Cyclic consolidation and axial friction for seabed pipelines

Finite-element analyses of the axial sliding resistance of a seabed pipeline embedded in soft normally consolidated modified Cam clay are presented. The study demonstrates the rise in axial resistance associated with episodes of movement with intervening periods of consolidation. It is shown that the excess pore pressures generated during undrained axial movements lead to significant consolidation and strength gain in the surrounding soil. For modified Cam clay, using properties representative of kaolin, the resistance rises exponentially with cycles and reaches the drained limit within approximately 10–20 episodes of movement and consolidation. The rate of gain in resistance is shown to be controlled by the volumetric stiffness ratio, κ/λ, for soil that is initially normally consolidated. Simple relationships are proposed that will assist in utilising this beneficial phenomenon in design practice. The increase in axial sliding resistance due to consolidation may mean that mitigation requirements against cyclic axial pipe walking, caused by thermal expansion, may be reduced or eliminated.