Ship Section Research Articles

The Atlantic Subtropical Cells Inferred from Observations

AbstractThe Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres, they are characterized by equatorward transport at thermocline level, upwelling at the equator, and poleward Ekman transport in the surface layer. This study uses recent data from Argo floats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward flow and the thermocline equatorward flow. The latter is bounded by the 26.0 kg m−3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ± 1.1 Sv from the southern hemisphere compared to 2.9 ± 1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ± 3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ± 0.8 Sv at 10°S and 6.4 ± 1.4 Sv at 10°N). A diapycnal transport of about 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.

Experimental studies of a damaged ship section in forced heave motion

This work documents a detailed series of experiments performed in a wave flume on a thin walled prismatic hull form. The model consists of a rectangular opening located on the side. The length of the model is slightly smaller than the flume breadth to achieve two-dimensional (2D) behavior in the experiments. Forced oscillatory heave tests in calm water have been carried out by varying the model-motion parameters and examining both intact and damaged conditions. Video recordings, measurements of the wave elevation inside the damaged compartment and of the force on the model were performed in all the experiments. The effect of damage opening in the model on hydrodynamic loads is examined by comparing with an intact section. A theoretical analysis is used to explain the behavior of added mass and damping coefficients in heave for a 2D damaged section. The presented results demonstrate occurrence of sloshing and piston mode resonances in the tests and their influence on the hydrodynamics loads of a damaged ship. Detailed physical investigations are presented at these resonance frequencies for the damaged section. Effect of filling level in the damage compartment, damage-opening length and air compressibility in the airtight compartment is examined. Nonlinear effects are documented and appear dominant, especially, for lowest filling level where we have shallow-water depth conditions in the damaged compartment. Resonance phenomena that can lead to significant local loads are identified for the shallow water condition. Air compressibility in the airtight compartment and floodwater act as a coupled system and influence inflow/outflow of floodwater in the compartment. It has a significant effect on local floodwater behavior in the damaged compartment.

Numerical investigation of piston-modal wave resonance in the narrow gap formed by a box in front of a wall

Piston-modal wave resonance between a ship section and a bottom mounted terminal is studied by employing a numerical wave flume based on OpenFOAM® package. A systematic investigation on the piston-modal behavior is performed to characterize the influence of fluid viscosity and flow rotation. Around the resonant frequency, the fluid viscosity and flow rotation not only dissipate the wave amplitude in the narrow gap, but also increase the wave amplitude in the upstream of the box. The dynamic mechanism behind the phenomenon is found to be the interaction between the energy dissipation induced by the fluid vortical flow and energy transformation associated with free surface motion. The increased incident wave amplitude can cause the normalized wave amplitudes and wave forces to deviate more from the potential flow results, while the variation of reflection coefficient is dependent on box-wall geometries. All of these phenomena imply a more significant effect of fluid viscosity and flow rotation with the increase of incident wave amplitude, but the energy dissipation is not the only factor in piston-modal resonance.

Interannual Variability of the Atlantic North Equatorial Undercurrent and Its Impact on Oxygen

AbstractThe North Equatorial Undercurrent (NEUC) has been suggested to act as an important oxygen supply route toward the oxygen minimum zone in the eastern tropical North Atlantic. Observational estimates of the mean NEUC strength are uncertain due to the presence of elevated mesoscale activities, and models have difficulties in simulating a realistic NEUC. Here we investigate the interannual variability of the NEUC and its impact onto oxygen based on the output of a high‐resolution Ocean General Circulation Model (OGCM) and contrast the results with an unique data set of 21 ship sections along 23° W and a conceptual model. We find that the interannual variability of the NEUC in the OGCM is related to the Atlantic meridional mode with a stronger and more northward NEUC during negative Atlantic meridional mode phases. Discrepancies between the OGCM and observations suggest a different role of the NEUC in setting the regional oxygen distribution. In the model a stronger NEUC is associated with a weaker oxygen supply toward the east. We attribute this to a too strong recirculation between the NEUC and the northern branch of the South Equatorial Current in the OGCM. Idealized experiments with the conceptual model support the idea that the impact of NEUC variability on oxygen depends on the source water pathway. A strengthening of the NEUC supplied out of the western boundary acts to increase oxygen levels within the NEUC. A strengthening of the recirculations between NEUC and the northern branch of the South Equatorial Current results in a reduction of oxygen levels within the NEUC.

