Head Sea Waves Research Articles

Numerical study of nominal wake fields of a container ship in oblique regular waves

Accurate estimation of the actual nominal wake behind the ship in waves is important for propeller designers when improving efficiency and minimizing cavitation. The present CFD study investigates the nominal wake fields of the Kriso Container Ship (KCS) in regular waves with wavelength equal to the ship length in five different headings. The ship is sailing at design speed and the wave steepness is 1/60. The results show that when sailing in the studied waves, the nominal wake fraction fluctuate up to 39% of the mean nominal wake fraction. The mean nominal wake fraction is higher than in calm water for all headings besides head sea waves. The maximum mean nominal wake fraction is found to occur in stern quartering sea waves, with a 16% higher mean nominal wake fraction than in calm water. It is also found that the transient bilge vortex and shadow from the skeg have a significant influence on the nominal wake field. Finally the study shows that the modified advance angle on the r/R=0.7 circle in the propeller plane varies 3.5 degrees more in port stern quartering than in calm water, increasing the risk of cavitation.

Hydroelastic Modeling of Wet Deck Slamming on Multihull Vessels

Slamming against the wet deck of a multihull vessel in head sea waves is studied analytically and numerically. The theoretical slamming model is a two-dimensional, asymptotic method valid for small local angles between the undisturbed water surface and the wet deck in the impact region. The disturbance of the water surface as well as the local hydroelastic effects in the slamming area are accounted for. The elastic deflections of the wet deck are expressed in terms of "dry" normal modes. The structural formu­lation accounts for the shear deformations and the rotatory inertia effects in the wet deck. The findings show that the slamming loads on the wet deck and the resulting elastic stresses in the wet deck are strongly influenced by the elasticity of the wet deck structure.

Numerical and experimental study on the parametric roll resonance for a fishing vessel with and without forward speed

The parametric resonance in roll motion (known shortly as parametric roll) was studied for a fishing vessel in regular waves. This is an instability and resonance phenomenon that can lead the roll to reach very high oscillation amplitudes at its natural frequency, depending on the damping level involved. In the worst cases it is responsible for vessel capsize. Here the problem was investigated numerically and experimentally, performing dedicated physical tests on a typical Norwegian fishing vessel with blunt hull and small length-to-beam ratio.Experimentally, a dedicated study was carried out on the SFH112 fishing vessel in scale 1:10 in regular head-sea waves with different wave frequencies and steepnesses. The tests were performed without and with forward speed, with corresponding Froude number Fn = 0.09 and 0.18. Numerically, a blended method was developed based on a 6-DOF 3D hybrid method where the radiation and diffraction potentials were computed for zero forward speed by WAMIT and used in the STF strip theory to obtain speed dependent loads. The convolution integrals were used to account for the effect of radiation free-surface memory effect. Nonlinearities in the Froude-Krylov and restoring loads were accounted for by integrating the corresponding pressure terms on the instantaneous wetted-hull surface defined by the incident waves and body motions. Use of the weak-scatterer hypothesis in radiation and diffraction loads has also been considered. The method was applied to reproduce and complement the experiments on the SFH112 fishing vessel. The numerical simulations showed good agreement with the experimental results. For the cases near the instability border of a 1-DOF Mathieu-type instability diagram, the physical and numerical predictions were different in terms of parametric roll occurrence. The instability borders for the experimental cases are also different from the instability borders of 1-DOF Mathieu-type instability diagram. The instability region for the experiments and 6DOF simulations cover a wider range of frequency ratio and the threshold value of metacentric height variation amplitude to have parametric roll seems to be lower than predicted by the 1-DOF Mathieu instability diagram. The results also show that the instability region for the cases with forward speed shifts to the lower frequency ratios(natural roll frequency to encounter frequency ratio) compared to the cases without forward speed. The effect of the weak-scatterer hypothesis on the results is also assessed and shown to be important for parametric roll. The results show that the simulations without this hypothesis tend to underestimate the occurrence and severity of parametric roll especially in longer and steeper waves.

