Wave-piercing Catamaran Research Articles

Slamming Kinematics, Impulse and Energy Transfer for Wave-Piercing Catamarans

Slamming Kinematics, Impulse and Energy Transfer for Wave-Piercing Catamarans

Effect of Slam Force Duration on the Vibratory Response of a Lightweight High-Speed Wave-Piercing Catamaran

When the surface of a ship meets the water surface at an acute angle with a high relative velocity, significant short-duration forces can act on the hull plating. Such an event is referred to as a slam. Slam loads imparted on ships are generally considered to be of an impulsive nature. As such, slam loads induce vibration in the global hull structure that has implications for both hull girder bending strength and fatigue life of a vessel. A modal method is often used for structural analysis whereby higher order modes are neglected to reduce computational effort. The effect of the slam load temporal distribution on the whipping response and vertical bending moment are investigated here by using a continuous beam model with application to a 112 m INCAT wave-piercing catamaran and correlation to full-scale and model-scale experimental data. Experimental studies have indicated that the vertical bending moment is dominated by the fundamental longitudinal bending mode of the structure. However, it is shown here that although the fundamental mode is dominant in the global structural response, the higher order modes play a significant role in the early stages of the response and may not be readily identifiable if measurements are not taken sufficiently close to the slam location. A relationship between the slam duration and the relative modal response magnitudes is found, which is useful in determining the appropriate truncation of a modal solution.

Effect of Slam Force Duration on the Vibratory Response of a Lightweight High-Speed Wave-Piercing Catamaran

Effect of Slam Force Duration on the Vibratory Response of a Lightweight High-Speed Wave-Piercing Catamaran

Pressure Distribution due to Stern Tab Deflection at Model Scale

"Trim tabs form an important part of motion control systems on high-speed watercraft. By altering the pitch angle, significant improvements in propulsion efficiency can be achieved by reducing overall resistance. For a ship in heavy seas, trim tabs can also be used to reduce structural loads by changing the vessel orientation in response to encountered waves. In this study, trials have been conducted in the University of Tasmania hydraulics laboratory using a closedcircuit water tunnel to measure model scale trim tab forces. The model scale system replicates the stern tabs on the fullscale INCAT Tasmania 112 m high-speed wave-piercer catamaran. The model was designed for total lift force measurement and pressure tappings allowed for pressures to be measured at fixed locations on the underside of the hull and tab. This investigation examines the pressures at various flow velocities and tab deflection angles for the case of horizontal vessel trim. A simplified two-dimensional CFD model of the hull and tab has also been analysed using ANSYS CFX software. The results of model tests and CFD indicate that the maximum pressure occurs in the vicinity of the tab hinge and that the pressure distribution is long-tailed in the direction forward of the hinge. This accounts for the location of the resultant lift force, which is found to act forward of the tab hinge."

Low Reynolds Number Performance of a Model Scale T-Foil

"Submerged T-foils are an essential forward component of the ride control systems of high speed ferries. A model scale T-Foil for a 2.5m towing tank model of a 112m INCAT Tasmania high-speed wave-piercer catamaran has been tested for both static and dynamic lift performance. The tests were carried out using a closed-circuit water tunnel to investigate the lift and drag characteristics as well as frequency response of the T-Foil. The model T-Foil operates at a Reynolds number of approximately 105, has an aspect ratio of 3.6 and a planform which is strongly tapered from the inboard to outboard end. All of these factors, as well as strut and pivot interference, influence the steady lift curve slope of the model T-foil which was found to be 61% of the value for an ideal aerofoil with elliptic loading. The T-foil dynamic performance was limited primarily by the stepper motor drive system and connection linkage. At the frequency of maximum motion of the 2.5 m catamaran model (about 1.5Hz) the model T-foil has approximately 5% reduction of amplitude and 15 degrees of phase shift relative to the low frequency response. Only very small limitations arose due to the unsteady lift as predicted by the analysis of Theodorsen. It was concluded that the model scale T-foil performed adequately for application to simulation of a ride control system at model scale."

Slam Characteristics of a High-Speed Wave Piercing Catamaran in Irregular Waves

Slam characteristics of a 112m INCAT wave piercing catamaran in a range of realistic irregular sea conditions are presented in this paper. Towing tank testing of a 2.5 m hydroelastic segmented catamaran model was used to gather a database of slam events in irregular seas. The model was instrumented to measure motions, centrebow surface pressures and forces, encountered wave elevations and wave elevations within the bow area tunnel arches. From these measurements characteristics of the vessel slamming behaviour are examined: in particular relative vertical velocity, centrebow immersion, archway wave elevations and slam load distributions. A total of 2,098 slam events were identified over 22 different conditions, each containing about 80 to 100 slam events. The data, although inherently scattered, shows that encounter wave frequency and significant wave height are important parameters with regard to centrebow slamming. Relative vertical velocity was found to be a poor indicator of slam magnitude and slams were found to occur before the centrebow arch tunnel was completely filled, supporting the application of a two-dimensional filling height parameter as a slam indicator.

