Currents past gravity anchors astride offshore pipelines : a direct numerical simulation

Consisting of large arch-shaped precast concrete blocks positioned at intervals astride the pipeline, the gravity anchor system is engineered to provide secondary stabilization for large diameter offshore pipelines operating in severe metocean conditions. Bottom-seated on the seabed, however, the gravity anchor could subside into scour pits formed around it, imposing integrity risks on pipeline operations. With scour processes around gravity anchors correlating closely with the hydrodynamic behavior of flow, the present study concentrates on characterizing flow mechanisms around gravity anchors by means of direct numerical simulation. The three-dimensional Navier-Stokes equations were solved using the Petrov-Galerkin finite element method. Currents with a 90 degree angle of attack past a gravity anchor astride pipelines resting on a rigid bed was simulated at a pipe Reynolds number of 1000. The computed amplification of bed shear stresses was presented first, and compared with laboratory observations of scour patterns. Then, the responsible mean flow topology has been identified in two areas, comprising the horseshoe vortex upstream of the anchor and the lee-wake vortices.