Multi-axial Fatigue Life Assessment Of Revolving Platform Weld For Offshore Crane

Revolving platform, welded box-type structure, is one of the key components of offshore crane, whose safety and stability are critical to the whole crane. Since it is mainly subjected to combined compress and bending, the weld of revolving platform is under complex multi-axial stress state. Additionally, stress of revolving platform weld fluctuates cyclically owing to frequent operation of offshore crane coupled with atrocious marine environment (wind, waves, etc.). Therefore fatigue failure is one of the main failure modes for the weld. Consequently it is meaningful that revolving platform weld is evaluated by multi-axial fatigue assessment method.30t-level offshore crane revolving platform is selected and studied in this paper. For estimating multi-axial fatigue life of the revolving platform weld, a new multi-axial fatigue life assessment approach, which is called Modified Wohler Curve Method (MWCM), is adopted and analyzed with Equivalent Hot Spot Stress (EHSS). The main works of this paper are as follows:(1) An approach based on relevant fatigue assessment codes to calculate Hot Spot Stress (HSS) and a method based on ANSYS to determine Equivalent Hot Spot Stress (EHSS) are proposed. Then an assessment algorithm is established in combination with MWCM and HSS or EHSS.(2) The multi-axial fatigue lives of several transversely loaded butt welds are estimated through MWCM&HSS and MWCM&EHSS. To verify the accuracy of the algorithm this paper proposed, a comparison between predicted life and experimental life is made. Additionally, the fillet weld widely used in engineering is selected and its multi-axial fatigue life is estimated through MWCM&HSS and MWCM&EHSS. Then the influence of combined loading ratio on multi-axial fatigue life between MWCM&HSS and MWCM&EHSS is investigated and the rationality of EHSS acted as reference stress was verified.(3) In accordance with API 2C-2012 Offshore Pedestal-mounted Cranes, the loads of objective offshore crane is calculated and the boundary condition of revolving platform is then determined after calculating loads of revolving platform. Furthermore, FEA is employed to find the critical location and solid element is utilized to model the weld of the critical location. To obtain the stress distribution of the weld in detail, shell-solid mixed FE model of revolving platform is constructed by MPC technique and FEA is made in succession, which lay a solid foundation of acquisition of EHSS.(4) Fatigue assessment points are determined and EHSS of these points is calculated based on detailed stress distribution of the weld in dangerous region. Finally the multi-axial fatigue life of revolving platform weld for objective offshore crane is estimated through MWCM&EHSS which can be a guide for maintenance and protection of offshore crane.