Dynamics of ships running aground

A comprehensive dynamic model is presented for analysis of the transient loads and responses of the hull girder of ships running aground on relatively plane sand, gravel, or rock sea bottoms. Depending on the seabed soil characteristics and the geometry of the ship bow, the bow will plow into the seabed to some extent. The soil forces are determined by a mathematical model based on a theory for frictional soils in rupture and dynamic equilibrium of the fluid phase in the saturated soil. The hydrodynamic pressure forces acting on the decelerated ship hull are determined by taking into account the effect of shallow water. Hydrodynamic memory effects on the transient hull motions are modeled by application of an impulse response technique. The ship hull is modeled as an elastic beam to determine the structural response in the form of flexural and longitudinal stress waves caused by the transient ground reaction and hydrodynamic forces. A number of numerical analysis results are presented for a VLCC running aground. The results include bow trajectory in the seabed, time variation of the grounding force, and the maximum values of the sectional shear forces and bending moments in the hull girder.     

考虑弹塑性土体作用的船舶搁浅损伤特性研究

船舶软搁浅是指船舶搁置在由较软粘土构成的海床或者在柔软的浅滩上的事故。航运业的发展与船舶的大型化趋势使船舶发生软搁浅的风险大大增加,研究船舶的软搁浅性能对船舶设计与制造具有重要意义。本文基于有限元分析软件ABAQUS,选取典型的软搁浅场景,以12,000DWT油船舱段结构为分析对象,研究舱段结构在搁浅过程中的变形、吸能、加速度响应等性能参数。通过系统分析不同速度对船体结构的搁浅性能的影响,全面分析其搁浅性能。本文的相关结论可以为分析船舶软搁浅事故及设计抗冲击舱段结构有一定指导作用。     

高能搁浅下双层底结构的损伤特性研究

船舶搁浅事故会引起船体破损、环境污染和人员伤亡等严重后果.研究船舶搁浅,不仅有利于海上生命安全、防止海洋污染,还可为船体结构的抗冲击设计及规范航运繁忙区域中船舶的航速、操作规程提供一定的依据.本文用数值仿真法研究了船舶高能搁浅中的内部力学问题,分析了典型双层底结构的损伤变形、受力和能量耗散等结果,提出了一种新式的抗搁浅YF双层底结构,并与原结构进行了比较.研究表明,损伤变形集中于结构与礁石相接触的区域,高能搁浅内部力学问题的研究可以主要考虑局部的船体结构;肋板的存在显著增加了船底结构的抗搁浅能力;高能搁浅过程中,由于垂直方向的接触力,礁石对双层底的垂向贯入量会略有减小;当纵桁远离搁浅区域时,它的吸能能力无法发挥,抗搁浅作用很弱;YF双层底结构比原结构具有更大的吸能能力和抗搁浅力.     

Grounding experiments on soft bottoms

To verify a theoretical analysis procedure for calculation of the hull girder response of ships running aground, a series of large-scale ship grounding experiments was performed on an artificial island made of engineered fill. The tests were conducted by running a condemned fishing vessel up on selected beaches of the island with velocities ranging between 2m/s and 5.5m/s. During the tests, surge, heave, and pitch accelerations were measured and also the deformations of the beach and the ship bow. Based on these accelerations, rigid body velocities and motions were determined. The forces arising from the interaction between the bow of the vessel and the seabed were determined by solving the equations of motions. This article describes the analysis of the measured results and a comparison of the results from the full-scale measurements with results from a recently developed analysis procedure for grounding on soft bottoms. Presented at the International Conference on Technology for Marine Environment Preservation (MARIENV '95), Tokyo, Japan, September 24–29, 1995.     

A simplified method to predict grounding damage of double bottom tankers

This paper presents a set of analytical expressions for the calculation of damage opening sizes in tanker groundings. The simplified formulas were given for the grounding force, longitudinal structural damage and the opening width in the inner and outer plating of a tanker's double bottom. The simplified formulas derived are based on a set of numerical simulations conducted with tankers of different dimensions- 120, 190 and 260 m in length. The simulations were performed for five penetration depths and for several rock/ground topologies.The formula for the horizontal grounding force was derived provided the grounding force is proportional to the contact area and the contact pressure. By use of regression analysis it was shown that the contact pressure for any combination of ship and rock size can be expressed with a single normalized polynomial. The actual contact pressure was found by scaling the normalized pressure with the structural resistance coefficient. Given the formulation for the normalized contact pressure, the actual contact force for a ship can be found as a product of average contact pressure and the contact area.The longitudinal length of the damage was evaluated based on the average contact force and the kinetic energy of the ship. The damage opening widths in the outer and inner bottom of the ship were derived separately for two ranges of relative rock sizes as they have strong influence on the deformation mode. The damage widths were given as a function of rock size, penetration depth and double bottom height. To improve the prediction of the onset of the inner bottom failure, a critical relative penetration depth as a function of the ratio of the rock size and the ship breadth was established.Comparison to the numerical simulations showed that the derived simplified approach describes the horizontal grounding force and the damage length well for the penetration depths above 0.5 m. For the range of specified relative rock sizes, the damage width in the inner and outer bottom deviates from numerical simulations approximately up to 25%, which was considered sufficient for the analyses where rapid damage assessment is needed. Comparison was also made to real accidental damage data and to the results of several simplified formulas.     

