A review of intact ship stability research and criteria

An investigation on capsizing of a vessel in a seaway is of paramount importance from the point of view of safety of life at sea. The present paper analyzes the major research efforts made throughout the world regarding intact stability and the establishment of stability criteria. Attempts have been made by the authors to draw conclusions from this analysis and to provide guidelines for future research efforts in the field of intact stability of ships.     

A neural network approach to ship track-keeping control

This paper presents an on-line trained neural net work controller for ship track-keeping problems. Following a brief review of the ship track-keeping control development since the 1980's, an analysis of various existing backpropagation-based neural controllers is provided. We then propose a single-input multioutput (SIMO) neural control strategy for situations where the exact mathematical dynamics of the ship are not available. The aim of this study is to build an autonomous neural controller which uses rudder to regulate both the tracking error and heading error. During the whole control process, the proposed SIMO neural controller adapts itself on-line from a direct evaluation of the control accuracy, and hence the need for a “teacher” or an off-line training process can be removed. With a relatively modest amount of quantitative knowledge of the ship behavior, the design philosophy enables real time control of a nonlinear ship model under random wind disturbances and measurement noise. Three different track-keeping tasks have been simulated to demonstrate the effectiveness of the training method and the robust performance of the proposed neural control strategy     

A rational approach to seepage flow effects on bottom friction beneath random waves

The bottom friction beneath random waves is predicted taking into account the effect of seepage flow. This is achieved by using wave friction factors for rough turbulent, smooth turbulent and laminar flow valid for regular waves together with a modified Shields parameter which includes the effect of seepage flow. Examples using data typical to field conditions are included to illustrate the approach. The analytical results can be used to make assessment of seepage effects on the bottom friction based on available wave statistics. Generally, it is recommended that a stochastic approach should be used rather than using the rms values in an otherwise deterministic approach.     

A new approach to the overturning stability of floating structures

The increasingly widespread use of floating offshore production facilities has led to a need for a better understanding and accurate prediction of dynamic behavior. The approach described in this paper uses an approximate criterion for safe, forced, roll oscillations of a typical semi-submersible based on a stability margin. This stability margin is developed from a comparison between the total energy of the steady-state oscillating system and the total energy required to cause instability in the corresponding unforced, undamped system. The accuracy and utility of the new approach is assessed by comparing the loss of stability of numerical simulations with the approximate criterion.     

A method for breach assessment onboard a damaged passenger ship

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

Calculation of ship collision avoidance manoeuvres: A simplified approach

The moment at which in a two-ship encounter a collision avoidance manoeuvre should be initiated in order to achieve a specific distance at the closest point of approach has been calculated. The calculations were based on the kinematics of encounter and simplified equations of motion. Mathematical equations which relate the maneouvring distance, the turning direction and the extent of course change for any desired passing distance and any given turning rate are determined. Knowledge of these quantities is essential for the safe conduct of the ship with automatic, computer-assisted navigation as well as with ordinary, manual navigation if the navigator no longer contents himself with estimates but wishes to know precisely in advance the outcome of any specific manoeuvre he chooses.     

An efficient and robust approach to predict ship self-propulsion coefficients

Achieving a reliable and accurate numerical prediction of the self-propulsion performance of a ship is still an open problem that poses some relevant issues. Several CFD methods, ranging from boundary element methods (BEM) to higher-fidelity viscous Reynolds averaged Navier–Stokes (RANS) based solvers, can be used to accurately analyze the separate problems, i.e. the open water propeller and the hull calm water resistance. However, when the fully-coupled self-propulsion problem is considered, i.e. the hull advancing at uniform speed propelled by its own propulsion system, several complexities rise up. Typical flow simplifications adopted to speed-up the simulations of the single analysis (hull and propeller separately) lose their validity requiring a more complex solver to tackle the fully-coupled problem. The complexity rises up further when considering a maneuver condition. This aspect increases the computational burden and, consequently, the required time which becomes prohibitive in a preliminary ship design stage.The majority of the simplified methods proposed in literature to include propeller effects, without directly solve the propeller flow, in a high-fidelity viscous solver are not able to provide all the commonly required self-propulsion coefficients. In this work, a new method to enrich the results from a body force based approach is proposed and investigated, with the aim to reduce as much as possible the computational burden without losing any useful result. This procedure is tested for validation on the KCS hull form in self-propulsion and maneuver conditions.     

