moment opposing the rolling. They can produce a reduction in roll amplitude of more than a third. Their effect is generally enhanced by ahead speed. They are aligned with the flow of water past the hull in still water to reduce their drag in that state. When the ship is rolling the drag will increase and slow the ship a little. Figure 6.6 Bilge keel SEAKEEPING 117 Passive tanks These use the movement of water in specially designed tanks to oppose the rolling motion. The tank is U-shaped and water moves from one side to the other and then back as the ship inclines first one way and then the other. Because of the throttling effect of the relatively narrow lower limb of the U joining the two sides of the tank, the movement of water can be made to lag behind the ship movements. By adjusting the throttling, that is by 'tuning' the tank, a lag approaching 90° can be achieved. Unfortunately the tank can only be tuned for one frequency of motion. This is chosen to be the ship's natural period of roll as this is the period at which really large motions can occur. The tank will stabilize the ship at zero speed but the effect of the tank's free surface on stability must be allowed for. Figure 6.7 Stabilizer fin 118 SEAKEEPING Active fins This is the most common of the active systems. One or more pairs of stabilizing fins are fitted. They are caused to move by an actuating system in response to signals based on a gyroscopic measurement of roll motions. They are relatively small although projecting out further than the bilge keels. The whole fin may move or one part may be fixed and the after section move. A flap on the trailing edge may be used to enhance the lift force generated. The fins may permanendy protrude from the bilge or may, at the expense of some complication, be retractable, Figure 6.7. The lift force on the fin is proportional to the square of the ship's speed. At low speed they will have litde effect although the control system can adjust the amplitude of the fin movement to take account of speed, using larger fin angles at low speed. Active tank This is similar in principle to the passive tank system but the movement of water is controlled by pumps or by the air pressure above the water surface. The tanks either side of the ship may be connected by a lower limb or two separate tanks can be used. Figure 6.8 shows a system in which the air pressure above the water on the two sides is controlled to 'tune* the system. The air duct contains valves operated by a roll sensing device. The system can be tuned for more than one frequency. As with the passive system it can stabilize at zero ship speed. It does not require any projections outside the hull. The capacity of the stabilization system is usually quoted in terms of the steady heel angle it can produce with the ship underway in still water. This is then checked during trials. It is possible to use modern theories to specify performance in waves but this would be difficult to check contractually. SUMMARY It has been shown that a ship's motions in irregular ocean waves can be synthesized from its motions in regular waves. Roll, pitch and heave responses in regular waves have been evaluated and the effects of added mass and damping discussed. The energy spectrum has been shown to be a powerful tool in the study of motions as it was in the study of waves. Factors limiting a ship's seakeeping capabilities, including the degradation of human performance, have been discussed and it has been seen how they can be combined to give an overall assessment of the probability that a ship will be able to undertake its intended Figure 6.8 Tank stabilizer [...]... structural elements fail in their turn the result can be loss of the ship Failure may be due to the structure: (1) Becoming distorted due to being strained past the yield point This will lead to permanent set and the distortion may lead to systems being unable to function For instance, the shafts may be unable to turn (2) Cracking This occurs when the material can no longer sustain the load applied and... 40.008 + 2 .56 5 For the whole section the Z values are: 136 STRENGTH Table 7.1 Calculation of properties of simplified section in Figure 7.8 Item Scantlings Area (m 2 ) Lever about keel (m) Upper deck Second deck Side shell Tank top Bottom shell Centre girder Summations 6 6 13 10 10 1 .5 X 0.022 X 0.016 X 0.014 X 0.018 X 0.020 X 0.006 0.132 0.096 0.182 0.180 0.200 0.009 0.799 13 10 6 .5 1 .5 0 0. 75 Moment... 22.308 9.600 7.690 0.4 05 0 0.0 05 40.008 0 0 2 .56 3 0 0 0.002 2 .56 5 If the bending moments for this structure are those calculated in Example 7.1, the stresses can be found The still water stresses are: In the deck In the keel The wave bending stresses are: In the deck In the keel This gives total stresses in the deck of 280.0 or 14.6MN/m2 compression, and in the keel stresses of 1 85. 5 or 9.7MN/m2 tension,... hogging The sagging stresses would be too high for a mild steel ship and some action would be needed One way would be to spread the central 50 00 tonne load uniformly over the whole ship length This would reduce the stresses to 132.7MN/m2 in the deck and 87.9 MN/m2 in the keel If it was desired to increase the section modulus to reduce the stresses, the best place to add material would be in the keel... outer to the inner bottom Longitudinal stiffening of the bottom is by rolled section or plating called longitudinal girders or simply longitudinals The central longitudinal keel girder is one of considerable importance It is continuous fore and aft, extending from the flat keel to the tank top or inner bottom Sided longitudinal girders are intercostal That is, they are cut at each floor and welded to. .. moment is: Putting x = 70 the wave bending moment at amidships is found to be 718MNm sagging With the wave crest amidships the wave moment would be of the same magnitude but hogging The total moments are obtained by adding the still water and wave moments, giving: Sagging = 797+ 718 = 151 5MNm Hogging = 797-718 = 79MNm Had the mass of 50 00 tonnes been distributed uniformly over the whole ship length the still... the condition assumed in the standard calculation It is useful for an operator to be able to assess readily the effects on longitudinal strength of additions or removals of weight relative to the standard distribution For small weight changes, influence lines can be used to show the effect on the maximum bending moment due to a unit weight added at any point along the length Lines are drawn for the... manganese ore Although this was a relatively old ship the lesson is there to be learnt A ship's ability to withstand very high occasional loading is ensured by designing to stress levels which are likely to be met perhaps only once in the life of the ship Failures in ship structures are much more 121 STRENGTH likely to be due to a combination of fatigue and corrosion These cumulative failure mechanisms... is distorted so as to be concave up it is said to sag 2nd the deck is in compression with the keel in tension When the ship is convex up it is said to hog The deck is then in tension and the keel in compression High still water forces and moments, besides being bad in their own right, are likely to mean high values in waves as the values at sea are the sum of the still water values and those due to a... results can be used to indicate the maximum bending moments the ship is likely to experience in waves The choice of wave height is important To a first order it can be assumed that bending moments will be proportional to wave height Two heights have been commonly used L/20 and 0.607(£) 05 where L is in metres In recent years the latter has been more generally used because it was felt to represent more . due to the structure: (1) Becoming distorted due to being strained past the yield point. This will lead to permanent set and the distortion may lead to systems being unable to . length of bottom roughly parallel to the sea surface. It is amplified if the bottom has a low rise of floor. The pressure acting in a slam can be shown to be proportional to the square . world in which the design is to operate, and to take into account their probability of occurrence. LIMITING FACTORS IN SEAKEEPING A number of factors, apart from its general