Motions and slamming impact on catamaran

Prediction of craft motions and the dynamic loads acting on a catamaran hull are of great importance to the designer. This paper presents the motions of a Vosper International catamaran in head seas with and without forward speed. Two approaches are used—strip theory and the 3D pulsating source method. A method to predict slamming loads acting on this catamaran section using Computational Fluid Dynamics is presented. The loads acting on catamaran hulls and the cross structure are illustrated.     

Numerical and experimental study of a flapping foil generator

The energy extraction performance of a flapping foil generator is studied through experiment and numerical simulation. A practical flapping foil generator has been proposed. The heave motion and the pitch motion of the foil are adjusted through a crankshaft-like structure. The heave and pitch motions of the foil are transferred to the rotational motion of the main shaft. A pair of gears is adopted to increase the pitch angle. A prototype with pitch amplitude θ₀ = 60^∘ has been built and the experiment is carried out in a tunnel. The overall performance of the mechanism has been analysed. Good agreement of numerical results and experiment data has been found. Further simulations with larger pitch amplitudes are carried out. It is found that higher efficiency can be achieved with larger pitch amplitude at medium frequency.     

Time domain simulation of side-by-side floating bodies using a 3D numerical wave tank approach

The coupled system of two side-by-side fixed and/or floating bodies interacting with a large amplitude nonlinear wave is studied using a direct time domain solution method. The numerical model is based on a three-dimensional mixed Eulerian–Lagrangian (MEL) method under certain simplifying approximations permitting Rankine panel scheme to be implemented over a time-invariant boundary surface to solve the boundary value problem for the unknown velocity potentials. A 4th order Adams–Bashforth–Moulton scheme is used for time marching of rigid-body motion histories of the individual bodies and evolution of the free-surface including the gap region in which large resonant fluid motions occur. A systematic study has been carried out to evaluate the performance of the developed time domain method in simulating the forces and motions as well as the fluid motion in the gap region for the two body system under various arrangements and in different wave-headings. At first, the computed numerical results have been validated and verified with computational and experimental results available in literature for standard geometries such as vertical truncated cylinders and rectangular boxes. Secondly, effectiveness of the damping lid model which is introduced to suppress wave resonance in the gap region is investigated including its influence on maximum sway forces on fixed and floating rectangular barges in side-by-side configurations. Thirdly, comparative studies on absolute and relative motion response for two cases (two rectangular barges, and a FLNG-FPSO + shuttle tanker) in side-by-side arrangement are detailed to bring out the importance of nonlinearities arising due to steep nonlinear incident waves. Finally, coupled motions of the two-body system of an FPSO and a shuttle tanker floating in side-by-side configuration in a steep nonlinear wave field are studied in which the two bodies are connected through hawsers, and also the FPSO is moored to the ground. Additionally there is a fender between the two bodies.     

A nonlinear model of parametric rolling stabilization by anti-roll tanks

The present paper describes a mathematical model in which the fluid motion inside a U-tank is nonlinearly coupled to the heave, roll and pitch motions of the ship. The main purpose of the investigation is centred on the control of roll motion in the case of parametric resonance in longitudinal waves. A transom stern small vessel, known to be quite prone to parametric amplification, is employed in the study. Four tank designs are employed in order to study the influence of tank mass, tank natural frequency and tank internal damping on the control of parametric rolling at different head seas conditions. Additionally, the influence of the vertical position of the tank is also investigated. The main results are presented in the form of limits of stability, with encounter frequency and wave amplitudes as parameters. Distinct dynamical characteristics are discussed and conclusions are drawn on the relevant parameters for the efficient control of the roll amplifications in head seas.     

