Ship Structural Design Research Articles

Study on the Wave Loads and Structural Stresses of Container Ship

In this article ship motions and wave loads are analyzed by using three dimensional motion of the ship program. As an example of a 5500TEU container ship, the heading angles of maximum motions and loads, amplitude of movement under the action of unit regular wave, load conversion functions, conditions and RAO of middle section at different angles have been gotten. The maximum dominant load parameters are also obtained. Then the long-term wave loads are calculated through the spectrum theory, and the results are compared with the values of classification rules. At the end forces and stresses distribution of the ship hull structure under dominant wave loads are also calculated by the dynamic load method with ANSYS software. The conclusions can be used to analyze ultimate loads of the container ships, and can also be the reference of ship structural design.

Ultimate Strength Assessment of Hull Girder under Cyclic Bending Based on Smith’s Method

Ultimate strength is a critical and fundamental consideration in the design of ship and offshore structures. Accurate assessment of the (residual) ultimate strength of a ship hull is important not only for the initial design, but also for the operation, maintenance, repair, and even for disaster management, rescue, and salvage assessment of the ship hulls. In fact, under ocean wave conditions, the hull girder is likely to withstand extreme cyclic bending. In this article, a simplified method based on Smith's method is proposed to assess the ultimate strength of the ship hulls under extreme cyclic bending. To obtain the nonlinear average stress-average strain relationship (σ-ε curve) of the basic structural element (consisting of a stiffener with its attaching plating) under cyclic in-plane compression and tension, systematic calculations are carried out by using a combination of the Beam-Column analysis and the nonlinear finite element method (NLFEM). By introducing the cyclic σ-ε curves of basic structural elements to a simplified calculation program, which is developed by the authors and based on Smith's method, the ultimate strength of hull girders exposed to extreme cyclic bending is subsequently determined. The ultimate strength of three box girder models and an actual ship are obtained under cyclic bending by using the authors' developed procedures including the simplified calculation program and are compared with the corresponding results from NLFEM calculations and also the box girder experiments. 1. Introduction Ultimate strength is a critical and fundamental consideration in the design of ship and offshore structures. An appropriate level of the ultimate strength of a ship hull is perhaps its most important guarantee of structural safety. Accurate assessment of the ultimate strength and/ or residual ultimate strength of a ship is pertinent not only for the initial design, but also for the operation, maintenance, repair, and even for disaster management, rescue, and salvage assessment of the ship hulls in accidental situations. Perhaps the first attempt to calculate ship hull girder ultimate strength was by Caldwell (1965), who applied a type of rigid plastic mechanism analysis to evaluate strength. Since then, naval architects have gradually realized various technology developments and advances of importance to the ultimate strength assessment of ship structures. Over the last two decades, the ultimate bearing capacity of ship structures (including plates, stiffened plates, and hull girders) in their many different modes of possible behavior has been widely studied, and a great deal of results developed, which have made possible an increasingly clearer understanding of the ultimate strength behavior of ship structures. However, because of the complexity and importance of the problem, and the need for continuing development is still felt, for example, as described in the 19th International Ship and Offshore Structures Congress (ISSC) report, which emphasizes that (even) "with these developments ultimate strength assessment is now becoming a more important issue to ensure the safety of ship structures."

Climate change and safe design of ship structures

The paper addresses projected changes of wave climate in the North Atlantic and their impact on the safe design of ships, with a particular focus given on associated uncertainties. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) uses four scenarios for future greenhouse gas concentrations in the atmosphere called Representative Concentration Pathways (RCP). Two of these scenarios are applied to investigate how sensitive the future North Atlantic wave climate is to the emissions they represent. Winds obtained from six global climate models have been used to simulate waves for a historical period at the end of last century and to project waves for a future period towards the end of this century for these two scenarios. Based on these projections, possible changes in extreme wind and waves are investigated and the associated uncertainties are discussed. The occurrence of rogue-prone sea states which may trigger generation of rogue waves in the past and future climate is also studied. It is shown how the scientific findings on uncertainties related to climate change projections and rogue waves can be incorporated in the risk-based approach used in current design practice of tankers, and ship structures in general. The potential effect of climate change on the safety level of current design practice for tankers is demonstrated. Finally, the paper discusses how structural design of ships can be upgraded to account for climate change and rogue waves without necessarily leading to significant economic consequences.