Research on Calculation Method of Wave Load and Mooring Force Based on Time Domain Potential Flow Theory

In order to predict the wave force of moored ships quickly and accurately, based on the unsteady time domain flow theory, the calculation method of wave load and mooring force of LNG ship under full load and ballast conditions is studied. Through short-term forecasting and long-term forecasting, the wave bending moment and wave force of each cross section of the LNG ship, as well as the motion response of LNG ship which have the greatest influence on the wave load are obtained. Based on this, the wave force and motion response is performed, and the mooring point is set according to the actual environmental conditions when the LNG ship is moored at the dock. The mooring force of each cable that meets the specifications and the number of mooring lines required are finally obtained. The research results in this paper can provide a technical support for dock mooring setup and mooring force calculation for similar ship types.

A Cartesian grid based multiphase flow model for water impact of an arbitrary complex body

A Cartesian grid based multiphase flow model is developed to simulate water impact problems. This model is capable of simulating complex moving bodies interacting with a highly non-linear free surface such as jet flow or air cushion. A radial basis function based ghost cell method (RBFGCM) is developed to treat the arbitrary moving body on a fixed Cartesian grid. The complex moving boundary is tracked with the RBF and ghost cells are identified based on the signed function property of the RBF. To capture large deformation of the free surface, a gradient-augmented level set (GALS) method is used. Sub-grid resolution is obtained by simultaneously evolving both the level set (LS) function and its gradient information. Also, a simple distance function assignment method is developed to treat the contact boundary between the free surface and the solid surface. The accuracies of the ghost cell method (GCM) and the GALS method are validated by inline oscillation of a cylinder and horizontal sloshing cases, respectively. Then, the water impact of an arbitrary body is simulated. The cases include the water entry of a free falling multihull and the water entry of a bow-flare ship section with various roll angles. The accuracy of the proposed multiphase flow model and its capability are examined by comparing the present results to experimental and numerical results. Also, the results show that the present method can more accurately predict the slamming load in the presence of flow separation and air cushion than the smoothed particle hydrodynamics (SPH) based single-phase flow model and the boundary element method (BEM). Furthermore, the influence of roll angles on the slamming load and the free surface are studied.

Approximation Method for Flare Slamming Analysis of Large Container Ships in Parametric Rolling Conditions

Since the research of flare slamming prediction is seldom when parametric rolling happens, we present an efficient approximation method for flare slamming analysis of large container ships in parametric rolling conditions. We adopt a 6-DOF weakly nonlinear time domain model to predict the ship motions of parametric rolling conditions. Unlike previous flare slamming analysis, our proposed method takes roll motion into account to calculate the impact angle and relative vertical velocity between ship sections on the bow flare and wave surface. We use the Wagner model to analyze the slamming impact forces and the slamming occurrence probability. Through numerical simulations, we investigate the maximum flare slamming pressures of a container ship for different speeds and wave conditions. To further clarify the mechanism of flare slamming phenomena in parametric rolling conditions, we also conduct real-time simulations to determine the relationship between slamming pressure and 3-DOF motions, namely roll, pitch, and heave.

Direct calculation of buckling and fatigue strength of large Ro-Ro ship section

Objectives A Roll-on/Roll-off(Ro-Ro)ship is a modern ship type with high technology and high added value. Due to the specification of its structure, a consensus on buckling and fatigue checking for Ro-Ro ships has not been reached among the different classification societies of the world. Methods On the basis of analyzing the structural characteristics of Ro-Ro ships, the present study considers the direct calculation of buckling strength, advanced buckling analysis and fatigue analysis of a selected section of a large Ro-Ro ship. The calculations are conducted on the basis of self-developed software from the SJTU KSHIP Project. The pretreatment of buckling and fatigue assessment are established in accordance with the DNV standard Strength Analysis of Hull Structure of Roll-on/Roll-off Ships and Car Carriers, and the related CCS rules. Results The calculated buckling strength, advanced buckling strength and fatigue strength of the target cabin meet the requirements of the related rules. Conclusions The contents of this paper have a certain reference value for the related structural analysis of Ro-Ro ships.

