Collapse Strength of Damaged Plating

In this paper, based on Finite Element (FE) simulation, the ultimate strength of damaged continuous plating is studied considering the combined effect of residual deflections and stresses. Firstly, damage over plating is simulated resulting to residual deflection in prescribed shape and magnitude accompanied by residual stresses. Then compressive loading is applied and the effect of simultaneous presence of both residual deflections and residual stresses on the plating ultimate strength is studied. It is found that (1) the compressive strength may be significantly overestimated when the damage related residual stresses are not considered, (2) a larger magnitude of initial dishing does not necessary result to a lower ultimate strength. It is shown that the simplified methods in the Common Bulk Carrier Rules need further improvements.

A Comparative Numerical Study on the Bending Response of Aluminium Stiffened Plates With Fixed/Floating Frames

In this paper, different structural arrangements of the transverse frames in an orthogonally stiffened plate are investigated from the bending response point of view. The transverse frames are assumed to be either fixed or floating. Other alternate placements of the transverse frames are also included in the comparative calculations. Stress and deflection contours are obtained via finite element analyses. Finally, some recommendations are outlined comparing the results of maximum stress and deflection with the allowable limits.

Modelling the Seasonality of Extreme Waves in the Gulf of Mexico

Statistics of storm peaks over threshold depend typically on a number of covariates including location, season and storm direction. Here, a non-homogeneous Poisson model is adopted to characterise storm peak events with respect to season for two Gulf of Mexico locations. The behaviour of storm peak significant wave height over threshold is characterised using a generalised Pareto model, the parameters of which vary smoothly with season using a Fourier form. The rate of occurrence of storm peaks is also modelled using a Poisson model with rate varying with season. A seasonally-varying extreme value threshold is estimated independently. The degree of smoothness of extreme value shape and scale, and the Poisson rate, with season, is regulated by roughness-penalised maximum likelihood; the optimal value of roughness selected by cross-validation. Despite the fact that only the peak significant wave height event for each storm is used for modelling, the influence of the whole period of a storm on design extremes for any seasonal interval is modelled using the concept of storm dissipation, providing a consistent means to estimate design criteria for arbitrary seasonal intervals. Characteristics of the 100-year storm peak significant wave height, estimated using the seasonal model, are examined and compared to those estimated ignoring seasonality.

Multidisciplinary Design Optimization: A New Way to Bring Softer Bows

It is well known that sharp bulbous bow has a good performance on ship resistance reduction, but it is also threatens the struck ships and the environment greatly. For their own economy profit, ship owners would like the bulbous bow to be designed sharp and rigid. However, from the viewpoint of environmental protection, the bulbous bow should be designed blunt and soft. Multidisciplinary Design Optimization (MDO) is a prosperous design concept and technique, to reconcile this problem effectively. The basic concept and theories of MDO are introduced in this paper. An optimization analysis is accomplished on the bulbous bow design for a container ship, using Collaborative Optimization Method. The characters of the bulbous bow on resistance reduction, collision force density and structural strength requirement are all considered at the same time. A compatible bulbous bow can be obtained by this way.

Interaction Between the Underwater Explosion Bubble and the Structure

Based on the potential-flow assumption, BEM is applied to simulate the dynamic characteristics of underwater explosion bubble near boundaries and solve the interaction of bubble and elastic-plastic structure by coupling with FEM. A complete 3D program of underwater bubble analysis (UBA) is developed and the calculated error is within 10%. With this program, flat plate, cylinder and other simple structures are analyzed; the damages caused by retarded flow, pulsating pressure and jet and other loads on the structures are calculated, including different cases with free surface or without free surface. Results show that bubbles can cause great damage, and the specific cases can even cause greater damage. From the wall pressure and the stress curves of typical elements on the structure, it can be seen that the pressure peak occurs when the bubble collapses, which proves that the pressures caused by the bubble’s collapse and jet can result in great structure’s severe damage. It can provide reference for the research on the dynamic characteristics. The research in this paper aims to provide references for the correlated research on the dynamics of the underwater bubble.