Rapid swing and spin of [deep] taut-wire-moored instruments

Rapid ‘swing’, compass variations O(10°) in O(10 s), and ‘spin’, complete rotations around the vertical axis within a few minutes, are a concern of acoustic current meters moored in-line. Observations are used from fast sampling, at once per 1 and 30 s, instrumentation on deep-ocean moorings mainly outside surface wave and bottom boundary influences. Such instruments do not require a vane common to some historic mechanical current meters and they are often moored in a much easier to handle sub-surface buoy or mounting rack, without vanes. In their mountings they are nearly symmetric, so that they can spin freely in (turbulent; shear) flows. A comparison is made between noise levels of such free spinning instrumentation with those of instruments mounted in a fixed bottom-frame and with those of instruments equipped with a vane to one side. Typical spinning has a single rotation varying between 40 and 200 s. Spinning is shown to be highly binary: on or off. Its effects are found negligible on estimates of ocean currents, provided compass updates are adequate as in existing instrumentation. Acoustic noise is O(10) times larger than noise due to spinning. Some effects of spinning are noticed in the acoustic echo amplitude showing higher noise at frequencies >100 cpd, cycles per day. The character of this noise changes dramatically due to spinning. However, it is mainly in the ocean turbulence range and does not affect measurements of internal waves or periodic zooplankton motions.     

Assessment of URANS surface effect ship models for calm water and head waves

Surface effect ship (SES) air cushion and seal models are implemented in an URANS hydrodynamics solver. The air cushion is modeled either as a prescribed pressure patch, or as a compressible isothermal/adiabatic ideal stagnant air with fan and leakage flows. The seals are either discretized as hinged bodies or modeled as 2D planing surfaces with hydrodynamic interaction. Verification and validation studies are performed using T-Craft experimental data for calm water resistance, sinkage and trim at Froude number (Fr) = 0.1–0.6; impulsive heave and pitch decay at Fr = 0; and wave-induced resistance and motion predictions in head waves at Fr = 0 and 0.6. The compressible air cushion model with fan and leakage flows perform better than those without the fan and leakage flows and the prescribed pressure patch model. The hinged seal model performs better than the 2D planing surface model, but is computationally expensive for time accurate simulations. Therefore, the 2D planing surface model is used for the validation studies. SES simulations on grids with 5.3 M cells show grid verification intervals of 6%, which are comparable to those reported for displacement and semi-planing hull studies on similar grid sizes. On an average calm water and impulsive motion predictions compare within 8.5% of the experimental data, and wave-induced motion predictions show somewhat larger error of 13.5%. The errors levels are mostly comparable to those for displacement and semi-planing and planing hulls. The study identifies that most critical advancement needed for SES simulations is the seal modeling including fluid structure interaction.     

The deep-water motion through the Lifamatola Passage and its contribution to the Indonesian throughflow

In order to estimate the contribution of cold Pacific deep water to the Indonesian throughflow (ITF) and the flushing of the deep Banda Sea, a current meter mooring has been deployed for nearly 3 years on the sill in the Lifamatola Passage as part of the International Nusantara Stratification and Transport (INSTANT) programme. The velocity, temperature, and salinity data, obtained from the mooring, reflect vigorous horizontal and vertical motion in the lowest 500 m over the ～2000 m deep sill, with speeds regularly surpassing 100 cm/s. The strong residual flow over the sill in the passage and internal, mainly diurnal, tides contribute to this bottom intensified motion. The average volume transport of the deep throughflow from the Maluku Sea to the Seram Sea below 1250 m is 2.5 Sv (1 Sv=10⁶ m³/s), with a transport-weighted mean temperature of 3.2 °C. This result considerably increases existing estimates of the inflow of the ITF into the Indonesian seas by about 25% and lowers the total mean inflow temperature of the ITF to below 13 °C. At shallower levels, between 1250 m and the sea surface, the flow is directed towards the Maluku Sea, north of the passage. The typical residual velocities in this layer are low (～3 cm/s), contributing to an estimated northward flow of 0.9–1.3 Sv. When more results from the INSTANT programme for the other Indonesian passages become available, a strongly improved estimate of the mass and heat budget of the ITF becomes feasible.     

Numerical investigation of the seakeeping behavior of a catamaran advancing in regular head waves

A numerical study was undertaken in order to assess the capability of an unsteady RANS code to predict the seakeeping characteristics of a high-speed multi-hull vessel in high sea states. Numerical analysis includes evaluation of ship motions, effects of wave steepness on ship response, catamaran natural frequency and added resistance in waves. Computations were performed for the DELFT 372 catamaran by the URANS solver CFDSHIP-Iowa V.4. The code was validated with encouraging results for high ship speeds (0.3≤Fn≤0.75) and high wave amplitudes (0.025≤Ak≤0.1). Comparison with strip theory solutions shows that the RANS method predicts ship motions with higher accuracy and allows the detection of nonlinear effects. Current computations evidence that heave peaks occur at resonance for all Fn, and reach the absolute maximum at Fn=0.75. Maximum pitch occurs at frequencies lower than resonance, for each speed, and absolute maximum occurs at medium Fn=0.6. Maximum added resistance, R_aw, was computed at Fn=0.45, which, interestingly, is near the catamaran Fn_coincidence. Overall, we found similar results as Simonsen et al. (2008) for KCS containership, though, herein, a multi-hull geometry and higher speeds were tested. Also, our results are useful to further evaluate the exciting forces and their correlation with f_e and λ/L_pp.     

