SUMMARY MOTIONS AND LOADS OF A HIGH-SPEED CRAFT IN REGULAR WAVES: PREDICTION AND ANALYSIS F Prini, School of Marine Science and Technology, Newcastle University, UK R W Birmingham, School of Marine Science and Technology, Newcastle University, UK S Benson, School of Marine Science and Technology, Newcastle University, UK H J Phillips, Royal National Lifeboat Institution, UK P J Sheppard, Royal National Lifeboat Institution, UK J Mediavilla Varas, Lloyd s Register, UK M Johnson, Lloyd s Register, UK R S Dow, School of Marine Science and Technology, Newcastle University, UK The paper presents a comparison of results from numerical and experimental seakeeping motion tests completed on the Severn Class lifeboat of the Royal National Lifeboat Institution (RNLI). A numerical model was previously developed to predict the motions and loads that the craft is likely to experience throughout its operational life [1]. Model-scale seakeeping tests have now been carried out at Newcastle University s towing tank to validate the numerical model. Two models were tested, of which one was designed and built to be segmented and held together through a strain-gauged backbone beam. Results are presented in terms of Response Amplitude Operators (RAOs) and compared against the motions from the hydrodynamic simulations. The results give confidence that the numerical model accurately predicts the craft behaviour at low speed, whilst, as expected, its accuracy at high speed becomes questionable. The validation plan will also involve towing tank tests with global loads measurement and extensive full-scale sea trials in real operational conditions. Results will contribute to improving the RNLI s design and operational practice and could be of use to others interested in the behaviour of high-speed craft in heavy seas. NOMENCLATURE β CG CM FP λ LCG LOA LR RAO RNLI SMP SAR Wave direction Centre of Gravity Continuous Model Forward Perpendicular Wavelength Longitudinal Centre of Gravity Overall Ship Length Lloyd s Register Response Amplitude Operator Royal National Lifeboat Institution Segmented Model Preliminary segmented model prior segmentation Search And Rescue 1. INTRODUCTION High-speed motor craft have to operate in severe conditions, meeting challenging structural requirements. Amongst these, Search and Rescue craft face some of the most difficult situations. A research project addressing structural design approaches of search and rescue craft is ongoing by the authors. The research is undertaken on the Severn Class Lifeboat of the Royal National Lifeboat Institution (RNLI), although results are of interests to the wider marine industry. 1
The RNLI has been operating the Severn Class since the 1990s and is in the process of considering an extension program to lengthen its service life to 50 years [2,3]. A research project has been undertaken by the RNLI and Newcastle University, with the support of Lloyd s Register, to investigate the loads that this craft is likely to experience in its operational life and its consequential structural response. A systematic approach was developed to investigate seakeeping motions, loads and structural response through numerical and experimental methods. A global finite element model was built with the engineering package MAESTRO [1]. Motion responses, global loads and bottom pressure envelopes were computed using three different codes based on potential theory and the results were initially compared with the experimental tests by Fridsma [4]. Model-scale seakeeping tests, as shown in Figure 1, are being carried out to validate the numerical model. This paper presents motion data from the initial tests together with the numerical predictions from [1]. Figure 1 Severn Class Lifeboat (left). Relative FE structural model (middle) and model-scale seakeeping test (right). 2. NUMERICAL SIMULATION MODEL A full-ship structural finite element model of the Severn Class was developed with the design software MAESTRO, as presented in [1]. The model integrates hydrodynamic and structural analysis. The potential flow solver MAESTRO- Wave was used to predict bottom pressures, motions and hull-girder loads in regular waves, at a range of speeds, headings and wavelengths. The simulations were run in the frequency domain with two strip theory codes and a panel method code. MAESTRO FE will be used in future analyses to compute the structural response to the loads predicted with the hydrodynamic simulations. Solvers based on potential theory are, in naval architecture, the most common design tools to predict the seakeeping motions and loads of a vessel. These tools require less computational resources than more advanced CFD methods, such as Euler and RANS equation solvers [5]. Potential flow codes are generally either strip theory or panel method codes. Strip theory solvers are computationally efficient and they are generally used for predicting motions and hullgirder loads. Panel method codes are considered more suitable for the computation of bottom pressure envelopes that can be applied to a finite element model for structural assessment [6]. 2
