Plastic mechanism analysis of structural performances for stiffeners on bottom longitudinal web girders during a shoal grounding accident

A theoretical model is introduced in this paper for structural performance of stiffeners on double-bottom longitudinal girders in a shoal grounding accident. Major emphasis is placed on establishing the characteristic deformation mechanism of stiffeners and identifying major energy dissipation patterns. Numerical simulations using the LS-DYNA nonlinear finite-element program were carried out to examine thoroughly the progressive deformation process during sliding deformation. Stiffener deformations were observed to fall into two categories: stiffeners fully contacted with the indenter, and stiffeners subjected to indirect deformation due to energy transfer from attached girders. Grounding performance of stiffeners is substantially influenced by that of the attached plating, and therefore a review of the existing deformation models of longitudinal girders (i.e. Simonsen 1997, Midtun 2006 and Hong 2008) was included. Hong's model of bottom girders was found not capable of representing the effects of stiffeners, and a new model of girders was thus developed. Based on observation of the numerical deformation process and the new analytical girder model, a kinematically admissible model of stiffeners on bottom longitudinal girders was built. Using the methods of plastic mechanism analysis, simplified analytical expressions for energy dissipation by girder-attached stiffeners, both fully contacted and noncontacted, were formulated, and equations for grounding resistance were subsequently obtained. The theoretical expressions agree favorably with results from nonlinear finite-element simulations and capture two significant characteristics of the problem: that energy varies little with indentation for stiffeners that fully contacting the indenter, and that energy is independent of slope angle for indirectly deformed stiffeners. The proposed theoretical model helps to predict analytically shoal grounding performance of stiffeners on longitudinal girders with reasonable accuracy.     

Verification of a simplified analytical method for predictions of ship groundings over large contact surfaces by numerical simulations

In this paper, a verification is presented of a simplified analytical method for the predictions from numerical simulations of structural performance during ship groundings over seabed obstacles with large contact surfaces and trapezoidal cross-section. This simplified analytical method was developed by Lin Hong and Jørgen Amdahl and calculates grounding characteristics, such as resistance and distortion energy, for double-bottomed ships in shoal grounding accidents. Two finite-element models are presented. One was built for a hold, and the other was built for a hold and a ship hull girder and also considers sectional properties, ship mass, added mass and the hydrodynamic restoring force. The verification was completed by comparing horizontal and vertical resistances and the distortion energy between seven numerical-simulation cases and a set of corresponding cases computed by a simplified analytical method. The results show that the resistances obtained by the simplified analytical method are close to the mean values of the resistance curves obtained by numerical simulations. The comparisons prove that the energy dissipation-prediction capability of the simplified analytical method is valuable. Thus, the simplified analytical method is feasible for assessing ship groundings over seabed obstacles with large contact surfaces and trapezoidal cross-section. Furthermore, studies of the influence of ship motion during groundings ascertained that ship motion affects structural performance characteristics. Resistances are lessened at the end of the grounding due to the reduction of indentations caused by heave and pitch motions of the ship hull girder. Finally, a new method for predicting the structural performance of the time-consuming complete-ship model by applying a combination of normal numerical simulations and ship-motion calculations is proposed and proven.     

Rapid prediction of structural responses of double-bottom structures in shoal grounding scenario

This study presents a simplified analytical model for predicting the structural responses of double-bottom ships in a shoal grounding scenario. This solution is based on a series of analytical models developed from elastic-plastic mechanism theories for different structural components, including bottom girders, floors, bottom plating, and attached stiffeners. We verify this simplified analytical model by numerical simulation, and establish finite element models for a typical tanker hold and a rigid indenter representing seabed obstacles. Employing the LS-DYNA finite element solver, we conduct numerical simulations for shoal-grounding cases with a wide range of slope angles and indentation depths. In comparison with numerical simulations, we verify the proposed simplified analytical model with respect to the total energy dissipation and the horizontal grounding resistance. We also investigate the interaction effect of deformation patterns between bottom structure components. Our results show that the total energy dissipation and resistances predicted by the analytical model agree well with those from numerical simulations.     

双层底船搁浅场景下结构响应的快速预报（英文）

This study presents a simplified analytical model for predicting the structural responses of double-bottom ships in a shoal grounding scenario. This solution is based on a series of analytical models developed from elastic-plastic mechanism theories for different structural components, including bottom girders, floors, bottom plating, and attached stiffeners. We verify this simplified analytical model by numerical simulation, and establish finite element models for a typical tanker hold and a rigid indenter representing seabed obstacles. Employing the LS-DYNA finite element solver, we conduct numerical simulations for shoal-grounding cases with a wide range of slope angles and indentation depths. In comparison with numerical simulations, we verify the proposed simplified analytical model with respect to the total energy dissipation and the horizontal grounding resistance. We also investigate the interaction effect of deformation patterns between bottom structure components. Our results show that the total energy dissipation and resistances predicted by the analytical model agree well with those from numerical simulations.     

