Stress Analysis of Pipe-In-Pipe Systems under Free Span for Deep Water Pipeline Applications

This study examined the phenomena of free span for a pipe -in- pipe (PIP) system for pipeline application. Two different span length of 8 and 30 meters are modelled and simulated using nonlinear stress analysis. The effect of pressure, temperature and gravity on the PIP system are determined and compared with conventional single pipeline. From the results obtained, it is clear that the finite element analysis (FEA) results correlated very well with those calculated using analytical methods. Percentage differences were generally less than 10%, with some discrepancies which were due to assumption of thin-walled theory which assumes a radial stress equals to zero, whereas the FEA calculates a non- zero radial stress. The key finding in this study demonstrated the strong potentials of PIP system in terms of structural reliability for deepwater pipeline application. Specifically, the 30m single pipe in free span (with pressure and temperature) deflected 205.1mm, more than double the corresponding PIP. This knowledge can be beneficial to selection and design considerations for pipeline system responses to both the gravity, thermal and pressure loading as well as the potential failure modes that may results in a typical scenario. Various theoretical calculations of stresses are used to validate the finding in this study of the single pipe and PIP models for flat seabed and free span.