INTRODUCTION

As an extension of the current twin-hull semisubmersible, a mono-hull crane barge semisubmersible has been developed based on a concept [1] of increasing the displacement together with an increase m waterplane area and a reduction of the width m order to maintain both the motion behaviour and the operation property.

The Immersed part of a crane barge semisubmersible consists of a single barge-shaped hull and struts standing on the top of the hull and supporting the work deck. Figure 1 shows the general hull configuration of a crane barge semisubmersible.

Two main theoretical methods may be considered for obtaining the hydrodynamic characteristics of such a structure. One of the methods has been suggested by Hooft. [2] This method has also been recommended by some rules and regulations. [3,4] It relies on two fundamental assumptions. One of the assumptions is the validation of the superposition principle for the added mass, wave damping and wave exciting forces of all the discretized substructures or elements. The other assumes that the diameter of the substructure is so small compared to the wavelength that a modified Monson equation can be used to calculate both inertia and damping terms, and that the interaction between substructures can be ignored. Furthermore, m most cases the damping due to wave radiation may also be negligible.

Fig. 1 General hull configuration of crane barge semisubmersible (available in full paper)

The other method originates from the slender body theory, and a 2D strip theory is utilized for computation, including the interaction between massive substructures. [5] In 1970 Tasar et al [6] introduced viscous damping as a correction to the linear wave-damping term for predicting the heaving motion of a SWATH ship, whereas viscous correction for all the six modes of motion was proposed by Matsushima et al.[7]. A more detailed formulation of the viscous effect has been implemented by Lee and Curphey. [8] The increasing attention being paid to viscous damping is really due to its significance for predicting the motion of a semisubmersible.

The former method may be more appropriate to the semisubmersible platform composed of a great number of small members and the latter to the usual twin-hull semisubmersible with fewer substructures. For a crane barge semisubmersible with a partially truncated water region on the top of the hull, however, the conventional strip method may be unsuitable. Because of the large scale of the struts in comparison with the hull and the considerable interaction effect between struts and hull, Hooft's method may be inadequate as well. Instead, a 3D diffraction method should be adopted. Moreover, the eddy-making resistance may be significant to a certain extent for a submerged flat body oscillating in water. Therefore, a method combining the 3D &action theory and a viscous effect estimation is used in the present study for hydrodynamic analysis and motion predictions of the mono-hull crane barge semisubmersible.

MOTION PREDICTION METHOD

The six-degree-motion equation with viscous effect for a semisubmerslble may be expressed as (formula available in full paper)

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