Sensitivity analysis of the probabilistic damage stability regulations for RoPax vessels

In the light of the newly developed harmonised probabilistic damage stability regulations, set to come into force in 2009, this article presents a systematic and thorough analysis of the sensitivity of the Attained Subdivision Index with reference to a wide range of related design parameters. The sensitivity of the probabilistic regulations was investigated for a typical large RoPax vessel, with variation of parameters, such as the number, positioning and local optimisation of transverse bulkheads; the presence and position of longitudinal bulkheads below the main vehicle deck; the presence of side casings; and the height of the main deck and double bottom. The effects of water on deck and of operational parameters (draught, centre of gravity and trim) were also investigated. The results of the study, presented in graphical form, can provide valuable assistance to the designer when determining subdivision characteristics at the very early stage of the design process, resulting in optimal, efficient and safe ships

Optimal control of the heave motion of marine cable subsea-unit systems

One of the key problems associated with subsea operations involving tethered subsea units is the motions of support vessels on the ocean surface which can be transmitted to the subsea unit through the cable and increase the tension. In this paper, a theoretical approach for heave compensation is developed. After proper modelling of each element of the system, which includes the cable/subsea-unit, the onboard winch, control theory is applied to design an optimal control law. Numerical simulations are carried out, and it is found that the proposed active control scheme appears to be a promising solution to the problem of heave compensation

The effect of lift on the wave-making resistance of multi-hull craft

A potential based panel method is presented to estimate the wave-making characteristics of multi-hull craft. In order to simulate the lifting potential flow around the sub-hulls, the method adopts mixed source/doublet distributions on the sub-hulls and their wake surface, while sources are distributed on the main hull and the free surface. In this way, the asymmetric flow characteristics of the sub-hull are properly simulated, i.e., a Kutta condition is satisfied at the trailing edge of the sub-hull. Comparison is made between the numerical and model experimental measurements, and a good correlation has been found. The wave-making characteristics and pressure distributions on the sub-hull predicted by the present method can differ from those based on a distribution of sources alone, especially the pressure distributions at the stern of the sub-hulls

A seakeeping analysis method for an air-lifted vessel

A seakeeping analysis in the frequency domain is presented to predict the motion response of an airlifted vessel (ALV) in waves. The ALV is supported by pressurised air in two separate cushion chambers; the pressure variation in the cushions has a significant effect on the motions of the vessel. The adiabatic gas law is used to couple cushion pressure and the free-surface elevation of water inside the chamber. Attention is focused on the waves generated by the pressure, and a method is presented to compute the corresponding free-surface elevation. New numerical schemes are proposed for calculating the threedimensional free-surface elevation for the four wave numbers. Numerical results of the free-surface elevation, escape area, escape volume and motion responses of the ALV are provided. & 2008 Elsevier Ltd. All rights reserved

Superlinear convergence for PCG using band plus algebra preconditioners for Toeplitz systems

AbstractThe paper studies fast and efficient solution algorithms for n×n symmetric ill conditioned Toeplitz systems Tn(f)x=b where the generating function f is known a priori, real valued, nonnegative, and has isolated roots of even order. The preconditioner that we propose is a product of a band Toeplitz matrix and matrices that belong to a certain trigonometric algebra. The basic idea behind the proposed scheme is to combine the advantages of all components of the product that are well known when every component is used as a stand-alone preconditioner. As a result we obtain a flexible preconditioner which can be applied to the system Tn(f)x=b infusing superlinear convergence to the PCG method. The important feature of the proposed technique is that it can be extended to cover the 2D case, i.e. ill-conditioned block Toeplitz matrices with Toeplitz blocks. We perform many numerical experiments, whose results confirm the theoretical analysis and effectiveness of the proposed strategy

Safety level of damaged RoPax ships : risk modelling and cost-effectiveness analysis

