A medium-resolution wave hindcast study over the Central and Western Mediterranean Sea

The present study is aimed at determining the confidence limits of design wave parameters derived from numerical modeling—for both extremes and operational conditions—over the Central and Western Mediterranean Sea. The paper presents the methodology and results of an extensive validation activity conducted on a chain of medium-resolution third-generation wave models used for hindcast purposes. The stringent requirements of state-of-the-art coastal and offshore engineering applications over this area make the adoption of medium- or high-resolution hindcast wave and wind models almost mandatory because of the complex coastal geometry, bathymetry, and orography that in turn lead to large variations of the design wave parameters even within small regions. The chains of nested meteorological and wave models used in this hindcast study belong to the ETA and WaveWatch III families, respectively. In this study the wind and wave numerical models have been run over the past 20 years, with increasing resolutions of the wave models from 0.2° up to 0.04°. The results presented herein have 0.1° resolution for both wind and wave models. The wave data obtained are compared with available measurements from 14 wave buoys in coastal zones in the Central and Western Mediterranean Sea.     

On the vibrational characteristics of a two-tier scaled container stack

It is estimated that around 10,000 containers are lost during maritime transportation every year, representing an economic loss to the liner industry. Regulations and norms used to calculate values to secure them to the ship’s deck account for static loads only, neglecting more realistic conditions. This paper describes an approach to simulate a two-tier scaled model of a 20-ft ISO freight container and its linking connectors, denominated twist locks, subject to a dynamical load induced by its base. To analyze this problem two methods were employed: a shaking table test and finite-element analysis. Results of this study indicate that the numerical model built to simulate two-tier container stack dynamics is a promising tool for further studies. Moreover, the model is able to predict conditions close to real situations faced by container stacks while stored on deck.     

Study on a procedure for propulsive performance prediction for CRP-POD systems

The experimental procedure to predict the full-scale performance of the CRP-POD propulsion system is studied. In the CRP-POD system, the RPM ratio of the two propellers is not mechanically fixed, in contrast with conventional CRP systems. Therefore the existing procedure for conventional CRP systems is not appropriate for evaluating the performance of each propeller. In this paper, the characteristics of the CRP-POD system, designed for a 9,600 TEU class container carrier, are studied experimentally. Based on this study, a procedure for propulsive performance prediction for CRP-POD propulsion ships is suggested.     

Verification and validation study of URANS simulations for an axial waterjet propelled large high-speed ship

The accurate prediction of waterjet propulsion using computational fluid dynamics (CFD) is of interest for performance analyses of existing waterjet designs as well as for improvement and design optimization of new waterjet propulsion systems for high-speed marine vehicles. The present work is performed for three main purposes: (1) to investigate the capability of a URANS flow solver, CFDSHIP-IOWA, for the accurate simulation of waterjet propelled ships, including waterjet–hull interactions; (2) to carry out detailed verification and validation (V&V) analysis; and (3) to identify optimization opportunities for intake duct shape design. A concentrated effort is applied to V&V work and performance analysis of waterjet propelled simulations which form the focus of this paper. The joint high speed sealift design (JHSS), which is a design concept for very large high-speed ships operating at transit speeds of at least 36 knots using four axial flow waterjets, is selected as the initial geometry for the current work and subsequent optimization study. For self-propelled simulations, the ship accelerates until the resistance equals the prescribed thrust and added tow force, and converges to the self propulsion point (SPP). Quantitative V&V studies are performed on both barehull and waterjet appended designs, with corresponding experimental fluid dynamics (EFD) data from 1/34 scale model testing. Uncertainty assessments are performed on iterative convergence and grid size. As a result, the total resistance coefficient for the barehull case and SPP for the waterjet propelled case are validated at the average uncertainty intervals of 7.0 and 1.1%D, respectively. Predictions of CFD computations capture the general trend of resistance over the speed range of 18–42 knots, and show reasonable agreement with EFD with average errors of 1.8 and 8.0%D for the barehull and waterjet cases, respectively. Furthermore, results show that URANS is able to accurately predict the major propulsion related features such as volume flow rate, inlet wake fraction, and net jet thrust with an accuracy of ~9%D. The flow feature details inside the duct and interference of the exit jets are qualitatively well-predicted as well. It is found that there are significant losses in inlet efficiency over the speed range; hence, one objective for subsequent optimization studies could be maximizing the inlet efficiency. Overall, the V&V work indicates that the present approach is an efficient tool for predicting the performance of waterjet propelled JHSS ships and paves the way for future optimization work. The main objective of the optimization will be reduction of powering requirements by increasing the inlet efficiency through modification of intake duct shape.     

