Showing papers in "Journal of Marine Science and Technology in 1998"

A B-spline Galerkin scheme for calculating the hydroelastic response of a very large floating structure in waves

Abstract: This paper presents an effective scheme for computing the wave-induced hydroelastic response of a very large floating structure, and a validation of its usefulness. The calculation scheme developed is based on the pressure-distribution method of expressing the disturbance caused by a structure, and on the mode-expansion method for hydroelastic deflection with the superposition of orthogonal mode functions. The scheme uses bi-cubic B-spline functions to represent unknown pressures, and the Galerkin method to satisfy the body boundary conditions. Various numerical checks confirm that the computed results are extremely accurate, require relatively little computational time, and contain few unknowns, even in the region of very short wavelengths. Measurements of the vertical deflections in both head and oblique waves of relatively long wavelength are in good agreement with the computed results. Numerical examples using shorter wavelengths reveal that the hydroelastic deflection does not necessarily become negligible as the wavelength of incident waves decreases. The effects of finite water depth and incident wave angle are also discussed.

Finite-volume simulation of flows about a ship in maneuvering motion

Abstract: A finite-volume method of computing the viscous flow field about a ship in maneuvering motion was developed. The time-dependent Navier-Stokes equation discretized in the generalized boundary-fitted curvilinear coordinate system is solved numerically. A third-order upwind differencing scheme, a marker and cell (MAC)-type explicit time marching solution algorithm and a simplified subgrid scale (SGS) turbulence model are adopted. The simulation method is formulated, including the movement of a computational grid fitted to the body boundary that allows computation of the flow field around a body under unsteady motion. To estimate the maneuvering ability of a ship, the accurate prediction of the hydrodynamic forces and moments of the hull is important. Therefore, experimental methods of finding the hydrodynamic forces of a ship in maneuvering motion, such as the oblique towing test, the circular motion test (CMT) and planar motion mechanism (PMM) test, were established. Numerical simulation methods for those captive model experiments were developed introducing computational fluid dynamics (CFD). First, numerical methods for steady oblique tow and steady turn simulation were developed and then extended to unsteady forced motion. Simulations were conducted about several realistic hulls, and the results were verified by comparisons with measured results obtained in model experiments. Hydrodynamic forces and the moment, the longitudinal distribution of the hydrodynamic lateral force, and the pressure distribution on the hull surface showed good agreement.

Locomotion by mechanical pectoral fins

Abstract: The objectives of this study were the development of a new device for maneuvering an underwater vehicle using the mechanism of a fish swimming, an experimental and theoretical analysis of the hydrodynamic characteristics of the device, and its application to maneuvering a fish robot. Observations and experimental analysis of the pectoral fins of a black bass (Micropterus salmoides) revealed that the locomotion of the fish, such as swimming forward at low speed, swimming backward, and turning in a horizontal plane is generated by using a combination of a feathering motion and a lead-lag motion of the pectoral fins. A mechanical pectoral fin making a feathering motion and a lead-lag motion generates a thrust force in a range of phase differences between both motions. The unsteady vortex lattice method, including the effect of viscosity, can express fairly well the unsteady forces acting on the mechanical pectoral fin in the range of phase differences where it exerts the thrust force. The fish robot, consisting of a model fish body and a pair of mechanical pectoral fins, can not only swim forward and turn in almost the same position, but can also swim in a lateral direction without changing the yaw angle.

Experimental study on microbubble ejection method for frictional drag reduction

Abstract: The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2–10 m/s It was confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. According to previous experiments, the size of the bubbles is an important factor in frictional drag reduction by microbubble ejection. Usually bubbles larger than a certain diameter (for example 1 mm) have no effect on frictional drag reduction. Three different methods were proposed and tested to generate smaller bubbles. Among them, a 2D convex (half body of an NACA 64-021 section) with ejection holes at the top was the best and most promising. The diameter of the bubbles became about one-third the size of the reference ejection on a flat plate. Moreover, the bubble size did not increase with increasing flow rate. This is a favorable characteristic for practical purposes. The skin friction force was measured directly with a miniature floating element transducer, and decreased drastically by microbubble ejection from the top of the 2D convex shape.

