Offshore pipe-in-pipe systems require high performance thermal insulation to maintain high fluid temperature at arrival and to avoid hydrate formation during the cool-down process that follows a pipeline shut-down. At field joints, it might be difficult to achieve the design insulation performance due to installation challenges. In these cases, the insulation layer partially fills the gap between the inner and outer pipes and thus "cold spots" could potentially arise at field joints during the pipeline operation and cool-down. In this paper the impact on the thermal performance of partially insulated pipe-in-pipe field joints is evaluated through Computational Fluid Dynamics (CFD). Thermal convection is included in the fluid model for the pipe content and the air gap between the inner and outer pipes. Comparison is also made between the numerical analysis and simplified lumped-parameter models. Results from numerical simulations show that for the case considered no cold spot arises due to a lack of field joint insulation and length-averaged Overall Heat Transfer Coefficient (OHTC) can be used to predict the pipeline cool-down time. Numerical predictions have been compared to simulated service test results, which confirm the length-averaging effect on the OHTC. Further studies are recommended to assess potential cost savings that could be achieved for uninsulated field joints.
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