Ship Structural Members Research Articles

A new model for predicting thickness loss due to both corrosion wear and mechanical wear

Thickness loss due to corrosion wear generally occurs on ship structural members in corrosive environments. For corrosion wear, a probabilistic model can be successfully identified from the thickness measurements data to evaluate and estimate wear conditions. On the other hand, thickness loss due to mechanical wear may occur in some ship structural members in addition to thickness loss due to corrosion wear. In this study, a new probabilistic model for the case where mechanical wear is superimposed to corrosion wear is proposed. The condition observed in the actual structure is a mixture of corrosion wear condition and condition of corrosion wear with mechanical wear. When identifying a probabilistic model based on the plate thickness measurements data, it is impossible to distinguish whether the measurements data represent a corrosion condition due to corrosion wear or corrosion wear with mechanical wear. Therefore, a method is examined to obtain a model that would be the maximum likelihood by classifying the data using latent variables. By classifying the data and identifying both of a probability model of corrosion wear and a probability model of corrosion wear with mechanical wear, respectively, the posterior distribution of the latent variables is updated. These processes are repeated until the likelihood of the mixed probability models is maximized. For verification, a set of corrosion data consisting of mixed wear conditions were simulated and analyzed. It shows that both probability models could be properly identified with the proposed method, and in particular, the amount of diminution corresponding to high cumulative probability could also be properly estimated. Furthermore, this method was applied to thickness measurements data of ship structural members in the lower cargo holds of bulk carriers, where mechanical wear is also a concern, and it was confirmed that there was no difference in the corrosion wear condition of the members in the lower part of cargo holds of bulk carriers, however, significant differences were observed in the degree and extent of mechanical wear, indicating that the influence of mechanical damage is very significant in the wear condition of these members.

Numerical simulation on the ice-induced fatigue damage of ship structural members in broken ice fields

A fatigue damage assessment for an ice-going ship navigating through broken ice fields was carried out using a numerical model to simulate the interactions between ice floes and structure. The stress history was obtained by a numerical model and the corresponding fatigue analysis results are presented. The stress time series required for fatigue analysis could be extracted directly using an efficient interaction model, the application of a periodic media analysis method, and the convolution integral. A 2-parameter Weibull model was applied to describe the probability distribution of the stress amplitude, and the fatigue damage was obtained using the Palmgren-Miner rule. Finally, the fatigue damage considering the actual ice conditions of the Baltic Sea was calculated using the proposed method and LR method, and each result was compared.

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.

On the non-classical mathematical models of coupled problems of thermo-elasticity for multi-layer shallow shells with initial imperfections

Mathematical modeling of evolutionary states of non-homogeneous multi-layer shallow shells with orthotropic initial imperfections belongs to one of the most important and necessary steps while constructing numerous technical devices, as well as aviation and ship structural members. In first part of the paper fundamental hypotheses are formulated which allow us to derive Hamilton–Ostrogradsky equations. The latter yield equations governing shell behavior within the applied hypotheses and modified Pelekh–Sheremetev conditions. The aim of second part of the paper is to formulate fundamental hypotheses in order to construct coupled boundary problems of thermo-elasticity which are used in non-classical mathematical models for multi-layer shallow shells with initial imperfections. In addition, a coupled problem for multi-layer shell taking into account a 3D heat transfer equation is formulated. Third part of the paper introduces necessary phase spaces for the second boundary value problem for evolutionary equations, defining the coupled problem of thermo-elasticity for a simply supported shallow shell. The theorem regarding uniqueness of the mentioned boundary value problem is proved. It is also proved that the approximate solution regarding the second boundary value problem defining condition for the thermo-mechanical evolution for rectangular shallow homogeneous and isotropic shells can be found using the Bubnov–Galerkin method.

シェルーソリッド混合解析を用いた船体構造部材に発生する表面き裂の三次元破壊力学解析に関する研究

Applications to three-dimensional fracture mechanics analyses for surface cracks in welded joints of ship structure are presented. To evaluate mechanical behaviors of ship structures, shell finite elements are often used in finite element analyses (FEA). It is difficult to solve three-dimensional crack problems such as surface cracks using the shell elements because the plate thickness is assumed in the shell element formulation. A shell-solid mixed analysis is performed to evaluate stress intensity factors (SIFs) of three-dimensional surface cracks in ship structural members. The whole analysis region is modeled by triangle or quadrilateral shell elements, and the local region near the surface crack is modeled by tetrahedral solid elements. The shell model and the solid model are connected using RBE3 (Rigid Body Element 3) connection in MSC.Nastran. In addition, a semi-auto RBE3 connection technique is developed to enhance the shell-solid mixed analysis. VCCM (Virtual Crack Closure-integral Method) using tetrahedral finite elements are adopted to calculate the SIFs of the three-dimensional surface cracks. The SIFs calculation and the evaluation of the surface cracks in the welded joints of the ship structure are demonstrated.

