ABSTRACT

When a ship moves forward in water and is decelerated by some obstacle or structure. it is not only the mass of the ship that is decelerated but also a certain amount of water moving along the ship. It is this so-called hydrodynamic added mass which is the crucial factor in the design calculations. The magnitude of this added mass is influenced by: dimensions and shape of the underwater hull. underkeel clearance. spring characteristics of the obstacle and collision mode.

In order to identify more precisely the influence of all the different contributing parameters. a very extensive model test program has been performed at the Netherlands Ship Model Basin. The program was initiated for two reasons:

  • For the design of fender structures and associated jetties. the need for experimental back-up data was felt.

  • For the validation of computational programs developed at NSMB a good deal of experimental results was needed.

Therefore. a series of some 750 test runs has been performed. During which the sensitivity of the hydrodynamic mass for fender stiffness. collision mode and collision speed was tested.

One of the outcomes of the test series was that the accepted and frequently used hydrodynamic added mass values appear to be too low. a finding which is of direct consequence to the designers.

INTRODUCTION

The most common way to obtain the design loads on structures and fendering systems is to estimate the amount of kinetic energy of the approaching ship. For this estimate some assumptions have to be made. The biggest ship approaching the structure is selected. the top collision speed and the mode where under contact is made. These quantities are then identified as known quantities.

However when a ship with a certain approaching speed will be decelerated. also a certain amount of water which is moving along with the ship has to be decelerated. Therefore. the mass to be taken into account for the determination of the kinetic energy is not only the mass of the ship; there is a certain additional mass. The so-called hydrodynamic added mass.

The designer. when designing some structure. usually takes the added mass as a constant value added to the ship I s mass. This value is then taken independent of all factors affecting it; see Saurin [21] and Vasco Costa [23] and [24] Thoresen [22] takes possible eccentricities into account. while Giraudet [12] accounts for underkeel clearance only. In recent years the possibility of an accidental collision between a ship and an offshore flatform has drawn considerable attention ([ 7],[11],[17]. [19] and [20]). in which basically the structural aspects were considered.

An advanced computer program. dealing with all mentioned parameters affecting the added mass. has been developed at NSMB. This program was presented for the first time at the OTC by Van Oortmerssen [18] in 1974. In the following years up till now. the program has been refined and validated. The results, presented in this paper. are meant to provide more data to check this program.

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