An inverse hull design approach in minimizing the ship wave

The Levenberg–Marquardt Method (LMM) and a panel code for solving the wave-making problem are utilized in an inverse hull design problem for minimizing the wave of ships. A typical catamaran is selected as the example ship for the present study. The hull form of the catamaran is described by the B-spline surface method so that the shape of the hull can be completely specified using only a small number of parameters (i.e. control points). The technique of parameter estimation for the inverse design problem is thus chosen. The LMM of parameter estimation, which is the combination of steepest descent and Newton’s methods, has been proven to be a powerful tool for the inverse shape design problem. For this reason it is adopted in the present study.In the present studies, the inverse hull design method can not only be applied to estimate the hull form based on the known wave data of the target ship but can also be applied to estimate the unknown hull form based on the reduced wave height. The optimal hull forms of minimizing wave for a typical catamaran in deep water at service speed and at the critical speed of shallow water are estimated, respectively. Moreover, a new hull form with the combining feature of the optimal hull forms for deep water and shallow water is performing well under both conditions. The numerical simulation indicates that the hull form designed by inverse hull design method can reduce the ship wave significantly in comparison with the original hull form.     

Discrete elementmodeling of ice loads on ship hulls in broken ice fields

Ice loads on a ship hull affect the safety of the hull structure and the ship maneuvering performance in ice-covered regions. A discrete element method （DEM） is used to simulate the interaction between drifting ice floes and a moving ship. The pancake ice floes are modelled with three-dimensional （3-D） dilated disk elements considering the buoyancy, drag force and additional mass induced by the current. The ship hull is modelled with 3D disks with overlaps. Ice loads on the ship hull are determined through the contact detection between ice floe element and ship hull element and the contact force calculation. The influences of different ice conditions （current velocities and directions, ice thicknesses, concentrations and ice floe sizes） and ship speeds are also examined on the dynamic ice force. The simulated results are compared qualitatively well with the existing field data and other numerical results. This work can be helpful in the shil3 structure design and the navigation securitv in ice-covered fields.     

GPU-Based DEM Simulations of Global Ice Resistance on Ship Hull During Navigation in Level Ice

The ice resistance on a ship hull affects the safety of the hull structure and the ship maneuvering performance in icecovered regions.In this paper,the discrete element method(DEM)is adopted to simulate the interaction between level ice and ship hull.The level ice is modeled with 3D bonded spherical elements considering the buoyancy and drag force of the water.The parallel bonding approach and the de-bonding criterion are adopted to model the freezing and breakage of level ice.The ship hull is constructed with rigid triangle elements.To improve computational efficiency,the GPU-based parallel computational algorithm was developed for the DEM simulations.During the interaction between the ship hull and level ice,the ice cover is broken into small blocks when the interparticle stress approaches the bonding strength.The global ice resistance on the hull is calculated through the contacts between ice elements and hull elements during the navigation process.The influences of the ice thickness and navigation speed on the dynamic ice force are analyzed considering the breakage mechanism of ice cover.The Lindqvist and Riska formulas for the determination of ice resistance on ship hull are employed to validate the DEM simulation.The comparison of results of DEM,Lindqvist,and Riska formula show that the DEM result is between those the Lindqvist formula and Riska formula.Therefore the proposed DEM is an effective approach to determine the ice resistance on the ship hull.This work can be aided in the hull structure design and the navigation operation in ice-covered fields.     

船体曲面几何表达及水动力性能计算的NURBS方法

采用非均匀有理B样条(NURBS)对船体曲面形状进行几何表达,所生成的网格直接用于后续的有关船体水动力性能计算工作中。对Wigley船型的线性兴波阻力和斜航操纵运动的船体,结合近似的Kutta条件作了相应的数值计算,从与试验结果的比较来看,该方法具有较好的工程精度,对船舶及海洋工程领域中的计算机辅助设计(CAD)与计算流体动力学(CFD)之间的相互集成,具有推动与促进作用。     

Hull form reliability-based robust design optimization combining polynomial chaos expansion and maximum entropy method