갑판지지 웨브 구조에서의 개구부 상세설계기준 정립

There are a lot of openings, holes and slots in ship structural members. It is not easy to solve the troubles around the openings adequately at the detail design stage, because there are a lot of concerning locations. There are not also clear design rules of classification societies and it is not possible to apply direct calculation for all the concerning members. Therefore, it is necessary to set up simplified approach such as a standard or guidance in order to decide the opening design quickly. For this study, guidance and regulations of each classification and several companies were surveyed. Grillage analysis and the refined mesh method were used to evaluate the strength around hole considering boundary condition and more detailed member arrangement. As a result, the standard for opening design was established and verified.

変動振幅荷重を受ける構造体の疲労き裂伝播シミュレーション

In a design stage of marine structures, the S-N approach is commonly used for fatigue strength evaluation, where fatigue failure is often considered as the penetration of the plate-thickness of structural members. It is sometimes observed that the cracks having penetrated the plate-thickness of ship structural members exhibit relatively slow growth rates, which may be due to the high structural redundancy and/or the compressive residual stress. Therefore, it is important to predict the crack propagation when it is detected during in-service inspections, which can give us the proper maintenance schedules of marine structures. From the viewpoint of such aspects, the authors have developed a simulation program, "CP-System", for multiple cracks propagating in a three-dimensional stiffened panel structure. It can predict fatigue crack lives and paths, where through-the-thickness crack propagation is formulated as a two-dimensional in-plane problem, and the crack propagation behavior is simulated by step-by-step finite element analyses. In order to accurately evaluate the fatigue lives of ship structures, it is necessary to take into account wave-induced load histories, which are generally a kind of clustered loading with variable stress range. In the present paper, a new crack opening/closing simulation method is developed utilizing the crack tip stress field parameters evaluated by finite element analyses, and the effective stress intensity range, ΔKRP, which precisely correspond to the tensile plastic deformation ahead of the crack tip, is obtained by taking into account of the plastic wake induced by fatigue crack propagation. The usefulness of the developed method is demonstrated by the simulation of fatigue crack propagation in a ship structure under a wave load sequence.

Corrosion fatigue phenomena learned from failure analysis

The purpose of this paper is to present corrosion fatigue phenomena learned from failure analysis. The emphasis is focused upon the importance of the role of corrosion pit in the corrosion fatigue crack initiation process and intergranular fracture surface in the corrosion fatigue crack propagation process of steam turbine blade. The hydrogen effect is also stressed in the crack propagation process in the corrosion fatigue crack propagation of the cargo oil tank ship structural members for very large crude oil carrier. Finally the future problems should be solved in corrosion fatigue failure are touched in brief.

Study on Fatigue Design Loads for Ships Based on Crack Growth Analysis

Abstract Although ship structural members are designed to survive 20 years, fatigue crack damage still occurs, often starting very early in the ship's life, with some cracks growing quickly to considerable size. The primary factor causing variability is the sequence of the variable amplitude load cycle, and the others are welding irregularities on the weld toe and variations in mechanical properties in the material. In this paper, the load sequence effect on fatigue life variability is examined by the use of a fatigue design procedure proposed by the authors. It is found that the fatigue lives of ship structural members vary from several months to 20 years, strongly dependent on the sequence of sea state which ships encounter during ocean-going service. The worst possible loading sequence is elucidated for the fatigue life of the ship's hull. Sensitivity study of fatigue life is discussed.