Numerical and experimental study of submerged supple nets: Applications to fish farms

Abstract: Our aim is to investigate the behavior of submerged supple nets. This work generates many problems owing to the discontinuous and highly flexible nature of the nets. Only the action of external forces can bring an infinitely flexible structure like a net into a definite shape. When considering supple nets immersed in a fluid, these external forces themselves depend on the net geometry. A numerical method to solve this fluid-structure coupling problem is proposed, and is applied to fish farms. In order to validate the calculation model of the hydrodynamic forces on the mesh sides, we measured the hydrodynamic forces on a plane panel of netting spread across a transverse current. We thus proved that the Landweber model modified according to the Richtmeyer formula as regards friction gives good results. The calculated shape of the fixed net cage is qualitatively in accordance with flume tank observations. We have adapted the algorithm to the study of the dynamic behavior of floating fish farms.

Fatigue crack propagation and computational remaining life assessment of ship structures

Abstract: The fitness for serviceability of structural members of marine structures in which fatigue cracks might be found during in-service inspection is investigated in order to prevent instantaneous failures of ships, as well as a loss of serviceability such as the oil- and/or watertightness of critical compartments. The essential features of fatigue crack propagation and the remaining life assessment are discussed in the first part of the paper, where the effects of weldment, complicated stress distributions including stress biaxialities at three-dimensional structural joints, structural redundancy, and crack curving are found to be of primary importance. The second part of the paper contains a discussion of an advanced numerical simulation method for the remaining life assessment, in which the above-mentioned effects of fatigue crack propagation are taken into account. The simulated crack paths and the fatigue crack propagation lives are found to be in fairly good agreement with the experimental results.

Response-based extreme value analysis of moored offshore structures due to wave, wind, and current

Abstract: Floating moored offshore structures have a significant future in offshore operations as an attractive economic alternative to fixed structures in deep waters and/or in areas where there is no existing infrastructure This paper describes an analysis procedure based on the structure variable approach to estimate load and response values of a moored offshore platform at a given return period by taking into account the joint occurrence of wave, wind, and current The results show that the most severe mooring loads may not occur when wind, wave, and current are collinear and are at their maximum design values, ie, the 50- or 100-year case It is recommended that the extreme mooring design loads for moored offshore systems should be determined through a range of physical or numerical simulations where wave, wind, and current are noncollinear and act with less severe magnitudes than the 50- or 100-year case This recommendation has also been adopted in the ITTC/Ocean Engineering Committee recommendations to the ITTC Conference held in September 1996

The drag on an airplane taking off from a floating runway

Abstract: The motions of an infinitely long, two-dimensional runway subjected to the dynamic moving load imposed by an airplane taking off are investigated. The runway is assumed to be floating in an inviscid fluid and is initially at equilibrium before the plane takes off. The deformation of the runway resulting from the take-off is wave-like and moves in the same direction as the plane. The maximum drag occurs when the plane catches up with the first wave. Three different runway configurations were considered: a baseline and ones which were ten and one hundred times more flexible than the baseline. For these runways, the added drag to the aircraft was very small, ranging from 1% for the stiffest to 10% for the most flexible runway.

Analysis of the accident of the MV Nakhodka. Part 2. Estimation of structural strength

Abstract: In the early morning of January 2, 1997, a Russian tanker, the MVNakhodka, broke in two in the Sea of Japan. The fore part of the vessel drifted and was stranded on the coast of Japan, and the aft part sank. The coast of Japan was seriously polluted by spilled heavy oil. Following this disaster, the Japanese Government established a Committee for the Investigation of the Causes of the Casualty of theNakhodka. This paper deals with the structural strength of MVNakhodka at the time of the accident. First the structural characteristics of theNakhodka are described, and the reduction in thickness of the structural members are estimated based on the data measured on the fore part of the vessel which drifted ashose. Then the ultimate longitudinal strength of the hull girder at the time of the accident is evaluated by applying Smith's method, and the possibility of break-up collapse due to excess loads is discussed. The mechanism of fracture at the bottom plate is also discussed based on the observed fracture surfuce of the cross section. Finally an FEM (finite element method) simulation of the break-up of the hull girder is performed. It is shown that buckling/plastic collapse took place at the deck plate near Fr.153, which was followed by the successive buckling collapse of the side shell plate of the hull girder. Right after the collapse of the deck structure, the bottom plate fractured just in front of the transverse bulkhead at Fr.153.

