ABSTRACT

AS offshore structures become larger and heav1er, larger launching barges are being used to transport them. The conventional method of launching offshore structures at the job site consists of pulling or pushing the structure in the longitudinal direction of the barge using jacks and/or winches. This requires an extremely large launching capac1ty, and thus large-scale jacks and other units. In addition, excessive stresses tend to be produced in the barge hull.

With a view to finding a way to launch large structures without these problems, we conducted model tests on side launching -- a method whereby the structure is launched sideways in a direction transverse to the barge.

A twin-barge installation method has been proposed by J.B. Graham, et. al. In this paper, we propose two other launching methods based on twin-barge installation.

In all, we conducted model tests on five different launching methods. Our paper will present the results of those tests, and discuss the practicability of the different launching methods as evaluated by analytical methods, including computer programs.

INTRODUCTION

As offshore structures become longer, the convent1onal launching method has come of present the following problems in perform1ng the safe launching of the structures:

  1. Enormous capacity jacks or winches are required to move a structure on skid rails. Moreover, to maintain smooth launching after the structure has been set in motion, the winches need a relatively longer cable, or the jacks need more efficient shifting equipment.

  2. During launching, the greater weight of a structure inducts a larger long1tud1nal moment in the barge which acts as a long beam. Therefore, a stronger barge is required.

  3. As the rocker arms must also support a larger reaction during launching, they also require more strength.

  4. As structures become longer, there is a possibility that the buoyancy of the 1mmersed part of the structure will keep the structure in balance, thus resisting launch1ng.

  5. Due to a bad distribution of buoyancy there 1S the danger that the launching trajectory might be steepened so sharply that the end of the structure could ram into the seabed.

In order to solve these problems, we conducted model tests and analyses. The major purposes of the tests were the following:

  1. To determine the amount of launching force needed to initially set the structure into motion and then to keep it moving, taking the barge trim and draft conditions into consideration.

  2. To estimate the necessary strength of the rocker arms and the barge hull during launching.

  3. To evaluate the difference in trajectories caused by single-hinged and double-hinged rocker arms.

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