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Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil’s undrained strength under long term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in reconstituted kaolin and Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil.

The touchdown zones of steel catenary risers and lazy wave risers are fatigue hotspots, where the risers interact continuously with the seabed due to hydrodynamic loading exerted on the host vessel. The whole-life interactions can range from small amplitude daily motion cycles to motions that involve large amplitude cyclic interaction with the seabed during storm events. A key design challenge that affects the fatigue life of these risers is the accurate modelling of the evolution of the riser-soil stiffness, throughout the whole-life of the riser and for different soil conditions, including overconsolidated conditions that may occur due to the geological history, aging or biochemical processes of the sediments. This paper describes centrifuge model pipe tests simulating whole-life riser-soil interaction in normally consolidated and uniform overconsolidated clay samples, under successive sequences of cyclic motions. Results confirm that the whole-life soil stiffness evolution depends strongly on cyclic amplitudes, with reconsolidation induced soil stiffness recovery after heavy remoulding, and is also influenced by the soil overconsolidation ratio, with a reduced tendency for soil hardening at higher soil overconsolidation ratios. This study provides insights into the relevant cyclic soil stiffness to consider when assessing the whole-life design of risers interacting with overconsolidated seabed sediments.

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