Engineering Critical Assessment Of The Key Weld In Semi Submersible Platform

With the increasing number of large oil and gas reserves found in deep water,more and more drilling platforms begun to work in the abyssalzone, and the semi-submersible platform has become the best choice for the exploration of petroleum resources in those areas. So China had invested heavily to build the most advanced semi submersible platform in the world. However, the environmental loads such as winds, waves and currents are much larger than that in shallow sea, and the working conditions of the platform are very bad. As a result, the service life and the maintenance cycle are brought into sharp focus by these issues. Since the platform was designed based on the theory of zero defects and the influence of welding defects on the platform service life was not considered, it seemed necessary to reexamine the influence of welding defects using the method of Engineering Critical Assessment based on fracture mechanics, which would be discussed in this paper.Firstly, the welding process of a weld in the welding zone between column lateral bracing and pontoon was simulated using ANSYS software. A new method called "Local to Overall to Local" was put forward to define the boundary conditions, which greatly simplify the calculation. The simulation results showed that when the local model was small compared to the whole model, the constraint of the whole model could be ignored. There were just a few differences between the results obtained by free boundary and constrained boundary. When a force of 30 MPa was applied to the structure, the transverse residual stresses along the weld direction decreased from 182 MPa to 152 MPa, which would be used as secondary stress during the ECA.Secondly, the statistical analysis was done on the CTOD of EH36 steel tested under the temperature of 0? and the welding method is FCAW. It was found that the CTOD of weld seam with probability of 95% was 0.6577 mm and that of HAZ was 0.7468 mm. And then the value of 0.6577 mm was used to do the fracture and fatigue assessments. The conclusion drawn through the assessments was that when there existed a surface crack the safety allowance of structure was 145 MPa while when there existed an embedded crack the safety allowance was 154 MPa, which is limited by the strength of materials. It was also found that when the surface crack at weld toe was under the most dangerous alternating load, the fatigue life was just about 4 years far below the design life of 30 years. So the maintenance cycle must be shorter so that the defects could be repaired in time. The fatigue life of embedded crack was about 173 years under the same condition.Lastly, sensitivity analysis was carried on for parameters of the Engineering Critical Assessment and the importance ranking was obtained. Fatigue load, defect shape and location were the three most important factors; Other factors,sequentially arranged according to the size of influence on the service life, were crack height, crack length, residual stress, fracture toughness and environmental limit load.