Effects of recreational diving on early colonization stages of an artificial reef in North-East Atlantic

Increasing interest for recreational SCUBA diving worldwide is raising the concern about its potential effects on marine ecosystems. Available literature is still much focused either on impacts on coral reefs of tropical regions or on diver’s behaviour underwater. In this study we analysed, through photo-quadrats, the benthic community composition in a section of a decommissioned Portuguese navy ship that was sunk for touristic purposes. The ship broke down and became separated in two sections enabling a Control versus Impact sampling design, as one section is less attractive for diving. Gorgonians (mainly belonging to the species Leptogorgia sarmentosa and Eunicella verrucosa) were the taxa more negatively affected in the dived ship section, with smaller coverage and size. More resilient species such as the acorn barnacle Amphibalanus amphitrite were positively correlated with the Impact samples. In the case of the study area, according to the available data, 70% or more of the total amount of dives are now on the sunken ships. From these results, lessons can be taken to apply on natural reefs and related management plans.

Prediction of Forces and Pressures on a Wedge and a Ship Section with CIP-Based Methods

Prediction of Forces and Pressures on a Wedge and a Ship Section with CIP-Based Methods

Development of an Algorithm for Calculation and Application of Conformal Mapping Methods on the Calculation of Hydrodynamic Coefficients

The multipole method was first developed by Ursell [1]. His method consists of the superposition of potential functions that satisfy the Laplace equation, the free-surface boundary condition, and the condition at infinity. The potential functions represent a source and horizontal dipole at the origin, which give the radiated waves at infinity, and a series of multipoles that die off rapidly as one moves away from the origin. The strengths of the source, dipole and multipoles are all determined so that the body boundary condition is met. Ursell used this method to solve the problem of a heaving circular cylinder. For sections that are not circular in shape, conformal mapping is used.In the multipole method, the mapping function that transforms the ship section into a semi-circle is found. The mapping function can then be used in conjunction with Ursell’s known solution for a circular cylinder to find the solution for the actual ship section.The difficulty in the technique is to determine the proper mapping function for each cross section. Various methods have been proposed to find this mapping function. The most common mapping uses the so-called Lewis-forms [2], [3] and [4].It is recognized that this method gives smooth solutions over all frequency range (no irregular frequencies). On the other hand, sharp corners are not well represented, and sections with very low sectional area coefficient may not be well represented as well. Even though, for first rough estimates in initial stages of ship design, this method may give results that agree reasonably well with the other more computational demanding methods, in terms of order of magnitude and trend.Considering the difficulty to find good charts of the Lewis forms data – to the best of our knowledge, the best known are those published by Bhattacharyya [5] - and even more difficult to find the data in digital form. The main purpose of this work is to present a computational method of computing the data given through the Lewis forms and apply it to naval ship sections in order to find rough estimates of the hydrodynamic coefficients in heave.

The Assessment of the Ultimate Hull Girder Strength of RO-RO Ship after Damages

Many accidents of Ro-Ro ships happen in Indonesia such as collision and grounding. When the collision or grounding takes place on the Ro-Ro ship, the ultimate strength of hull structure after damage becomes decrease. Car and passenger decks are critical location since collision and/or grounding occur. In the present study, the assessment of the ultimate hull girder strength is conducted. The cross section of Ro-Ro ship is taken to be analyzed. The collision and grounding damages are assumed to be palced on the side and bottom area, respectively. The damages are created by removing the element from the side shell and bottom part. Finally, the result obtained is compared with one another.

