Quartering Seas Research Articles

Measurement and prediction of wave loads on a high-speed catamaran fitted with active stern tabs

Measurements of unsteady loads within the structure of an 86 m high-speed catamaran ferry have been made for a variety of speeds and encountered sea direction and the increase in unsteady bending moment with wave height determined. The measurements are compared with the predictions of a time domain strip theory which predicts unsteady loads and motions. There is a significant non-linear dependence on wave height for both motions and loads. Good agreement is found between computed and measured unsteady bending moments at speeds from 12.5 to 37.5 kt in bow quartering seas. Rather larger computed values were found in head seas and much smaller computed values in beam seas. This is because the direction of encountered seas was not always exactly in the centre of each directional sector, where the computed values are determined.

Nonlinear analysis of parametric rolling in longitudinal and quartering seas with realistic modeling of roll-restoring moment

Parametric rolling of a containership in longitudinal and quartering seas is examined by applying nonlinear dynamics to a 1DOF mathematical model with realistic modeling of the wave effect on roll-restoring moment. In our previous work, we confirmed that a mathematical model with a roll-restoring moment in waves calculated with the Froude–Krylov assumption could considerably overestimate the danger of capsizing associated with parametric rolling. Therefore, in the present work, all numerical calculations based on nonlinear analysis were carried out with the direct aid of a measured roll-restoring moment in waves. For this purpose, captive model experiments were conducted for various sets of wavelengths in longitudinal seas. This experiment demonstrates that the Froude–Krylov prediction could not explain the wavelength effect on restoring moment as the wave-steepness effect. Using the numerical model with the aid of this measured roll-restoring moment, the Poincare mapping technique was applied to identify bifurcation structures of roll motions not only in longitudinal seas, but also in quartering seas. As a result, it was confirmed that capsizing associated with parametric rolling is more likely to occur in following seas than in quartering seas. However, period-doubling and chaos appeared in quartering seas. Finally, an averaging method assuming a period-2 orbit was applied to the same model with the same conditions as the Poincare map. Reasonably good agreement was obtained between the numerical results with a Poincare map and those with the averaging method in longitudinal seas, but the averaging method has limited capability in quartering seas.

소형 트롤 어선의 예망중 동요특성

This paper deals with an experimental study on the dynamical characteristics of ship motion during trawling job on the real sea. The experiments were carried out on the small stern trawler, and roll and pitch motion of the ship according to the wave directions were simultaneously recorded by P/C in the same sea conditions. From these data, the statistical properties and power spectra were obtained, and then the analysis of ship motion in trwal job was made. As a result, it was found that the pitch motions in trwal job were displayed low amplitudes on the whole, but the rolling motions were displayed high amplitudes relatively, and very high value on the beam and quartering sea especially. Also, the trial ship has a high extinction coefficient in the roll motion, compared with the rule of stability, but when wave height takes 2.5m over, it can induce the safety of ship to scare occasionally. Therefore, a usefull countermeasure for the safety of ship has to be made adequately.

An experimental study of motion behaviour with an intact and damaged Ro-Ro ship model

Six degrees of freedom motion response tests of a Ro-Ro model have been carried out in regular waves for intact and damaged conditions. The stationary model was tested in different wave heights and wave frequencies for the head, beam and stern quartering seas in order to explore the effect of damage and varying wave heights on the motion responses of the model. Analysis of the results indicates that damage has an adverse effect on the motion responses of the model depending upon the directionality of the waves and frequency range applied.

