Hull loads and response, hydroelasticity
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1 Transactions on the Built Environment vol 1, 1993 WIT Press, ISSN Hull loads and response, hydroelasticity effects on fast monohulls E. Jullumstr0 & J.V. Aarsnes Division of Marine Vehicles, Norwegian Marine Technology Research Institute, N-7002 Trondheim,, Norway ABSTRACT The dynamic response of a correctly scaled hydroelastic monohull is presented. An experimental techniques for proper modelling of the important structural parameters as mass distribution, stiffness and damping is outlined. Different sensor types were used to measure the shear forces and bending moments. Model tests have been carried out with a model of a 110 m full scale ship. The experimental results for bending moment and shear forces are compared with calculated results using the FASTSEA computer program. INTRODUCTION Dimensioning of large, high speed vehicles demand knowledge and methods to determine the limiting environmental loads, operational aspects and structural strength. To achieve good design load predictions, appropriate theoretical tools and model test techniques must be developed. Model test is also required for verification and calibration of theoretical methods and numerical codes. In this paper we describe the work performed in order to develop an experimental technique for elastic models of monohulls. The model is made of dyvinicell foam and glassfibre as a shell model. During the tests different techniques for measuring the structural response were tested. The bending moment and shear forces were measured using piezo- electric sensors, fibreoptics and strain gauges. The slamming forces were measured using piezo film. Results from the different sensors are compared and their feasibility for measurement of responses for an elastic model is discussed. This include procedures for calibration of the different sensors and methods for analyzing the measured responses.
2 Transactions on the Built Environment vol 1, 1993 WIT Press, ISSN The test program include decay test in air (dry tests) and in water to determine the structural properties (damping and natural frequencies) and zero speed added mass and damping for the dominating modes. Further the model was tested with forward speed (Froude number 0.40) in regular and impulse waves. Resonances induced by waves for the lowest natural flexible modes and their effect upon the structural loads are compared to the rigid body loads. The experimental results for bending moment and shear forces are compared with calculated results using the FASTSEA computer program. MODEL TESTS TECHNIQUE Structural Modeling Technique The modelling of the elastic properties of a ship gives several additional problems compared to the modelling for dynamics of rigid ships. Additional requirements to the model can be summarized as follows: * Correctly scaled global structural stiffness (at least for the dominating elastic modes) * Structural damping must be similar to full scale values. * Longitudinal mass distribution In cases where slamming loads are important for the elastic response the local stiffness of plating should also be correctly scaled due to the possible influence of local stiffness on the actual slamming load. In the modelling of elastic models two different solutions have so far been used (see Maeda Ref 5): * Backbone model * Fully elastic model In Fig. 1 the two alternatives are shown. For the backbone modelling the elasticity of the model is represented by an elastic keel to which rigid segments are connected. Using this modelling technique it is relatively easy to model the stiffness and materials as steel or aluminum with stable and well documented properties can be used. Further it is easy to modify the structural properties and the structural damping is low. One problem with this modelling is the gap between the different section. They may be closed using an elastic membrane, but it is difficult to avoid
3 transfer Transactions of on the tension Built Environment trough vol 1, 1993 WIT the Press, membrane. ISSN If the gaps are open each sections have to be built water tight. Further the dynamic pressure within the gaps may to some extent influence the results. For ship models with forward speed the gaps will give additional problems due to the influence on the flow field around the ship. The model production costs will be significantly higher for a backbone model than for an fully elastic model as outlined below. The fully elastic model is built up using cross section with one or two different layers with different elasticity as shown in Fig. 1. Backbone model Elastic beam /Segment Fully elastic model Divinycsll foam Glassfibere resin Figure: 1. Methods for modelling of an elastic monohull In the present tests the fully elastic model was used. The model was made of dyvinicell foam and glassfibre/polyester resin. The thickness of both layers was varied in order to achieve the correct elastic properties of the model. Geometrical similarity with the full scale ship and constant thickness of the foam is obtained by numerical controlled milling of the model both outside and inside. Correct scaled elasticity of the model is obtained by the resin which was laid on the inside of
4 the Transactions hull. on the Built Prior Environment to vol 1, 1993 the WIT Press, model production ISSN the correctly scaled elastic properties of the model were established from F.E.M calculations for the full scale ship. Instrumentation-and calibration The main intention with the test program was to investigate which measuring technique that was most reliable for measuring global loads for an elastic model of a monohull. In the following the different techniques for measuring the bending moment and shear forces will be discussed. A complete description of the instrumentation of the model is given by Jullumstrtf and Aarsnes Ref. 4. The bending moment and shear forces were measured using piezo- electric sensors, fibre optics and strain gauges. The piezo-electric sensors and the fiber-optic sensors are bonded into the dyvinicell material by means of epoxy. The strain gauges are glued to the surface of the glassf ibre/polyester