Three-dimensional Numerical Investigation on Thickness-through Fatigue Crack Closure

In order to establish the calculation method of the fatigue crack opening stress level for the surface cracks in ship structural members, the validity of various crack-node release schemes and opening / closure stress definitions are investigated by 2- and 3-dimensional F.E. analysis of the crack closure behavior of a thickness-through crack. As results, followings are found;1) The measured and calculated opening stresses defined by the compliance method agree well when the crack tip node is released at the maximum load and the element edge length at the crack tip is chosen so that it is about 1/10 the size of the plastic zone.2) The variation of the crack propagation rate with the depth which gives a good account of the changing nature of the crack edge shape of a real thickness-through crack can be derived when the opening stress is defined as the remote stress at the time the compressive local stress at the crack tip node changes to a tensile stress.3) It is advisable to define the opening stress by the sign change of the local stress at the crack tip whenKopis evaluated locally by 3-dimensional F.E. analysis.

The Study of Fatigue Design Method based on Fatigue Crack Growth Analysis (3rd Report)

In the previous paper, authors have proposed a new fatigue design method based on storm model in the North Pacific using crack growth analysis. Following results were obtained: fatigue strength of ship structural members is strongly influenced by the number, the order, and the size of sea state. In order to develop the storm model for ocean route, many data are needed such as wave scatter diagram, a number of storm, a duration of storm and relative angle in storms. When we can obtain these data, we can make storm model according to our proposed procedure. However, in general, there are few data in ocean route.In this paper, authors propose a procedure to develop storm model in any ocean route where there is only wave scatter diagram. Fatigue crack growth life is analyzed under following three different cases of relative angle and duration of storm: 1) relative angle is kept constant during a sea state, 2) relative angle is kept constant until reaching maximum wave height and then is reversed by 180 degree, 3) relative angle is changed every wave, and for duration of storms, 2 days, 3.5 days, and 4.7 days. Referring to the results of crack growth simulation, authors propose a procedure to develop storm model in any ocean route.

The study of storm loading simulation model for fatigue strength assessment of ship structural members

Tomita et. al. proposed a storm loading simulation model for a ship hull structural fatigue strength assessment. They analyzed encountered wave data collected from large number of ships and introduced the random loading model. The model simulates the encountered wave state history of an ocean going ship. But, the statistical character of wave data is not consistency with a wave frequency table to calculate a long-term distribution of wave-induced load, such as Hobgen's wave frequency table.The purpose of this study is to introduce a new storm loading simulation model which is consistent with a long-term wave frequency table. Applying the storm loading model to calculate a ship structural response in the ocean, the long-term distribution of the response can be obtained directly. And also a fatigue strength analysis employing a crack propagation low can be examined.

The Study of Fatigue Design Method based on Fatigue Crack Growth Analysis (2nd Report)

In the previous paper, authors have proposed a new fatigue design method based on storm model in the North Pacific using crack growth analysis. Following results were obtained: fatigue strength of ship structural members is strongly influenced by the number, the order, and the size of sea state. In order to develop the storm model for ocean route, many data are needed such as wave scatter diagram, a number of storm, a duration of storm and relative angle in storms. When we can obtain these data, we can make storm model according to our proposed procedure. However, in general, there are few data in ocean route.In this paper, authors propose a procedure to develop storm model in any ocean route where there is only wave scatter diagram. Fatigue crack growth life is analyzed under following three different cases of relative angle and duration of storm: 1) relative angle is kept constant during a sea state, 2) relative angle is kept constant until reaching maximum wave height and then is reversed by 180 degree, 3) relative angle is changed every wave, and for duration of storms, 2 days, 3.5 days, and 4.7 days. Referring to the results of crack growth simulation, authors propose a procedure to develop storm model in any ocean route.

The Study of Fatigue Design Method based on Fatigue Crack Growth Analysis

Nowadays, the fatigue design of ship structural members is performed based on each classification society rule. However, the considerable size of crack damage is reported sometimes soon after the service. And in the same type of ships, fatigue damage occurs in one and not occurs in the other. These facts cannot be analyzed and explained by the current fatigue design method.Then, authors have proposed new fatigue design method. The differences from the current fatigue design method are the following three points.(1) Use of fatigue crack growth analysis as the basis for fatigue strength evaluation instead of the Miner rule.(2) Implementation of author' s new storm model for the time history of stress.(3) Proposal of the new sailing model of the ship instead of the all headings model.The developed method and proposed fatigue strength charts are applicable to the initial fatigue design instead of the current fatigue design method, and also the residual fatigue life prediction in cracked structural members when fatigue cracks are found after a certain service period.