Time–variant reliability analysis of novel deck and bottom panels

There has been an increasing interest in using reliability–based approaches in ship structural design. The methods and numerical tools presently available to solve structural reliability problems are capable of representing in a rational manner the uncertainties involved in the design of structures and their consequences on the safety level. With these methods, the uncertainties in the design variables involved in a given strength requirement can be rationally quantified through probabilistic models. As a result, the probability of failure, or the probability of violation of the strength requirement can be computed and used as the safety measure.The objective of the present paper is to use novel longitudinals of Y–stiffener profiles instead of the conventional T–stiffeners profile in double hull oil tanker's bottom and deck panels under axial compressive loads considering the structural degradation due to corrosion effects. The Y–stiffener profile is a fabricated section and simply can be produced by attaching the lower end the conventional stiffener's web to the top of the hat part. The Y–stiffener utilizes the advantages of having more effective plate that allows increasing the stiffener spacing, which in turn drops the required number of stiffeners. Time–variant reliability analyses were performed using VaP (Variable Processor) software via applying the Second–Order Reliability Method (SORM) for both conventional and novel Y–stiffener profiles. The ultimate strength and the applied load formulations were derived based on the International Association of Classification Societies–Common Structural Rules (IACS–CSR) for oil tankers. With the Y–stiffeners, a significant improvement in the panel's safety level was achieved in comparison with the conventional profiles in terms of providing higher safety index, larger safety index to weight ratio over the vessel entire lifetime and more structural weight saving.

Experimental study on collapse of cracked stiffened plate with initial imperfections under compression

The collapse mechanics and ultimate strength of crack damaged stiffened plates are investigated by testing stiffened plates with an initial crack. A series of stiffened plates with initial distortions and with a crack are subjected to in-plane compression until achieving their ultimate strength. The crack position, length and the angle between the crack and longitudinal stiffener are changed for determining the influence of the crack on the plate ultimate strength. The ultimate strength and the strain response of the panels are reported. Finally, the ultimate strength reduction related with the presence of the crack and the failure mode are discussed for establishing a damage tolerant procedure for the design of ship structures.

Simulation Based Structural Analysis of an Oil Tanker Bulkhead

Traditional way of ship structural design is to follow the rule books of classification societies. These rule books certainly ensure enough structural integrity of ships. But in this age of scientific and engineering progress, reliable structural analysis programs and tools are available. This paper shows how these programs can be used to verify classification society rules and to accurately design in small details. A bulkhead of an oil tanker is chosen for this investigation. The analysis is done taking Von Misses stress and factor of safety as design criteria. The results of this investigation aim to help the traditional structural engineers to design ship structures in a more effective and optimized way.

Ultimate limit state-based multi-objective optimum design technology for hull structural scantlings of merchant cargo ships

As the ultimate limit state is now a primary criterion for ship structural design, multi-objective optimization techniques for both minimizing weight and maximizing safety are routine practices in the structural design of naval ships as such techniques are critical for determining the structural weight needed to meet the functional requirements associated with naval armaments. However, a comparative approach is still used in the structural design of merchant cargo ships, based on measurements from existing as-built reference ships. This comparative approach obviously needs to be upgraded, because it requires more man-hours and causes more design errors than a fully automated design procedure. The comparative approach may also lead to inadequate design results, in which some structural members are too strong, and others barely satisfy the strength criteria, which can lead to catastrophic failures in some cases. The aim of this paper is to develop a fully automated methodology for the optimum design of hull structural scantlings for merchant cargo ships that are modelled by plate-shell finite elements. A full optimization technique with multi-objectives is applied for minimizing structural weight and maximizing structural safety, as per design constraints associated with the ultimate limit states of the plate panels, support members and hull girders. The developed procedure is applied to the hull structural scantlings of a very large crude oil carrier (VLCC), and this test demonstrates the procedure's capacity to meet the strength requirements of common structural rules. A comparison between the new design and an as-built reference ship is made, confirming that the proposed procedure reduces the number of man-hours required by about 20%, lightens the structural weight by 3% and improves the safety factors for the critical members.

Ultimate hull-girder-strength-based reliability of a double-hull oil tanker after collision in the Adriatic Sea

ABSTRACTThe aim of this paper is to present a methodology for the assessment of the structural reliability of an oil tanker damaged in a hypothetical collision accident in the Adriatic Sea. Monte Carlo simulation is employed to generate random damage scenarios. Assumption of the damage size is based on the numerical simulation of the ship–ship collision. Residual ultimate strength and still-water bending moment distribution are determined based on the size and location of the damage. Structural reliability analysis is employed to determine the safety index with respect to the ultimate hull-girder failure. Histograms of safety indices are thus obtained, representing new measures for the performance assessment of the damaged ship. If extended to more severe design wave environments and collision scenarios, the presented methodology may be used for general reliability-based comparison of different alternative designs of marine structures with respect to the accidental failure modes or for improvement of ship structural design rules.

