Seabed characterisation and models for pipeline-soil interaction

Pipelines and flowlines represent major cost items in the development of deepwater fields. Accurate modelling of the axial and lateral pipe-soil resistance can lead to significant cost reductions by optimising design. Critical design issues include axial motion, or walking, of pipelines due to cyclic thermal changes, lateral buckling due to thermal expansion, and fatigue damage to risers in the touchdown region. Traditionally, interaction between a pipeline and the seabed has been simplified into frictional models for axial and lateral resistance during walking or buckling. Improving these models is a priority, but is hampered by (i) difficulties in characterising the behaviour of very low strength, near surface, seabed soils and (ii) a lack of detailed understanding of the soil mechanics of pipe-soil interaction. The cylindrical geometry of a pipeline invites comparison with the behaviour of tubular piles. Recent advances in pile design methods generated by considering the underlying soil mechanics indicate that the same potential exists for improving the understanding of pipeline behaviour. The paper describes recent advances in measuring the low shear strengths associated with near-surface seabed soils, using both in situ methods in the form of cylindrical (T-bar) and spherical penetrometers, and laboratory shear tests at very low effective stresses. The relationship between penetration resistance and the vertical and lateral resistance of pipelines is explored, taking account of the depth of burial, and cycles of movement. New approaches for assessing the axial and lateral resistance of on-bottom pipelines are described. Future trends and recent developments are summarised.