Key features impacting soil-conductor lateral behaviour as illustrated by centrifuge tests

Conductors are a type of pile used during subsea drilling operations to prevent hole collapse and to provide axial support to the well. The response of the conductor to lateral movement, as induced by environmental conditions, contributes to the assessment of fatigue damage of the entire wellhead system. Such assessment requires soil-structure interaction analysis, typically performed by modelling the soil-conductor lateral behaviour as non-linear springs called p-y curves. While bespoke approaches do exist, current industry practice often involves the use of p-y curves given in API RP2GEO, which were originally developed for foundation piles. Recent studies have shown that these curves do not adequately capture the soil-conductor response, especially at small lateral displacements. In addition, no account is given to load-history effects. This paper presents results from centrifuge testing of a rigid length of conductor installed in reconstituted samples of carbonate silt and subject to cycles of lateral displacement, with focus on identifying key features that influence soil-conductor behaviour. The results show that the degraded secant stiffness is impacted by load history – for example, after applying cycles of large amplitude displacement, the secant stiffness at smaller amplitude cycling will be significantly lower than if it had not previously experienced the more onerous loading. Furthermore, pore pressure dissipation between or during cyclic events can result in secant stiffness increasing. The results presented in this paper are part of an ongoing research project, aimed at improving fatigue design of subsea wells.