Rack Phase Difference or RPD may occur during the installation phase of a jack-up rig when it is jacking up at a location. Instances of failure of leg members of a jack-up have been reported in the past. Therefore, this is an important design issue. Among the many causes of high RPD, important ones are when one of the jack-up spudcans becomes eccentrically supported on the bottom or has lateral offset due to preexisting soft spots in the soil during installation. The resulting moment and shear on the leg is carried essentially by the horizontal guide reactions that may induce high stresses leading to failure of a diagonal brace in extreme cases.

In a previous OMAE paper written by the authors [1] the importance of a 3D model of the jack-up that uses non-linear analysis methods by including large deformations and rotations was highlighted. In this paper it was also emphasized that the meaning of RPD has to be understood properly in relationship with moment or shear carrying capacity at the spudcan for a given jack-up.

The present paper includes material plasticity effects in addition to the other nonlinearities in the 3D analysis to investigate the failure mechanism of a critical brace. Both applied moments due to the vertical load eccentricity at the spudcan and lateral displacement effects of it are considered. The behavior of a trussed leg jack-up depends heavily on the bracing pattern, chord and bay spacing as well as the size of the braces. For a well-designed jack-up the reserve strength beyond the initial exceedance of the allowable limits of member stress following the codes to the actual physical failure is considered to be important. This reserve strength could prevent actual damages to the rig. This paper discusses these aspects using one particular jack-up as example. The particular jack-up has shown very high RPD and reserve strength when a brace fails due to a large spudcan moment being applied or base deflection. The failure of the brace, however, does not indicate that the ultimate strength of the rig has been reached which is expected to be much higher. Although the analysis is for one particular jack-up, it discusses the parameters that in general could make the reserve strength large indicative of good design. These observations and the overall methodology of analysis used here could be beneficial to other rigs.

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