Ship Centerline Research Articles

Turbulence characteristics of the ship air-wake with two different topside arrangements and inflow conditions

We investigate the effect of the Atmospheric Boundary Layer (ABL) and vessel topside arrangement on ship air-wake flow. The investigation is performed by comparing experimental results with two inflow conditions: a simulated ABL and a smooth-wall flat-plate turbulent boundary layer; and two simplified ship models: Simplified Frigate Ship model 2 (SFS2) and NATO-Generic Destroyer (NATO-GD). Particle Image Velocimetry (PIV) and point measurements over the landing deck center-line show that the ABL inflow increases the flow fluctuations but reduces the length of the wake. Furthermore, we observed an enhancement of turbulence energy on the ship landing deck when the ABL is simulated. The data also show that the topside arrangement, especially at the hangar elevation upstream of the landing deck, plays an essential role in the air-wake over the ship center line. There are significant differences in the flow re-circulation regions, turbulence structure and energy content. Based on a turbulence structure and energy spectrum analysis, we highlight regions with a possible increased safety risk for both models. Over the SFS2 center line, regions with an increased safety risk are located directly above the landing deck. Meanwhile, on the NATO-GD center line, these regions are located above the hangar elevation.

Optimal Time Selection for ISAR Imaging of Ship Target via Novel Approach of Centerline Extraction With RANSAC Algorithm

Optimal time selection for inverse synthetic aperture radar imaging of ship target is significant for the determination of image project plane, and the performance of it is dependent on the accuracy of ship centerline extraction. In this article, a novel optimal time selection method based on the random sample consensus (RANSAC) technique is proposed, which can extract the ship centerline with high accuracy in the case of ship superstructure and low signal to noise ratio. First, a necessary distance threshold is determined for the RANSAC algorithm combined with the ship body width estimation. Then, a novel cost function is proposed according to the amplitude characteristic of radar image, which can alleviate the influence of randomness. Finally, the robustness of the optimal time selection method is demonstrated by the results of simulated and real measured data.

Maneuvering hydrodynamic derivatives and course stability of a ship close to a bank

Captive model tests are conducted for a pure car carrier in the proximity of a sloped bank with variations in water depth, distance between ship hull and bank, hull drift angle and heel angle. Using the hydrodynamic derivatives obtained, the check helm and hull drift angle required at equilibrium conditions are estimated. Course stability of the ship sailing in the proximity of the bank is also studied based on a simplified analysis method. In h∕d=1.2 where h∕d means ratio of water depth h to ship draft d, the check helm of the ship reaches 35° near η0∕L=0.27 where η0 is the distance between the ship center line and the bank. Although this ship is course unstable in case of no rudder control, the ship becomes course stable in any water depths by employing the autopilot with appropriate control gains. The present analysis method is useful for conventionally assessing the course stability of a ship in the proximity of a bank.

Construction strength analysis of landing craft tank conversion to passenger ship using finite element method

This research aims to determine the longitudinal strength of passenger ship which was converted from Landing Craft Tank with 54 m of length as stated by BKI (Biro Klasifikasi Indonesia / Indonesian Classification Bureau). Verification of strength value is done to 4 (four) loading conditions which are (1) empty load condition during sagging wave, (2) empty load condition during hogging wave, (3) full load condition during sagging wave and (4) full load condition during hogging wave. Analysis is done using Finite Element Analysis (FEA) software by modeling the entire part of passenger ship and its loading condition. The back and upfront part of ship centerline were used as the boundary condition. From that analysis it can be concluded that the maximum stress for load condition (1) is 72,393 MPa, 74,792 MPa for load condition (2), 129,92 MPa for load condition (3), and 132,4 MPa for load condition (4). Longitudinal strength of passenger ship fulfilled the criteria of empty load condition having smaller stress value than allowable stress which is 90 MPa, and during full load condition with smaller stress value than allowable stress which is 150 MPa. Analysis on longitudinal strength comparison with entire ship plate thickness variation of ± 2 mm from initial plate was also done during this research. From this research it can be concluded that plate thickness reduction causes the value of longitudinal strength to decrease, while plate thickness addition causes the value of longitudinal strength to increase.