Diagnosis of vertical velocities with the QG omega equation: an examination of the errors due to sampling strategy

Vertical motion at the mesoscale plays a key role in ocean circulation, ocean-atmosphere interaction, and hence climate. It is not yet possible to make direct Eulerian measurements of vertical velocities less than 1000 m day⁻¹. However, by assuming quasi-geostrophic (QG) balance, vertical velocities O (10 m day⁻¹) can be diagnosed from the geostrophic velocity field and suitable boundary conditions. Significant errors in the accuracy of this diagnosis arise from the necessary compromise between spatial resolution and synopticity of a hydrographic survey. This problem has been addressed by sampling the output of a numerical ocean model to simulate typical oceanographic surveys of mesoscale fronts. The balance between the number of observations and the synopticity of observations affects the apparent flow and in particular the diagnosed vertical motion. A combination of effects can typically lead to errors of 85% in the estimation of net vertical heat flux. An analytical two-layer model is used to understand components of this error and indicate the key parameters for the design of mesoscale sampling.     

Numerical investigation into the application of response conditioned waves for long-term nonlinear fatigue analyses of rigid hulls

Previous studies of response conditioned wave methods have been focused on their applicability to the prediction of extreme nonlinear wave-induced load effects. The results showed that theses methods can be used to accurately and efficiently predict the nonlinear short-term probability distributions for rigid hull responses. This has led us to investigate how response conditioned wave methods can be used for long-term nonlinear fatigue analyses, and with which accuracy this can be done. In this paper we present the results from our investigation. The studies were performed using a container vessel with a length between perpendiculars of 281 m. Calculations were done with a nonlinear strip theory method in which the hull of the vessel was assumed to be rigid. The most likely response wave (MLRW) method was used to condition the waves. Only head seas were considered. We found that the MLRW method under-predicted the long-term fatigue damage by 3%. The method, however, required a simulation time that was approximately three orders of magnitude less than that required for a conventional long-term nonlinear analysis based on random irregular waves. A preliminary investigation showed that due to lacking springing and whipping contributions the MLRW method under-predicted the fatigue damage for a flexible hull by approximately 50%. Several comments about a more accurate extension of the proposed method to flexible hulls are included.     

Investigation of motions of catamarans in regular waves—I

In this paper an analytical technique based on the two-dimensional Green function method associated with a cross-flow approach for taking viscous effects into account to estimate the motion response of catamarans in the frequency domain is presented. In order to validate this method, the numerical results are compared with experimental values obtained for two different catamarans (ASR5061 [Wahab, R., Pritchett, C. and Ruth, L.C. 1971. On the behaviour of the ASR catamaran in waves. Marine Technology, 8, 334–360] and Marintek [Faltinsen, O., Hoff, J.R., Kvalsvold, J. and Zhao, R. 1992. Global loads on high speed catamarans. 5th Int. Symp. on Practical Design of Ships and Mobile Units, University of Newcastle-upon-Tyne, 1.360–1.373]).In the second part of the paper the tests carried out with a third catamaran configuration at the Hydrodynamics Laboratory of the University of Glasgow are presented to evaluate the non-linear effects. These test results cover different speeds and wave heights at a wide range of wave frequencies. The paper concludes that the two-dimensional method correlates very well with measurements of small amplitude motions. For large amplitude motion tests, the non-linear effects become significant when the model speed and wave amplitudes increase. The peak values of heave and pitch motions measured around the resonance frequency are smaller than those obtained from the linear theory.     