Use of added mass and shear flow in thin-walled sections of ships for modal analysis using finite element method

At new ships’ design stages, an important requirement is to grant the structure safety to give to the shipowner and the crew a greater tranquility during ships’ operation. In this scenario, it is relevant to make vibration analysis to identify possible resonance conditions between the structure and the main excitation sources. To trustly predict the ship’s natural frequencies, it is necessary to use a methodology which involves the model to confidently represent the structure and its interaction with the fluid medium. A 3D model that takes into account all the elements with relevance in the calculation of the vertical natural frequencies was developed. To achieve a simpler modeling and short computational effort, a 1D model based on Timoshenko’s beam and thin-walled beam shear flow theories was also developed. Additionaly, the added mass effects were considered using several methods. The methodology was verified with experimental results obtained for a Panamax type ship from which vibration data was acquired during a sea trial. The results showed that both models confidently represent the structure in the vertical vibration analyses, but the computational effort in 1D model is much smaller than that required by the 3D model.

Study on ultimate strength of ship’s hull considering cross section and beam finite element.

The objective of the present research is to study the ultimate strength of ship’s hull considering cross section and beam finite element under longitudinal bending. The single hull bulk carrier and double hull oil tanker are taken to be analysed. The one-frame space of ship is considered in the calculation. The cross section of ship’s hull is divided into element composed plate and stiffened plate. The cross section is assumed to be remained plane and the simply supported is imposed to both side of the cross section. The longitudinal bending moment is applied to the cross section for hogging and sagging condition. The Smith’s method is adopted and implemented into the in-house program of the cross section and beam finite element to calculate the ultimate strength of ship’s hull. The result of the ultimate strength for hogging and sagging condition obtained by considering the cross section and beam finite element is compared with one another.

Research on Outfitting Virtual Assembly based on Ergonomics

To improve the present situation where the assembly work of ship outfitting depends on manual experiential operation, the thesis, taking outfitting assembly operation as the research object and optimal assembly path as the target, puts forward the simulation method and procedure of outfitting virtual assembly considering operational posture, operational load, reachability and visibility and other ergonomic factors, taking pipe assembly operation of ship section as the application example. The results show that the method can obtain better assembly operation path, avoid interference of workers’ operation process and improve the efficiency of assembly operation.

Bow Flare Water Entry Impact Prediction and Simulation Based on Moving Particle Semi‐Implicit Turbulence Method

Prandtl’s mixing length method and the k‐epsilon method are introduced into the Moving Particle Semi‐Implicit (MPS) method for the purpose of modeling turbulence effects associated with water entries of two‐dimensional (2D) bow flare section. The presented numerical method is validated by comparing its numerical prediction with experimental data and other numerical results obtained from the Boundary Element Method (BEM). The time histories of the pressure and the vertical slamming force acting on the dropping ship section subjected to various conditions with different dropping velocity and inclined angles are analyzed. The results show that both the pressure and the vertical slamming force are in good agreement with the experimental data.

Effect of the Roll Center Position on the Roll Damping of a Ship Section

Effect of the Roll Center Position on the Roll Damping of a Ship Section

Strength assessment of an intact and damaged container ship subjected to asymmetrical bending loadings

The objective of this work is to analyse the effect of structural damage and associated neutral axis translation and rotation of the residual load carrying capacity of a container ship hull subjected to asymmetrical bending loading. The assessment is performed by a finite element analysis (FEA) and a formulation based on the Common Structural Rules (CSR). A container ship is analysed in intact and damaged conditions. The position of the neutral axis of the mid-ship section of the container ship is identified at each load step based on FEA and CSR. An update to the progressive collapse approach stipulated by CSR is proposed and compared with the finite element solution. Finally, some conclusions are presented.

Comparative Study of Water-Impact Problem for Ship Section and Wedge Drops

Comparative Study of Water-Impact Problem for Ship Section and Wedge Drops

Calculation of hydrodynamic coefficients of ship sections in roll motion using Navier-Stokes equations

The flow in the vicinity of 2D ship sections carrying out forced roll motions is simulated using the solution of the Navier-Stokes equations implemented in OpenFOAM. A hybrid Lagrangian-Eulerian adaptive mesh scheme is applied to solve forced motions on ship sections oscillating in calm water as well as the free surface capturing problem. Three sections of a containership, with differing geometrical properties, are investigated by comparing viscous unsteady results with potential flow solutions. The method presented in this study has been validated by comparing numerical results with experiments performed on a rectangular section. The influence of viscosity, particularly on vortex shedding, on the roll motion is investigated for an array of oscillation frequencies.