International Shipbuilding Progress - Volume 52, issue 4

Authors: Claessens, E.J. | Pinkster, H.J.M. | Dallinga, R.P.

Article Type: Research Article

Abstract: The quantification of the performance of ships and marine structures is multidisciplinary in character. It requires input from the areas of ship hydrodynamics (for the resistance, the inherent motion characteristics and mean environmental forces), the area of ship-operations (for an operational scenario that describes the master's reactions on the anticipated and encountered behaviour with related criteria for tolerable behaviour) and oceanography (for a description of the interacting wind and waves). The most complete description …of the operational performance up to now is based on a statistical approach, using long-term wind and wave statistics for the operational area or route. In this approach it is not possible to account for spatial variations of the wave characteristics along coastal routes, the coherence of wind speed, wind direction and their persistence on one hand and the characteristics of the generated waves (height, period, spectral characteristics) on the other. Regarding the predicted performance statistical methods have difficulties in accounting for complex operational scenarios and the effects of wind and wave persistence on the performance at the mission level. A method that is currently under development is based on time domain "scenario simulations". The wind and wave input for these simulations (several years of hourly characteristics of wind and waves) is obtained through a hindcast for the operational area. This hindcast accounts for the natural evolution of the spectral shape (thus accounting for separate wind-driven and swell-wave components), the directional spread and coastal effects (such as current and sheltering) on wave growth and decay. The performance is established by evaluating the progress on a mission in time; simulating a large number of missions yields the performance statistics at a mission level. The present paper describes the techniques underlying this method. The paper focuses on the wave input for the scenario simulations. Based on results of sample simulations it is concluded that wind-wave models provide a practical basis for a performance analysis. It is also shown that the variable spectral shape as accounted for in these simulations can have a considerable influence on the performance prediction. Show more

Keywords: hindcasts, scenario simulations, seakeeping, ship behaviour, wind-wave climate, wind-wave model, validation, operational performance of marine structures

Citation: International Shipbuilding Progress, vol. 52, no. 4, pp. 297-323, 2005

Authors: Bulian, Gabriele | Francescutto, Alberto

Article Type: Research Article

Abstract: A re-analysis of the Collision Database used in the development of harmonised subdivision and damage stability rules recently proposed by SLF47 has been undertaken, aimed at clarifying the grey area identified in the zone of large ships and in particular the large passenger cruise vessels. The results of the analysis confirm indeed the validity of HARDER for ships up to a medium size, say 175 m, above which some ship typology appears heavily penalised. It appears …that, also in view of the characteristic ratio between the main ship dimensions, maintaining present nondimensional probability density functions for the damage length and penetration up to that length and then taking the parameters of the pdf as constant in dimensional form for larger vessels could remove the problem without modifying the general structure of the probabilistic harmonised approach. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 4, pp. 325-356, 2005

Authors: Sutulo, S. | Soares, C. Guedes

Article Type: Research Article

Abstract: An analysis of the problem of the conversion of frequency-domain model for linear seakeeping hydrodynamic forces into the corresponding time-domain representation is carried out. After a short consideration of the memory-functions method, the study is focused on the so-called Method of Auxiliary State Variables whose advantage is that the original mechanical system consisting of the ship and the surrounding fluid and having distributed parameters is approximated with a simpler lumped parameters system described by …a relatively simple set of linear ordinary differential equations. An algorithm for determination of the coefficients of those equations based on the fractional approximation of the added masses and damping coefficients curves is proposed and tested. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 4, pp. 357-384, 2005

Authors: Balasubramanyam, Y. | Natarajan, R.

Article Type: Research Article

Abstract: Now-a-days, a large number of marine vehicles are being developed to meet the increased demand for inland and coastal transportation services. Among these marine vehicles, the catamaran type is very popular due to its large deck area, adequate stability, good resistance and sea-keeping qualities. Since the propulsive power of any marine vehicle mainly depends on its resistance characteristics, it is essential to develop a suitable hull form of the marine vehicle in order to obtain better …resistance characteristics which contemplate primarily on fuel economy. In the initial design stage, the propulsive power is usually predicted analytically by any one of the existing power prediction methods. But, however, it is necessary to conduct model tests to obtain the final hull form. Hence, for the present study, a 1:12 scale catamaran model was fabricated with change in bow and stern shapes and tested for three hull spacings in the towing tank at Department of Ocean Engineering, Indian institute of technology Madras, Chennai-600 036, India. The model test results are analyzed and presented with discussions in this paper. Show more

Keywords: Catamaran, demihulls, bow and stern shapes, gap ratio, resistance, interference factor

Citation: International Shipbuilding Progress, vol. 52, no. 4, pp. 385-400, 2005