Sensitivity Analysis on Fatigue Reliability and Inspection of Ship Structural Members

Sensitivity analysis on fatigue reliability and inspection is carried out for a typical structural member of bulk carrier. Fatigue properties of the member and probability of crack detection at the field inspection are obtained by the questionnaire asked to the naval engineers. Changing stress level (fatigue life) of the member, crack growth curve and inspection interval as parameters around a standard condition, failure probabilities of member during 20 years' service are calculated by the Markov Chain Model. The result of analysis can suggest the solutions for design requirements such as improvement of reliability level, reduction of structural weight and the simplification of inspection.In the latter part of the paper, effectiveness of combined inspection method and sampling inspection method is discussed through the sensitivity analysis assuming member sets having different number of initiated cracks. It is found that combined inspection has more advantage than sampling inspection, when the number of members having initiated crack is a few in the inspecting member set.

Fatigue strength assessment of scallops —an example for the application of nominal and local stress approaches

Scallops, i.e. small cut-outs in ship structural members, are frequently subjected to high cyclic stresses, which require fatigue strength to be considered during structural design. The paper summarizes the results of several fatigue tests for various types of scallops under axial loading. Further types and the effect of complex load combinations are evaluated using local approaches for the fatigue strength assessment, in particular the notch stress approach for plate edges and the structural or hot-spot stress approach for welded connections. After a review of the approaches, the analysis of local stresses by the finite element method and by stress concentration factors (SCFs) is described. Regression formulae for SCFs, derived from parametric stress analyses, are presented for elliptical and rectangular scallops with rounded corners. The approaches are illustrated by examples from which conclusions for the practical application are drawn.

Fatigue crack growth behavior under random loading model simulating real encountered wave condition

The purpose of this study is to clarify a random loading model for fatigue strength analysis of ship structural members, and to examine fatigue crack growth behavior under the above random loading condition. We analyzed data collected from 38 ships that sailed in the North Pacific and 11 ships in the Japan-Indian Ocean over a period of 14 years; and 6 naval ships in the North Pacific near Japan. The main points are as follows 1. 1) the distribution function of long-term wave-induced load and stress can be described by the Weibull distribution function, with an expected shape parameter value close to unit; 2. 2) the time history of wave conditions during service sailing can be divided into two types—calm sea condition and various storm conditions. These two kinds of conditions appear in random order; 3. 3) based on the above results, a random loading model—‘storm model’ for the fatigue design of a ship is proposed. Then, welded joints simulating side longitudinals of ship structural member were tested under constant and storm loading model. Fatigue crack growth behavior such as crack growth rate, retardation phenomenon etc. were examined.

Stochastic Characteristic of Long Term Distribution of Wave-Induced Loading and Simulation Method for Fatigue Strength Analysis of Ship Structural Member

Stochastic Characteristic of Long Term Distribution of Wave-Induced Loading and Simulation Method for Fatigue Strength Analysis of Ship Structural Member

Statistical Characteristics of Long Term Distribution of Wave-Induced Loading and the Simulation Method for Fatigue Strength Analysis of Ship Structural Member (2nd Report)

Statistical Characteristics of Long Term Distribution of Wave-Induced Loading and the Simulation Method for Fatigue Strength Analysis of Ship Structural Member (2nd Report)

Statistical Characteristics of Long Term Distribution of Wave-Induced Loading and the Simulation Method for Fatigue Strength Analysis of Ship Structural Member (1st Report)

Fatigue strength is one of the most important factors to be taken into consideration in the design of ships. When analyzing the fatigue strength of ship hull structures, it is necessary to obtain both the long term distribution and the time history of wave-induced loading. Recent investigations have found that short term distribution over a short period, about one hour, is closely approximated by the Rayleigh distribution, while long term distribution, which consists of a large number of short term random condition, has not yet been completely clarified. Up to now, designers have assumed it to be an exponential distribution. The time history of wave-induced loading during actual sailing condition has not been made clear at all.The purpose of this study is to clarify the two not well known factors mentioned above by analyzing data collected from 38 ships that sailed the North Pacific over a period of 14 years (1976-1989).The main points made in this study are as follows.1) Various wave conditions were recorded in the same sea zone during the same period to clarify their scatter band.2) The distribution function of long term wave-induced loading can be analytically described by the Weibull distribution function, with an expected shape parameter value close to 1.03) The time history of wave conditions during real sailing can be divided into two types. One is calm sea condition, and the other consists of various storm conditions. These two kinds of conditions appear altermately in random order.