선미식 트롤선의 추파중 선체동요특성에 관한 연구

불규칙 해면에 있어서 선미식 트롤선의 추파중의 선체 응답의 특성을 얻기 위하여 3척의 실선운동을 해상에서 계측하고, 이것을 통계적 방법과 이론적인 방법을 이용하여 분석하였으며, 그 결과를 요약하면 다음과 같다. 1. 각 선박의 종동요의 응답의 에너지분포는 추파와 추사파에서 모두 대체로 낮으며, 특히 선박의 크기가 클수록 매우 낮았다 응답스펙트럼의 대역폭은 모두 넓은 편이며, 응답 peak의 주기는 분명하지 않으며, 한 개 이상 나타나는 때도 있다. 2. 각 선박의 횡동요 응답의 에너지 분포는 추파와 추사파에서 모두 크며, 특히 소형선에서는 매우 크다. 응답스펙트럼의 대역폭은 종동요의 응답에 비해 비교적 좁은 편이며, 응답의 peak 주기는 분명하고 한개만 나타난다. 3. 실선실험과 이론계산에 의한 세 선박의 추파와 추사파 중의 종동요와 횡동요응답의 크기와 peak주기는 대체로 양자가 잘 일치하고 있는 것으로 나타나고 있다. 따라서 이론추정치로부터 어선의 응답특성을 평가하는데 큰 문제는 없는 것으로 생각된다. 4. 소형선의 경우 종동요 응답만 고려하면 선미쪽으로 파를 받는 것이 유리하나, 횡동요의 경우에는 오히려 불리하게 되므로 선미쪽으로 파를 받는 경우에는 유의파고와 평균파주기에 따른 자선의 복원성을 고려하는 등 안전에 보다 세심한 주의가 필요하다. 이상과 같이 추파와 추사파 중 선박 크기에 따른 종동요와 횡동요의 응답 특성을 고찰해 보았으나, 이 결과는 제한된 해상의 조건에서 얻어진 것이므로 어선의 일반적인 응답 특성을 도출해내기 위해서는 보다 다양한 해상 조건과 선박을 이용하여 더 많은 연구가 이루어져야 할 것으로 사료된다. In the field of research of sea keeping quality, much development has been made in recent years using the method of calculation based on the strip theory. It is very important to investigate the hull response of a fishing vessel in waves to ensure the safe navigation and fishing operation in rough seas by preserving excellent sea keeping qualities. For this purpose, the author measured various responses of three fishing vessels in waves using real sea experimental measuring system and analyzed the experimental data The results obtained can be summarized as follow. 1. The amplitudes of pitching motion in the experiments appeared low values with more than one peak occasionally in following sea and quartering sea, and the band width of those was found to be wide relatively. 2. The amplitudes of rolling motion in the experiments appeared high values with only one peak in following sea and quartering sea regardless of ship's tonnage, and the band width of those was found to be narrow relatively. 3. The comparisions of theoretical results with those of experiments for the pitching motions and rolling motion in following sea and quartering sea show that the theoretical values are higher slightly than those of experiments in both directions and the period at which the peak appears in the calculations and the experiments has good agreement approximately 4. The calculated responses of two vessels under a assumed wave of 2.2m height and 5.0sec period showed that the response of pitching motion of ship-A are 2.2 times bigger than those of ship-C in following sea and quartering sea, and the response of rolling motion of ship-A is 4.2 times bigger than that of ship-C in quartering sea.

入港時の船速の違いと港口での出会い波との関係に関する基礎的研究

Many habors have been constructed facing to the open sea front in Japan. In a rigorous season the shipping activities are often interrupted due to strong wind and heavy seas. However, these severe situation is not recognized for the poor information. So, we developed measurement system of ship motions and we carry out a filed observation of fishery boat entering into harbor under quartering sea condition. Furthermore, we observe wave heights and wave directions using wave sensors installed on the sea bottom of harbor entrance. The obtained results are summarized as follows : (1) The encounter waves can be calculated by wave heights observed at a fixed point, the two-dimensional ship positions and the main wave direction obtained from the wave direction sensor data. (2) As an example, the period of the observed waves is about 8.4 seconds, whereas that of the encounter waves is about 20 seconds. As the breakwater is constructed in order to shelter the main waves, the entering boat receives the waves almost from behind, and the period of encounter waves becomes longer than that of the actual waves. (3) It is found that the water velocity along running boat direction correspond with the changing of boat speeds beneath the encounter waves.