Marine accident analysis for collision and grounding in oil tanker using FTA method

In this study, collision and grounding data registered in GISIS (Global Integrated Shipping Information System) were investigated for oil tankers. The database includes the information of the collision and grounding accidents during the period between 1998 and 2010 in oil tankers. The risk assessments were carried out using fault tree analysis (FTA) programme for the incidents as collision and grounding occurred in oil tankers. In this study, we were able to investigate first the potential problems which cause the collision and grounding accidents have been determined, second, the occurrence of accidents has been shown with causal factors by the FTA method, and, finally, the significance degree of the initial events causing occurrence of accidents have been put forth. Collision in oil tanker resulted in economical loss (81%), pollution (6%) and death or injury (13%). Grounding in oil tanker resulted in economical loss (91%) and pollution (9%). According to the FTA results, the main reason for the accidents originating from human error is as follows: for collision accidents, Convention on the International Regulations for Preventing Collisions at Sea (COLREG) violation and the lack of communication between vessels; and for grounding accidents, the interpretation failure of the officer on watch and lack of communication in the bridge resource management.     

台湾海峡水域搁浅事故的方差分析

船舶搁浅事故造成的经济损失和海洋环境污染是台湾海峡水域主要危害之一.为减少搁浅事故的发生,使台湾海峡水域的船舶航行更安全、海洋环境更清洁,以台湾海峡水域搁浅事故为例,在统计分析船舶搁浅事故特征的基础上,选用方差分析法对搁浅事故因素相关性和差异性进行了系统的分析.提出了台湾海峡水域搁浅事故中人为因素所占比例最高,水深不足...     

Longitudinal strength of ships with accidental damages

This paper presents an investigation of the longitudinal strength of ships with damages due to grounding or collision accidents. Analytical equations are derived for the residual hull girder strength and verified with direct calculations of sample commercial ships for a broad spectrum of accidents. Hull girder ultimate strengths of these sample vessels under sagging and hogging conditions are also calculated, based on which correlation equations are proposed. To evaluate a grounded ship, using the section modulus to the deck would be optimistic, while using the section modulus to the bottom would be conservative. On the contrary, to evaluate a collided ship, using the section modulus to the deck would be conservative, while using the section modulus to the bottom would be optimistic. The derived analytical formulae are then applied to a fleet of 67 commercial ships, including 21 double hull tankers, 18 bulk carriers, 22 single hull tankers and six container carriers. The mean values, standard deviations and coefficients of variation for the coefficients in these new analytical formulae are obtained. The ship length exhibits little influence on these coefficients because they are close to the mean values although ship length spans from 150 to 400 m. The ship type shows some influence on the residual strength. Uniform equations are proposed for commercial ships which do not depend on a ship's principal dimensions. These formulae provide very handy tools for predicting the residual strength in seconds, without performing step-by-step detailed calculations, an obvious advantage in cases of emergency or salvage operation.     

逐步破坏法破损油船剩余强度研究

基于IACS共同规范研制了逐步破坏法计算完整油船极限强度和破损剩余强度的程序.考虑了船体发生搁浅碰撞后,其剩余有效剖面是非对称的,船体还可能发生不同程度倾斜的实际情况,计算了双壳油船在不同破损情况下破损船体的剩余强度,并给出了实例及结果分析.     

Behavior of a double hull in a variety of stranding or collision scenarios

A series of nine tests was conducted to investigate the behavior of a double hull in a variety of stranding or collision scenarios. Cones of five different nose radii were made to model accident scenarios ranging from grounding on a sharp rock to stranding on a relatively flat seabed or shoal, and collision with a sharp bulbous bow of a fast ship to collision with a large bow of a VLCC. Three sub-series were designed in which the cones pressed shell plating, main supporting members and intersections of main supporting members. The test results reveal that the nose radius and the location of penetration have a very strong influence on the behavior of a double hull. Therefore, careful definition of accident scenarios is of crucial importance to assess the strength of ship hulls in accidents, and it is necessary to base the assessment on probability of accidents. Characteristics of the response of structural members were identified and idealized as simple theoretical models. Analytical formulae were derived and discussed. Primary damage mechanisms include membrane stretching of shell panel, onset of rupture, crack propagation, folding of main supporting members, and crushing of intersections of main supporting members. The new plate punching model captures the phenomenon that the load-carrying capacity of a plate depends on the size of the striking object. The plate perforating model predicts the reduced strength of plates with cracks. It reflects the observed test phenomenon that loads do not drop to zero even after rupture occurs in shell plating. A simple analytical method was developed to calculate the global strength of a double hull. The method takes geometrical parameters of seabed rocks or bulbous bows into account, and can be used for a wide range of different accident scenarios. Calculations using this method compared satisfactorily with the test results. This method can be easily incorporated into a probability-based framework to properly assess structural performance for a variety of damage scenarios. Similar to the Wang et al. (J Ship Res 41 (1997) 241) paper on raking damage, which uses only four analytical models, this method also requires only a common calculator to carry out the calculations.     