In the ship design optimization process, the neglect of the unavoidable uncertainty of parameters in the actual navigation and experimental observations, may lead to a bad result with some hidden dangers in practical applications. Considering the influence of the uncertainty, a new and effective hull form reliability-based robust design optimization (RBRDO) framework, including the hull form modification module and RBRDO module, has been developed and tested in the present work. Radial basis function method is utilized as the parametric hull surface modification technique to generate a series of smooth hull forms while combining polynomial chaos expansions (PCE) method with maximum entropy method (MEM) to conduct the uncertainty analysis for the prediction of the mean and the standard deviation of the objective and the failure probability of constraints. To verify the validity of the method, hull form design optimization of the bow of KCS model is implemented under the influence of the uncertainty. Numerical results indicate that the proposed RBRDO framework is effective compared with traditional Monte Carlo method. Meanwhile, compared with traditional DO case, RBRDO case has higher adaptability to the environmental uncertainty with the lower failure probability, which ensure the robustness and reliability of the optimal hull form.     

The evaluation method of total damage to ship in underwater explosion

Whipping response will happen when a ship is subjected to underwater explosion bubble load. In that condition, the hull would be broken, and even the survivability will be completely lost. A calculation method on the dynamic bending moment of bubble has been put forward in this paper to evaluate the impact of underwater explosion bubble load on the longitudinal strength of surface ships. Meanwhile the prediction equation of bubble dynamic bending moment has been concluded with the results of numerical simulation. With wave effect taken into consideration, the evaluation method of the total damage of a ship has been established. The precision of this evaluation method has been proved through the comparison with calculation results. In order to verify the validity of the calculation results, experimental data of real ship explosion is applied. Prediction equation and evaluation method proposed in this paper are to be used in ship structure design, especially in the preliminary prediction of the ultimate withstanding capability of underwater explosion damage for the integrated ship in preliminary design phase.     

An optimization approach for fairing of ship hull forms

This paper presents a numerical fairing procedure to be used at the preliminary design stage to create high-quality ship hull form geometry. The procedure is based on a variational optimization approach in which a fairness measure related to the surface curvature is the objective function to be minimized subject to a set of geometric constraints to ensure that the final form has the required geometric characteristics. The optimization variables are selected as the control points of a B-spline surface representing the initial hull form. A nonlinear direct search technique is employed to solve the problem. The methodology is applied for typical ship forms to indicate that, provided that the designer can specify appropriate design objectives and geometric constraints, the methodology can produce alternative hull forms with significantly improved fairing characteristics. The choice of the fairness objective function is shown to have a crucial effect on the quality of the hull surface. Highly nonlinear exact fairness functionals yield surfaces of high quality at the expense of high-computerized effort.     

Design sensitivity analysis for vertical free vibration of hull girder

This paper presents a calculation method of derivatives of natural frequencies and mode shapes to parameters affecting vertical hull girder vibration based on design sensitivity analysis. The method premises free vibration analysis of hull girder using the transfer matrix method. Governing sensitivity equation is derived from the direct differentiation of state vector and transfer matrix to design variables. Derivatives of natural frequencies and mode shapes are determined after two trial calculations of the equation. By using the obtained derivatives, the changes of natural frequencies can be rationally and efficiently predicted in case of ship design modification and loading variation.     

Optimization Design of Minimum Total Resistance Hull Form Based on CFD Method

In order to reduce the resistance and improve the hydrodynamic performance of a ship, two hull form design methods are proposed based on the potential flow theory and viscous flow theory. The flow fields are meshed using body-fitted mesh and structured grids. The parameters of the hull modification function are the design variables. A three-dimensional modeling method is used to alter the geometry. The Non-Linear Programming (NLP) method is utilized to optimize a David Taylor Model Basin (DTMB) model 5415 ship under the constraints, including the displacement constraint. The optimization results show an effective reduction of the resistance. The two hull form design methods developed in this study can provide technical support and theoretical basis for designing green ships.     

Simultaneous optimization of propeller–hull systems to minimize lifetime fuel consumption

In traditional naval architecture design methodologies optimization of the hull and propeller are done in two separate phases. This sequential approach can lead to designs that have sub-optimal fuel consumption and, thus, higher operational costs. This work presents a method to optimize the propeller–hull system simultaneously in order to design a vessel to have minimal fuel consumption. The optimization uses a probabilistic mission profile, propeller–hull interaction, and engine information to determine the coupled system with minimum fuel cost over its operational life. The design approach is tested on a KCS SIMMAN container ship using B-series propeller data and is shown to reduce fuel consumption compared to an optimized traditional design approach.     