Motions of a Floating Elastic Plate in Waves (2nd Report)

In the first report, we have developed a new modal analysis method for estimating wave-induced motions of a mat-like very large floating structure (VLFS), and verified it by comparing the numerical results with the experimental ones with respect to the motion amplitudes of the elastic mode in head sea condition. Furthermore, we applied it to a large floating module, and investigated the motion performances of it and disturbance waves around it in head and quartering seas. Finally we showed the response amplitude operators of the first 10 modes in all heading waves with various wave lengths.In this paper, we deals with motion performances of two adjacent mat-like structures in waves which are assumed to be under construction of VLFS. We have applied our method to a Γ-shaped structure and a small rectangular-shaped structure which will be connected to construct VLFS in open seas. First, several lower eigenmodes of the Γ-shaped structure with a length of 600 m and a breadth of 120 m are evaluated. Then its motion responses and wave profiles around it are calculated. In the next place, we investigate the motion performances of the Γ-shaped structure and the small rectangularshaped structure which will be connected and wave profiles around them. Furthermore, we discuss the variations of vertical gap between them and rigidity effects of the structures on the motion performances of these structures in waves.

Motions of a Floating Elastic Plate in Waves

This paper deals with three-dimensional responses of a very large floating structure in head and quartering seas. First we estimate three-dimensional dry-eigenmodes of a completely free plate by employing a finite element method. Then, hydrodynamic pressures exerted on the plate are evaluated by combining a pressure distribution method and the dry-eigenmodes. Wave-induced motions of the plate are finally determined by solving a set of equations of wave-induced motion in the principal coordinates. Present modal analysis method is verified through comparisons of calculated amplitudes of vertical motions and instantaneous shapes of a plate model in head seas with available measured data. They show reasonable agreements. Next, we apply this method to the large floating module of 300mx60m, and investigate the motions of it and the disturbance waves around it in head and quartering seas. Finally, the response amplitude operators of the first 10 modes in all heading waves with various wave lengths are shown.

Surf-riding and oscillations of a ship in quartering waves

The behavior of a ship encountering large regular waves from astern at low frequency is the object of investigation, with a parallel study of surf-riding and periodic motion paterns. First, the theoretical analysis of surf-riding is extended from purely following to quartering seas. Steady-state continuation is used to identify all possible surf-riding states for one wavelength. Examination of stability indicates the existence of stable and unstable states and predicts a new type of oscillatory surf-riding. Global analysis is also applied to determine the areas of state space which lead to surf-riding for a given ship and wave conditions. In the case of overtaking waves, the large rudder-yaw-surge oscillations of the vessel are examined, showing the mechanism and conditions responsible for loss of controllability at certain vessel headings.

An Analysis of a Ship Capsizing in Quartering Seas

This paper is concerned with stability curves and capsizing of a ship in severe quartering waves. The effects of wave height, wave length and heading angles of ship to waves on stability curve are analytically investigated for a container ship. A time domain numerical simulation program for motions and capsizing has been used to investigate motions in a variety of wave configulations. Numerical simulations based on a mathematical model are carried out to predict critical situation leading up to capsizing of a container ship in severe quartering waves. Various phisical mechanism that could be responsible for capsize are pointed out by numerical experiments.

Model studies of the motion response of a damaged four column semisubmersible in regular and irregular waves

A brief review of the stability requirements of semisubmersibles is presented. Results of the dynamic response of a damaged, twin pontoon, four column semisubmersible at a scale of 1:100 are discussed. The tests involved both regular and irregular waves with the model oriented in head, beam and quartering directions for both intact as well as damage conditions. Four damage conditions representing partial damage to one column were simulated: two in windward (positive) direction and two in leeward (negative). The “light damage” condition represented about 9% flooding of the damaged column, while for the “moderate damage” the flooding was equivalent to about 18%. For moderate damage conditions, regular wave studies showed that the motions are essentially nonlinear, although for light damage conditions this cannot be said with certainty. Model tests showed certain asymmetry in the motions of the damage semisubmersible with respect to the position of the damaged column. Moderate damage conditions seemed to produce significant subharmonic response of the vessel in a frequency range which is twice the natural frequency of the vessel in heave. These observations were confirmed from the results of irregular wave studies. Irregular wave studies showed that the quartering sea pitch and roll motions in windward damage conditions are as significant as those in the leeward conditions which was not the case for regular wave studies. The energy due to the motion of the semisubmersible was concentrated in the frequency of 0.7–1.3 Hz, which corresponds to the energetic range of the normal sea state. The natural frequencies of the vessel in damaged condition in pitch, roll and heave are higher than the corresponding frequencies in the intact condition of the vessel. The natural frequency in heave, for both intact and damage conditions, is higher than either those of pitch and roll in similar conditions. These natural frequencies in pitch and roll begin to approach that of the natural frequency in heave as the damage condition increases. This is true irrespective of the position of the damaged column or the sea state. The value of the natural frequency in heave itself increased much more slowly with increase in damage condition. It was inferred that the nonlinear wave pressure term played only a minor role in the asymmetry of motions of the vessel, while the mooring characteristics had a more dominant influence.