Waves in the nearshore waters of northern Arabian Sea during the summer monsoon

Waves at 15 m water depth in the northern Arabian Sea are measured during the summer monsoon for a period of 45 days and the characteristics are described. The significant wave height varied from 1.1 to 4.5 m with an average value of 2.5 m. 75% of the wave height at the measurement location is due to the swells arriving from the south-west and the remaining is due to the seas from south-west to north-west. Wave age of the measured data indicates that the waves in the nearshore waters of northern Arabian Sea during the summer monsoon are swells with young sea.     

Energy performance evaluation of fishing vessels by fuel mass flow measuring system

A new fuel consumption monitoring system was set up for research purpose in order to evaluate the energy performance of fishing vessels under different operating conditions. The system has been tested on two semi-pelagic pair trawlers in the Adriatic Sea with an engine power of around 900 kW, and with length overall of around 30 m. Both vessels work with a gear of similar design and size, the differences between the two vessels are in the propeller design and the hull material: the first with a controllable pitch propeller (CPP) and a metal hull, the second with a fixed pitch propeller (FPP) and a wooden hull. The fuel monitoring system conceived at CNR-ISMAR Ancona (Italy) consists of two mass flow sensors, one multichannel recorder and one GPS data logger. The working time duration, the vessel speed, the total fuel consumption and the instant fuel rate were logged by the system. A typical commercial round trip for a semi-pelagic trawler consists of several fishing operations (steaming, trawling sailing, etc.). Fuel consumption rate and vessel speed data were used to identify energy performance under different vessel-operating conditions. The highest fuel demands were during the trawling (130 l/h at 4.4 kn) and the steaming (100–130 l/h at 11 kn) phases. Fuel savings of up to 15% could be obtained by reducing the navigation speed of half a knot.     

Coupled heave and pitch motions of planing hulls at non-zero heel angle

Current paper presents a mathematical model based on 2D-asymmetric wedge water entry to model heave and pitch motions of planing hulls at non-zero heel angles. Vertical and horizontal forces as well as heeling moment due to asymmetric water entry are computed using momentum theory in conjunction with added mass of impact velocity in vertical and horizontal directions. The proposed model is able to compute sway and yaw forces, roll moment, as well as heave and pitch motions in calm water and regular waves. Validity of the proposed model is verified by comparing the results against existing experimental data in both symmetric and asymmetric conditions. Ultimately, different parametric studies are conducted to examine the effects of non-zero heel angle on dynamic vertical motions. The resulting sway and yaw forces due to asymmetric motion are also derived and effects of heel angle on these side forces are investigated.     

Design of a prototype ocean current turbine—Part I: mathematical modeling and dynamics simulation

The “C-Plane” is a submerged variable depth ocean current turbine that is tethered to the sea floor and uses sustained ocean currents to produce electricity. As part of the development of a $\frac{1}{30}$th scale physical model of the C-Plane, a mathematical model and dynamics simulation of the prototype was developed and is presented in this paper. This three-dimensional mathematical model represents the C-Plane as a rigid body with moveable control surfaces that is moored with three linear elastic cable elements. Gravitational, buoyancy, hydrodynamic, cable, gyroscopic, and inertial forces are included and a PC-based dynamics simulation is created. The simulation demonstrates that the C-Plane is stable and capable of changing depth in all expected operating conditions. The C-Plane prototype can fly level from a height of 3 to 6 m using the configuration suggested in this paper. The maximum ascent rates of the C-Plane with a water speed of 0.3 m/s are 0.015 m/s when the pitch is fixed at 0° and 0.030 m/s when the pitch is fixed at 4°. The maximum descent rates of the C-Plane are 0.018 m/s when the pitch is held at 0° and 0.031 m/s if the pitch is held at −4°.     

Numerical prediction of green water incidents

Green water loads on moored or sailing ships occur when an incoming wave significantly exceeds the freeboard and water runs onto the deck. In this paper, a Navier–Stokes solver with a free surface capturing scheme (i.e. the VOF model; [Hirt and Nichols, 1981]) is used to numerically model green water loads on a moored FPSO exposed to head sea waves. Two cases are investigated: first, green water on a fixed vessel has been analysed, where resulting waterheight on deck, and impact pressure on a deck mounted structure have been computed. These results have been compared to experimental data obtained by [Greco, 2001] and show very favourable agreement. Second, a full green water incident, including vessel motions has been modelled. In these computations, the vertical motion has been modelled by the use of transfer functions for heave and pitch, but the rotational contribution from the pitch motion has been neglected. The computed water height on deck has been compared to the experimental data obtained by [Buchner, 1995a] and it also shows very good agreement. The modelling in the second case was performed in both 2-D and 3-D with very similar results, which indicates that 3-D effects are not dominant.     