Slam occurrences and loads of a high-speed wave piercer catamaran in irregular seas

In order to optimise the structural design of large lightweight high-speed catamarans, a thorough understanding of the structural loads is required. Not only is slam severity an important factor in the structural design, but also the occurrence rates of such events in realistic sea conditions is vital for long-term load estimations. The slamming behaviour of a 2.5-m hydroelastic segmented model representative of an Incat wave piercer catamaran was investigated over a range of realistic irregular sea conditions. The model was instrumented with strain gauges to record centrebow slam loads, pressure transducers, wave probes and linear variable displacement transducers. More than 2000 slam events were identified over 22 test conditions, providing an extensive database of slam events. Slam events were identified from the pressure transducer measurements and two important slam parameters investigated: occurrence rates and slam severity. The slam magnitudes were found to be scattered; numerous outliers were detected with magnitudes up to 4 times the median observed in every tested condition. Slam occurrence rates and severity are generally greater in conditions where motions are large. A slam occurrence threshold was identified by extrapolating the experimentally measured occurrence rates. For a 112-m vessel with speeds between 20 and 38 knots and with a modal wave period of 8.5 s, slams are shown not to occur at significant wave heights less than 1.5 m. The slam loads and occurrence data obtained through scale model testing can be used by high-speed catamaran ferry designers to assist the development of structural load cases and operation limits for future designs.

An insight into the slamming behaviour of large high-speed catamarans through full-scale measurements

The slamming behaviour of a large high-speed catamaran has been investigated through the analysis of full-scale trials data. The US Navy conducted the trials in the North Sea and North Atlantic region on a 98 m wave piercer catamaran, HSV-2 Swift, designed by Revolution Design Pty Ltd and built by Incat Tasmania. For varying wave headings, vessel speeds and sea states the data records were interrogated to identify slam events. An automatic slam identification algorithm was developed, considering the measured rate of change of stress in the ship’s structure coupled with the vessel’s pitch motion. This has allowed the slam occurrence rates to be found for a range of conditions and the influence of vessel speed, wave environment and heading to be determined. The slam events have been further characterised by assessing the relative vertical velocity at impact between the vessel and the wave. Since the ship was equipped with a ride control system, its influence on the slam occurrence rates has also been assessed.

Wave slamming loads on wave-piercer catamarans operating at high-speed determined by hydro-elastic segmented model experiments

Catamaran vessels operating at high-speed can be exposed to deck diving and bow damage and one resolution of this problem is the wave-piercer design of INCAT Tasmania. Owing to the complexity of the unsteady non-linear flow in the bow area during large wave encounter model testing has been undertaken to identify the peak dynamic slam loads on the ship structure. This paper provides experimental benchmark information relating to the wave slam loads on wave-piercing catamaran ferries. Since the time frames of transient slam loadings and whipping vibration of the entire hull in its first bending mode are similar it is important that the test model replicates the whipping response and therefore needs to be a hydro-elastic model. A 2.5 m hydro-elastic segmented catamaran model has been developed based on the 112 m INCAT Tasmania wave-piercer catamaran to establish the peak wave slamming loads acting on the full-scale vessel. Towing tank tests were performed in regular seas at a maximum full-scale operating speed of 38 knots. The model was instrumented to measure the dynamic slam loads acting on the centre bow and vertical bending moments acting in the demihulls of the catamaran model as a function of wave frequency and wave height. Peak slam loads measured on the centre bow were found to approach the total weight of the model, this being a broadly similar result to the peak loads measured at full-scale. It was found that global dimensionless heave and pitch accelerations peaked in the same range of encounter frequency as did the peak slam load.

Optimization of a 17m Catamaran Based on the Resistance Performance

Based on Computational Fluid Dynamics (CFD) method and the wave theory, the analysis of hydrodynamic characteristics is made for a wave-piercing catamaran (WPC) in this paper. The resistances of the WPC with different demi-hull spacing are calculated by use of FLOW-3D, and the relationship of the wave resistance with the demi-hull spacing is also discussed. The comparison of results show that the computational resistance matches the test date well in the design speed. This computational method can predict the coefficient of WPC accurately and can be used as a guidance of WPC initional design and optimization on the demi-hull spacing.

