In the last 10 years, to transport natural gas, deep water research projects of laying pipelines on the seabed have been carried out. In the long-distance transportation of natural gas, the pipeline is required to have compressive strength against external water pressure under the deep sea, so UOE steel pipes are generally used. The manufacturing method of UOE steel pipe is the cold stamping forming method, and the strength of the steel pipe is anisotropic. To predict the compressive strength of UOE steel pipes and clarify the crushing mechanism of steel pipes, Nippon Steel conducted numerical analysis simulations integrating steel pipe forming-performance evaluation. The numerical analysis simulation consists of a two-dimensional forming model of the steel pipe and a three-dimensional crushing model of the steel pipe reflecting the formed shape and residual stress. Through experiments, the wall thickness and strength anisotropy in the circumferential direction position of the steel pipe were measured, the residual stress was measured, and the validity of the numerical analysis model was evaluated based on the actual compressive strength of the steel pipe.
1. Strength Anisotropy and Residual Stress of UOE Steel Pipe
It is well known that the factors affecting the compressive strength of steel pipes are poor shape (roundness and uneven wall thickness of steel pipes), yield strength (YS), and residual stress. The compressive yield strength and residual stress in the circumferential direction have a great correlation. The yield strength distribution of the wall thickness section measured by the round bar and cylindrical specimens (both 6 mm in diameter) shows that the decrease in the compressive yield strength in the outer circumferential direction of the steel pipe is particularly obvious. A comparison of the S-S curves in the wall thickness position shows that a circular S-S curve appears on the outside due to the Bauschinger effect of elastic deformation starting from the center of the wall thickness. According to the comparison of UOE steel pipe and seamless steel pipe for oil well, the residual stress of the two steel pipes tends to be compressed on the inner surface, but the residual stress value of UOE steel pipe is small.
2. Numerical analysis simulation
During the numerical analysis, an integrated model was used to evaluate the forming-compressive strength of UOE steel pipes. In the forming model (two-dimensional plane deformation element) of UOE steel pipe, the S-S curve of the plate is used, and the residual stress is applied to the crushing model (three-dimensional solid element). Since it is difficult to accurately predict the change of the S-S curve from the plate to the steel pipe only by numerical analysis simulation, a semi-experimental method (simulation deformation test) is used to predict the S-S curve. That is, the calculated equivalent plastic strain hysteresis is applied to the round bar specimen sampled from the plate, and then the resulting compression S-S curve is defined for each wall thickness position.
3. Results and Research
3.1. Validity of the crush model
The prediction accuracy is dominated by the number of element combinations, pressure increment value, and convergence judgment value of the model. If these influencing factors are corrected, the prediction error of this model is estimated to be about 5%. By correcting the error, the prediction accuracy can be further improved. After comparing the crushing values of the comprehensive model and the ellipse approximation model when the same roundness is given, it is found that there is no large average difference between the two, so it can be seen that the roundness will depend on the maximum outer diameter and the smallest inner diameter By making parameters similar to those of an ellipse, the outer diameter distribution of UOE steel pipes with local curvature changes can be represented by a model. Comparing the crushing value predicted by the ellipse model with the calculated value of the common formula for predicting the compressive strength of UOE steel pipes, it is found that the predicted values of different D/t (outer diameter/wall thickness) and roundness are the same as those predicted by the common formula, so it is presumed that the same result can be obtained by using the forming-crushing comprehensive model. Therefore, it can be said that the comprehensive model can analyze the crushing mechanism and can be applied to quantify the influence of forming conditions on compressive strength.
3.2. The crushing mechanism of UOE steel pipe
Investigated the stress-strain relationship when the equivalent plastic strain hysteresis predicted in the UOE steel pipe production process was simulated by using a round bar sample, and compared the predicted S-S curve with the simulated curve. The results showed that the predicted S-S curve is more consistent with the S-S curve of the actual steel pipe, even if it is a wall thickness section subjected to different strain hysteresis, its YS is also the same as the measured value. The drop in wall thickness outer YS in strain hysteresis under this forming condition is dominated by the tensile strain load during U stamping. In addition, almost no drop in compression YS due to the Borgesine effect of elastic deformation is observed inside the steel pipe. Using the simulated strain test proposed above, the strength in the circumferential direction of the actual steel pipe can be more accurately predicted.
Post time: Aug-10-2023