The high thermal insulation potential of a pipe-in-pipe system makes it the preferred solution for challenging flow assurance conditions. Due to the higher bending stiffness of a pipe-in-pipe system, longer free spans would be expected for pipelines resting on uneven seabed. However, there are no clearly defined standard formulae for the calculation of structural response of free spanning pipe-in-pipe system exposed to vortex induced vibration (VIV) and the resulting fatigue damage. If the same method as for a single wall pipe was applied, the combined equivalent pipe properties would be assumed and the VIV response and stresses of the equivalent pipe could be obtained. However, the longitudinal stresses in the inner and outer pipe to be used for the fatigue assessment of the girth welds would not be easily obtained, especially for sliding pipe-in-pipe systems.
Based on previous experience and development work for pipe-in-pipe systems, a numerical model for VIV assessment of sliding pipe-in-pipe systems is proposed giving improved interpretation of individual pipe characteristics. Modal analyses of sliding pipe-in-pipe systems are performed by using this numerical model. The natural frequencies and mode shapes are extracted. According to the numerical analysis results, the longitudinal stress ranges due to VIV are obtained and fatigue assessment of the pipeline girth welds for the inner and outer pipes are performed. In order to understand the interaction between the outer and inner pipe, the effect of friction and initial gap between the centralizers and outer pipe surface are studied.