The Use of X-Bracing in Fixed Offshore Platforms
Authors: Knapp A.E. 
Published: December 2007

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, w 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 2 percent.

Introduction

The production of hydrocarbons offshore has always centered about the fixed platform. Such platforms when first developed for shallow waters 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. Today these concepts are still being utilized for platforms in 200 to 400 ft of water. Present offshore platform designs are often heavy Present offshore platform designs are often heavy and bulky. Whereas the need for an innovative look at platform configurations for 1,000 ft of water has been recognized, innovative thinking also needs to be applied to current designs for shallower water.

The proper use of X-bracing can result in the design of more efficient structures. Several platforms incorporating X-bracing have been installed platforms incorporating X-bracing have been installed in the Gulf of Mexico. Such usage by Amoco has proved very satisfactory as the structures were proved very satisfactory as the structures were lighter, less expensive to fabricate, and easier to install. The benefits from X-bracing may become more significant in platforms designed for even deeper waters and more hostile environments. Occasionally designers 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. Curves presented in this paper provide a designer with the appropriate values of the effective length factor, K, to be used in conjunction with the AISC specifications. Equations for the elastic stability of the X-brace, from which these curves have been prepared, are derived in the Appendix. Examples prepared, are derived in the Appendix. Examples are presented that illustrate the advantages of X-bracing and demonstrate the use of the effective length factor curves.

Principle of the X-Brace

In order to understand the behavior of X-bracing, it is necessary to recognize that the jacket is basically a three-dimensional truss. Loads applied to the structure are reacted primarily by axial forces in the members, although bending and torsional moments are also developed due to wave loads on individual members and due to joint translations and rotations at the ends. Consider the X-bracing shown in Fig. 1. Customarily, both members ABC and DBE will have the same cross-sectional properties, and similar lengths L and a. One may first visualize this X-brace as consisting of two independent single-diagonal members. The shear through the bent is distributed between both members; one member is always in tension whenever the other is in compression. If, for example, member ABC in Fig. 1 is the compression member, buckling in any lateral direction may occur.