Pile Running Risks for Offshore Foundations in Clay

Abstract

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. 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.