漁船の甲板上海水打ち込みについて : 自然の風波中における相対水位変動

It is noteworthy that the shipping water onto deck which occures when the relative wave elevation along the side of a boat gets being over the upper deck side line or bulwark top line is one of the important factors of the capsize of fishing boats. The experiments with a 2m radio controlled model boat in the wind waves were caried out to clarify the statistical properties of the relative wave elevation and the shipping water on deck. As the results of these experiments, the statistical model of the relative wave elevation and occurance of the shipping water were estimated by the analysis of the results of these experiments. The characteristics of these phenomena could be clarified as follows. 1) It was deduced that the variances of the relative wave elevation were large at the foreword of a ship hull in head and bow seas and at the sternward in the following and quartering seas and also, they were larger at weather side than at lee side in the bow, beam and quartering seas. 2) The probability of occurance of the shipping water has the similar tendency to the values mentioned above, however, there is no significant difference between at weather and lee side in their magnitude. 3) It is remarkable that the values of a frequency-band width parameter ε for the relative wave elevation is very large i.e. 0.6〜0.9.

Repeated Assessment of Stress, Mood and Performance Changes Resulting from Exposure to Vessel Motions at Sea

A preliminary study was conducted with six experienced male Coast Guardsmen and a small monohull vessel (95' Coast Guard Patrol Boat) to evaluate the feasibility of a proposed experimental paradigm as well as the sensitivity of an array of performance, physiological and affective state measures to vessel motions and motion sickness. Performance measures (e.g. Navigation Plotting, Critical Tracking, Visual Search, Complex Auditory Monitoring, Grammatical Reasoning, etc.), physiological measures (e.g., motion sickness severity, stress hormone excretion, urine output and specific gravity), and affective state measures (e.g., mood dimensions) were sampled continuously for eight hours each day for three consecutive days. All variables were compared between control (dockside, engines running) and steaming conditions (four-hour octagonal steaming patterns were repeated twice each eight hour day). Results show all physiological measures examined to be sensitive to the Influence of vessel motions or motion sickness. Motion sickness severity was found to rise and fall depending upon the encounter direction of the vessel to the movement of the primary swell; steaming courses with head or bow seas produced significantly greater degrees of illness than did courses possessing stern or quartering seas. Vessel motions led to significant increases in crew fatigue and changes in concentration. Some performance tasks (e.g. Navigation Plotting and Visual Search) were degraded at sea despite the potential contribution of practice effects, habituation to stress and motivation to perform. The Implications of such factors are discussed In terms of past, current and future performance assessment paradigms utilizing repeated testing.

Model Experiments on Capsize and Its Prevention For a Small Fishing Boat in Waves

There have been frequently occured miserable accidents of capsize for small fishing boats in the northern sea area near Japan, which were attended in the most cases with loss of human lives. Such accidents occured on the salmon fishing boats in high percentage in comparison with others.The purpose of this paper is to investigate the important factors affecting to capsize for such a small fishing boat as well as the effectiveness of the mean for preventing the accidents, through the model experiments in heavy seas.In the experiments, the 1/7 scale model was navigated in regular and irregular waves of high steepness generated in the model basin so that the capsize of the model could be expected to occur at some special conditions of the model and sea.The model ship was equipped with the weight shifting device for simulating cargo shift which could occur by heavy rolling motion of ship and would promote the capsize, while, on the other hand the inflatable rubber bulges were also attached on the both sides of the model hull which will lend an additional buoyancy and the righting moment to the ship in the emergency cases.A great number of running was carried out in the experiments under the various ship conditions as well as sea conditions, and as the results some capsizing phenomena were observed in case of poor stability condition. It was deduced through analysis of the experiments that the trapped water on deck with cargo shift act most effectively to capsize the vessel in beam and quartering seas, while the inflated bulges were so efficient to prevent the capsize that no capsize was observed when the inflation of the bulges was timed in spite of the trapped water and weight shift.The theoretical analysis of ship motions, such as rolling, accelerations and relative motions in irregular waves can explain well the experiment results of capsizing phenomena which attracted attention to the danger of navigation in quartering seas having high steepness.