基于ABAQUS的船舶搁浅数值仿真研究

船舶搁浅性能研究是船底结构设计的一个重要环节。应用国际上通用的动力有限元程序ABAQUS／EXPLICIT对船舶搁浅动力学过程进行仿真研究。通过系列仿真试验,对不同搁浅位置及摩擦系数的情形下船底结构的搁浅性能,包括结构的损伤变形、搁浅力和能量的吸收分别进行对比和分析,得到一部分重要的结论,为船舶搁浅性能的评估及结构优化设计提供重要依据。     

船舶搁浅于刚性斜坡数值仿真的模型化技术

显式非线性有限元分析已逐渐成为研究船舶搁浅问题的重要方法,但其模型化技术是实现数值仿真的关键。本文将搁浅船处理为可变形结构,用弹塑性材料模拟并进行整船建模,斜坡处理为刚体,用刚性材料模拟,船舶与斜坡之间定义为主从接触,船艏定义为自接触。通过仿真计算,获得并讨论了搁浅过程中的船体运动情况、搁浅接触力曲线、能量转化及船舶损伤变形情况。算例表明本文的模型化技术可以用于船舶搁浅的仿真计算。     

Investigation on structural performance predictions of double-bottom tankers during shoal grounding accidents

An investigation is carried out in this paper for the predictions of structural performance of double-bottom tankers during ship grounding over the “shoal” type seabed obstacles. Hong and Amdahl developed a simplified analytical model for the unstiffened double bottom. This method is carefully studied, verified and then used as the first stage of our prediction. The second stage is concerned with stiffeners since stiffeners are indispensable components for double-bottom tankers. A prevailing way to handle is to smear stiffeners onto their attached plating known as the smeared thickness method. However, the effective ratio in this method is dubious in such shoal grounding accidents. Proper values of this parameter are determined in stage two, and then together with the method in stage one, constitute a reliable and efficient tool for structural performance predictions of double-bottom structures in shoal grounding accidents.A double-bottom tanker is chosen as object for the case study. Finite element models of the hold both stiffened and unstiffened are created for numerical simulations using the LS_DYNA software. Simulation cases cover a wide range of slope angles of the indenter and indentations. Numerical results show that Hong and Amdahl's model in stage one is capable of predicting energy dissipation with high precision but poor accuracy for grounding resistances, and a possible reason may be the neglect of vertical resistance. The updated smeared method proposed in stage two is also proved to be capable of grasping major characteristics of stiffeners. Results and conclusions drawn from this paper can be conveniently applied for assessments of the performance of ship double-bottom structures during shoal sliding grounding scenarios, and will benefit the application of accidental limit state design concept in the ship design stage.     

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.     

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.     

Analysis of structural crashworthiness of double-hull ships in collision and grounding

A conceptual design framework for collision and grounding analysis is proposed to evaluate the crashworthiness of double-hull structures. This work attempts to simplify the input parameters needed for the analysis, which can be considered as a step towards a design-oriented procedure against collision and grounding. Four typical collision and grounding scenarios are considered: (1) side structure struck by a bulbous bow, (2) side structure struck by a straight bow, (3) bottom raking, (4) bottom stranding. The analyses of these scenarios are based on statistical data of striking ship dimensions, velocities, collision angles and locations, as well as seabed shapes and sizes, grounding depth and location. The evaluation of the damage extent considers the 50- and 90-percentile values from the statistics of collision and grounding accidents. The external dynamics and internal mechanics are combined to analyse systematically the ship structural damage and energy absorption under accidental loadings.     

Analytical modelling of ship bottom grounding considering combined surge and heave motions

This paper provides a new contribution to the analytical treatment of ship grounding accidents. New formulations are proposed to assess the resisting force of outer/inner bottom plating and transverse floors when the vessel undergoes combined surge and heave motions during the grounding event. Considering shallow and sharp rocks described by parabolic functions, analytical solutions are derived from plastic limit analysis and validated by comparison to non-linear finite element simulations. A failure criterion is also proposed to trigger the rupture of the bottom plating and all the derived closed-form expressions are implemented into an in-house solver. The solver is then coupled to a 6-DOFs external dynamics program, which allows to account for the action of the surrounding water. Resulting tool is first validated on a full scale cruise ship by comparison to finite element results. It appears than although some discrepancies arise, especially in the response of transverse floors after rupture, the bottom damage distribution seems to be well predicted. Finally, the developed tool is used to quickly predict the grounding response of different types of ships and the influence of their mass and hydrodynamic properties on the damage extent is investigated.     