Analysis of wave slam induced hull vibrations using continuous wavelet transforms

Large high-speed wave-piercing catamarans are subject to continuous wave induced hull vibrations during their lifetime of operation. In severe sea conditions, the vessel experiences high load impacts, known as slamming, accompanied by high frequency structural response giving rise to fatigue effects. Classical vibration analysis techniques such as the Fourier Transform fail to identify the exact response frequencies of slamming events due to the transformation to the frequency domain and the loss of temporal information about these transient events which is of great importance to fatigue analysis. The work presented in this paper introduces, describes, applies and recommends the continuous wavelet transform as an effective means to investigate the wave induced hull vibrations in both the time and frequency domains simultaneously.

規則波中を航走する波浪貫通型双胴船に発生する不規則船体運動に関する研究

Since wave-piercing catamarans were developed based on the concept to reduce wave-induced ship motions in high-speed, in Australia, the ships have special features, like two slender demi-hulls with sharp bow to reduce reserve buoyancy above the water surface, a center bow to provide reserve buoyancy in heavy waves and so on. Therefore, it is easily imagined that these features may generate nonlinearity or irregularity of ship motions in rough seas. In order to investigate the nonlinearity and the irregularity in ship motions of the ship, measurements of ship motions in various wave directions, heights and periods by using a model of the ship were carried out. The experimental results demonstrate that nonlinear features to the change in wave height appear in ship motion amplitudes in regular waves and suggest that linear ship motion theories widely used in a design stage of ship building do not work well. In shorter waves, irregular ship motions and twice period ship motions appear even in regular waves. Since the nonlinearity of ship motions is not so strong, the effect of the nonlinearity on the operation of such ships has not been pointed out as a serious problem.To investigate the cause by which the irregular ship motions appear in regular waves, ship motions were calculated by a nonlinear strip method. The results indicate that irregular ship motions of the ship were caused by drastic change of restoring force which acting on the cross deck structure of the ship.

波浪貫通型双胴船におけるデッキ上の車両転倒に関する研究

波浪貫通型双胴船におけるデッキ上の車両転倒に関する研究

Computation of wet deck bow slam loads for catamaran arched cross sections

A computational model for catamaran wet deck slamming is developed on the basis of the variation of added mass as the hulls enter the water. In the case of a wave-piercing catamaran the bow cross section has a double arch cross section and slamming occurs when the arches fill. Residual air is entrained at the top of the arch due to bubble formation by turbulent mixing and this modifies the effect of the water added mass on the hull. The computational model therefore introduces a soft connection between the water added mass associated with the slam and the hull. The method has been evaluated by comparison with two-dimensional model drop tests in terms of the maximum forces and acceleration imposed on the hull, the variation of velocity during the slam event and the depth of penetration into the water. It is concluded that the added mass computation is adequate for slam modelling in global motions and loads calculations since it gives a good representation of the maximum total forces on the section and their duration.

大波高中における波浪貫通型双胴高速船の運動特性に関する実験的研究

大波高中における波浪貫通型双胴高速船の運動特性に関する実験的研究

Global and slam loads for a large wavepiercing catamaran design

Prediction of design loads for small high speed commercial craft has traditionally been governed by classification society, rule based formulae. Larger high speed vessels, particularly those greater than 100 m in length, require validation of rule based design loads. Validation methods include model tests or hydrodynamic studies using computer based motion and loads software. This paper presents the results of studies into the calculation of global loads for a 112 m, wavepiercing catamaran design. In particular, the preliminary design approach for assessment of loads is discussed. Longitudinal bending and pitch connection moment loads are derived from linear and nonlinear motion and loads software packages FASTSEA, SWAN and BESTSEA. The computed results are compared with rule based loads and empirically derived loads based on full-scale measurements from similar craft. Local wet deck slamming loads are discussed with reference to current rule based formulae and results of full scale tests on similar vessels. The effect of high local slam loads is discussed and how these loads may influence the design process. Conclusions are drawn on the presented results concerning the interaction of global and local loads in the design of a 112 m wavepiercing catamaran for Incat shipbuilders in Hobart, Australia.

Dynamic Wave Loads On a High Speed Catamaran Ferry Fitted With T-Foils and Stern Tabs

Measurements of unsteady stress have been made on an INCAT wave-piercing catamaran during an extended period of passenger operations under varying sea conditions, vessel speed and heading relative to the sea. It has been found that the stress fluctuations are consistent with unsteady bending of the hull cross section. The measured results are compared with the predictions of a time domain, spatially fixed, strip theory method suitable for computation at high Froude number. It is found that there is a progressive nonlinear reduction of global bending moment and sectional shear fluctuations relative to wave height as the wave height increases. The intensity of the observed stress fluctuations is shown to be consistent with the loading predictions of the time domain method. © 2005: Royal Institution of Naval Architects.

