A Finite Element Model for the Analysis of Pile Driving

MOUNIR  MABSOUT

  One of the main considerations in the design of offshore structures is the transfer of the load from the superstructure (platform, equipment, legs) to the soil underneath the water. Although rigid mat foundations have been used to support gravity‑type platforms in shallow water, piles remain a key element in the design of offshore foundations.

 The function expected of the piles depends on the platform type. While piles driven through the legs of jacket‑type platforms sustain compressive (gravity) and lateral (wind, earthquake) loads, piles for tension‑leg platforms (TLPS) are essentially required to resist uplift forces which tend to pull the pile out of the soil. Still, all piles, regardless of their functions are installed by boring or driving. In the latter case, the pile ‑drive equipment consists typically of a hammer system, and, occasionally, of a force generator. The equipment used for driving piles for offshore structures is the same as that employed for onshore applications (buildings, retaining walls,...), except in very deep water where underwater hammers are used. Likewise, the same general pile‑driving analysis applies to offshore as well as onshore pile driving.

 The feasibility of conducting a detailed analysis of pile driving using a finite element technique is examined in the present study, taking into account the nonlinear behavior of undrained clayey soil and tracing the penetration of the pile into the soil. A three-dimensional model is used for this purpose, which is handled by two‑dimensional analysis due to axisymmetric nature of the problem. A nonlinear time‑domain dynamic analysis is performed in which the hammer blows on the pile are represented by a periodic forcing function, and the pile penetration is treated using a frictional contact slide‑line algorithm. The pile driveability, integrity, and capacity are discussed , as well as the evolution of the state of stress in the soil and the development of pore‑water pressure in the course of driving.