Predictions of porpoising inception for planing vessels

The inception of porpoising is theoretically predicted for planing vessels. Two different approaches are presented. First, a linear stability analysis is applied to find the porpoising limits while the hydrodynamic coefficients, i.e. added mass and damping coefficients, are determined by either a simplified method or a numerical method. Another approach is to seek the porpoising limits by performing nonlinear time domain simulations. Either the simplified method or the numerical method is used in the simulations. In the numerical method, a 2D+t theory together with a boundary element method is employed. The trim angle limits for porpoising are determined by changing the longitudinal position of the centre of gravity (COG) of the vessel and keeping the forward speed constant. The predicted porpoising limits are compared with Day and Haag’s (Planing boat porpoising, Thesis, Webb Institute of Naval Architecture, 1952) experimental results. The influences of parameters such as the load coefficient, the vertical position of COG and the radius of gyration of the ship are investigated by varying those parameters in the linear stability analysis. In the nonlinear time-domain simulations, by trying different longitudinal position of COG, one can find the critical trim angle when the porpoising commences. The obtained trim limits agree generally with those predicted by the linear stability analysis. Bounded oscillations for the unstable cases near the critical trim angle can be seen in the time-domain simulations due to the nonlinear effects.     

A finite element model for flexible pipe armor wire instability

The constructive disposition of metallic and plastic layers confers flexible pipes with high and low axial stiffness respectively when tensile and compressive loads are applied. Under certain conditions typically found during deepwater installation or operation, flexible pipes may be subjected to high axial compression, sometimes accompanied by bending. If not properly designed, the structure may not be able to withstand this loading and fails. From practical experience observed offshore and in laboratory tests two principal mechanisms, which will be discussed in this paper, have been identified regarding the configuration of the armor wires. When the pipe fails by compression the armor wires may exhibit localized lateral or radial deflections, consequently permanent damage is observed in the armor wires with a sudden reduction of the structure’s axial stiffness. The pressure armor may also unlock, thus causing potential fluid leakage.In this work a finite element model is developed to estimate the critical instability load and failure modes. An axi-symmetric model is constructed employing a complex combination of beam and spring elements. For each armor layer only one wire needs to be modeled, hence the computational cost is minimized without compromising the phenomenon characterization. A parametric case study is performed for a typical flexible pipe structure, where the friction coefficient between the wire armors and the external pressure are varied, and the critical instability loads and failure modes are obtained and results are discussed.     

通过形状差异修正模式方法进行的损伤识别试验(英文)

An experimental program was undertaken to test the feasibility to detect the occurrence of structural damage using a modified mode shape difference technique. The vibration response of a steel beam fixed at one end and hinged at the other was obtained for the intact and damage conditions. Modal analysis was performed to extract the frequencies and mode shapes. The method shows a good potential in detection of occurrence and location of damage.     

Income-contingent user preferences in policy evaluation: application and discussion based on multi-agent transport simulations

Standard economic policy evaluation allows the realization of projects if the aggregated economic benefit outweighs their costs. The use of one single aggregated welfare measure for evaluating and ranking projects has often been criticized for many reasons. A major issue is that differentiated effects on individuals or subgroups of the population are not taken into consideration. This leads to the need for transport planning tools that provide additional information for politicians and decision makers. The microscopic multi-agent simulation approach presented in this paper is capable of helping to design better solutions in such situations. In particular, it is shown that the inclusion of individual income in utility calculations allows a better understanding of problems linked to public acceptance. First, individual income-contingent utility functions are estimated based on survey data in order to describe human mobility behavior. Subsequently, using the MATSim framework, the implementation is tested in a test scenario. Furthermore, and going beyond Franklin (2006), it is shown that the approach works in a large-scale real world example. Based on a hypothetical speed increase of public transit, effects on the welfare distribution of the population are discussed. It is shown that the identification of winners and losers seems to be quite robust. However, results indicate that a conversion or aggregation of individual utility changes for welfare analysis is highly dependent on the functional form of the utility functions as well as on the choice of the aggregation procedure.