Lifting/Installation analysis of spool - Comparison between brace and spreader solution from a structural, operational and economical point of view

Abstract

As large sections of production pipe are lifted and installed, forces act on the structure. If initial analyses of the structural integrity show that the pipe will fail, support elements have to be introduced. The two most used solutions are the spreader beam and the brace bar, which deals with the problem in somewhat different ways. A pipe section, called a spool, is analyzed using both of the solutions to reveal differences in the structural impact. The result of this is that the spreader beam shows considerably lower utilization of the pipe capacity, mostly due to a significant reduction of bending moments found in the brace bar analysis. The structure is also exposed to buckling issues in the areas where compression loads occur. The standard method for buckling analysis is to use the Euler theory. This basic method uses only fixed and moment free restraints for the beam members, and might not be sufficient to represent the true conditions for the spool. Therefore, a method based on elastic restraints is used for the buckling analysis in this thesis. It turns out that the method in general gives higher buckling lengths for the spool than if Euler theory is used. A parametric study of varying buckling lengths shows an interesting behavior of the utilization results provided by the software program Staad.Pro. The study suggests that the program overestimates the capacity of the pipe, if the results are compared to Euler theory. Operational and economical issues are compared and discussed, highlighting advantages and disadvantages with the two solutions.