Pile Running Risks for Offshore Foundations in Clay

Large offshore structures such as deep-sea platforms and offshore wind turbines are frequently dependent on foundation systems requiring significant penetration into the seabed. Typical deep offshore foundations include large-diameter open-ended monopiles and spudcans capable of supporting considerable structural loads. Drop-fall hazards, large settlements under self-weight and pile running are all major hazards during foundation installation. Each occurs when driving forces exceed the shaft resistance and end bearing capacity. Assessing the risks posed by drop-fall are compounded by the presence of soils whose properties are heterogeneous and vary rapidly with depth.
This paper assesses drop-fall and pile running rates for large offshore piles in the presence of spatially variable clayey deposits, where the end-bearing resistance of the pile is negligible. A simplified analytic procedure is used to determine the velocity and run depth, based on a combination of well-accepted bearing capacity equations and Newton's second law. The method differs from traditional static bearing capacity analyses due to the consideration of continuous pile running velocities with depth. The role of depth-dependent variations in undrained shear strength has the capability of both initiating and arresting drop-fall. In the case of large driven piles whose bearing capacities at depth are primarily controlled by shaft friction, it is shown that drop-fall distances are largely robust to variations in shear strength, while velocity profiles at the near-surface where the end bearing resistance dominates are susceptible to sizeable fluctuations in drop-fall velocity. A set of worst-case scenarios is presented whereby piles penetrate under driving or self-weight from the mudline, such that velocities increase before the accumulation of shaft resistance. Despite pile skin friction providing the primary resistance compared to end-bearing resistance in the cases presented, the run depths and velocities considered continue to show surprising variability. Comparative results of clayey and sandy soils are presented with reference made to the importance of accurate in-situ test results in assessing velocity scales of fluctuation.

Keywords: Drop-fall velocity, Pile running, Driven pile, Spatially variable soil resistance.