Cause and countermeasure of long-period oscillations of moored ships and the quantification of surge and heave amplitudes

Long-period oscillations of moored ships whose periods are about 1 or 2 min cause many troubles in many ports and harbours. It is necessary to investigate these phenomena and verify their causes and countermeasures in each case because they are strongly dependent on the environment of each port and harbour. From this point of view, long-period oscillations of moored ships in the Port of Shibushi in Japan were investigated by means of wave observations, the image processing of moored ship motions using the video camera and motion-capture software and numerical simulations. From observation results, the relationship between offshore long-period waves and long-period oscillations of moored ships was recognized and surge and heave amplitudes were quantified by using wave data in order to forecast moored ship motions. Furthermore, from observation and numerical results, it was revealed that long-period waves with the peak period of 120 s from the offshore typhoon kept or exaggerated the local harbour oscillation of 60–70 s and it caused long-period oscillations of moored ships. Numerical results in case of reducing the reflection coefficient of the target berth implied that it ceased the local harbour oscillation and it would give an effective countermeasure to reduce long-period oscillations of moored ships in the Port of Shibushi.     

Inertial and tidal shear variability above Reykjanes Ridge

Yearlong 75 kHz acoustic Doppler current profiler (ADCP) data were obtained well above Reykjanes Ridge (northern extension of the Mid-Atlantic Ridge (MAR)). The area is characterized by relatively large semidiurnal tidal (‘D₂’) currents that have (at lunar M₂) more than half a decade larger variance than inertial (f) currents. However, despite the relatively weak near-inertial kinetic energy, its vertical current shear shows larger magnitudes than at M₂ in an otherwise flat f−D₂ band limited between frequencies 0.74 and 1.35f, which equals the inertio-gravity wave bounds [σ_min, σ_max](N=f). N represents the buoyancy frequency. The shear in this band dominates all shear computed at 20 m effective vertical scale. As the kinetic energy spectrum peaks at M₂, but not (significantly) at S₂ and N₂, a difference in tidal (and inertial) scales and hence sources is observed. M₂-tides contribute mostly to large-scale coherent motions. The dominant incoherent f−D₂ shear is highly variable in time (∼2-day periodicity). Furthermore, inertial and tidal shear are more or less completely separated in space and time, each occurring in different layers in the vertical. The thin shear layers reflect the rapidly varying short vertical scale N_s profile, to within the ∼20 m limitation of ADCP data, rather than the large-scale smooth N_L. In each of large-N_s layers Ri≈1, probably. The yearlong smoothed shear magnitude follows N_L, but only as stable Ri≈5. The shear polarization is more circular than rectilinear, albeit varying with time, and highly symmetric around f. During transitions, e.g., between stratified and homogeneous layers and between waves from varying sources, near-circular motions can generate near-rectilinear shear in the direction of wave propagation (in the direction of the minor axis of the current ellipse). This contrasts with the possibility of near-rectilinear barotropic oscillatory motions generating near-circular shear under viscosity in shallow seas.     

Effect of waves on cavitation and pressure pulses of a tanker with twin podded propulsion

There is increasing interest in optimizing ships for the actual operating condition rather than just for calm water. In order to optimize the propeller designs for operations in waves, it is essential to study how the propeller performance is affected by operation in waves. The effect of various factors that influence the propeller is quantified in this paper using a 8000 dwt chemical tanker equipped with twin-podded propulsion as a case vessel. Propeller performance in waves in terms of cavitation, pressure pulses, and efficiency is compared with the performance in calm water. The influence of wake variation, ship motions, RPM fluctuations and speed loss is studied. Substantial increase in cavitation and pressure pulses due to wake variation in the presence of waves is found. It is found that the effect of other factors is relatively small and easier to take into account as compared to wake variation. Therefore, considering the wake variation at least in the critical wave condition (where the wavelength is close to ship length) in addition to calm water wake is recommended in order to ensure that the optimized propeller performs well both in calm water and in waves.