Ship Responses In Waves Research Articles

Difference in ship response in waves between strip and 3D methods based on integrated formulation

Numerous benchmark calculations have been performed on ship motions and loads in waves. However, there are usually significant differences between codes, even for programs based on the same theory, and it is often extremely difficult to identify the factors that cause such differences. In this paper, the authors present a formulation of the strip method and 3D method in such a way that the common parts between the two are emphasized, and then develop an integrated program that can handle the two methods (STFM and zero-speed Green's function method) based on this formulation. This approach enables the extraction of the differences in the ship response between the two methods caused by the pure difference in the hydrodynamic force of them, i.e., the 3D effect. A comparison using tank tests under a wide range of conditions is performed to clarify the estimation accuracy of the two methods and the tendency of the 3D effect for ship motion, hull-girder sectional force, and hydrodynamic pressure for a container ship and a VLCC.

Study on simplified estimation formula for long-term predictions of ship response in waves – Application to vertical bending moment

This paper proposed a simplified formula for the long-term maximum wave-induced vertical bending moment (VBM) of ships. First, as a general theory independent of the type of ship response, the relationship between the long-term response and the peak value of the response amplitude operation (RAO) was discussed and its governing factors were clarified, and the methodology of the simplified formula development approach was presented. In particular, if the RAO has two properties, “band parameter-invariance” and “peak value-wavelength correspondence,” the long-term response can be expressed by only the peak value of RAO. Based on this approach, a simplified formula for the long-term maximum VBM was presented. First, focusing on the characteristics of the VBM-RAO in the case of zero speed, a formula with zero speed was derived, and based on this, a formula with a speed of 5 knots was derived. These formulae were validated by comparison with the results of a numerical analysis using 154 actual merchant ship models. The proposed method for developing the simplified formula is physically clear and is valid for all types of ships and responses.

A new mathematical hull-form with 10-shape parameters for evaluation of ship response in waves

This study presents a new mathematical hull-form that is expressed as an explicit function with 10 hull-form parameters, which is called the Matsui hull-form in this study. The proposed hull-form was developed by expanding the modified Wigley hull-form so that the following 10 hull-form parameters can be independently varied: main dimensions L, B, d, fineness coefficients {C}_{b}, {C}_{mathrm{m}}, {C}_{mathrm{w}}, second moment of waterplane area coefficient {C}_{mathrm{w}2}, longitudinal center of buoyancy LCB and floatation LCF, and a parameter beta related to anterior–posterior asymmetry. The main purpose of this hull-form is that it is utilized for the following two objects: the first is the simple evaluation of the seakeeping performance and wave loads in the early ship designing stage without any detailed offset data, and the second is a systematical study on the influence of a ship’s dimensions on the ship response in waves. This paper presents the derivation of the Matsui hull-form and the applicability of the proposed hull-from was confirmed by comparing the ship response in waves with the actual ships. Moreover, a sensitivity analysis of the ship response in waves was conducted as an example of the application of the proposed hull-form.

Coupling of dynamic reaction forces of a heavy load crane and ship motion responses in waves

ABSTRACT The conventional approach to dynamic analysis of ship motion response with shipboard operation equipment is usually done by establishing combined equations of motion of the multi-body system. The weakness of such methods is usually associated with the effectiveness of modelling and the simulation efficiency of computing the dynamic ship responses in waves in the time domain. In recent years, time domain simulation of nonlinear ship motion response in waves has become more and more popular. In this paper, we present coupled simulation of a heavy load crane with interactive ship motion responses in waves. The static gravitational forces of the crane system and dynamic excitation forces from the payload are applied to the ship as external forces on varying attack points of the hull during crane operations. Simulation of the crane operation is implemented in the digital twin ship platform and demonstrated meaningful physical behaviours.