Numerical Analysis of a Surface Ship in Waves

Abstract Ship's resistance and engine power to sustain ship's speed in seaways are augmented due to complex non-linear interactions between the ship and the ambient sea (waves). Ship designers, in early design stage, use an ad hoc "sea margin" to account for the effects of seaways in selecting propeller and engine. A numerical tool capable of accurately predicting added resistance and power of a ship cruising in waves would greatly help select its powering (margin) requirement and determine the optimal operating point that can maximize the energy efficiency. For seakeeping analysis, strip theory-based methods have long been used. More recently, nonlinear time-domain three-dimensional (3D) panel methods have started being used widely. A more physics-based avenue to seakeeping analysis is offered by coupled solutions of two-phase unsteady Reynolds-Averaged Navier-Stokes equations and six degrees-of-freedom rigid-body motion (RBM) equations. The URANS approach also avails itself of including the effects of propulsors, either explicitly or approximately. By accounting for all the nonlinear effects in hydrodynamic forces and moments and the resulting ship motions, and the effects of fluid viscosity and turbulence, the coupled URANS-RBM method is believed not only able to predict added resistance and speed loss more accurately, but also to provide valuable insights into the physical mechanisms underlying added resistance and power. The objectives of this study are: (1) to validate a coupled URANS-RBM solver developed for high-fidelity prediction of added resistance, speed loss and added power on ships cruising in regular head sea and irregular waves, and (2) to conduct a detailed analysis of the interactions among ship hull, propeller and waves for a 1/49 scaled model of the ONR Tumblehome (ONRT) (Model 5613) in order to shed light on the physical mechanisms leading to added resistance, speed loss and added power. Figure 1 depicts the ONRT self-propellers with two 4-bladed propellers in regular waves. The main flow features such as the free surface, the hub vortices and blade-tip vortices from the propeller, as well as vortices generated by the sonar dome, shafts, shaft brackets and bilge keels are captured.

Verification of application of the 2.5D method in high-speed trimaran vertical motion and added resistance prediction

Abstract For the motion prediction of a trimaran, the hydrodynamic interference between main hull and outriggers is a significant factor and need to be taken into consideration. The high-speed slender body potential flow theory (2.5D method) is a high-efficient method to predict the seakeeping performance of ships in waves and has been validated for mono-hull ships. In this paper, the 2.5D method has been applied to predict the vertical motion (heave and pitch) and added resistance of a trimaran in head-sea waves. In addition, a trimaran model test was carried out to analyses the relationship between the travelling speed, outrigger layout, incident waves and heave, pitch amplitude, added resistance of a trimaran. The experimental results are also compared with the numerical results in order to verify the 2.5D method in the application of trimaran motion and added resistance prediction. The comparison results have shown that between Fn = 0.353 and 0.471, the 2.5D method can give out accurate prediction results of vertical motion and added resistance of a trimaran. Meanwhile, the calculation efficiency is high enough which makes it applicable to massive computations.

Slam loads and pressures acting on high-speed wave-piercing catamarans in regular waves

Slamming loads and pressures on high-speed catamarans with a centre bow (CB) differ from those on conventional catamarans with flat deck structures. The latter are well covered by class rules, which provide empirical formulae to calculate the design slamming pressure. An experimental study was therefore performed to quantify slamming pressures in the archways between bow and main hulls and the CB slamming force on a 112 m wave piercing catamaran. The CB length was systematically varied on a 2.5 m hydroelastic segmented catamaran model, which was tested in regular head sea waves at a speed of 2.89 m/s, full-scale equivalent of 38 knots. Slamming pressures were measured by 18 pressure transducers fitted into the CB, while data obtained from CB accelerometers and load cells enabled identification of the slamming force. The results indicate that slamming loads increase significantly with increasing CB length while the maximum peak slamming pressures varies to a lesser extent. It was also found that wave encounter frequency has a strong effect on the location of maximum pressure along the CB, considerably more so than any influence of CB configuration. The distribution of the peak pressures within the CB archway shows that the inboard peak pressures are larger than those at the top of the arch and the outboard locations.

