The collapse pressures of cold formed pipes with rounded stress vs. strain curves in the circumferential direction are evaluated using collapse tests and finite element analyses. A novel prediction method to estimate the collapse pressures of cold formed pipes is discussed. A new index of the collapse pressure, or a dominant offset stress that has a high correlation with collapse pressures, was derived instead of 0.2% offset stress by evaluating collapse pressures obtained from finite element analyses. The novel prediction method is capable of accurately estimating experimented collapse pressures with a prediction error of 3%.
Cold formed pipes, for example HF-ERW, UOE, and PB pipes, have different stress vs. strain curve in the longitudinal and circumferential directions. In addition, the stress vs. strain curve obtained by a tensile test is different from that obtained by a compression test. Figure 1 shows the characteristics of plastic anisotropy of 0.2% offset stress measured from HF-ERW pipes. L-T-YS, L-C-YS, C-T-YS, and C-C-YS represent the longitudinal YS under tension, longitudinal YS under compression, circumferential YS under tension, and circumferential YS under compression, respectively, wherein C-C-YS has the highest value because it is strengthened by the work hardening effect during the sizing in HF-ERW forming process. During the sizing process, HF-ERW pipes are subjected to compressive stress in the circumferential direction to adjust the outside diameter and improve the ovality. It is important to measure the circumferential stress vs. strain curve under compression because this curve is one of the dominant factors of collapse pressures.
Collapse pressure is an important design factor for oil & gas wells and offshore pipelines. Some conventional formulas to predict collapse pressures were established for heat-treated seamless pipes (Tamano, Inoue and Mimaki, 1989; ISO, 2004). Nagata and Tsuru (2016) proposed the prediction method of heat-treated HF-ERW line pipes reel-laid in deep water using the formula derived by DET NORSKE VERITAS (DNV) (2007). However, an investigation of the applicability for cold formed pipes with rounded stress vs. strain curves is not necessarily apparent from previous researches.