小水线面船单点搁浅损伤特性分析

小水线面船吃水较深，极易与海底擦碰而搁浅，从而带来搁浅强度问题。小水线面船搁浅时，潜体和连接桥结构受载的严酷程度以单点搁浅最为严重且单点搁浅模式（即单点搁浅模式）发生的概率最大。基于非线性有限元理论，应用商用大型有限元软件建立小水线面的整船搁浅有限元模型，研究小水线面船分别触礁石和泥沙底以单点搁浅模式搁浅时的搁浅特性。通过仿真分析，获得小水线面船搁浅的时序损伤特性、搁浅载荷及小水线面船各部分结构在搁浅载荷作用下的强度性能。本文对SWATH的搁浅特性进行了初步的研究，所得结论对小水线面船结构设计和小水线面船搁浅强度的评估具有一定的参考价值。     

Simplified analytical method for evaluating web girder crushing during ship collision and grounding

The paper presents a simplified analytical method to examine the crushing resistance of web girders subjected to local static or dynamic in-plane loads. A new theoretical model, inspired by existing simplified approaches, is developed to describe the progressive plastic deformation behaviour of web girders. It is of considerable practical importance to estimate the extent of structural deformation within ship web girders during collision and grounding accidents. In this paper, new formulae to evaluate this crushing force are proposed on the basis of a new folding deformation mode. The folding deformation of web girders is divided into two parts, plastic deformation and elastic buckling zones, which are not taken into account for in the existing models. Thus, the proposed formulae can well express the crushing deformation behaviour of the first and subsequent folds. They are validated with experimental results of web girder found in literature and actual numerical simulations performed by the explicit LS-DYNA finite element solver. The elastic buckling zone, which absorbs almost zero energy, is captured and confirmed by the numerical results. In addition, the analytical method derives expressions to estimate the average strain rate of the web girders during the impact process and evaluates the material strain rate sensitivity with the Cowper-Symonds constitutive model. These adopted formulae, validated with an existing drop weight impact test, can well capture the dynamic effect of web girders.     

试样极限塑性和船舶触礁模型试验校准计算

船舶碰撞与搁浅时的结构损伤是一个复杂的动态非线性过程。国际船舶与离岸结构会议碰撞与搁浅委员会（ISSC－V.3)组织其成员进行了两项专题研究：（1）材料极限塑性极限应变试验数据用于有限元模型计算时的尺度修正系数；（2）各种计算方法用于船舶搁浅（触礁）计算的准确性。介绍了作者承担上述两项研究的成果，给出了极限塑性极限应变的尺度修正系数的计算方法和结果，论证了非线性有限元显式积分方法及MSC／Dytran软件用于搁浅计算的准确性。     

舰船直升机接地装置设计

直升机在舰船上的停放、维护、补给和悬停等状态下,其喷气燃料系统在岸补或海补过程中易产生静电。操作人员在直升机维护和燃料补给过程中,静电可能引起电路工作异常和燃料爆炸燃烧,这不仅会造成较大的经济损失,还会威胁舰船和操作人员的安全。通过研究应用直升机的舰船接地装置,采取有效的接地措施,解决静电问题,对提高舰船和操作人员安全性具有重大意义。     

舰船接地技术与EMI控制

论述了近十年国内外舰船的主要接地技术和在控制电磁干扰方面的应用.具体分析了舰面设备或部件上感应电压产生的原因和相应的控制技术,介绍了接地技术在提高屏蔽电缆屏蔽效能和减小耦合方面的应用,讨论了舰用数字计算机接地系统的基本组成和安装要求,提出了舰船系统接地的基本原则.     

船舶搁浅的模糊概率计算

目前，计算船舶搁浅的概率时常采用故障树和事件树方法，以确定的概率数表示底事件的发生概率，这种方法的一个不利之处在于无法考虑基本事件发生概率的不确定性，所以无法了解船舶搁浅概率的不确定性，本文引入了合适的模糊数表示基本事件的发生概率，通过模糊计算算法得到了船舶搁浅的模糊概率，并进行了敏感分析，确定了减小船舶搁浅事故发生的措施，与其他方法的计算结果比较表明，本文采用的方法更有利于了解船舶搁浅的概率分布规律。     

船底座礁实验数值模拟中选择参数影响研究

非线性有限元方法是分析船舶碰撞和搁浅问题的一个强有力工具,但是数值模拟结果的可靠性很大程度上依赖于对工程问题的恰当处理和有限元软件中主要参数的准确控制.本文以某单壳船底结构准静态座礁实验结果为例,用非线性有限元软件LS-DYNA进行数值模拟,研究了下列选择参数对单壳船底结构抗撞性的影响:边界条件;船底结构的材料模型;壳单元类型;船底结构与礁石模型之间的摩擦系数;船底结构的残余应力.通过比较计算结果的碰撞力曲线,能量吸收曲线来评价这些参数对数值模拟的影响并给出了一些建议.     

船舶中压交流综合电力推进系统接地故障分析

为避免因接地故障而引起的电力系统过电压、设备损坏,需要对接地系统进行合理设计,设定合适的接地电流保护阈值。本文对船舶中压交流综合电力推进系统接地故障模式进行了分析,以实船为例,完成了中性点高阻接地装置的选型分析计算和验证。