Design of Offshore jacket platform

[Abrar_1512]

In design of Offshore jacket platform, I am having wave & current loads and ship impact load as lateral loads. Fr analysis purpose there are two types one is linear static analysis without effect of ship impact and another is ship impact analysis (nonlinear static pushover) without wave loads. If I m doing linear static then the structure is overdesigned in UC check. But for ship impact nonlinear static analysis the structure doesn't meet the requirements. If anyone have knowledge about this then please help me out with this.
Thanks

 

[geoff_mcdonald]

The criteria for a ship impact analysis is quite different than a linear static wave loading analysis. Linear static wave analysis checks for the elastic capacity of the structure whereas the ship impact analysis checks for energy absorption capacity of the structure. I will focus on the ship impact analysis since it seems to be more relevant to your question. Energy absorption is determined by work done on the structure (and possibly the vessel) through elastic and plastic deformation. Most of the energy will be absorbed through plastic deformation of the structure which means that the members must yield and deform. In order to improve the energy absorption capacity of the structure redundancy is key. Allowing a member to yield and deform typically means that the load must be transferred to another member. So adding additional bracing and/or supports will significantly improve the behavior of the structure. That is a big reason why you typically see X-bracing for a lot of offshore jackets.

I noticed that you said you were performing a pushover analysis. Are you actually incrementing a load or are you performing a nonlinear analysis which stops at the desired energy level. I am a SACS user and I know that SACS calls all nonlinear analyses "pushover analyses" in the analysis type selector. You can also use the ship impact analysis specifically designed to perform nonlinear dynamic ship impact analyses.

 

[Abrar_1512]

I am performing pushover analysis by incrementing load upto impacted member failure and getting the total energy absorption.

What I understood is that I have to design the structure for only one lateral load i.e. ship impact load. Other loads will be taken care off as ship impact will be always higher than other lateral loads eg. wave and current loads. This was the ans from one of the source that I got. Is it true?