3. MODEL-SCALE TESTS 3.1 Motions and Loads Measurement 24 th International HISWA Symposium on Yacht Design and Yacht Construction Seakeeping tests with motion measurement have been carried out extensively in the past. These tests consist of measuring motions on a scaled model of the prototype vessel. More challenging is the measurement of structural loads at small scale. Due to the practical complexities in satisfying the structural similarity at model-scale, designers have developed different approaches to investigate hull girder loads through model tests. The most common method has been the use of a segmented model [7]. Two scaled models of the Severn Class were designed and built for the seakeeping tests: a continuous model and a segmented model. The first is a conventional model for measuring rigid body motions. The second is a rigid segmented model for measuring hull girder loads. The segmented model was built as a whole, as continuous, and preliminary tests were carried out. The aim of these tests was to ensure that both models have the same seakeeping characteristics prior to one being segmented. The same tests will also be carried out after the segmentation to assess its effect on the model behaviour. Hull girder loads will then be measured by means of a strain gauged backbone beam connecting the various segments. This paper reports on the preliminary test carried out to compare the hydrodynamic characteristics of the two models with the numerical results. 3.2 Facility The tests were performed at Newcastle University s towing tank, whose principal particulars are shown in Table 1. The tank is fitted with a wave maker capable of generating regular and irregular waves and a series of 2-wire resistance wave probes located along the tank. Table 1 Newcastle University's Towing Tank main particulars. TOWING TANK SPECIFICATIONS Length 37 m Width 3.7 m Maximum Water Depth 1.25 m Maximum Carriage Speed 3.5 m/s WAVEMAKER CAPABILITIES Wave Period 0.5-2.0 s Wave Height (period dependent) 0.02-0.12 m 3.3 Scaled Models 3.3 (a) Construction Details The full-scale vessel is the Severn Class lifeboat designed and operated by the RNLI. The Severn is a 17-metre monohull of composite construction. It has a hard chine soft vee hull form with propeller tunnels [8]. Equipped with twin screw propellers it is capable of achieving 25 knots maximum speed. 3
A scaling factor of 13.5 was used for the model design. The model size, identified based on the scope of the experiment and the size of the facility, allows most of the tests to be conducted clear from tank wall interference, blockage and shallow water effects. Table 2 reports the main particulars of both the full-scale vessel and the models. Figure 2 shows the two models built for the tests. Table 2 Severn Class main particulars at full-scale and model-scale. SEVERN CLASS PARTICULAR FULL-SCALE MODEL-SCALE Scale Factor 1 13.50 Length Overall [m] L OA 17.00 1.259 Length Waterline [m] L WL 15.57 1.153 Beam Overall [m] B OA 5.62 0.416 Beam Waterline [m] B WL 5.00 0.370 Depth [m] D 2.52 0.187 Draught (at amidships) [m] T 1.46 0.108 LCG (aft of amidships) [m] LCG 1.30 0.096 LCG (from transom) [m] LCG 6.45 0.478 Displacement [kg] Δ 43170 17.12 Speed max [kn] V 25 6.80 Speed max [m/s] V 12.86 3.50 Wet Surface Area [m 2 ] WSA 94.73 0.520 Figure 2 Continuous and segmented models of the Severn Class Lifeboat used for the towing tank tests. The continuous and the segmented model, have a different structural layout, since the latter accommodates additional structural members to hold the hull segments together and measure loads in correspondence of the segmentation points. Achieving geometrical similarity and the same hydrodynamic and seakeeping behaviour between the two models was an important consideration during the design and construction stage. Moreover, a lightweight construction was required to give enough flexibility when adjusting the ballast to achieve the target inertial properties. It was therefore decided to realise both models with the same mould and in advanced fibrereinforced composites. The advantages of this construction technique are high accuracy, light weight and high stiffness. 4
Two female moulds were used: one for the hull shell and one for the deck. Both moulds were CNC- machined out of high-density foam blocks. The models are manufactured in carbon fibre, epoxy resin and foam core. Hull shell and deck are of vacuum bagged sandwich construction. The central skeg was laminated separately and attached to the hull at a later stage. The bilge keels were 3D-printed in plastic and bolted onto the hull shell. The models were also fitted with a spray deflector to avoid water entering. Whilst the continuous model hull shell is stiffened with three bulkheads, the internal structural layout of the segmented model is more complicated. For this, three sets of bulkheads stiffen the hull shell in correspondence of the segmentation cuts. An intercostal top hat stiffener and a channel beam of monolithic construction run along the model length, as shown in Figure 3. Figure 3 Segmented model. CAD drawings (left) and details of the internal structure connecting the segments (right). 