A study on flow field around full ship forms in maneuvering motion

Abstract: To estimate the maneuvering ability of a ship, an accurate estimation of the hydrodynamic forces and moment acting on the ship's hull is indispensable. For the purpose of developing a numerical method of computing the viscous flow field around a hull and evaluating its validity, the hydrodynamic pressure on the hull and the velocity field were measured. Two full ship models with different hull forms in the aft part were used for the experiment. From the results of pressure measurements, the distribution of hydrodynamic lateral forces was obtained. The simulation method is a numerical solution of the Navier-Stokes equation based on a finitevolume method and applied to the maneuvering motion. The measured and computed results agree qualitatively well, and the method is a valuable tool for estimating the maneuvering ability of a ship. The typical characteristics of the flow field in the steady turning condition are revealed by the numerical simulation, and the mechanism of the relations between hull form, flow field, and hydrodynamic forces are clarified.

Design system for optimum contra-rotating propellers

Abstract: A new type of contrarotating propeller (CRP) system has been developed through the cooperative research work of five shipbuilding companies in Japan (Hitachi Zosen Corporation; Kawasaki Heavy Industries, Ltd.; Mitsui Engineering and Shipbuilding Co., Ltd.; NKK Corporation; and Sumitomo Heavy Industries, Ltd.). This paper describes a design system for an optimum CRP, which is one of the numerous outcomes of this work. The optimum design system is composed of three theoretical programs: (1) the design program of the optimum CRP; (2) the steady lifting surface program of the CRP; (3) the unsteady lifting surface program of the CRP. These theoretical programs will be discussed in the first part of the paper, and the design system supported by these theoretical programs will then be verified by comparing calculated and experimental results.

Analysis of the accident of the MV Nakhodka . Part 1. Estimation of wave loads

Abstract: MVNakhodka collapsed and broke in two on January 2, 1997 in the Sea of Japan, giving rise to a serious and disastrous oil spill on the coastline of Japan. During the inquiry into the cause of the accident, one of the main tasks of the inquiry committee was to identify the external loads which made the ship structure collapse. Among the several possible scenarios for the accident, after careful examination, the wave loads in the heavy weather at the time of the accident were taken to be the most plausible cause. The results of that research are described in two papers, and the present paper deals with the way in which wave loads on theNakhodka at the moment of the accident were estimated. We first describe the details of the accident such as the location, the sea condition, the loading condition, and the ship's heading speed. Then the static loads resulting from the distribution of the cargo weight and the buoyancy are given to show that the static bending moment at the time of the accident caused extreme sagging. Next the wave loads in the irregular waves, calculated by the nonlinear time domain simulation program SRSLAM, are presented. It is shown that the bending moment in the waves reaches 1 087 800 KN*m at maximum SS 6.9, which is where the hull girder collapse took place. It was concluded that the ship broke in two because the bending moment exceeded the hull girder strength which had been reduced due to corrosion, taking into account conclusions derived from the research dealing with the structural strength aspect. We also discuss the stochastic significance of the estimated value and nonlinear nature of the peak distribution, in addition to the effects of the ship speed and wave heading on the wave load estimation.

Vibration reduction of main hulls using semiactive absorbers

Abstract: A semiactive-type absorber for vibration reduction of main hull girders was investigated. The semiactive absorber system includes a moving mass, support springs, dynamic dampers, and a control system. Only a small electrical power supply is needed for control of the damper valve and the operation of the control system. In this paper, the dynamics of the ship's hull and the constraints of the semiactive absorber are described first. Then, a suboptimal operation law is derived based on the properties of the absorber and the theory of optimal vibration reduction. The numerical simulation results show that the semiactive absorber is more efficient in hull vibration reduction than the passive absorber during critical periodical excitation from the propeller. The vibration caused by multifrequency excitation can also be suppressed by the semiactive absorber. In terms of effectiveness, the semiactive absorber is almost as effective as the active absorber. In particular, the performance of the semiactive absorber is excellent in the reduction of high-frequency fluctuations.

Numerical analysis on free surface waves and stern viscous flow of a ship model

Abstract: This paper deals with numerical techniques for computing the viscous flow past a ship hull with and without a free surface using a Reynolds-averaged Navier-Stokes solver with global conservation. In the first technique, a coarse grid is used to find an approximate solution to the free surface problem. Interpolation of a fine grid is subsequently carried out, and a more exact solution, particularly in the boundary layer and wake, is obtained. In the second technique, a modified Baldwin-Lomax model is introduced to compute the viscous flow with and without a free surface. These numerical techniques are applied to simulations of the flow around a Series 60 and an SR196C ship model. The results are compared with measurement data, and the usefulness of the numerical techniques is demonstrated.