A Hybrid 2.5-Dimensional High-Speed Strip Theory Method and Its Application to Apply Pressure Loads to 3-Dimensional Full Ship Finite Element Models

As the three-dimensional (3D) finite element model (FEM) has become the de facto standard for ship structural design, interest in accurately transferring seakeeping loads to panel-based structural models has increased dramatically in recent years. In today's design practices, panel-based hydrodynamic analyses are often used for mapping seakeeping loads to 3D FEM structural models. However, 3D panel-based hydrodynamic analyses are computationally expensive. For monohull ships, methods based on strip theories have been successfully used in the industry for many years. They are computationally efficient, and provide good predictions for motions and hull girder loads. However, many strip theory methods provide only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which are difficult to apply to 3D finite element structural models. Previously, the authors have proposed a hybrid strip theory method to transfer 2D strip theory-based seakeeping loads to 3D FEM. In the hybrid approach, the velocity potentials of strip sections are first calculated based on the ordinary 2D strip theories. The velocity potentials of a finite element panel are obtained from the interpolation of the velocity potentials of the strip sections. The panel pressures are then computed based on Bernoulli's equation. Integration of the pressure over the FEM wetted panels yields the hydrodynamic forces and moments. The equations of motion are then formulated based on the FEM. The method not only produces excellent ship motion results, but also results in a perfectly balanced structural model. In this article, the hybrid approach is extended to the 2.5D high-speed strip theory. The simple Rankine source function is used to compute velocity potentials. The original linearized free surface condition, where the forward speed term is not ignored, is used to formulate boundary integral equations. A model based on the Series-64 hull form was used for validating the proposed hybrid method. The motion response amplitude operators are in good agreement with VERES's 2.5D strip theory and with experimental results. Finally, an example is provided for transferring seakeeping loads obtained by the 2.5D hybrid strip theory to a 3D FEM.

Development of Pareto strategy multi-objective function method for the optimum design of ship structures

It is necessary to develop an efficient optimization technique to perform optimum designs which have given design spaces, discrete design values and several design goals. As optimization techniques, direct search method and stochastic search method are widely used in designing of ship structures. The merit of the direct search method is to search the optimum points rapidly by considering the search direction, step size and convergence limit. And the merit of the stochastic search method is to obtain the global optimum points well by spreading points randomly entire the design spaces. In this paper, Pareto Strategy (PS) multi-objective function method is developed by considering the search direction based on Pareto optimal points, the step size, the convergence limit and the random number generation. The success points between just before and current Pareto optimal points are considered. PS method can also apply to the single objective function problems, and can consider the discrete design variables such as plate thickness, longitudinal space, web height and web space. The optimum design results are compared with existing Random Search (RS) multi-objective function method and Evolutionary Strategy (ES) multi-objective function method by performing the optimum designs of double bottom structure and double hull tanker which have discrete design values. Its superiority and effectiveness are shown by comparing the optimum results with those of RS method and ES method.

양축 면내 압축하중 하의 이중판보강 선박판부재의 설계시스템 구축

Because of the importance of steel material saving and rational ship structural design due to the rapid increase in steel prices, a ship structural design system was developed for plate members reinforced by doubler plates subjected to biaxial in-plane compressive loads. This paper mainly emphasizes the design system improvement and upgrade according to the change in the in-plane loading condition of the doubler plate from the single load discussed in a previous paper to the biaxial in-plane compressive load discussed in this paper. A direct design process by a structural designer was added to this developed optimized system to increase the design efficiency and provide a way of directly inserting a designer's decisions into the design system process. As the second stage of preliminary steps of doubler design system development, design formulas subjected to these biaxial loads used in the doubler plate design system were suggested. Based on the introduction of influence coefficients <TEX>$K_t_c$</TEX>, <TEX>$K_t_d$</TEX>, <TEX>$K_b_d$</TEX> and <TEX>$K_a_d$</TEX> based on the variations in the doubler length, breadth, doubler thickness, and average corrosion thickness of the main plate reinforced by the doubler plate, respectively, the design formulas for the equivalent plate thickness of the main plate reinforced by the doubler plate were also developed, and a hybrid design system using these formulas was suggested for the doubler plate of a ship structure subjected to a biaxial in-plane compressive load. Using this developed design system for a main plate reinforced by a doubler plate was expected to result in a more rational reinforced doubler plate design considering the efficient reinforcement of ship plate members subjected to these biaxial loads. Additionally, a more detail structural analysis through local strength evaluations will be performed to verify the efficiency of the optimum structural design for a plate member reinforced by a doubler plate.