Research on Design Method of the Full Form Ship with Minimum Thrust Deduction Factor

In the preliminary design stage of the full form ships, in order to obtain a hull form with low resistance and maximum propulsion efficiency, an optimization design program for a full form ship with the minimum thrust deduction factor has been developed, which combined the potential flow theory and boundary layer theory with the optimization technique. In the optimization process, the Sequential Unconstrained Minimization Technique(SUMT) interior point method of Nonlinear Programming(NLP) was proposed with the minimum thrust deduction factor as the objective function. An appropriate displacement is a basic constraint condition, and the boundary layer separation is an additional one. The parameters of the hull form modification function are used as design variables. At last, the numerical optimization example for lines of after-body of 50000 DWT product oil tanker was provided, which indicated that the propulsion efficiency was improved distinctly by this optimal design method.     

Strength Analysis of Turret Mooring Productive/Storage Tanker

The longitudinal strength of turret mooring productive/storage tanker is studied.A numeri-cal example has been implemented according to the method presented in this paper to give practical il-lustration.From the results of the numerical example,it is concluded that the turret hole located near theforward of the amidships has small effect on the longitudinal strength of the ship hull.As for design ex-treme value of wave bending moment of storage tanker,statistic method is a more reasonablemethodology,especially with the consideration of the servere environmental conditions.The primary esti-mation of design section modulus of turret storage tanker can be determined by this design bending mo-ment.     

A Simple Empirical Formula for Predicting the Ultimate Strength of Ship Plates with Elastically Restrained Edges in Axial Compression

An investigation is conducted on the static ultimate limit state assessment of ship hull plates with elastically restrained edges subjected to axial compression. Both material and geometric non-linearities were considered in finite element(FE) analysis. The initial geometric imperfection of the plate was considered, while the residual stress introduced by welding was not considered. The ultimate strength of simply supported ship hull plates compared well with the existing empirical formula to validate the correctness of the applied boundary conditions, initial imperfection and mesh size. The extensive FE calculations on the ultimate strength of ship hull plates with elastically restrained edges are presented. Then a new simple empirical formula for plate ultimate strength is developed, which includes the effect of the rotational restraint stiffness, rotational restraint stiffness, and aspect ratios. By applying the new formula and FE method to ship hull plates in real ships, a good coincidence of the results between these two methods is obtained, which indicates that the new formula can accurately predict the ultimate strength of ship hull plates with elastically restrained edges.     

Practical evaluation of sinkage and trim effects on the drag of a common generic freely floating monohull ship

A practical method to account for the influence of sinkage and trim on the drag of a freely floating (free to sink and trim) common monohull ship at a Froude number F ≤ 0.45 is considered. The sinkage and the trim are estimated via two alternative simple methods, considered previously. The drag is also estimated in a simple way, based on the classical Froude decomposition into viscous and wave components. Specifically, well-known semiempirical expressions for the friction drag, the viscous pressure drag and the drag due to hull roughness are used, and the wave drag is evaluated via a practical linear potential flow method. This simple approach can be used for ship models as well as full-scale ships with smooth or rough hull surfaces, and is well suited for early ship design and optimization. The method considered here to determine the sinkage and the trim, and their influence on the drag, yields theoretical predictions of the drag of the Wigley, S60 and DTMB5415 hulls that are much closer to experimental measurements than the corresponding predictions for the hull surfaces of the ships in equilibrium position at rest. These numerical results suggest that sinkage and trim effects, significant at Froude numbers 0.25 < F, on the drag of a typical freely floating monohull ship can be realistically accounted for in a practical manner that only requires simple potential flow computations without iterative computations for a sequence of hull positions.     

The influence of hull form on the motions of high speed vessels in head seas

Prediction of ship motions at high Froude number is carried out using a time domain strip theory in which the unsteady hydrodynamic problem is treated in terms of the motion of fixed strips of the water as hull sections pass through it. The Green function solution is described and the integration of the ship motion carried out by an averaging method to ensure stability of the solution. The method is validated by comparison with tank data for conventional slender hulls suitable for catamarans, small water area twin hull (SWATH) forms and hulls suitable for high-speed monohulls. Motion computations are then carried out for 14 designs with an operating speed of 40 kts and a displacement of 1000 tonnes. The vessels are assumed not to be fitted with motion control systems for the purposes of this comparative study. Motion sickness incidence is predicted to rise to between 42 and 72% depending upon the hull design in 3 m head seas of average period 7.5 s. MSI values reduce in smaller seas with a shorter average period to be less than 15% in all cases in 1m seas with an average period of 5.5 s.     