This paper elaborates on results of a recent risk analysis study for RoPax vessels, carried out as part of the activities of the SAFEDOR Integrated Project, targeting possible improvements on safety levels following large scale flooding. The study is based on a comprehensive analysis of accident statistics for the period 1994-2004, through which a high-level risk model (in the form of event trees) is established. This is then used to determine the current safety level of RoPax vessels (in various risk metrics, such as individual risk, potential loss of life and on an F-N curve), reconfirming that even though safety levels are improving, risk is still 'high in the ALARP region'. In search of ways to further improve the situation possible risk control options are examined, by performing a sensitivity analysis on the effects of the Attained Index of Subdivision A onto the safety levels and by evaluating their cost-effectiveness

Comparison of diesel-electric with hybrid-electric propulsion system safety using system-theoretic process analysis

Cruise ship industry is rapidly developing, with both the vessels size and number constantly growing up, which renders ensuring passengers, crew and ship safety a paramount necessity. Collision, grounding and fire are among the most frequent accidents on cruise ships with high consequences. In this study, a hazard analysis of diesel-electric and hybrid-electric propulsion system is undertaken using System-Theoretic Process Analysis (STPA). The results demonstrate significant increase in potential hazardous scenarios due to failures in automation and control systems, leading to fire and a higher number of scenarios leading to propulsion and power loss in hybrid-electric propulsion systems than on a conventional cruise-ship propulsion system. Results also demonstrate that STPA enhancement is required to compare the risk of two propulsion systems

Tissue Doppler imaging following paediatric cardiac surgery : early patterns of change and relationship to outcome

In this study, tissue Doppler imaging (TDI) was used to assess changes in ventricular function following repair of congenital heart defects. The relationship between TDI indices, myocardial injury and clinical outcome was explored. Forty-five children were studied; 35 withcardiac lesions and 10 controls. TDI was performed preoperatively, on admission to paediatric intensive care unit (PICU) and day 1. Regional myocardial Doppler signals were acquired from the right ventricle (RV), left ventricle (LV) and septum. TDI indices included: peak systolicvelocities, isovolumetric velocities (IVV) and isovolumetric acceleration (IVA). Preoperatively, bi-ventricular TDI velocities in the study groupwere reduced compared with normal controls. Postoperatively, RV velocities were significantly reduced and this persisted to day-1 (PreOp vs. PICU and day-1: 7.7+2.2 vs. 3.4+1.0, P < 0.0001 and 3.55+1.29, P < 0.0001). LV velocities initially declined but recovered towards baseline by day-1 (PreOp vs. PICU: 5.31+1.50 vs. 3.51+1.23, P < 0.0001). Isovolumetric parameters in all regions were reduced throughout the postoperative period. Troponin-I release correlated with longer X-clamp times (r=0.82, P < 0.0001) and reduced RV velocities (r=0.42, P=0.028). Reduced pre- and postoperative LV velocities correlated with longer ventilation (PreOp: r=0.54, P=0.002; PostOp: r=0.42, P=0.026). This study identified reduced postoperative RV velocities correlated with myocardial injury while reduced LV TDI correlated with longer postoperative ventilation

CFD modelling and numerical investigation of a large marine two-stroke dual fuel direct injection engine

This study aims at developing a CFD model for large marine two-stroke dual fuel engine with gaseous fuel direct injection at high pressure. For the gaseous fuel, the shock tube theory and the pseudo-diameter concept are employed to model the injection, jet penetration and air entrainment processes, whereas its non-premixed combustion is represented by a steady diffusion flamelet model along with a pilot fuel ignition kernel. Following this model validation, a large marine two-stroke dual fuel engine closed cycle is simulated for both the gas and diesel modes at 75% load, and the involved phenomena are comparatively assessed. The derived results demonstrate that the gas mode combustion takes place in lower maximum temperature and leaner conditions compared to the diesel mode, resulting in lower NOx emissions. This study is expected to benefit the development of future engine designs and the engine settings optimisation for reducing emissions and increasing efficiency