Effect of Reeling on Small Umbilical Tubing Fatigue

Umbilicals use steel tubing of different sizes to carry various pressurized fluids. During manufacturing, transportation and installation of the umbilicals, the tubing is subjected to reeling and unreeling operations, resulting in a cumulative amount of plastic strain. The amount of strain varies with different cross-section designs and umbilical manufacturers. This plastic strain may limit the fatigue performance of the tubing in dynamic umbilicals, thereby limiting the number of reeling operations of umbilicals in deepwater. A previous paper presented a novel experimental methodology to simulate reeling and its effect on the fatigue of 57.6-mm ID × 3.4-mm WT made of super duplex steel. Results indicated that reeling can significantly degrade the fatigue performance of the welded tubing as the cumulative reeling strain increases up to 20%. This paper discusses an experimental program aimed at assessing the size effect on fatigue of reeled tubing. Ten 10 additional fatigue tests were conducted with 12.6-mm ID × 1.46-mm WT tubing reeled to 20% strain. Results indicate that the smaller tubing, once reeled to 20%, its fatigue performance degrades to the same level as that of the larger tubing. However, the combined fatigue data do not support current design criteria DnV-RP-C203. Therefore, given the uncertainty of all of the variables involved, fatigue qualification for specific applications is considered necessary.