resin. The calibration of the different bending moment and shear force sensors was performed for the dry model connected vertically at two longitudinal positions. A vertical point load was used to give a known shear force and vertical bending moment distributions. During the calibration different longitudinal positions were used both for the vertical load and the connection points. The following loading conditions was used: * Static loads of different magnitude * Harmonic oscillating loads covering typical encounter periods and moment amplitudes * Step function loads (cut of a wire carrying a known weight) One reason for this quite comprehensive calibration program was to verify the use of the two layer material for production of elastic models. This include evaluation of possible nonlinear effects (e.g. amplitude dependency, difference between tension and compression, hysteresis) and frequency dependency in the model. Using a number of different loading conditions is also required in order to ensure that the same bending moment or shear force gives the same signal from the transducers. For the strain gauges and fiber optics it can be assumed that the sensors are frequency independent and static values can be measured. The piezo sensors will be frequency dependent and shows a typical high-frequency band-pass characteristic due to the intrinsic sensor capacitance and resistances in the instrumentation system. In the present case the cut-off frequency was found to be about 0.1 Hz. This
5 value Transactions can on the probably Built Environment vol be 1, 1993 improved WIT Press, by modifying ISSN the amplifiers but leakage current in the system will limit the possible improvement. The resulting response transfer function of the piezo cable system is shown in Fig. 2. It is observed that for frequencies less than about 0.2 Hz the frequency dependence is significant. i " Frequency [Hz] Figure 2: Amplitude and phase of the piezo cable sensor transfer function One problem with this frequency dependent behavior of the piezo sensors is that static tests can not be used for calibration. After testing several methods it was found that using a step function loads (cut of a wire carrying a known weight) was an accurate method for calibration of piezo cable sensors. However the method require that the natural frequency of any mode of motions which could be excited must be much higher than the cut-off frequency. The strain gauges are suitable for measurements of both static and dynamic loading. The calibration is therefore simpler than for the piezo sensors. The strain gauges gives a measurement of the strain over a relatively small length and the measurements may therefore be more sensitive to loading dependent stress concentration. Based on the experience so far, the fibreoptic sensors seems to be the most promising for measurements of global loads for fully elastic models, without any of the drawbacks mention for the piezo cable and strain gauges. However the required equipment is still relatively expensive and further work with problems as temperature stabilization and
6 methods Transactions for on the analyzing Built Environment vol the 1, 1993 measured WIT Press, signal ISSN is required before a robust and reliable measuring technique is available for regular use. NUMERICAL METHOD The theoretical calculations for the monohull have been performed using the program FASTSEA. The numerical method have been described by Faltinsen and Zhao /!/ and /2/. The program is developed for high speed, non-planing vessels for Froude number above about 0.4. The program can handle both monohull and multihull vessels. The unsteady flow problem is based on linear potential theory and are solved in the frequency domain. The program has recently been applied for the calculation of global loads on a catamaran. The agreement with model tests was found to be satisfactory, see Faltinsen at.al. /3/ for further details. The present version of the program is based on rigid body motions and the effect of the structural elasticity on the dynamic response are therefore not be included. At present work is in going on for including the effect of hydroelasticity. This work cover both the steady state wave induced response at the elastic natural frequency (springing) and a time domain solution including slamming forces and the resulting transient elastic response of the vessel. TESTS AND TEST RESULTS The model tests were carried out in the main towing tank at Marintek. The principal dimensions of the tank is 260 m long, 10.5 m wide and 5/10 m deep. It is equipped with a computer controlled double-flap wave maker at the end. During the tests the model was mounted into a f ree-to-surge rig in the front of the towing carriage. The model was also free to heave and pitch. Test Conditions An elastic model of an 110 m monohull is tested. The main particulars of the model are as follows : Length between perpendiculars Lpp = m Beam at waterline a midship B^ = m Draft, even keel T = m Displacement V = m^ Block coefficient Cg = 0.50 The used model scale was 1: Further details about the model is given by Jullumstrgf and Aarsnes During the test program only head sea was applied. The towing speed was 25 kn, corresponding to a Froude
7 number Transactions of on the Built Environment The test vol 1, 1993 program WIT Press, cover regular ISSN waves and impulse waves. The impulse wave is characterized by one single short-duration wave group passing the model. The wave group consist of a broad range of wave frequencies. The technique relies on a high level of computer controlled wave generation. An obvious advantage of this method is that it is fast, giving a high resolution RAO at little test time. This method has earlier been used in model testing of floating structures at zero speed. A high forward speed makes the application of the impulse wave method more complex due to the difficulties to hit the wave exactly at the position (in space and time) where the generated wave train merges into one single wave. Decay Tests The test program include decay test in air (dry tests) and in water to determine the structural properties of the model. The decay tests in air were performed connecting the model in vertical springs to represent the water plane stiffness. The following main results were obtained from the dry decay tests for the 2-node mode: Natural frequency: (full scale) %i = 2.88 Hz Logarithmic damping coefficient 6 =0.16 The results was obtained from analysis of the measured vertical bending moment at midship. The above damping represent the structural damping only. At full scale a typical structural damping for the first elastic mode is logarithmic damping coefficient 6=0.1. The obtained structural damping is therefore slightly to high. The decay test in water were performed for zero speed and using the same excitation as for the dry tests. The following main results were obtained from the wet decay tests for the 2-node mode: Natural frequency: (full scale) f^i =1.80 Hz Logarithmic damping coefficient 8 =0.18 Hydrodynamic damping coeff. 