통합공통구조규칙 기반 중형 살물선의 중량 절감에 관한 연구

H-CSR(Harmonized Common Structural Rules) integrating CSR-BC(Common Structural Rules for Bulk Carriers) and CSR-OT(Common Structural Rules for Double Hull Oil Tankers) came into effect in July of 2015, so that bulk carrier and double hull oil tanker should comply with this rules. So far, several studies for trend analysis of requirements of structure scantling based on H-CSR have been carried out briskly. However, those studies are rare to apply H-CSR in actual structural design of ships, especially bulk carriers. In this study, an automated system for compartment arrangement is used to search the design case that minimizes still water bending moment(S.W.B.M) in 38k bulk carrier designed by Far East Ship Design & Engineering Co. Ltd. Also, various structural design cases are considered by changing arrangement of structural members to reduce ship weight. The SeaTrust-Hullscan software developed by Korean Register is used to perform structural design of ships based on mother ship and proper design cases are selected by user. The DSA(Direct Strength Analysis) is performed to evaluate structural safety for the yielding and buckling analysis by using MSC Nastran software. The effect of weight reduction is verified by comparison of ship weight between mother ship and the selected design cases.

유빙 및 평탄빙의 충돌에 의한 빙하중과 선체구조응답 해석기법

2 The most important factor in the structural design of ships and offshore structures operating in arctic region is ice load, which results from ice-structure interaction during the ice collision process. The mechanical properties of ice related to strength and failure, however, show very complicated aspect varying with temperature, volume fraction of brine, grain size, strain rate and etc. So it is nearly impossible to establish a perfect material model of ice satisfying all the mechanical characteristics completely. Therefore, in general, ice collision analysis was carried out by relatively simple material models considering only specific aspects of mechanical characteristics of ice and it would be the most significant cause of inevitable errors in the analysis. Especially, it is well-known that the most distinctive mechanical property of ice is high dependency on strain rate. Ice shows brittle attribute in higher strain rate while it becomes ductile in lower strain rate range. In this study, the simulation method of ice collision to ship hull using the nonlinear dynamic FE analysis was dealt with. To consider the strain rate effects of ice during ice-structural interaction, strain rate dependent constitutive model in which yield stress and hardening behaviors vary with strain rate was adopted. To reduce the huge amount of computing time, the modeling range of ice and ship structure were restricted to the confined region of interest. Under the various scenario of ice-ship hull collision, the structural behavior of hull panels and failure modes of ice were examined by nonlinear FE analysis technique.

Sea state conditions for marine structures' analysis and model tests

The study reviews, based on the state-of-the-art findings, some uncertainties associated with wave data and models currently used in design and operation procedures of ship and offshore structures. Although the same basic principles prevail for hydrodynamic loads on ships and offshore structures, actual problems and methods for assessing these loads in the design and operation stage are not the same. Different wave data and models are used for specifying design and operational criteria for these two types of platforms and different uncertainties are related to them. Wave data and models used to define sea state characteristics are discussed and particular attention is given to the associated sources of uncertainties. Some weaknesses of wave input used in design and operational procedures for marine structures are pointed out. Focus is also given on uncertainties related to model tests as tank testing is an important supporting tool for design and operation. Impact of some selected uncertainties on wave description and wave loads is demonstrated by examples.

Development of Reliability-Based Damage-Tolerant Optimal Design of Ship Structures

AbstractWhile in service, ship structures are continuously subjected to environmental loading and deterioration, which could lead to increased inspection and maintenance costs during a vessel’s lifecycle. The design strategy presented in this study considers structural damage, such as corrosion and fatigue, and makes some provision for their impact on structural integrity. If adopted during the conception and design stage, such a strategy could lead to a reduction in lifecycle costs. The approach synergistically combines advanced computational methods, damage models, probabilistic tools, and optimization techniques to enhance the robustness and overall structural reliability of ship structures. The study first presents a formulation of system reliability-based damage-tolerant optimal design models. The design strategy is illustrated through an example problem involving the redesign of a tanker structure for optimal performance and reliability. The sensitivity of the optimal design to uncertain design para...