船体構造設計のための波浪中応答長期予測値の簡易算式に関する研究

船体構造設計のための波浪中応答長期予測値の簡易算式に関する研究

On the development of a fast modeling of floodwater effects on ship motions in waves

The motions of a flooded ship in waves are difficult to evaluate. They are affected by complex phenomena involving ship and floodwater dynamic interactions. Several numerical methods have been proposed to estimate the dynamic behavior of a damaged ship in waves. In this article, a fast simulation tool is developed to tackle the problem, aiming at a simplified method with an acceptable accuracy of the results. A novel approach in simulating floodwater effects on ship motions is presented. The method allows modeling the floodwater motions (seen as lumped mass) out of phase from ship motions. This technique is based on the basic fluid mechanics knowledge for a liquid in a uniformly accelerated tank. The study intends to analyze the behavior of a flooded ship in regular beam waves, for compartment fillings that involve shallow and intermediate fluid depths. The nonlinear model for the roll damping moment of a ship is implemented. An attempt to model viscous effects in the floodwater dynamics is proposed and applied. Two sets of applications are carried out on two different hull models. The comparisons of the intact and of the flooded ship responses in waves are conducted and discussed.

Prediction of non-linear ship responses in waves considering forward speed effects

This article describes the development of a non-linear time-domain boundary element method to determine non-linear ship responses (motions and loads) in waves. The general approach by Cummins was used to express the equation of motion in the time domain. Hydrodynamic forces were split into inertia, radiation, diffraction, Froude–Krylov and restoring components. Radiation forces were determined in time domain by convolution of the impulse response of the ship with the motion velocity. The Froude–Krylov and restoring forces were computed over the instantaneous wetted surface, taking into account ship motions, the undisturbed wave and stationary wave system. Diffraction forces were computed from the complex force amplitude resulting from the linear problem of the incident wave diffraction. The impulse response of the ship can be determined using the linear hydrodynamic damping coefficients or added masses. In this work, these two approaches were compared. Ship motions calculated with the developed method were compared with model test and RANSE-based simulations as well as with linear frequency domain results for three different ship types at various forward speeds. It was shown that the convolution integral represents the radiation forces satisfactorily. The convolution integral based on damping coefficients showed distinct advantages at zero speed. With increasing forward speed, the added mass-based convolution integral leaded in some cases to better results.

A preliminary experimental investigation into the influence of compartment permeability on damaged ship response in waves

After damage rapid and accurate guidance is necessary to safely recover vessels. An improvement in the understanding of ship behaviour after damage will increase chances of recovery of the vessel and safety for the ship crew and cargo, with corresponding benefits to the maritime environment. Whilst a number of studies have investigated static behaviour of damaged ships with permeability previous research at forward speed considers the damaged compartment to be empty. Novel aspects of this research include the study of a variety of permeabilities and internal arrangements of the damaged compartment for a ship moving in waves.Results from experiments carried out in a towing tank using a segmented ship model show that permeability appears to have a large effect on the pitch and heave motion responses when the vessel is travelling at forward speed, with reduced effect for a stationary ship. Furthermore, results indicate that internal arrangement of compartments has less effect than compartment permeability on the damaged ship responses. Finally, the influence of damaged orifice size on the motion responses is investigated and the results show that at forward speed the responses are significantly affected and that they are largest for the minimum orifice size tested.

Study on Short-term Variability of Ship Responses in Waves

Short-term variability of ship responses is investigated by cross-spectrum analysis. In a steady state condition, it is well known that a certain length of sampled data is required for stable results of the spectral analysis. However, the phase lag between responses, in terms of the phase angle of the cross-spectra, has not been discussed in detail. Using long stationary time series, the transition of amplitudes and relative phase angles of the cross-spectra has been investigated by iterative analyzes with a few seconds of time shifting. In the results, the short-term variability of the relative phase angle was observed. In effect, the variability may compromise the accuracy of the wave buoy analogy.