Response of an Aluminum Stiffened Plate Under Extreme Slamming Loadings1

The slamming phenomenon often occurs when the bottom of a ship hits the seawater with a relatively high velocity in irregular waves. In extreme sea conditions, the slam-induced dynamic loads can be quite large resulting in local structural damages and/or global deformation of the panels, in particular when ships face head sea waves with high forward speed. In this respect, high-speed vessels are affected by the slamming phenomenon as they are built with thin steel plates or aluminum. The paper presents finite element simulations performed with the explicit ls-dyna solver of an aluminum stiffened plate subjected to extreme slamming loadings so as to evaluate the large plastic deformation and failure of the stiffened plate and the effect of the heat affected zone (HAZ) due to the welding using the mechanical properties of an aluminum alloy grade 5083-H116. The outcomes of this study identify the parameters that have importance when estimating the extent of structural deformation due to slamming loads.

2DOF numerical investigation of a planing vessel in head sea waves in deep and shallow water conditions

Analysis of a craft with two degrees of freedom (2DOF) consumes time more than simulation of a craft with a fixed trim condition; therefore in most of the previous researches fixed trim condition is taken into account to analyze the flow field around a craft in shallow water and head sea wave conditions. In this paper numerical simulation of Reynolds Average Naiver Stokes (RANS) equations are used to analyze the motion of DTMB 62 model 4667-1 planing vessel in calm water and head sea waves in both deep and shallow water with two degrees of freedom (heave and pitch). For this purpose, a finite volume ANSYS-FLUENT code is used to solve the Navier-Stokes equations for the simulation of the flow field around the vessel. In addition, an explicit VOF scheme and SST k-ω model is used with dynamic mesh scheme to capture the interface of a two-phase flow and to model the turbulence respectively in the 2DOF model.Regarding the results, reducing the wavelength and also the depth of the water can increase the drag force. Also comparing the results of a fixed trim vessel with the results of a free to sink and trim one in calm water shows a difference of approximately 50% in the drag force in shallow water.

The effect of centre bow and wet-deck geometry on wet-deck slamming loads and vertical bending moments of wave-piercing catamarans

An experimental study was performed to determine the influence of centre bow length and tunnel height on the magnitude of the wave slamming loads and bending moments acting on a 112 m Incat wave-piercer catamaran vessel. A 2.5 m hydroelastic segmented catamaran model was tested in regular head sea waves at a high model speed in multiple test series, whilst five centre bow (CB) and wet-deck configurations were considered, designated here as the parent, low, high, long and short CBs. The model global motions, centre bow slam loads, accelerations, and slam induced vertical bending moments of the catamaran model in waves were measured. It was found that the slamming force, the centre bow entry force and slam induced bending moment all increase as the centre bow length increases. Increasing the wet-deck height increased the motions but reduced the maximum slam load in moderate waves. It was seen that the short CB was the best design for the alleviation of slam loads. The high CB was the second best choice for operation in moderate waves but it was the worst configuration in terms of heave and pitch motions among various CB configurations tested.

Numerical study on the manoeuvring of a container ship in regular waves

ABSTRACTShips operate in seaways, but ship manoeuvrability is usually studied in calm water conditions, thus ignoring the effects of wave on the ship hydrodynamic behaviour. So, the assessment of surface ship manoeuvrability in wave environment is more realistic and accurate than its estimation in calm water condition. In the present study, captive dynamic ship model tests are numerically simulated in a computational fluid dynamics environment in regular head sea waves and the hydrodynamic derivatives are derived from the estimated force/moment time series using the Fourier series expansion method. These derivatives and wave excitation forces are fitted in the manoeuvring equations of motion and are solved to simulate the ship standard manoeuvres in head sea waves. Parameters of these definitive manoeuvres in wave condition are compared with those in still-water condition.

URANS predictions on the hydrodynamic interaction of a conceptual FLNG-LNG offloading system in regular waves

Using an unsteady Reynolds-Averaged Navier-Stokes solver, numerical investigations of hydrodynamic interactions of a conceptual FLNG-LNG offloading system in regular head sea waves are presented. Initially, a verification study is performed on estimating the numerical uncertainties within the URANS model. Based on the validated computational setup, the gap wave responses and wave loads on the FLNG and LNG vessels are studied for different wave frequencies and varying lateral separations. URANS predictions on the gap wave responses are compared with that from the experiments and demonstrate better accuracy over potential flow calculations especially at relatively high wave frequency conditions where gap wave resonance occurs. From the data gathered, it is seen that the gap wave resonance appears when the incident wave frequency approaches the natural frequency of the gap fluid, resulting in significant variation of wave loads in the directions of sway, heave, pitch and yaw. Meanwhile, reduction in the lateral separation shifts the occurrence of gap wave resonance to a higher wave frequency and brings more significant exaggerations on the gap waves and wave loads. Furthermore, when comparing model and full scale wave loads and gap wave responses, the two series of data correlate well implying insignificant influence of scale effects.

