John

I believe that this “60% rule”, in all but an exceptional case, is quite conservative if the total number of cycles is used. The problem is that the number of cycles is very high and usually in excess of the 2,000,000 limit currently in B31.3. Note that a typical platform natural period is on the order of 5 seconds, hence the high number of cycles.

This fatigue case is caused by platform deflections induced by wave action on the platform legs. In the life of the facility it will experience a large range of wave sizes with the largest waves in storm conditions. The problem is if you took the peak deflection from storm conditions and applied it to the full number of cycles (say 10^8) you would be excessively conservative. In reality the number of cycles experienced with anywhere near this load would be less than 10^3 cycles. The problem is that a large proportion of the fatigue damage can be caused at a displacement of 1/3 full displacement due to the high number of cycles.

If you know enough about the distribution of loading (deflections in this case) one can calculate the cumulative damage across the range of loads. In fact DNV RP-C203 is often used to this ends.

Given the detail of information required to perform this type of analysis it would be good to have a rule of thumb to use in the early stages of a project, hence my interest in this “60% rule”. Is anyone out there using such a rule of thumb or know of its origin?

All the numbers above are typical only and vary depending of conditions, location, the type of facility, etc are only quoted as example only and should not used.


Richard

Be careful with design of rolling support elements as in a marine environment these will be high maintenance items and after a few years would be unlikely to work.

For utility lines I have seen hoses used quite often. The hose is installed in a long U shape. In process lines I have never seen any sort of expansion joint or flexible.