K-Factor Solution for Combined Braced–Unbraced Offshore Jacket Frames

Abstract

Many offshore jackets have no braces between the top of jacket and the bottom of topsides deck. This can be either due to the installation requirement and/or an effort to reduce wave loads. This kind of jacket bay is a portal frame. For the design of portal frame columns, the current offshore standards point to the unbraced frame alignment chart solution from onshore standards such as the American Institute of Steel Construction (AISC). In this paper, we will show that the AISC alignment chart unbraced frame K-factor solution is conservative for offshore applications, and perhaps too conservative. This is because the traditional alignment chart assumes that the entire structure is a moment frame. In offshore applications, the unbraced frame is almost always combined with braced frames from above and/or below. In this paper, we will derive a K-factor solution that is suitable for a braced/unbraced frame combination. This solution is validated with structural buckling finite element method (FEM) analyses of typical offshore frames. Design implications from using the traditional alignment chart versus current new K-factor solution are also discussed.