Method for determination of escort tugs main dimensions and characteristics at conceptual design stage

Abstract

The paper is devoted to the development of a method for determination of main dimensions and characteristics of escort tugs designed for escorting tankers, gas carriers and other types of vessels with hazardous cargoes. Classification Societies require a reasonable preliminary calculation of the maximum holding force acting on a tug at the escort speed of 8 and 10 knots to be submitted in order to assign an escort class to a tug. The method for determination of hydrodynamic forces acting on the ASD escort tug during stationary escort operations is described. The method is based on processing of the data collected during towing tests of such tugs in the towing tank of the National University of Shipbuilding (NUOS). Processing of the experimental data allowed to obtain regression dependencies for stationary hydrodynamic forces acting on the escort tug on the drift angle ranging from 0 to 90 degrees. Towing tests were carried out on request of design organizations, and escort tug projects by TransShip Corporation were mainly used. These projects are TUG40, TUG50, TUG55, TUG55TA, TUG60, TUG60L, TUG60LA and TUG70 by E. D. Demidov, General Designer. The data obtained based on using the developed dependencies was compared with the data of full-scale tests of escort tugs of this series. Obtained dependences are included in the equations of motion of escort tug operational purpose, which is an integral part of the optimization problem of determination of escort tug main dimensions and characteristics. A method for solving the transcendental equations of the tug stationary motion during escort operations is proposed, similar to the method for solving an optimization problem. Using the obtained method for determination of hydrodynamic forces and the operational purpose, the problem of determination of escort tug main dimensions is defined with solution oriented to optimize dimensions based on escort tug serving a random stream of vessels arriving at the seaport.