The influence of the centre bow and wet-deck geometry on motions of wave-piercing catamarans

The effects of tunnel height and centre bow length on the motions of a 112-m wave-piercer catamaran with an above-water centre bow were investigated through model tests. Five alternative centre bow configurations were considered, and multiple series of model tests were conducted in regular head sea waves. The results showed that both heave and pitch increased over a wide range of wave encounter frequency as the wet-deck height of the catamaran model increased. However, increasing the length of the centre bow showed an increase in the pitch but a decrease in the heave for a limited range of wave encounter frequency near the heave and pitch resonance frequencies of the catamaran model. The positions of minimum vertical displacement were found to be aft of the longitudinal centre of gravity, between 20% and 38% of the overall length from the transom. Increase in the wet-deck height and consequently the archway clearance between the main hulls and centre bow also resulted in an increase in the vertical displacement relative to the undisturbed water surface in the centre bow area. The results also indicated the vulnerability to wet-deck slamming for the different bow and wet-deck designs.

Influence of yaw-roll coupling on the behavior of a FPSO: An experimental and numerical investigation

An inconvenience in the experimental set-up of a FPSO in regular waves highlighted occurrence of parametric-roll events promoted by yaw-roll coupling and motivated a combined physical and numerical analysis on the relevance of this phenomenon on the roll resonance, as well as on the water shipping. The model tests examine the ship in head- and bow-sea waves in the zone of the first parametric resonance. Numerically, it is adopted a 3D Domain-Decomposition (DD) strategy combining a weakly-nonlinear potential-flow solver based on the weak-scatterer theory with a shallow-water approximation for the shipped liquid and with a bottom-slamming solution. Detailed comparisons against these and other seakeeping experiments validated the numerical method in its different aspects with global success.At first, a 2-dof equivalent linearized yaw-roll coupled system is examined and the measurements are used to estimate hydrodynamic coefficients required to complete the mathematical model of the problem. Then the DD method is applied to verify the instability occurrence and compared against the experiments. From the analysis, the parametric-roll instability does not occur if all nonlinearities in the roll restoring load are not accounted for. However the amplitude of the resonant roll is affected by the coupling with the other degrees of freedom. Especially the coupling with yaw tends to increase the steady-state roll amplitude. It also affects the water shipping with the trend in reducing its severity for the vessel, this is opposite to the influence of the parametric roll in head-sea waves on the water on deck, as documented in Greco et al. (2014) [4].

Linear seakeeping and added resistance analysis by means of body-fixed coordinate system

This paper presents an alternative formulation of the boundary value problem for linear seakeeping and added resistance analysis based on a body-fixed coordinate system. The formulation does not involve higher-order derivatives of the steady velocity potential on the right-hand side of the body-boundary condition, i.e., the so-called m j -terms in the traditional formulation when an inertial coordinate system is applied. Numerical studies are made for a modified Wigley I hull, a Series 60 ship with block coefficient 0.7, and the S175 container ship for moderate forward speeds where it is thought appropriate to use the double-body flow as basis flow. The presented results for the forced heave and pitch oscillations, motion responses, and added resistance in head-sea waves show good agreement with experiments and some other numerical studies. A Neumann–Kelvin formulation is shown to give less satisfactory results, in particular for coupled heave and pitch added mass and damping coefficients.