3.3 (b) Towing Point A towing arrangement was devised to tow both the models at the same point, account for the high pitch angles experienced during tests in waves and to avoid artificial trim moments that can be introduced by the towing force. Again, the main constraints were caused by the internal structural layout of the segmented model. A towing system was designed and manufactured in aluminium to connect the dynamometer to the model hull shell. This is shown in Figure 4. With this arrangement the model pitching point, and hence the towing point, are at the intersection of the propeller shaft line with the LCG, minimising artificial trim effects. 5
Figure 4 Details of the towing arrangement. 3.3 (c) Turbulence Stimulation Turbulence stimulation devices were fitted to ensure turbulent flow around the hull surface and the appendages. It was considered that turbulence stimulators mounted on the bow are not always effective for high-speed craft where the hydrodynamic lift causes the bow to emerge from the water. This effect is even more accentuated when the craft is running in waves. To stimulate turbulence flow around the hull shell, it was therefore decided to mount a probe piercing the water in front of the model and towed by the carriage. The probe was mounted with an inclination of 30 from the waterline and at a distance of 2.5 B OA from the model FP measured at the waterline. Around the bilge keels, which typically lie outside the hull boundary layer, turbulent flow was generated by trip wires bonded near the keel leading edge. 3.4 Models Setup Both models were trimmed and ballasted to achieve the same displacement, centre of gravity and mass moments of inertia. The required full-scale target values were computed with a Maxsurf model and the MAESTRO numerical model described in [1]. In view of measuring hull girder loads with the segmented model, the correct weight distribution and inertial properties were set both at a global level and for each segment. This was done by placing the major masses at their designated position and adjusting only the remaining ballast. For the centre of gravity, longitudinal, transverse and vertical position were adjusted. The radii of gyration for roll, pitch and yaw were adjusted using a bifilar swing. The centre of gravity position was checked again after swinging the model. 6
3.5 Test Apparatus and Procedure The experiment matrix involved two groups of tests: forward speed and zero speed. 3.5 (a) Forward Speed Tests - Resistance in smooth water at speeds ranging from 0.7 to 3.5 m/s (5 to 25 knots at full-scale). - Seakeeping in regular head waves at wavelength/hull length ratios in the range of 0.4 to 3.4; at speeds from 0 to 2.8 m/s (0 to 20 knots at full-scale). These tests were conducted with the model attached to a standard towing carriage free to heave and pitch, as shown in Figure 5 (left). Drag and side forces were measured with a dynamometer positioned at the bottom end of the heave post, whilst two potentiometers measured heave (or sinkage) and pitch (or trim). For seakeeping tests, the towing carriage and the data recording equipment were synchronised with the wave maker to minimise interference of waves reflected by the tank walls. 3.5 (b) Zero Speed Tests - Seakeeping in regular beam waves at wavelength/hull length ratios in the range of 0.4 to 3.4; at 0 m/s. These tests were carried out with the model positioned at the centre of the tank. A set of mooring lines, fitted with spring dampers, constrained the model from yawing and swaying along the tank whilst not constraining its motions in the other degrees of freedom. This setup is shown in Figure 5 (right). Rigid body motions in the six degrees of freedom were recorded with an optical tracking system. This system, consisting of a tracking device mounted on the model and two motion capture cameras, is described in detail by Bashir et al. [9]. For all the tests, the water surface was set to 1045mm and the water surface elevation was measured with a wave probe located at the tank midspan. At the current stage, the scope of the study is limited to: - Deep water only - Towed model (no self-propelled model) - One displacement condition - Model in the naked condition with centre and bilge keels (no trim tabs, rudders, shafts, A-brackets and propellers and no bow thruster opening) 7