基于贝叶斯网络方法对船舶搁浅概率的研究

针对船舶航行安全的薄弱环节,在故障树分析船舶搁浅事故成因的基础上,运用贝叶斯网络对事件之间的多态性和逻辑性进行了研究。得到了事件的后验概率值,分析了导致事故发生的薄弱环节,推理得出最可能引起事故发生的事件组合,结果与统计资料一致,为提高船舶运营的安全和运营管理提供了科学依据。     

A simplified methodology for evaluating alternative tanker configurations

Current MARPOL regulations for hypothetical oil ouflow and tank size limitations apply a deterministic approach which does not properly account for varying wing tank and double-bottom dimensions. Important considerations such as the influence of hydrostatic balance and tidal changes on the stranded vessel are not addressed in these regulations. This paper presents a more rational approach for evaluating oil outflow for double-hull and mid-deck tankers. This proposed methodology, while easy to apply, allows for the optimization of tankage configurations, and provides an effective means for indexing the environmental performance of tankers.     

Full six degrees of freedom coupled dynamic simulation of ship collision and grounding accidents

By taking advantage of the user-defined load subroutine (loadud) and the user common subroutine (usercomm) in LS-DYNA, the authors proposed a new coupled approach for simultaneously calculating structural damage and the planar 3DOF ship motions in ship collisions. The coupled procedure aimed at predicting the detailed structural damage together with reasonable global ship motions. This paper extends the method to consider the full 6DOF ship motions; thus, ship collision as well as grounding accidents can be properly handled. This method is particularly useful for design purposes because the detailed ship hull profile is not needed.A traditional ship maneuvering model is used for the in-plane surge, sway and yaw degrees of freedom with a series of nondimensional coefficients determined from experiments. It is assumed that the out-of-plane degrees of freedom are not coupled with the in-plane ship motions, and there is no coupling among roll, pitch and heave motions. The implementation is verified through free decay tests, and the obtained natural periods show good agreement with theoretical results.Several collision and grounding cases are simulated in which a supply vessel crashes into rigid plates with different orientations. The effects of the roll motion, the heave and pitch motions and the full 6DOF motions are studied. The results are compared with those from a 6DOF decoupled method. Ship motions through the proposed method compare reasonably well with SIMO results. It is found that several consecutive impacts may occur in the simulation of one collision case due to the periodic motions. This is not taken into account in the decoupled method, which makes this method unconservative.     

碰撞/搁浅事故中船体强桁材结构损伤机理解析预报方法研究

随着航运业的快速发展,海上航行的船舶越来越多.尽管人们做了许多努力避免海上意外事故的发生,但海难事故依然不可避免.为了降低上述事故造成的损失,需要在设计阶段快速并准确地预报船舶的结构耐撞性.本文以强桁材结构为研究对象,通过开展准静态冲压试验及相应的数值仿真,分析强桁材结构在面内冲压载荷作用下的变形机理,并基于试验与仿真所得到的结构变形特点,提出强桁材面内受压时的变形模式.以此为基础,运用塑性力学理论,推导出结构变形能、瞬时结构变形抗力及平均结构变形抗力的解析预报公式,并将计算结果与试验结果进行比较验证.研究得到的结构面内受压变形能和抗力解析计算公式,可以快速评估事故载荷下结构的响应情况,包括结构变形阻力及能量耗散,具有使用方便,计算速度快,计算结果相对可靠的优点,对船体耐撞结构设计及抗撞性能评估具有一定的指导意义.     

Strength of ship plates under combined loading

Strength of ship plates plays a significant role in the ultimate strength analysis of ship structures. In recent years several authors have proposed simplified analytical methods to calculate the ultimate strength of unstiffened plates. The majority of these investigations deal with plates subjected to longitudinal compression only. For real ship structural plating, the most general loading case is a combination of longitudinal stress, transverse stress, shear stress and lateral pressure. In this paper, the simplified analytical method is generalized to deal with such combined load cases. The obtained results indicate that the simplified analytical method is able to determine the ultimate strength of unstiffened plates with imperfections in the form of welding-induced residual stresses and geometric deflections subjected to combined loads. Comparisons with experimental results show that the procedure has sufficient accuracy for practical applications in design.     