Experimental and Theoretical Study of Motion of a Moored Barge

Abstract A 100-ft-square construction-type platform excited by unidirectional random surface gravity waves was studied in the field, at reduced scale in the laboratory, and by using the linear theory of rigid ships' motion. The platform was spread-moored in about 165 ft of water in the open Pacific Ocean. Water-level variations at three locations, ship rotations and accelerations, mooring forces, and wind velocity were measured in Sea States 2 and 3 (Neumann wind model). A Sea State 4 was simulated in the laboratory. Three recordings each (representing nearly beam-on, quartering and stem-on seas) of the prototype and model tests were analyzed using prototype and model tests were analyzed using time-series techniques to provide amplitude-response operators for all of the ship's motions and mooring forces. The amplitude-response operators were computed from a linear theory based on the slender body approximation. The in-line coupled equations of motion are formulated for the 6 degrees of freedom and solved literally and numerically. The results in the form of complex-response operators are compared with those obtained from the prototype and model measurements. Generally, the agreement in both amplitude and phase is excellent for the surge, sway, roll, heave and pitch motions. Discrepancies found for certain heading and wavelength (or wave frequency) combinations are easily explained by the presence of crosscoupling terms or values of damping coefficientsIt is found that the linear theory of ships' motion using the slender-body approximation is an adequate tool for predicting the behavior of moored platforms in Sea States of up to 3. Introduction Ships of conventional design seldom operate at zero speed in significant waves. As a consequence, only a trivial amount of data has been published on the motions of ships at zero speed. Zero-speed studies of models in the laboratory are difficult to conduct because of the requirement of eliminating wave reflections from the tank sides. Data on the behavior of moored ships, barges and special purpose platforms is scarcer than that for models. platforms is scarcer than that for models. Nevertheless, the recent and continuing demands for ocean construction underline the need for knowledge in this area. The present study is intended to contribute pertinent information. The present study consists of: present study consists of:Analysis of the measurements of the motions induced on an instrumented prototype barge by irregular waves as moored in the open Pacific Ocean in approximately 165 ft of water off San Clemente Island.Analysis of the measurements of the motions induced on a dynamically similar reduced-scale model barge by irregular waves.Determination of the motion response of the barge to representative sea states by application of the linear theory of ships' motion.Selected comparisons of the results obtained from Statements 1 through 3. Interest usually centers on the forces induced in the mooring lines. These forces may be obtained from the motions, once the latter are determined. PROTOTYPE PROTOTYPE The craft studied is a special purpose construction-type platform designated as Fishhook, plan views of which are shown in Fig. 1. The hull plan views of which are shown in Fig. 1. The hull outline is that of two standard (34 × 110 ft) Navy lighter barges (YFNB) separated by a distance of 22 ft, which gives the appearance of a flat-bottomed catamaran. Forward of Frame 5, the center section provides additional buoyancy, while aft of Frame 5, provides additional buoyancy, while aft of Frame 5, the barges are tied together by a structural framework, leaving an open well between the barges. The composite platform supports a massive tower and recovery mechanism whose mass distribution gives the Fishhook some unusual, if not unique, naval architectural and dynamic characteristics. The general location of the study was off of Wilson Cove on the north coast of San Clemente Island, about 65 miles south of Los Angeles. Figs. 2 and 3 present details of the mooring arrangement. The craft was moored by chains in about 165 ft of water. SPEJ P. 206