The Use of X-Bracing in Fixed Offshore Platforms
Authors: Knapp A.E. , Dixon David A. 
Published: October 2008

Elastic stability criteria are derived for an X-brace where both members are axially loaded. Solutions are obtained for both pinned and fixed-end conditions and for axial load conditions of one member in compression, while the other has compression, tension or zero axial load. The results are presented as curves that can be used by designers to determine an effective length factor, K, which reflects the degree of lateral support furnished to the primary compression member by the cross member. A comparison between an X-braced jacket and a conventional jacket reveals that, by the use of X-bracing, the weight of the primary bracing members can be reduced by 25 percent.

INTRODUCTION

The production of hydrocarbons offshore has always centered about the fixed platform. Such platforms when first developed for shallow waters of the Gulf of Mexico consisted of a low deck supported by bare piling. As water depths increased, it became necessary to brace the piling with a jacket. As production progressed further offshore, these same structural concept were applied in the greater water depths. Today these concepts are being utilized for platforms in 200 and 300 ft of water. Present offshore platform designs are generally heavy and bulky. Such designs result in structures that are difficult and expensive to fabricate and install Little effort has been expended on developing new structural configurations. Humble recognized the need for an innovative look at platform configurations for 1,000 ft of water. 1 Innovative thinking also needs to be applied to present designs for 200 to 300 ft of water. While there are certain advantages to improving the design of Gulf Coast platforms, much can be gained from developing more efficient structural configurations for use in other areas of the world where severe environmental influences are being encountered.

The authors propose that the proper use of X-bracing can lead to the design of more efficient structures. The use of X-bracing results in fewer members and longer spans, which in turn brings about more efficient use of material. Furthermore, X-braced structures possess greater reserve strength when compared to conventional structural configurations. Several platforms that have incorporated X-bracing have been installed in the Gulf of Mexico. Such usage by Amoco has proved very satisfactory as the structures were lighter, less expensive to fabricate, and easier to install. The benefits from X-bracing will increase in platforms designed for even deeper waters and more hostile environments.

Even when utilized, designers generally do not take full advantage of the X-brace. They fail to recognize that the "effective column length" of the compression member can be significantly reduced if support furnished by the cross member is taken into account.

Equations for the elastic stability of the X-brace are derived in the Appendix. Curves that have been prepared from the equations provide a designer with the appropriate values of the "effective length factor", K, to be used in conjunction with the AISC specifications.2

Examples are presented which illustrate the advantages of X-bracing compared to conventional bracing and demonstrate the use of the effective length factor curves.