Skid Helicopters on High-Speed Craft

Helicopter decks are common throughout the commercial and military shipping industries and also the offshore industry. Construction in steel and aluminum is common. Helicopter decks on high-speed craft are not common. The first known helicopter deck installed on a high-speed craft was on an Incat wave-piercing catamaran, HSV X1, Joint Venture, which saw service during the recent Gulf war and is currently still in service with the US Army. Following successful construction and NAVAIR certification of Joint Venture, a more advanced aluminum deck, certified also by NAVAIR and Naval Sea Systems Command (NAVSEA), has been fitted to HSV 2 Swift, which is the latest Incat 98 meter "SeaFrame" HSC. HSV 2 Swift is in service with the US Navy. This paper will focus on the design challenge that came about on HSV 2 Swift in the design of the deck to land and park skid type helicopters as opposed to a helicopter with pneumatic tires. High-speed craft are by their nature innovative, and new solutions to old problems are constantly being experimented with to ensure that the tenets of high speed and high efficiency are optimized. Weight minimization is the most critical performance aspect of a high-speed craft, and conservative or simplified analysis is not practical or economic. A high-speed craft relies on accelerating through hump or critical speed to obtain the high operating speeds of around 40 knots and greater. The ability to operate above hump speed is absolutely reliant on the weight of the vessel. Unnecessary weight on the vessel that does not have absolute mission or operation justification adversely affects the ability to operate above hump speed. Aluminum creates additional and very different challenges compared to a design in traditional steel. Alternative details and construction techniques are required for successful design in aluminum in terms of fatigue and ultimate strength. One innovation common in high-speed craft is aluminum extrusion of a top hat form. The top hat offers big savings in terms of ultimate strength and reduction in mass while keeping weight to a minimum. To aid in verifying the design of the Helo deck extrusion on HSV 2 Swift (Incat Yard 061) for the AH-1 and UH-1 helicopters (H-1 series skid type helicopters), analysis and physical testing were carried out. There had been some doubt that conventional hand calculations were suitable for a top hat style extrusion. The analysis and testing proved that extruded aluminum sections of top hat design are suitable for the H-1 series helicopter skid loading and that permanent deformation was negligible at the design load and even at significantly above the design load. The physical test is also further evidence to support the use of welded 6000 series extrusion in high-speed military vessels. Original design of the deck extrusion revolved around class rules, linear static finite element analysis (FEA), and military codes. Later analysis involved nonlinear FEA, further military code calculations, first principles hand calculations based on available text, and physical testing.

Investigation of Seakeeping Characteristics of High-Speed Catamarans in Waves

This paper presents the motion characteristics for high-speed catamarans travelling in waves. Two mathematical models, threedimensional translating-pulsating source distribution technique and three-dimensional pulsating source distribution technique, are used for predicting the motions of the wave-piercing catamaran in oblique waves. The comparison between numerical predictions and experimental data carried out in SSPA shows a good agreement except the one around the resonance regions of pitch motion. It may be due to the nonlinear effects of centre bow with large pitch motions in waves. The phase angles show some scatters in the high frequencies region at stern waves. The dynamic motions of the wave-piercing catamaran have been investigated by means of short-term statistical analysis with three-dimensional pulsating source distribution technique. The comparison of seakeeping performance between the conventional catamaran and the wave-piercing catamaran has been discussed.

Slamming Response of a Large High-Speed Wave-Piercer Catamaran

Commercial and military requirements for high-speed sea transportation has led to the evolution of large, fast, lightweight vessels. Knowledge of the effect of sea loads on the structure of such vessels is required in order to carry out structural design optimization. Wet deck slam events, which occur when the vessel's motion causes an impact between the cross-deck structure and the water surface when operating in large waves, are of particular importance for high-speed catamarans. Extensive full-scale hull stress, motion, and wave measurements have been conducted on an 86-m Incat high-speed catamaran ferry during a delivery voyage. A definition of a slam event, for this vessel, is proposed and used to identify slam events from the data records. The character and effects of these slamming events are investigated with respect to a number of factors, including structural loading, wave height and length, vessel speed and heading angle, relative vertical velocity, and frequency of occurrence. The results of the full-scale data analysis are presented. Particular attention is paid to the whipping response of the structure, with the principal structural response frequencies being identified through spectral analysis.