Speed loss due to seakeeping and maneuvering in zigzag motion

Ships are prone to experience a slight zigzag motion due to the interaction between steering, waves and other external disturbances. The zigzag motion will induce an added resistance and hence speed loss. This paper aims at addressing an effective tool to predict a ship's zigzag motion and azimuth propulsion system behavior in calm water and different sea conditions, and the resulting speed loss. Both model tests and numerical simulations have been carried out. Model tests with a freely running model were done in MARINTEK, Trondheim, Norway. The propulsion system was composed of two azipull thrusters, with autopilot control system to keep the power constant during the test process. Model speeds and the propulsion system behavior were recorded as time series when the models were freely running in calm water and regular head sea waves. Numerical simulations with simplified modular maneuvering model were conducted to verify the model test data. Good agreement between simulations and experimental results was found. The results also showed that when wave added resistance reach its peak value, the vessel has the most severe speed loss.

A numerical study of the second-order wave excitation of ship springing in infinite water depth

Assuming the wave steepness of the incident waves and the ship motions are small, the second-order weakly-nonlinear hydrodynamic problem of a ship moving with constant forward speed is studied numerically in a consistent way. The boundary value problem is formulated in a body-fixed coordinate system and the perturbation scheme is used. This formulation does not include any derivatives of the velocity potential on the right-hand side of the body-boundary conditions, and thus avoid the difficulties associated with the terms similar to the so-called mj-terms and their derivatives. The second-order sum-frequency wave excitation of ship springing is studied in both monochromatic and bichromatic head-sea waves. Different Froude numbers are considered. A time-domain Higher-Order Boundary Element Method based on cubic shape function is used as a numerical tool. An upstream finite difference scheme is used for longitudinal derivative terms in the free-surface conditions. For a modified Wigley hull in head-sea waves, it is found that the second-order velocity potential gives dominant contribution to second-order wave excitation of ship springing in the wave frequency region where sum-frequency springing occurs. Quadratic velocity terms in the Bernoulli equation have a relatively small contribution. The numerical results also demonstrate strong dependency of the second-order wave excitation of ship springing on the Froude numbers for small wave lengths. The effect of beam and draft is investigated.

Towards a fully 3D domain-decomposition strategy for water-on-deck phenomena

A numerical approach has been used to analyze the water shipping caused by head sea waves for a FPSO ship at rest. A 3D Domain-Decomposition (DD) strategy is used, where a linear potential-flow seakeeping analysis of the vessel is coupled with a local nonlinear rotational-flow investigation for the prediction of water-on-deck phenomena. The Navier-Stokes solver is applied in the region close to the ship bow. It combines a finite-difference spatial algorithm with a predictor-corrector time scheme. The sea and ship surfaces are tracked with a Level-Set (LS) technique and a hybrid Eulerian-Lagrangian algorithm. The inner solver receives the initial and boundary conditions in terms of velocity, pressure, sea-surface location and ship motions and provides the loads due to the nonlinear wave-ship interaction (including green-water loads) to the seakeeping method. Here the inner solver and its implementation within the DD are described in detail. Preliminary results in terms of water-on-deck occurrence are discussed and compared against 3D water-on-deck experiments.

Global Hydroelastic Response of Catamarans Due to Wetdeck Slamming

Global hydroelastic effects due to wetdeck slamming on a high-speed catamaran with forward speed in regular head sea waves are studied experimentally and theoretically. The elastic response is dominated by a two-node bending mode. Predictions of vertical shear forces, bending moments, and slamming force are in fair agreement. Experimental and theoretical error sources are discussed. An important uncertainty is associated with improper measurements of trim angles. The hydrodynamic model of the side hulls needs to be improved by considering hydrodynamic hull interaction.

Influence of Whipping on Long-term Vertical Bending Moment

This paper is concerned with estimating the response value corresponding to a long return period, say 20 years. Time domain simulation is required to obtain the nonlinear response, and long time series are required to limit the statistical uncertainty in the simulations. It is crucial to introduce ways to improve the efficiency in the calculation. A method to determine the long-term extremes by considering only a few short-term sea states is applied. Long-term extreme values are estimated using a set of sea states that have a certain probability of occurrence, known as the contour line approach. Effect of whipping is included by assuming that the whipping and wave-induced responses are independent, but the effect of correlation of the long-term extreme value is also studied. Numerical calculations are performed using a nonlinear, hydroelastic strip theory as suggested by Xia et al (1998). Results are presented for the S-175 containership (ITTC 1983) in head sea waves. The analysis shows that whipping increases the vertical bending moment and that the correlation is significant.

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.