Active control for installation of underwater structures

Abstract: The trend towards deepwater development requires a new approach to underwater installation of offshore structures. The present method using crane vessels has some drawbacks in operations at more than 2000 m depth. The natural period of the coupled system of the rigged structure and the crane vessel becomes longer, so that it is no longer possible to manipulate the cranes to achieve the desired positioning accuracy. This paper examines the application of an active control technique for underwater installations as one of the solutions to the present problems. An active control technique also has the advantage that it can deal with the structural flexibility which allows the structure to be large and light-weight. This structural flexibility imposes problems of suppressing the elastic responses and securing the stability of the control system. In this paper, anH ∞ controller combined with a low authority control/high authority control (LAC/HAC) feedback controller is designed for cases where structural flexibility cannot be ignored. A robust model-following controller is examined for cases where the structure can be treated as rigid. In order to confirm the control algorithm and verify the possibility of the active control installation method, basin tests are executed using two types of neutrally buoyant flexible models with ultrasound ranging systems and thrusters.

Fully three-dimensional ship seakeeping computations with a surge-corrected Rankine panel method

Abstract: A 3-d seakeeping code uses first-order Rankine panels with special numerical integration on the ship's hull and Rankine point source clusters above the free surface. The code computes the motions of the ship in regular waves of small height (linearized). The steady flow is captured without simplification by solving the fully nonlinear wave-resistance problem first. A special treatment of the surge motion considers the influence of periodic quantities on thrust and resistance, and improves surge motion predictions. Radiation and open-boundary conditions are enforced by staggered grids. Results for the ITTC standard test case S-175 containership agree well with experiments except for very long waves. The importance of capturing the three-dimensional steady flow contributions is also demonstrated.

Systematic study on the hull form design and resistance prediction of displacement-type super-high-speed ships

Abstract: Systematic theoretical and experimental studies were carried out to establish methods for hull form design, optimum dimension selection, and resistance estimation for displacement-type super-high-speed ships. In this study, a theoretical hull form design method for displacement-type super-high-speed ships was first developed using the minimum resistance theory and a sectionally varying hull form equation. Using an established hull form design method, a series of 60 hull forms were prepared with systematic variations of the most important design variables, and model tests were conducted for these ship models. Finally, regression analyses were performed for the results of the model tests. The study was very successful, and the prepared computer programs are now being actively used as efficient tools for the design of the displacement-type super-high-speed ships.

Basic studies on accuracy management systems based on estimating welding deformations

Abstract: Accuracy management concerns various aspects of all assembly industries, e.g., final product quality, manufacturing efficiency, and manufacturing costs. In the near future, accuracy management will be even more important in manufacturing systems because of mechanization and automation. This paper considers the concept of computer-aided accuracy management systems, which are based on the concept of integrated manufacturing systems. We propose two management systems, accuracy planning and accuracy estimation, and concentrate on accuracy planning. A model of welding deformations and an easy estimation method for such deformations by computer analysis using the finite element method (FEM) are described. A prototype system based on the accuracy management concept was implemented in the authors' integrated manufacturing systems, and some examples of estimating welding deformations are given here.

Distribution of void fraction in bubbly flow through a horizontal channel: Bubbly boundary layer flow, 2nd report

Abstract: A method of enveloping the hull with a sheet of microbubbles is discussed. It forms part of a study on means of reducing the skin friction acting on a ship's hull. In this report, a bubble traveling through a horizontal channel is regarded as a diffusive particle. Based on this assumption, an equation based on flow flux balance is derived for determining the void fraction in approximation. The equation thus derived is used for calculation, and the calculation results are compared with reported experimental data. The equation is further manipulated to make it compatible with a mixing length model that takes into account the presence of bubbles in the liquid stream. Among the factors contained in the equation thus derived, those affected by the presence of bubbles are the change of mixing length and the difference in the ratio of skin friction between cases with and without bubbles. These factors can be calculated using the mean void fraction in the boundary layer determined by the rate of air supply into the flow field. It is suggested that the ratio between boundary layer thickness and bubble diameter could constitute a significant parameter to replace the scale effect in estimating values applicable to actual ships from corresponding data obtained in model experiments.