The offshore Oil and Gas industry nowadays develops marginal fields, having more challenging field conditions. The high pressure and especially high temperature loads introduce a number of pipeline design issues for the development of these HP/HT fields. In the past decades, offshore pipe-in-pipe (PIP) systems have been adopted as one of the typical solutions for flow assurance issues. PIP systems typically transport high pressure and high temperature (HP/HT) oil and gas. One of the most important design challenges for HP/HT pipelines is upheaval buckling design. Similar to the buckling behavior of traditional single pipe systems (an insulated steel pipeline), also a PIP system could be subjected to upheaval buckling, when the effective axial force exceeds its critical buckling force. This buckling behavior could result in serious damage, necessitating use of accurate modeling methods by PIP system design engineers. Traditional analytical method for upheaval buckling analysis can only be applied for simplified PIP systems predicting system behavior; therefore finite element (FE) methods have been used in the present work. However, modeling of complex PIP systems for upheaval buckling simulations is quite challenging in order to obtain accurate calculation results. In order to build the most economic FE model for upheaval buckling simulation of PIP pipeline systems, various modeling aspects of upheaval buckling assessments were studied separately. Based on this work three types of FE models, each having different simplifications, have been established. These models have been built by different element types, mainly including beam elements, tube-to-tube elements, and PSI (Pipe Soil Interaction) elements. At the same time pipe soil interaction modeling and a non-linear buckling analysis method, as well as the Modified Riks method were used in the calculation. The upheaval buckling behavior of an actual PIP system has been analyzed using these models. The critical buckling forces were determined together with the corresponding stress levels for each of the Models and compared. Finally some useful suggestions regarding the modeling aspects are presented. These suggestions can also be considered for lateral buckling design of PIP pipeline.

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