Structural braces are provided between port and starboard columns or pontoons of semisubmersibles to reduce the bending moment at the column deck box connection created by squeeze/pry loads from hull-wave interaction. This hull-wave interaction is usually in the form of squeeze/pry action and bending caused by the relative longitudinal and vertical displacement between the pontoons caused by hull racking.
Traditionally, the braces have been designed as fixed end beam/columns for withstanding axial loads. The racking displacement of the brace ends results in brace end bending moment and induced stresses at the brace to hull connection. To handle the high bending moment, the brace wall thickness must be increased, which results in a stiffer connection. Because the racking deflection is controlled by the deck box, and not the stiffness of the braces, the magnitude of this deflection remains essentially constant regardless of brace stiffness. Thus the wall thickness increase simply attracts more bending moment, more stress and further increases in wall thickness. The additional reinforcement to resist the bending stresses typically results in substantially heavier structure than required by the compressive and tensile squeeze/pry loads alone, that they are intended to resist and control.
The authors have developed and analyzed an alternate structural arrangement that reduces the bending stiffness of the braces at their connection to the hull, allowing the braces to act more like pin ended column elements. The reduction in bending moment at the brace to hull connection results in reduced scantlings and a significant reduction in the weight of the braces and their associated hull structure.