Figure 5 Setup for forward speed test (left) and zero speed test (right). 4. DATA RECORDING AND ANALYSIS Resistance, motions, speed and surface elevation were recorded continuously at a sampling frequency of 100 Hz. For forward speed tests the measuring interval was adjusted according to the carriage velocity in order to maximise the amount of data sampled as well as obtaining time series trimmed to the portion of run to analyse. The results from resistance tests were derived by averaging the instantaneous measured values over the measuring interval. Tests at 3.5 m/s (25 knots full-scale) only allowed for 4 seconds of data recording. For these tests two consecutive runs were combined to double the measuring interval. For seakeeping tests, a Matlab code was developed to compute the mean values for motions and wave amplitude. The code first imports the recorded time series and trims the signal to only the portion of signal to analyse. The signal is de-trended to eliminate low-frequency components, then the absolute peaks are identified. The mean amplitude is computed by averaging the extrapolated peak amplitudes. An example of the graphical output is shown in Figure 6. Figure 6 Example of data analysis showing the time series of the original signal (top), the identification of the peaks (middle) and the estimation of the peaks amplitude (bottom). 8
5. RESULTS Model-scale test results are presented for both the continuous model (CM) and the segmented model prior its segmentation (SMP). 5.1 Resistance Figure 7 shows drag, sinkage and trim from resistance tests carried out at a range of speeds. Resistance tests were conducted with the main objective of assessing the correlation between the two models. Results from these tests are presented at model-scale as measured. Hence extrapolation to full-scale should be carried out in accordance to the standard procedures for the analysis of model-scale results from resistance tests. 5.2 Seakeeping in Head Seas Figures 8-12 show the heave and pitch motions from seakeeping tests in head waves at a range of speeds. Motions data from seakeeping tests are presented as RAOs (Response Amplitude Operators), normalised with the mean wave amplitude. During the data analysis process, the heave motion amplitudes of a few of the tests were found to lie out of the expected trend. These tests are highlighted in the plots by a text caption. The cause of these outliers is a clipped signal caused by the saturation of the potentiometer, which has a measuring range of just under one revolution. These tests will be run again during the next test session. For the seakeeping tests in head seas, the numerical results obtained through the hydrodynamic simulations presented in [1] are also plotted. These simulation were computed with three different linear potential theory codes: - 2D strip theory (2D) - 2.5 strip theory, which includes a forward speed correction term (2.5D) - 3D panel method (3D) The numerical results are plotted for the whole speed range tested. However, agreement between numerical and experimental data is only expected at low speed. As discussed in the following chapters, the linear potential theory underpinning the numerical results does not capture the planing hydrodynamics that characterises the high-speed regime. 5.3 Seakeeping in Beam Seas Figure 13 shows the roll motions from seakeeping tests in beam waves at zero speed. The numerical simulations with MAESTRO-Wave were initially run without roll damping correction. At the current stage any comparison with experimental results would be misleading, hence numerical results for seakeeping tests in beam seas are not plotted. This will be included in the future simulations. 9
Figure 7 Resistance tests. Drag, sinkage and trim at a range of speeds. Figure 8 Seakeeping tests. Heave and pitch RAOs in head waves at 0 knots. 10
Figure 9 Seakeeping tests. Heave and pitch RAOs in head waves at 5 knots. Figure 10 Seakeeping tests. Heave and pitch RAOs in head waves at 10 knots. 11
Figure 11 Seakeeping tests. Heave and pitch RAOs in head waves at 15 knots. Figure 12 Seakeeping tests. Heave and pitch RAOs in head waves at 20 knots. Figure 13 Seakeeping tests. Roll RAO in beam waves at 0 knots. 6. DISCUSSION 6.1 Comparison between the Models The segmented and continuous model show close resistance and seakeeping behaviour. The same resistance, sinkage and trim characteristics can be observed in the whole speed range, with an error usually within 4-5%. This gives the confidence that the two models are geometrically similar and that the correct displacement and CG position were achieved. The heave and pitch responses in head seas closely compare between the models, with an error generally within 2-3%. In addition to the outcome of the resistance tests, this suggests that also the longitudinal and vertical distribution of the ballast was correctly represented. 12