Implosion tests of aluminium-alloy ring-stiffened cylinders subjected to external hydrostatic pressure

In this paper, the dynamic implosion responses of ring-stiffened cylindrical tubes under external hydrostatic pressure are reported. The ring-stiffened cylinder models were fabricated from commercial seamless aluminium-alloy 6061-T6 tubes. Six external stiffened models were constructed: three models with three stiffeners, and the other three with five stiffeners. To quantify the effects of stiffening on the structural behaviour, implosion tests were also performed on another three unstiffened tube models. In this study, a total of nine tube models were tested until their failure. Implosion was initiated by increasing the hydrostatic pressure in the chamber until the tubes collapsed, and the effects of different types of pressurising media were investigated: (i) water only and (ii) water and nitrogen gas combined. The implosion pressure pulse near the collapsing tube was recorded during the event. The models with five stiffeners increased the implosion pressure pulse by approximately 3.5 times compared with the unstiffened model.     

Assessment of IACS-CSR implicit safety levels for buckling strength of stiffened panels for double hull tankers

The present study aims at applying structural reliability methods to assess the implicit safety levels of the buckling strength requirements for longitudinal stiffened panels implemented in the IACS Common Structural Rules (CSR) for double hull oil tankers. The buckling strength requirements considered are used in the initial stage of the hull girder scantlings’ design to control the buckling capacity of longitudinal stiffened panels subjected to the compressive loads induced by the hull girder vertical bending. The following buckling collapse failure modes are explicitly considered in the design formulation: uniaxial buckling of the plating between stiffeners, column buckling of stiffeners with attached plating and lateral-torsional buckling or tripping of stiffeners.The paper presents the procedure used to assess the implicit safety levels of the strength requirements for the three buckling collapse failure modes above mentioned, which includes the optimization of the scantlings of the plate panels and longitudinal stiffeners in order to reflect the minimum strength required by the formulation. A first order reliability formulation is adopted, and stochastic models proposed in the literature are used to quantify the uncertainty in the relevant design variables. A sample of five oil tankers representative of the range of application of the IACS-CSR design rules is considered. The effect of corrosion in the implicit safety levels is quantified based on the three corrosion levels of the Net Thickness Approach (NTA) adopted in the design rules. Sensitivity analyses are also performed to quantify the relative contribution or importance of each design random variable to the implicit safety levels.     

论船舶溢油污染及应急反应

近年来油船的数量和吨位越来越大,油船进出港口次数日渐增加,船舶发生海损事故的几率也随之增加。由于船舶碰撞等海难事故而发生的溢油,也成为海域污染的主要污染源之一。目前我国的海上溢油应急防治能力还是较低的,与国际上相关先进成果相比,理论和实用性方面均有一定差距,提高海上溢油应急反应能力刻不容缓。     

Fatigue damage calculation for oil tanker and container ship structures

The fatigue behaviour of longitudinal stiffeners of oil tankers and container ships, subjected to dynamic loads, is analysed. The following dynamic load components are considered: hull girder vertical wave bending moment, alone and combined with the horizontal wave bending moment, hydrodynamic pressure and inertial forces caused by cargo acceleration.

The spectral method was selected to calculate the fatigue damage, based on S—N curves and Miner's rule. Following this approach, the fatigue damage may be calculated as a function of a stress parameter Ω_p, which represents the cumulative effect of wave induced loads in the unit of time and incorporates the combined effects of stress level and its occurring frequency.

Simple formulas for Ω_p of oil tankers and container ships are given, obtained from the results of hydrodynamic analyses performed on several ships, in different wave environments.

Several examples show the applicability of the methods to real ship structures. The method, however, still needs to be calibrated because of the simplifying hypotheses introduced in the loading conditions.     

复杂应力状态下无筋板格极限强度研究

无筋板格是波浪中航行船舶的基本结构单元,精确评估无筋板格极限强度对保证船舶结构安全性具有重要意义.本文基于弹性大挠度理论分析和刚塑性分析,给出了复杂应力状态下无筋板格极限强度计算方法;定性的研究了各参数对无筋板格极限强度的影响;比较了剪应力与其它应力作用的合成分析方法和分离分析方法,验证了分离分析方法的可行性和高效性;开展了本文方法与经验公式及ABS规范公式的比较研究.     

船舶碰撞机理与耐撞性结构设计研究综述

研究船舶碰撞和触底事故的机理,以及如何提高船舶结构耐撞性是船舶碰撞研究领域的热点.文章介绍了解析法、数值仿真技术和风险分析法的发展与应用特点,阐述了近些年来船舶碰撞、船舶触底、缓冲船首设计、船桥碰撞和船舶与海洋平台碰撞等领域的研究成果,列举了一些降低船舶碰撞和触底事故风险的新型结构设计,并对今后的研究方向提出了若干建议.