A new heuristic approach to one-dimensional stock-cutting problems with multiple stock lengths in ship production

One-dimensional stock-cutting problem is a commonly encountered problem not only in shipbuilding and construction of coastal structures, but also in other engineering applications related to production and construction. The proposed approach achieves results using cutting patterns directly whereas analytical methods first need to establish a mathematical model. While obtaining ideal solutions of the analytical methods, the new approach limits the wastage to a minimum number of stock materials. In addition, the new approach allows the use of different sized stock materials while it creates various options for the use of single sized stock materials. Use of different sized stock materials broadens the point of view for the solution compared to the use of single sized stock material. Moreover, the new method yields integer results whereas the analytical methods using linear programming usually produce impractical non-integer results. To obtain integer results, the analytical methods need to solve the problem multiple times and screen the alternative solutions.     

Numerical simulation of ship stability for dynamic environment

The prediction of ship stability during the early stages of design is very important from the point of vessel’s safety. Out of the six motions of a ship, the critical motion leading to capsize of a vessel is the rolling motion. In the present study, particular attention is paid to the performance of a ship in beam sea. The linear ship response in waves is evaluated using strip theory. Critical condition in the rolling motion of a ship is when it is subjected to synchronous beam waves. In this paper, a nonlinear approach has been tried to predict the roll response of a vessel. Various representations of damping and restoring terms found in the literature are investigated. A parametric investigation is undertaken to identify the effect of a number of key parameters like wave amplitude, wave frequency, metacentric height, etc.     

船体结构腐蚀检测规划的成本及效益分析

建立了基于风险的船体结构腐蚀优化检测规划的成本-效益分析模型。以费效比作为选择最优检测策略的标准,最优的检测策略是在保证结构设计工作寿命期内的可靠指标大于最低可靠指标的基础上,使得结构生命周期内总的费效比最大。在此基础上,以受点腐蚀损伤的船体构件为例,对其检测策略进行了成本-效益评估,并对计算结果进行了敏感性分析。结果表明,基于风险的成本及效益分析方法可以将检测规划的经济性和可靠性有效地结合起来,能够在风险与成本之间达到一种平衡,它在优化检测策略时是有效的。     

The role of material efficiency to reduce CO2 emissions during ship manufacture: A life cycle approach

Low carbon shipping research focuses on energy efficiency and mitigation measures related to operations and technology. However, reducing energy use and greenhouse gas emissions associated with a vessel’s material production receives limited attention. Material efficiency is defined as “providing material services with less material production and processing”. The current business model for ship building and breaking does not embrace fully material efficiency principles. Exploring a case study of a vessel's steel hull, this study applies a Life Cycle Assessment approach to determine the effectiveness of material efficiency to reduce CO₂ emissions. When compared to Business as Usual, designing and manufacturing for 100% hull reuse provides an emissions reduction of 29% from 221,978 t CO₂ to 158,285 t CO₂; 50% reuse provides a 10% reduction (199,816 t CO₂). From a technical and safety perspective there remain key barriers that need addressing: a vessel’s hull would require to be designed for dismantling to improve reuse; the operation and maintenance schedule must ensure the value of the steel is retained and; data must flow between key stakeholders on the quality of the steel. Despite these challenges, progressing material efficiency would require different business models that reach out and integrate the whole supply chain. There is a need for public and privately funded demonstration projects at a range of scales and markets, to provide investors the confidence that there is retained value in the steel hull when it reaches its end-of-life.     