Numerical simulation of ship stability for dynamic environment

The prediction of ship stability during the early stages of design is very important from the point of vessel’s safety. Out of the six motions of a ship, the critical motion leading to capsize of a vessel is the rolling motion. In the present study, particular attention is paid to the performance of a ship in beam sea. The linear ship response in waves is evaluated using strip theory. Critical condition in the rolling motion of a ship is when it is subjected to synchronous beam waves. In this paper, a nonlinear approach has been tried to predict the roll response of a vessel. Various representations of damping and restoring terms found in the literature are investigated. A parametric investigation is undertaken to identify the effect of a number of key parameters like wave amplitude, wave frequency, metacentric height, etc.

Time Domain Analysis of Ship Responses in Waves

Time Domain Analysis of Ship Responses in Waves

Time Domain Analysis of Ship Responses in Waves

In the numerical simulations of ship maneuvering, a set of ordinary differential equations with constant coefficients is traditionally used. However, when the transient external forces due to rudder motions and waves are concerned, this type of equations of motions is not precise in a strict sense. But the research on the relationship between two types equations is sparse. Therefore, in this paper, the study has been made to clear the relations between them.For this purpose, based on the Strip Theory which is practicaly used to evaluate the seakeeping quality of a ship, the equation of motion described in the time domain with a convolution integral is derived. And by the analytical and numerical considerations, the comparisons of two types equations of motions have been made.

Effects of Axial Force on Deck Stress in Case of Slamming of Large Bulk Carriers

Slamming responses and the effects of axial force on deck stress are discussed regarding a large bulk carrier. Axial force may not be neglected in full-formed ship like bulk carriers particularly in rough seas, because of the high wave height and her large pitching and/or heaving motions. In rough seas, moreover, slamming may occur, so the deck stress is raised due to both the whipping vibrations and the axial force.In order to clarify the characteristics of slamming responses of a large bulk carrier in rough seas, model tests and numerical calculations are carried out : A series of tank tests for slamming is conducted in regular high waves with the use of an elastic model ship, and the deck strains are measured using strain gauges. On the other hand, numerical calculations are performed by virtue of a computer program for nonlinear ship responses in waves called TSLAM, to which a routine for calculation of axial force is newly added in the present paper.Comparing the results of experiments with the calculations and performing the numerical calculations for an actual bulk carrier, following conclusions are obtained : The effects of axial force on deck stress cannot be neglected for large bulk carriers in rough seas and are significant in the fore-body. Pressure force due to pitching motion is dominant in the axial force.

Computation of ship responses in waves using panel method

Hydrodynamic coefficients, Forces / Moments and Motions of a ship moving with a mean forward speed in six degrees of freedom are computed using Panel Method. In this study, an existing numerical model without speed consideration was modified by incorporating the speed parameters. Appropriate Green function was used to calculate the concern velocity potential. The accuracy of the developed numerical code employing the Panel Method has been validated by comparing the result with known/published results of a series 60 ship available in the literature. Based on the results presented in the paper, it can be concluded that the developed model is able to predict the responses of the ship with forward speed effect. Keywords: Motions, Green function, 3D Source distribution. Â doi: 10.3329/jname.v1i1.2037 Journal of Naval Architecture and Marine Engineering 1(2004) 35-46

“On Some Considerations on the Statistical Analysis of Ship Response in Waves”

The impulse response function, which had not been used so commonly in ship response, has come to be rather oftenly discussed in these days. This author has written about the usefulness of that function, and made clear the possibility of getting this statistically from the response in rough sea, through correlogram analysis. Here an example of this method is shown and the merit and the demerit of this method is considered.Secondly, the effect of nonlinearity of the response system on the computation of spectrum and the response function is estimated through an approximation method. The character of the truncation of the spectrum by this effect is introduced, and an example in rolling with square damping is shown.As an Appendix, the data processing scheme for the estimation of the frequency response of a ship are summarized from an effort to standardize the statistical analysis of frequency response function of a system.