6 = 0.02 Generalized added mass A^ji = 1.5 M M is the generalized mass for the first elastic bending mode. The natural frequency of the 3 -node mode and 4 -node mode was found to be 3. 0 Hz and 4.3 Hz respectively. The generalized added mass, A is determined based on the difference in natural frequency between the dry and wet decay tests. The results for the hydrodynamic damping is obtained as the difference between the wet and dry test damping
8 which Transactions is on two the Built almost Environment vol equal 1, 1993 WIT values Press, and are ISSN therefore rather uncertain. It is seen to be very small (as should be expected for this high frequency). Comparison with Rigid Body Results As noted the natural frequency of the first flexible mode was fjji=1.75hz. For excitation frequencies significant lower than this value (e.g f=0.25f#i) the dynamic amplification due to the structural elasticity of the response at the excitation frequency can be neglected. This imply that for encounter frequencies less than about 0.5 Hz the measured global load results can be compared with calculations based on rigid body assumption. In Fig 3 the heave motion transfer functions (RAO) is shown. For the heave motion the agreement between FASTSEA calculations and model test results are very good for wave period less than 10 s. At heave resonance (at about 12 s) the calculations gives somewhat higher response then what was obtained in the model tests. The main reason for this discrepancy is that the effect of viscous damping is not included in the calculations. The model test results from regular wave tests and impulse waves are generally very similar except at resonance where the effect of viscous damping are more pronounced for the regular wave results. 1.D 1 A T ^*_ T 1 O /! L_ j A. 1 o / / D n o 0 O A Oo 7 / P Q FASTSEA MnHcil Tcclo Model Tests, \ np Wove n n ^9^7 wcve per ioc i s -, Figure 3: Comparison of calculated and measured heave transfer function In Fig 4. the transfer functions for the vertical bending moment at midship are shown. The agreement between the FASTSEA calculation and model tests results is reasonable. It is observed that the measurements gives somewhat higher bending moment for wave lengths close to the ship length.
9 i Marine Engineering 71 Transactions on the Built Environment vol 1, 1993 WIT Press, ISSN Power spectrum (WAVE CARR) Power spectrum (BE.UO.S.G 23) n 01? n nin n OOR & o on? o nno j^, Free. (Hz) freo. (Hz) Figure 6: Energy spectrum for measured wave and vertical bending moment midship in a regular wave. I ^L. ' / jl v V j«-l.«v-«.«wocc. 10 f. iooo(.10 5 jc«oc 10».IOOCXMO J J x%-^ -^ /- ^ b»,^^ w- ~ Figure 7: Time history and spectral energy distribution of wave elevation and vertical moment midship in the case of an impulse wave.
10 Transactions on the Built Environment vol 1, 1993 WIT Press, ISSN Marine Engineering Time Isl Time Is] Figure 8: Result from filtering of measured vertical bending moment. Low frequency response and high frequency response.
11 Transactions on the Built Environment vol 1, 1993 WIT Press, ISSN The measured time series for the vertical bending moment has been analyzed using high-pass and low-pass filtering with a cut-off frequency equal to 1.0 Hz. The resulting time series are shown in Fig. 8. The low- pass filtered response (low-frequency response) cover the wave energy frequency range. In this frequency region the elastic effect is not important. The high pass filtered response (high frequency response) will be govern by the dynamic amplification due to the elastic response of the hull. For a rigid model this effect would have been lost. It is seen that the contribution from the elastic response is of the same order of magnitude as the contribution at the wave frequency. The main source of excitation of this transient elastic response is slamming or impact loads in the bow region of the ship when it hit the impulse wave. At present no theoretical method is available to determine the transient elastic response excited by bow flare or bottom slamming. The above results clearly indicate the importance of this kind of loading. CONCLUSIONS Model production techniques for hydroelastic modelling of mono- and multihull ships are developed. Instrumentation techniques for studying loads on and response of elastic models are developed, and the results are promising. Implementing the effect from structural elasticity on dynamic response in the FASTSEA program, gives together with the developed instrumentation / modeling technique a very powerful tool for dimensioning of further crafts. REFERENCES 1. Faltinsen, O.M. and Zhao, R., "Numerical prediction of ship motions at high forward speed," Phil. Trans R. Soc. Lond., Series A, Faltinsen, O.M. and Zhao, R., "Flow prediction around high-speed ships in waves", Mathematical approaches in hydrodyn., SIAM, Faltinsen, O.M., Hoff, J.R. Kv&lsvold, J. and Zhao, R., "Global loads on high-speed catamarans" PRADS-symposium, Jullumstr0, E. and Aarsnes, J.V., "Hull Loads and Response on high speed Craft", Int. conf. high speed passenger craft, RINA, London, 1993
12 Transactions on the Built Environment vol 1, 1993 WIT Press, ISSN Marine Engineering 5. Maeda, H "Modelling techniques for dynamic of ships", Phil. Trans R. Soc. Lond., Vol 334, pp
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