This paper addresses the results from a monitoring inspection as part of Petrobras regular plane of inspection of rigid pipelines (PIDR) and numerical Finite Element analysis (FEA) of two parallel HP/HT subsea pipelines operating in Brazilian deepwater subjected to lateral buckling and walking behaviors. The results obtained from inspection and numerical reanalysis furnishes important feedback for the integrity management of the two pipelines, reducing potential risks and lessons learned for future projects.
Deepwater pipelines submitted to high pressures and temperatures (HP/HT) are susceptible to global buckling due to axial compressive load. To guarantee pipeline and equipment’s integrity frequently is necessary to relieve high stresses and strains at buckle apex as well as to mitigate end expansion. Thus, the two parallel HP/HT pipelines were designed with single and double sleepers to trigger buckles at pre-determined locations and an anchoring system to prevent pipeline walking. Another important design aspect was to avoid undesirable buckles at the several crossings along the pipeline route applying a special device with stoppers to lock lateral displacements.
During a programmed inspection as part of regular Petrobras pipeline integrity program of rigid pipelines (PIDR), it has been verified that some double sleepers didn’t work as foreseen in design. Otherwise, some unplanned buckles on soil have been formed along the two pipelines changing the buckle apex stress levels and end expansions foreseen in detailed design.
In order to understand buckle formation behavior and guarantee long-term integrity of the HP/HT pipelines a Multi-Beam Echosounder Survey (MBES) was conducted in 2016 to build a representative Finite Element (FE) model. The temperature and pressure gradients from steady state and transient conditions were obtained from flow assurance simulations based on monitored platform operational historic data. The FE model was calibrated with buckle shapes and end displacements to assess pipeline behavior and its long-term integrity for load scenarios different from original design.