Stability and Dynamics of Semisubmersible After Accidental Damage
Authors: Clauss G.F. 
Published: October 2008

Stability tests with the 37 000 t-semisubmersible RS 35 have shown that low impact damages result in moderate angles of list owing to the subdividing of the outer compartments of the columns and the hull. In the event of collisions the distance between the outer and the inner shell provides sufficient margin for energy absorption. This distance amounts to 2.86 m at the columns and 1.8 m at the steroidal hull. Thus, the DnV low impact case with' a penetration depth of 1.5 m is well exceeded. In the event of such an' accident the deck structure is well above the water surface. The righting moment is simply calculated from the intact stability met centric height increased by the added stability effect. Any secondary damages are unlikely as the unique construction of the semi submersible RS 35 with a closed ring torus, supporting four columns, and an integrated box deck structure guarantees high structural integrity as vulnerable secondary members and diagonal bracing are avoided.

At catastrophic accidents, however, if outer and inner shells of the columns or the hull are destroyed, large quantities of leakage water will flow into the structure resulting in high angles of list. In these positions part of-the deck and its superstructure will immerse whereas the hull is surfacing on the opposite side. All elevate data, such as the positions of the centers of gravity and buoyancy as well as the moment of inertia of the rapidly changing water line area, show substantial variations. The leakage water inside the inner hull and columns with its free surface is moving unrestrained.

In these unlikely situations the damaged semi submersible with its watertight deck structure remains extremely stable. This safe behavior has also been observed in the seaway, even in highest waves. The results present the relevant motion transfer functions at various intermediate damage positions.

No hazards were caused by uncontrolled motions of the enclosed leakage water, capsizing was never observed.

INTRODUCTION

Several disastrous catastrophes with semi submersibles have focused research and development to stability characteristics of offshore platforms under adverse conditions. After several papers on intact stability of semi submersibles detailed investigations have been published in connection with the fatal capsizing of ALEXANDER KIELLAND. Based on these experiences DnV and the Norwegian Maritime Directorate have revised their stability requirements. With regard to these rules new platforms have been designed. As a first measure it is recommended to improve the structural integrity to avoid any loss of significant components of the platform, even at catastrophic damages. An excellent solution is a three dimensional framework stretching the columns between a closed underwater hull and an integrated box deck structure. Further, special attention has to be paid to carefully subdividing of the tank volumes.

This includes all hull, legs and deck spaces. Finally, the survival of the platform in any damaged condition has to be ascertained, especially in heavy seas.