The Fatigue Strength Analysis Of Jacking Structure Of Jack-up Units

As an important part of the Offshore Oil Industry exploration fleet, the structural safety performance of Jack-up Units is widely focused on. And with the increasing operating depth and the worse working condition,the fatigue strength of Jack-up Units is becoming more and more critical. As a connection of the main hull and the legs, jacking structure always withstands high-frequency alternating load. So there is more possibility that the jacking structure appears to be fatigue failure. Fatigue analysis of jacking structure is very necessary.This paper expatiates on elements of fatigue assessment and general methods for fatigue strength analysis of Jack-up Unit. The fatigue strength assessment method employed is based on a fully stochastic fatigue analysis using S-N curves.The paper calculates the fatigue strength of a certain type of Jack-up Unit with DNV software SESAM. The main aspects studied in this paper are as follows:1. Theoretical basis and basic methods for wave theory and fatigue strength analysis of offshore structures are expatiated on,with emphasis on procedure of spectral based fatigue assessment method,wave load calculation of Jack-up using Morison equation and linearisation of drag force.2. Detailed model of legs and jacking structure have been constructed as well as the simplified model of the main hull. Focus is also paid on the connection between legs and the jacking structure.3. After the quasi-static analysis of the unit and the analysis and processing of stress response spectrum, ABS S-N curves and Miner cumulative damage law have been used to calculate the fatigue damage of the jacking structure and evaluate platform structure fatigue life.4. The calculation of Dynamic Amplification Factor is based on SDOF method (single degree of freedom) and the structural dynamic modification is taken into account in the fatigue strength analysis.The paper has some reference value and offers an option on the modeling of fatigue strength analysis, the calculation of wave load and the procedure of fatigue strength analysis.