A Procedure For Evaluating The Stability Of A Barge Transporting A Large Offshore Structure
Authors: Sekita Kinji , Yamamoto Jun , Okamura Akira 
Published: October 2008

More and more offshore Structures are being built in very deep water. And these structures are becoming larger and heavier, many of them requiring transoceanic transportation to the job site aboard a launching or sinking barge. A barge with such a structure aboard has a very high center of gravity, which impairs its self-righting ability. Moreover, barges often are exposed to strong winds and rough ocean waves and sometimes even encounter the danger of overturning, or are forced to change their navigation routes.

To evaluate barge stability in accordance with ABS rules, we developed the NSC/TOW computer analysis programs system. This system analyzes barge stability under, the influence of winds and waves, as well .as the stresses caused in the structure by barge motion. Also, it can be used to evaluate the best methods for securing the structure to the barge.

This report explains the procedures for making these evaluations and shows typical analytical results. It also shows the results of both short and long-term forecasts made for the slamming into the water of the overhang portions of the structure being transported. Sea-state data along the navigation route are used in making the forecasts.

Further, structures under the influence of barge motion were analyzed to evaluate the safety of their structural members and tie-down braces.

INTRODUCTION

As offshore structures become larger in size, it becomes increasingly important to evaluate the stability of the ocean-going barges and the structures being carried aboard. These evaluations should include barge size, selection of the towing route and season, and the method of securing the structure to the barge.

This report presents analytical methods for solving these problems. First, it outlines a system of programs (NSC/TOW) that analyze barge stability against waves and winds during transportation of an offshore structure. Then it presents the results of analyses made on specific jacket structures. These sections are followed by evaluations of the wave induced slamming of the jacket structure, and then of stresses that occur in jacket members and tie-down braces while under tow.

OUTLINE OF NSC/TOW SYSTEM

This system consists of four programs,l as shown in Fig. 1. The first program, TOW/WEIGHT, calculates the weight of a structure, its center of gravity (COG), and three radii of gyration from data on the structural members and accessories used as the input. It also calculates the stub length of each tie-down brace and the length of the can of each tubular joint. All these calculations are performed in accordance with API RP-2A code.

TOW/WEIGHT also calculates the lumped mass of each node. This is used as input data in stress calculations by the TOW/STRESS program.

The TOW/STABILITY program checks, according to ABS rules, the stability of a barge carrying an offshore structure. If the barge has sufficient stability to withstand overturning moments caused by steady Winds, it has intact stability. This program also can check damaged stability.