The roll motions in beam seas show similar trend of the RAO curves whilst the error between the motion magnitudes varies with the wavelength. It is believed that for small λ/loa values, the mooring lines could have affected the vessel motions. However, the overall agreement between the roll motions of the two models was considered adequate to prove that the transverse and vertical ballast distribution was correctly represented. The motion signals recorded in high frequency waves (λ/loa = 0.6 1.0) are characterised by small motion amplitudes and high level of noise - the noise is likely to be introduced by the potentiometers used to measure heave and pitch, which fail to capture very small rotations. As a consequence, the motion responses of the two models can diverge significantly. This should not raise concern, since the motion magnitudes observed in these wave frequencies are small compared with the maximum values. 6.2 Comparison with the Numerical Simulations In the speed range 0-10 knots, the trend and amplitude of the RAO curves are similar between numerical and experimental results in the whole range of wavelengths tested. This shows that the hydrodynamic model adequately predicts the vessel motions. Pitch in head seas at 0 knots seems to be an exception, with experimental results up to 25% larger for waves of λ/loa> 1.5. The reason for such discrepancy is still under investigation. As expected, at higher speeds, from 15 knots upwards, the motion magnitudes predicted numerically are overestimating the actual response over part of the wavelength range tested. Possible reasons have been already discussed in [1] and can be summarised as follows: - Likely overestimation of the maximum motions by seakeeping software due to the poor damping of potential theory based solvers. - Lack of linearity at high speed. - Limit of linear theory, which is not capable of capturing the planing hydrodynamics. 7. CONCLUSIONS Towing tank tests are being carried out with two scaled models of the Severn Class: a continuous and a segmented model. The segmented model was built as a whole and preliminary tests were conducted to ensure that both models have the same seakeeping characteristics before one being segmented. The results obtained from these tests give confidence that the models were built and ballasted to show similar hydrodynamic behaviour, which makes the segmentation stage now possible. The comparison of motion RAOs between the experimental tests and the numerical model from [1], suggests that the hydrodynamic model accurately predicts the vessel behaviour at low speed. As expected, its validity becomes questionable at high speed, where the linear potential theory underpinning the model fails to capture the planing hydrodynamics. Towing tank tests with the segmented model will be carried out to investigate the seakeeping global loads of the vessel in waves. Full-scale seakeeping trials are also being conducted on a Severn Class lifeboat in a range of sea states, speeds and headings. The vessel has been instrumented to record motions, accelerations and structural response due to global and local loads. The results from these trials will provide a complete picture of the behaviour of the craft and of the loads experienced in real operational conditions to further validate scale and numerical models. They will also inform the predictive capability of the numerical model and make it possible to enhance its accuracy. 13
8. ACKNOWLEDGEMENTS 24 th International HISWA Symposium on Yacht Design and Yacht Construction This work is part of a research project funded by the RNLI and supported by Lloyd s Register. The authors thank the Technician Team of Newcastle University s Towing Tank for their assistance throughout the experiment process and Prof Mehmet Atlar for the original idea of the towing arrangement. REFERENCES [1] F. Prini, S. Benson, R. W. Birmingham, R. S. Dow, H. J. Phillips, P. J. Sheppard, and J. Mediavilla Varas, Seakeeping Analysis of a High-Speed Search and Rescue Craft by Linear Potential Theory, presented at the International Conference on Lightweight Design of Marine Structures, Glasgow, UK, 2015. [2] D. M. V. Roberton, R. A. Shenoi, S. W. Boyd, and S. Austen, A Plausible Method for Fatigue Life Prediction of Boats in a Data Scarce Environment, in Proceedings of the 17th International Conference on Composite Materials, Edinburgh, UK, 2009. [3] D. M. V. Roberton, Residual Life Assessment of Composite Structures: With Application to All Weather Lifeboats, PhD Thesis, University of Southampton, 2015. [4] G. Fridsma, A Systematic Study of the Rough-Water Performance of Planing Boats, Davidson Laboratory Stevens Institute of Technology, Report 1275, Nov. 1969. [5] V. Bertram, Practical Ship Hydrodynamics, Second Edition. Butterworth-Heinemann, 2012. [6] C. Zhao, M. Ma, and O. Hughes, Applying Strip Theory Based Linear Seakeeping Loads to 3D Full Ship Finite Element Models, in Proceedings of the 32nd International Conference on Ocean, Offshore and Arctic Engineering, Nantes, France, 2013. [7] Recommended Procedures and Guidelines - Global Loads Seakeeping Procedure, ITTC, 7.5-02-07-02.6, 2011. [8] F. D. Hudson, The Design and Development of Modern SAR Craft - A Personal View, presented at the International Conference on Surveillance, Pilot and Rescue Craft for the 21st Century, Gothenburg, 1997. [9] M. B. Bashir, L. Tao, M. Atlar, and R. S. Dow, Hydrodynamic Performance of a Deep-Vee Hull Form Catamaran in Regular Waves, in Proceedings of the 30th International Conference on Ocean, Offshore and Arctic Engineering, Rotterdam, NL, 2011. 14