A Comparative Study on the Plastic Formulations of Ship Shell Plating Under Patch Loading

The purpose of the present study is to further develop the response formulation of ship shell plating under general patch loading, considering the effect of finite, permanent deformation. A comparative study is conducted between various formulations for the load-carrying capacity of laterally patch loaded plates with the aid of nonlinear finite element method. The increasing public interest of transportation and resource exploitation in Arctic area gives rise to advances in ice strengthening of ship structures. As a result, the subject of ship shell plating under ice loads has been extensively studied. Similar to ice loads, wave slamming loads, wheel loads of vehicles and accidental loads, e.g. collision and grounding, are all likely to take place over a limited area of the plate, and can be termed as ‘patch loading’. Consequently, the resistance of ship shell plating under ‘patch loading’ is of significant interest. In this study, a recently developed plastic formulation for patch loaded plates is further extended for general patch loading condition, i.e. patch load with limited extension in both length and height direction. After a brief review of the development of the response and design formulations for plates under uniform lateral load and patch load, comparative studies are made in cases of limited and finite permanent deformation with the aid of nonlinear finite element methods. By allowing a certain level of permanent deformation, significant weight savings can be achieved. Some main findings will be concluded from the comparative study. The present formulation may be used as a versatile tool for predicting the resistance of plates under various types of patch loads, notably when finite, permanent deformations are accepted, e.g. in the Accidental (Abnormal) Limit States design.

Reliability of TLP Tethers Using Evolutionary Strategies

Tether system is a critical component for the TLPs, since its failure may lead to the collapse of the whole structure involving human lives, economic losses and damages to the environment. Due to this fact, reliability methods have been proposed to design TLP tethers and new codes are being developed to increase their safety level. The objective of this paper is to compare the probability of failure for TLP tethers considering the maximum tension limit state obtained with three methods, which are: a methodology based on Evolutionary Strategies and the Monte Carlo Importance Sampling, the First Order Reliability Method, and the Second Order Reliability Method. Von-Mises failure criterion is used as limit state function for the most loaded tether of a TLP submitted to different sea states. Efficiency of the ES algorithm to find design points and probabilities of failure obtained with the reliability methods are discussed.

Quality Control Issues in Estimating Wave Climate From Hindcast and Satellite Data

Accuracy of an estimated design wave condition such as the 100-year significant wave height is important information for risk assessment in design of marine structures. Assessment of accuracy of design conditions is already not simple when they have been derived from in-situ wave measurements at the project site. When using wave hindcast data this task is even less straightforward due to often a lack of sufficiently reliable measurement data near the project site for comparison. Different hindcasts may give significant discrepancies in prediction of extremes. Also, the quality of the hindcast may vary over time. In practice, we often do not much more than illustrating the quality of the data, for example by scatter plots of hindcast data versus measurements far away from the project site. In order to assess the quality of a design wave height from a hindcast systematically, errors in both the local hindcast data and in the extrapolation from these data need to be addressed. The paper discusses the overall idea and some building blocks for such an approach, while realising that there will not be one simple recipe applicable in all situations. One of the issues is the selection and use of observational data in quality control. Satellite wave data are an attractive data source for this purpose. For illustration, data from the seas around Norway are used.

Sea State Statistics and Extreme Waves Observed by Satellite

For the design of ships as well as for the investigation of ship accidents it is important to have knowledge about both the two dimensional spectral wave properties as well as extreme value statistics of ocean waves. Although numerical wave models have reached a high level of accuracy, they still have weaknesses with respect to the details of the 2-D wave spectrum. Furthermore standard models like WAM provide estimates of the 2-D wave spectrum, i.e., second order sea state statistics and therefore lack information on individual wave properties and the occurrence of extreme events. In this study the potential of global Synthetic Aperture Radar (SAR) wave mode data acquired by the European satellites ERS-2 and ENVISAT to investigate ship accidents is discussed and compared to altimeter data and ECMWF model results. These data are acquired independent of light and weather conditions on a global scale. A historic data set of ERS-2 wave mode data acquired between 1998 and 2000 is co-located with accidents which occurred during that time. ENVISAT ASAR wave mode data acquired since 2002 are considered, too. Different ocean wave parameters like significant wave height and wave periods are derived from the SAR data. The potential role of the respective wave conditions for some recent accident is discussed in detail. This includes in particular the analysis of cross sea conditions, groupiness and extreme events.

Emergency Towing: Field Exercises and Simulator-Based Training

Recently there have been a number of incidents involving drifting ships and offshore structures in the North Sea. Increasing traffic density combined with harsher weather and less experienced crews has led the Norwegian Coastal Administration to focus on improving the skills of personnel on board emergency response vessels operating off the coast of Northern Norway. This is order to reduce the consequences of incidents with disabled vessels sailing in the vulnerable coastal environment of the Norwegian and Barents Seas. In addition to field exercises involving dedicated Norwegian emergency response vessels and different types of vessels made available by shipping companies, training takes place on full mission bridge simulators. As part of an ongoing R&D project, MARINTEK and SMS invite stakeholders to participate in short workshops on topics related to the specification of functional requirements for emergency towing vessels and the sharing of experience from recent emergency towing operations.