Integrity of Buried Flexible Pipeline Using Numerical Approach

Abstract Unbonded flexible pipe is being adopted by many operators worldwide for short pipelines and where scrapping is not mandated from the perspective of service condition or practicality. Flexible pipes are also well suited for short connection subsea where field architecture does not permit rigid pipeline with large route radius. Utilisation of flexible pipes helps in combating corrosion which is one of the significant detriments for the oil and gas industry, and the cost of combating it can be substantially high. Corrosion not only affects the reliability and safe operation of the plant, but it can also impair the operator’s reputation. This paper outlines the extensive numerical investigations conducted on buried subsea unbonded flexible pipeline integrity through the understanding of its upheaval buckling behaviour. Currently, there are no simple guidelines to address the behaviour of buried flexible pipelines. Under hydrostatic and operational conditions, suitable recommendations for required cover depth are made vis-à-vis the envisaged vertical imperfections. A non-linear finite element analysis (FEA) method using commercial software simulates the buried flexible pipeline behaviour. In the absence of any precedent, the simulation is complicated since the flexible pipeline is modelled using a beam element. However, it must be able to capture the actual axial and bending moment behaviour in response to the hydrostatic and operational pressure and temperature loads under the condition where the flexible pipe is trenched/buried. The elongation of flexible pipe is design dependent and linked with the operating envelope. However, several vertical imperfections in the middle are modelled to assess the propensity of upheaval buckling. The model also considers that the backfill is uniform along the pipeline route for the buried section. The bending radius of the flexible pipe is checked along the imperfection section to ensure its integrity. The basis of behaviour and analysis for buried flexible pipelines differs from rigid pipeline systems. With a suitable technique, using non-linear finite element analysis while accounting for variation for initial seabed imperfection, buried flexible pipeline behaviour can be reasonably established. Also, with the inclusion of imperfection, buckle pressure/temperature becomes inversely proportional to imperfection heights. The larger imperfections require a smaller temperature to propagate upheaval buckling, and a more extensive cover depth would be needed to keep the pipeline in its position. The methodology presented in this study can be applied for flexible pipe with route curvature and thus expands on the classical view of buried flexible pipeline integrity which typically considers that the buried pipeline is straight. Also, it is possible to identify scenarios where laying of flexible on seabed would be a feasible option without any requirement for trenching/burial or stabilization and hence reducing capital expenditure (CAPEX).