Global Buckling of a Heated Pipeline in the Campos Basin Brazil in a Region With Free-Spans

This article presents the behavior a heated flowline in Campos Basin-Brazil that crosses an uneven seabed region with free-spans of different lengths. In that region the mean water depth is arround 900 m and the pipeline supports the highest temperatures. A regular inspection with ROV (Remote Operate Vehicle) showed that the flowline is interacting with the surrounding soil, evidencing movements due to thermal expansion. Geotechnical survey showed that the soil in the pipeline route is composed by normally consolidated clay where the strength properties were obtained by CPT interpretation. The principal question for the pipeline operator concerns the pipeline integrity despite of the evidence of thermal buckling occurrence. In order to answer this question, a 3D finite element model was generated considering lateral imperfections, free-spans and pipe-soil interaction along the pipeline route obtained by ROV. A complementary inspection was done with Side Scan Sonar Image obtained by AUV (autonomous Underwater Vehicle), mapping lateral buckles and the free-spans in the pipeline route. The bending moment and effective axial force in the pipeline obtained by 3D finite element global model were used to verify the pipeline integrity based on DNV OS-F101 standard [1]. In order to verify the vertical buckling in the free-spans, a parametric study was done with different free-spans and feed-in lengths based on geometry acquired from inspections. This study supplies the critical temperature variation that triggers thermal buckling in the free-spans, enabling the determination of the critical free-spans in the flowline route. The results obtained by finite element modeling was the definition of the maximum operating temperature and pressure that enables operate the flowline safety based on DNV OS-F101 code.