CFD approach to roll damping of ship with bilge keel with experimental validation

The most common method of reducing roll motion of ship-shaped floating systems is the use of bilge keel which act as damping elements. The estimation of the damping introduced by bilge keel is still largely based on empirical methods. The present work adopts the CFD approach to the estimation of roll damping, both without and with bilge keel and validates the results with experiments conducted in a wave flume. Specifically, free oscillation tests are conducted at model scale to obtain roll damping, both by experiments and CFD simulation and reasonably good comparisons are obtained. The experiments also include PIV study of the flow field and attempt has been made to correlate the measured flow field with that obtained by CFD. The CFD methodology has the potential to determine rationally the size and orientation of bilge keels in design with reasonably accurate estimate of the additional roll damping that it provides to ship's roll motion.     

3-D geometric modeler for rapid ship safety assessment

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

跨海桥梁船撞风险综合评估

近年来全世界范围内船撞桥事故时常发生,尤其是大型跨海桥梁,对其进行合理的船撞桥风险评估逐渐成为桥梁安全运营的重要保障之一。通过对近年国内外船撞桥案例调研分析,确定了影响事故的多重因素,建立包括4个二级风险评价指标和18个三级指标的层次化评价指标体系,并运用层次分析（AHP）法和熵权法,对各评价指标进行主客观综合赋权,明确各个风险因素对船舶撞击的重要性程度,基于模糊数学理论对船撞桥风险进行多层次综合评判。以浙江省舟山朱家尖跨海大桥为工程背景,结合该桥某年船舶通航统计数据,运用上述方法进行船撞桥风险评估,计算表明朱家尖跨海大桥平均风险评价值为4.22,属于可接受中风险水平,并提出了相应的风险控制对策及措施。     

A new approach for the solution of the two-dimensional guillotine-cutting problem in ship production

A new approach has been developed for the arrangement of rectangular pieces in plates for the guillotine-cutting problem as encountered in ship production and similar manufacturing processes. The algorithm is quite novel in the sense that instead of making preliminary specifications and ignoring all the alternative arrangements, as is the case for most of the current models, it selectively considers feasible arrangements by eliminating the majority of the probable arrangements that render the problem insolvable. Intermediate steps of the algorithm produce results which cannot be obtained through integer programming. Losses are distributed to the minimum number of plates. The number of arrangement plans is also minimized. Overall the solution of the problem has been drastically simplified to allow even hand calculations instead of long computer runs.     

An experimental study of motion behaviour with an intact and damaged Ro-Ro ship model

Six degrees of freedom motion response tests of a Ro-Ro model have been carried out in regular waves for intact and damaged conditions. The stationary model was tested in different wave heights and wave frequencies for the head, beam and stern quartering seas in order to explore the effect of damage and varying wave heights on the motion responses of the model. Analysis of the results indicates that damage has an adverse effect on the motion responses of the model depending upon the directionality of the waves and frequency range applied.     

An optimization approach for fairing of ship hull forms

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

Checking vulnerability to pure loss of stability in long crested following waves: A probabilistic approach

This paper presents a probabilistic methodology for the analysis of the vulnerability of a ship to the risk of inception of pure loss of stability events. A pure loss of stability failure is modelled as the persistence of the metacentric height below a critical level for a too long time. The metacentric height is modelled as a stationary Gaussian process with a spectrum obtained from the sea elevation spectrum. The time dependent failure index is obtained under the assumption of filtered Poisson process for the occurrence of critical events. The analysis separates cases where the fluctuation of the metacentric height is narrow-band from those where the bandwidth of the spectrum is wide, with an intermediate blending. In case of narrow-band processes appropriate approximate solutions to the problem are provided, while in the wide-band cases an exponential distribution for the persistence time below the critical level is employed. A rational development for the critical persistence time is also provided considering an approximation of the roll dynamics during periods of time where the metacentric height is negative. Monte Carlo simulations are performed to check the developed approximate distributions for the persistence time, and examples of application are provided for a sample ship.