Using cross-correlations of ambient vibrations for passive structural health monitoring of a high-speed naval ship

Previous studies have used the cross-correlation of ambient vibrations (CAVs) technique to estimate the impulse response (or Green's function) between passive sensors for passive imaging purposes in various engineering applications. The technique (CAV) relies on extracting deterministic coherent time signatures from the noise cross-correlation function computed between passive sensors, without the use of controlled active sources. Provided that the ambient structure-borne noise field remains stable, these resulting coherent waveforms obtained from CAV can then be used for structural monitoring even if they differ from the actual impulse response between the passive sensors. This article presents experimental CAV results using low-frequency random vibration data (>50 Hz) collected on an all-aluminum naval vessel (the HSV-2 Swift) operating at high speed (up to 40 kn) during high sea states. The primary excitation sources were strong wave impact loadings and rotating machinery vibrations. The consistency of the CAV results is established by extracting similar coherent arrivals from ambient vibrations between the pairs of strain gages, symmetrically located across the ship's centerline. The influence of the ship's operating conditions on the stability of the peak coherent arrival time, during the 7 days trial, is also discussed. [Sponsored by ONR, N00014-09-1-0440.]

Effects of the Welding Residual Stress on the Grounding Damage Analysis of a Tanker

Vessels are rarely subjected to accidental loads such as the collision, grounding and stranding. But these accidental loads cause a lot of damages to hull structure including a large deformation, fracture, tearing and so on. In case of carrying dangerous goods such as crude oil, these accidents can induce a serious environmental pollution. All ocean-going vessels were made by welding. The welding residual stress is a significant shortcoming of welding remains at the hull structure, even though welding technology in shipbuilding field has provided a variety of advantages, e.g. remarkable shortening of the shipbuilding time. The aim of this paper is to investigate the effects of the residual stress in a grounding accident. When a ship runs aground against a sea-obstacle like a rock, this accident can be classified as a grounding. Among parameters of grounding accident scenarios such as the ship speed, the initial striking point, and loading conditions of the ship, only ship speed varies from 10 to 15 knots under ballast condition with/without consideration of residual stress(this sentence is not clear to the read). The initial striking point is at the bow of the center line of ship. A series of nonlinear numerical simulations with large deformation and fracture were carried out using LS-DYNA. As a result, two cases with residual stress have longer damage length. The difference seems to be relatively small, but not negligible.

Study on the Effects of Side-Wall Interference on Added-Mass and Damping Coefficients by Means of Slender-Ship Theory

Slender-ship theory is applied to develop a new method of calculating with quantitatively good accuracy the effects of side-wall interference on the added-mass and damping coefficients of a ship in a waterway. The present paper is restricted to the radiation problem of heave and pitch motions with zero forward speed, but it may be easily possible to extend to the forward-speed case and the diffraction problem.In the outer region, 3-D Green function satisfying the boundary condition on the side walls is placed along the ship's centerline on the free surface to describe the outer solution. The side-wall boundary condition is satisfied by considering an infinite number of image ships, and then the summation of the resultant infinite series is analytically obtained in a closed form.In the inner region, the radiation condition and side walls are absent, and thus the inner solution is identical to that of Newman's unified theory : the strip-theory solution plus a homogeneous solution multiplied by a three-dimensional interaction coefficient. This interaction coefficient is determined by the compatibility requirement of the inner and outer solutions in an overlap region. Therefore through this coefficient, the side-wall effects are incorporated in the inner solution.Computations of the added-mass and damping coefficients are performed for a floating spheroid of length-beam ratio L/B=8 in a channel of breadth ratio B T/B=16, and of L/B in a channel of B T/B = 5. Numerical results are in virtually perfect agreement with the results by a newly-developed 3-D panel method. This suggests that the theory proposed can be quite effectively used also in estimating the tank-wall effects on the results of experiments in a towing tank with limited width.

On Single Way Type Launching Method

In usual launching method, we used two ways having the gauge of about 1/3 to 1/4 of ship-breadth.On the other hand, there was the single way launching method, in which only one way was placed at ship's centerline. We find many advantages in this single way launching, such as saving of man-hours, saving of material and wide-range for ship positioning, but its application was confined to smaller ship whose max launching weight was about 2, 000 t, because of the problems on ship stability, ship strength, and so on.The present paper concerns about the modified two way launching method developed by Mitsubishi Yokohama shipyard in which the gauge is so small that it is considered almost single way launching. This single way type launching methad was applied at first to smaller ship, but was extended to larger ship step by step after careful study on ship stability, ship strength and so on. Up to date, eleven ships were launched succsfully by this method including eight fish factory ships whose launching weight was about 8, 000 t and L×B×D 140 m × 24 m × 14.8 m.