An optimal design of wind turbine and ship structure based on neuro-response surface method

The geometry of engineering systems affects their performances. For this reason, the shape of engineering systems needs to be optimized in the initial design stage. However, engineering system design problems consist of multi-objective optimization and the performance analysis using commercial code or numerical analysis is generally time-consuming. To solve these problems, many engineers perform the optimization using the approximation model (response surface). The Response Surface Method (RSM) is generally used to predict the system performance in engi-neering research field, but RSM presents some prediction errors for highly nonlinear systems. The major objective of this research is to establish an optimal design method for multi-objective problems and confirm its applicability. The proposed process is composed of three parts: definition of geometry, generation of response surface, and optimization process. To reduce the time for performance analysis and minimize the prediction errors, the approximation model is generated using the Backpropagation Artificial Neural Network (BPANN) which is considered as Neuro-Response Surface Method (NRSM). The optimization is done for the generated response surface by non-dominated sorting genetic algorithm-II (NSGA-II). Through case studies of marine system and ship structure (substructure of floating offshore wind turbine considering hydrodynamics performances and bulk carrier bottom stiffened panels considering structure performance), we have confirmed the applicability of the proposed method for multi-objective side constraint optimization problems.

A framework for reliability assessment of ship hull damage under ship bow impact

Ship collision analysis outcomes are generally used in computational models to derive damage distributions. However, damage is usually assessed after the collision energy has been fully absorbed by structural members rather than at the onset of outer hull fracture. Furthermore, the deformation behaviour of ship structural members under load depends on uncertainty modelling through material, geometric, and structural considerations, captured in an appropriate reliability framework. To consider these significant missed opportunities in understanding the probability of ship structures meeting their performance targets during collisions, a novel stochastic framework is proposed in this paper. For efficient reliability computations, a plate resistance model is developed for hull damage assessment at the onset of failure. Stochastic modelling capabilities of Python scripting are interfaced with Abaqus® to compute the stochastic response. The reliability computations show that the probability of hull fracture increases as the hull deformation progresses, with maximum values occurring at the onset of outer hull fracture. The framework outcomes are useful in determining optimal ship structural design capabilities.

Reliability-Based Optimal Design of Steel Box Structures. II: Ship Structure Applications

AbstractTraditional design of ship structures relies on a combination of experience, sound judgment, and deterministic approaches and typically ignores the potential for design improvement and other benefits offered through the use of reliability methods and structural optimization strategies. Part I of this article outlines the underlying theories involved in incorporating reliability methods and structural optimization strategies into the initial design of ship structures, whereas Part II (this paper) discusses their application to two case studies, namely, (1) a simple ship structure and (2) a more complex ship structure in an attempt to achieve weight reduction in the face of constraints on ultimate strength and buckling capacity. Using the approach outlined in the companion paper, a weight reduction of 5.6% was realized in the case of the simple vessel, whereas a 2.0% reduction was achieved in the case of the more complex vessel. A reduction in weight reduction has the potential to minimize the lifec...

Structural topology design of container ship based on knowledge-based engineering and level set method

Knowledge-Based Engineering (KBE) is introduced into the ship structural design in this paper. From the implementation of KBE, the design solutions for both Rules Design Method (RDM) and Interpolation Design Method (IDM) are generated. The corresponding Finite Element (FE) models are generated. Topological design of the longitudinal structures is studied where the Gaussian Process (GP) is employed to build the surrogate model for FE analysis. Multi-objective optimization methods inspired by Pareto Front are used to reduce the design tank weight and outer surface area simultaneously. Additionally, an enhanced Level Set Method (LSM) which employs implicit algorithm is applied to the topological design of typical bracket plate which is used extensively in ship structures. Two different sets of boundary conditions are considered. The proposed methods show satisfactory efficiency and accuracy.

An Assessment of MSC Solutions for Ship Structural Design and Analysis

This paper presents assessment of ship structural analysis and design using MSC solutions. Linear and nonlinear finite element analyses are applied to 3D model describing in MSC.NASTRAN and MSC.MARC, respectively. The structural model analysis including geometric construction ability, size of meshing, boundary and loading conditions etc. are discussed. Comparisons of stress and strain between MSC and other solutions have been investigated. Reliable numerical results are adopted for direct strength analysis, ultimate strength as well as fatigue strength when applying the international association of classification society common structure rules (IACS CSR) and membership classifications. Actually, the finite element applications played an important in the large and ultra-ship structural analysis, the results obtained from numerical assessment help designer to predict the ultimate limit state, duration of ship building and operation.