Numerical Analysis of Mechanical Interaction of Pipe-in-Pipe Flowline

Pipe-in-pipe (PIP) flowline is a unique solution for long subsea tie-backs in deepwater and ultra-deepwater fields. This is because of its optimum thermal performance over wet insulation. However, pipelines are subjected to the highest loading condition during installation. Significant limitation imposed on existing installation vessel in deepwater, is peculiar to S-lay installation method. Contrary to the level of stress experienced with the S-lay installation method at specific locations such as overbend and sagbend region, this method is still widely utilized because of its high production rate. These regions are dominated by bending curvatures which are defined by different load conditions. Due to the composition of PIP system, it is important to understand the structural response of the flowline, the mechanical interaction occurring between various components and the amount of load transfer at this location. Although, the mechanical interaction within the PIP system are case specific. However, it has been observed that prior to case study analysis; simple pipe models are being developed to assess the mechanical interaction of this system. This paper addresses the impact of the centralizer material on the structural response and load transfer between the outer pipe and inner pipe. The numerical analysis was carried out using Ansys software and was based on Euler Bernoulli bending theory. The centralizer was clamped on to the inner pipe with the clearance between the centralizer and the outer pipe included in the model. The core of the analysis, was modeling the visco-elastic response of nylon rings (Polyamide 6), from which centralizers are made. The centralizer was spaced based on S-lay or J-lay installation criteria against heat sink. The results demonstrated the relationship between spacing of the centralizer and areas of first contact, amount of force transferred through the centralizer material, non-linearity introduced by contact formulation, alongside the time and temperature dependent behavior of visco-elastic material. The result correlated accurately with the bending principle. Different material model was assessed to determine accuracy of results obtained, in the absence of experimental test data to model visco-elastic response. In addition, the bending curvature was used to predict the mechanical interaction in installation and operation analysis, where limitations of explicitly modeling centralizers exist.