International Shipbuilding Progress - Volume 50, issue 4

Authors: Mahadik, A.S. | Jangid, R.S.

Article Type: Research Article

Abstract: The wave-induced dynamic force is one of the most important excitations to be dealt with in the design of offshore structures. In order to perform a reliable design of an offshore structure, it is important to obtain an exact evaluation of its dynamic response but also to examine the ways of reducing the response. This paper presents the response of offshore jacket platforms with an active tuned mass damper (TMD) under wave loading. The TMD is …installed at the top of the structure and the control force is evaluated using the classical linear optimal control algorithm. The governing equations of motion of the jacket platform with an active TMD are derived and their solution in the frequency domain is presented. The uni-directional random wave loading is expressed by the Pierson-Muskowitz spectrum. The response of the jacket platform with active TMD is compared with the corresponding response without a control system in order to investigate the effectiveness of the active control system. It is observed that active TMD can reduce the response of the jacket platform significantly. In addition, a parametric study had also been conducted to investigate the effects of important parameters on the efficiency of the active TMD. The important parameters considered include: the ratio of TMD mass to the structure mass, damping of the TMD, tuning frequency of the TMD and flexibility of the jacket platform. It had been observed that the above parameters significantly influence the response of the jacket platform with active TMD. Show more

Keywords: Active control, hydrodynamic force, jacket platform, active TMD, effectiveness, system parameters

Citation: International Shipbuilding Progress, vol. 50, no. 4, pp. 277-295, 2003

Authors: Artyszuk, Jaroslaw

Article Type: Research Article

Abstract: A detailed analysis of ship manoeuvring equations for the 'Esso Osaka' tanker is carried out in case both full scale motion data and experimental scale model force measurements are combined together. Some issues concerning the external excitations acting during real-world manoeuvring are raised, which enables an identification and/or validation of the manoeuvring mathematical models at the lowest possible level. The role of the ship sway added mass in the surge motion equation is emphasised. Its essential …reduction is necessary for an adequate rudder forces identification. Further implications concern the need of the relative and absolute adjustment of hull hydrodynamic sway force and yaw moment as measured in towing tanks. Show more

Citation: International Shipbuilding Progress, vol. 50, no. 4, pp. 297-315, 2003

Authors: Kara, F. | Vassalos, D.

Article Type: Research Article

Abstract: Numerical solutions for the hydrodynamic of bodies with forward speed are studied directly in the time domain using Neumann-Kelvin method. The exact initial boundary value problem is linearized using the free stream as a basis flow, replaced by the boundary integral equation applying Green theorem over the transient free surface Green function. The resultant boundary integral equation is discretized using quadrilateral elements over which the value of the potential is assumed to be constant and solved …using the trapezoidal rule to integrate the memory or convolution part in time. The steady problem is solved as the steady state limit of the impulsive velocity of the surge radiation problem with forward speed. Calculation and general character of the transient free surface source potential or Green function is presented. The unsteady and steady results for the hydrodynamic problem, which is tested using Wigley hull form and hemisphere show the acceptable agreement with experimental, analytical and other published numerical results. Show more

Citation: International Shipbuilding Progress, vol. 50, no. 4, pp. 317-332, 2003

Authors: Davis, Michael R. | Holloway, Damien S.

Article Type: Research Article

Abstract: A two dimensional Green function solution for the transient motion of water sections is used in a high Froude number strip theory to determine the motion response of high-speed vessels in head and oblique seas. The method is validated by comparison with published data for the heaving, pitching and rolling response of three catamaran hull forms at high Froude numbers representative of the Froude number of typical high speed catamaran ferries. A frictional damping term was …introduced to exactly match resonant motion maxima. The method is then applied to investigate the effects of hull spacing, sea direction and motion control systems. Finally the response of an 80m 1800 tonne vessel is investigated and the increase of motion sickness incidence with significant wave height and sea direction is investigated. In larger seas the maximum motions occur when the sea is taken on the bow, but in smaller, shorter period seas the maximum motions occur when the sea is taken on the quarter. Motions reduce rapidly as the sea direction approaches the beam. Show more

Citation: International Shipbuilding Progress, vol. 50, no. 4, pp. 333-370, 2003