The method suggested by Mak is a solution, but analysing the vessel as a continuous beam is a little tedious. There is a method known as 'Moment Distribution' also known and the 'Hardy Cross' method. This is an iterative process to get the moments and forces. Excel lends itself to this method. However, you will probably have to get hold of a structures book that describes this method.
The method was used by engineers before the advent of the computer to analyse structures that did not have pin jointed elements such as portal frames.
Alternatively, if you have more than four saddles, you can break up the model into sections. This method is not as accurate, but you can get a reasonable answer.
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Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM