Study On The Fracture Performance And The Safety Assessment Method Of Welded Joints Of The Steel Catenary Riser

Steel catenary risers (SCR), a new type of marine risers, have become more popular than top tension risers and flexible risers when exploiting petroleum and gas resource in deep sea areas. Risers connect to each other by welding, during which the defects are inevitable. Especially, when the cracks of risers start in the welded joints, the cracks will develop so long as a little energy exists. Combined with the mechanical heterogeneity of welding, the risers tend to easily fracture under the large bending moment. Compared with other loads, the bending moment has the greatest influence on the welded joints with circular cracks, and is considered as the most dangerous load. Therefore, it is significant and practical that study on the fracture performance and the safety assessment method of welded joints of the steel catenary riser.Considering the effect of the bending moment on the marine risers, based on the theory of fracture mechanics and commercial software ABAQUS, the welded joints with a crack of SCR are researched to gain the stress and strain fields around the crack tip region. According to the analysis of the effect of the diameter-thickness ratio, crack size and matched stack on the crack tip plastic region of welded joints, the stress distribution rule of the steel catenary riser is summarized. The results show that circular crack is the greatest threat to marine risers as a result of the biggest stress intensity factor K and the easiest crack growth. The crack tip’stress of the circular crack will reach its maximum value before the limit of crack extension. After the crack extension, the stress is decreased and tends to be steady. The change of plastic zone is significant when the matched stack factors of the welded joints of the riser are different. The plastic areas near to the parent metal of crack tip is smaller than that near to the welded areas under the low-matched stack, while under the high-matched stack, the opposite occurs.Based on the elastic-plastic fracture mechanics, the paper chose J-integral as the crack fracture parameter, calculated the fracture parameter of welded joint and discussed the crack depth, length, thickness ratio and weld pipe diameter have some influence on the fracture parameter of welded joint. Results show that the impact of these factors must be considered, and with the increase of load ratio, the difference is more obvious. The riser under the effect of large bending moment, the driving force of crack increased significantly. So the research of fracture properties under larger bending load is very important.On the basis of the above, according to the EPRI’s J-integral estimation method, dealed with the J-integral of crack, calculated the crack in the fully plastic solutions of coefficient h1, compiled the table of all the plastic coefficient h1for engineering. Through comparing FEA estimated result with formula calculated reault, the resalts can prove the engineering estimated method is rationality. Fitting the formulas for calculating the h1and processing the data of polynomial coefficients.This paper employs SINTAP to assess the safety of the risers with cracks under bending moment. The results show that the welded joint of the riser in this paper is safe. Additionally, the lower of the assessment level is, the higher of the conservation of the assessment curves is.