A method for breach assessment onboard a damaged passenger ship

Reliable analysis of stability and safety level in a flooding emergency onboard a damaged passenger ship is extremely important for making correct decisions on evacuation and abandonment. Thus there is demand for an automated system that detects flooding and analyzes the severity of the situation. This procedure requires estimation of the actual breach in the hull, and calculation of possible progressive flooding to undamaged compartments. A new approach to the breach assessment, based on measurement data from the flood level sensors, is presented. The developed method is complemented by time-domain flooding simulations in order to separate progressive flooding from direct inflow through the breaches in the hull of the damaged ship. The developed approach is tested and demonstrated with a large passenger ship design for various damage scenarios. The results show that the size and location of the breach can be evaluated with reasonable accuracy on the basis of the level sensor data, provided that there are enough well-placed, working level sensors.     

Ship Hull Slamming Analysis with Nonlinear Boundary Element Method

A boundary element method is developed for calculating the flare ship hull slammingproblem.The nonlinear free surface elevation and the linear element assumption are employed.The meth-od has been verified by comparisons with results for the water entry of wedges with various deadriseangles.Numerical results show that the pressure distribution varies greatly with the ship hull with differentcurvilinear equations,and the slamming features are also different.From the numerical simulation,the au-thors found that the structural damage of the flare hull might be caused by the increasing hydrodynamicpressure over an extensive area on the flare when the upper part of the flare comes into contact with water.     

3-D geometric modeler for rapid ship safety assessment

CAD systems are used broadly in the shipbuilding industry. CAD systems for naval architecture are a useful tool for hull form, internal arrangement and the structural design of ships. These systems require high precision and expertise for efficient use. Therefore, these systems are not appropriate in supporting emergency responses, which require rapid modeling even if it generates some errors.This paper describes a geometric modeler for rapid ship safety assessment. The modeler is developed based on the 3-D geometric modeling kernel ACIS. The definition of hull form, internal arrangement and major longitudinal structural members is a fundamental function of the modeler. The developed modeler is interfaced with other applications used for ship safety assessment such as hydrostatic calculation, ship motion analysis in wave condition, longitudinal strength analysis and so on. In addition, it can generate a new ship model by making variations in a previously defined ship model.     

Effect of ship structure and size on grounding and collision damage distributions

It has been argued that a major shortcoming in the International Maritime Organization (IMO) Interim Guidelines for Approval of Alternative Methods of Design and Construction of Oil Tankers in Collision and Grounding is that grounding and collision damages normalized by the main dimensions of the ship have the same probability density distributions regardless of a particular structural design and ship size.The present paper explores analytical methods for assessing the overall effect of structural design on the damage distributions in accidental grounding and collisions. The results are expressed in simple expressions involving structural dimensions and the building material of the ships. The study shows that the density distribution for collision and grounding damages normalized by the main dimensions of the ship depends on the size of the ship. A larger ship has a higher probability of a larger relative damage length than that of a smaller ship in grounding damage. On the other hand, the damages to the side structure caused by ship collisions are found to be relatively smaller for large ships.The main conclusion is that the existing IMO damage distributions will severely underestimate the grounding damages to the bottom structure of larger vessels and to a lesser extent overestimate collision damages to the side structure of the hull.     

A case study for the effect of a flow improvement device (a partial wake equalizing duct) on ship powering characteristics

A case study for the energy saving in the powering characteristics of a river going general cargo ship has been carried out. Two different hull forms were generated from the original hull form of the vessel to optimise the stern flow of the vessel. A possible energy saving concept, such as partial wake equalizing duct was investigated in this manner. Resistance, self-propulsion and flow visualization measurements were performed with the hull models to explore the effect of partial wake equalizing ducts on the powering characteristics of the hull form. Analysis of the results indicates that the partial wake equalizing duct concept with an appropriate stern design affect not only the flow characteristics at aft-end, but also the propulsion characteristics. In order to identify effect of each component (partial wake equalizing duct and stern form) a further investigation is needed.