Keywords: offshore foundations, concrete pile, finite element, wave loading.

Offshore piles have been commonly used as a foundation element of offshore structures, especially large structures such as Tension-Leg Platforms (TLP) or Spar. To perform an appropriate design of an offshore pile, the stress distribution within such a large structure is a dominating factor. In general, the external loading acting on an offshore pile comes from the axial (structural) and lateral forces (wave loading). Thus, it is a problem relating to structural and coastal engineering. However, researchers in these areas have limited their analyses in that structural engineering researchers consider vertical structural loads only, whereas coastal engineering researchers consider only lateral wave loads.

To produce a more accurate and effective design procedure for offshore foundation systems under axial and lateral wave loads, a finite element analysis is performed herein to determine the stresses and strains in a concrete pile under similar loading conditions.

In this study, a combined loading condition from both structures and waves are taken into account, thereby covering both structural and coastal engineering aspects. The vertical structural load is basically a static load that can be modelled as either a single concentrated point load or as a uniform load on the top of the pile. The lateral wave loading, in general, fluctuates in the time domain and is directly affected by the incident wave angle. To simplify such a complicated problem, a linear, dynamic wave load is used as the first approximation, which varies with time, and the first- order solution of wave loading on a vertical cylinder proposed by Zhu [1] is used to calculate the horizontal wave loads.

A finite element analysis software, STRAND6 [2], is used in this study. The pile can be modeled using brick elements, having a fixed support condition at the points where the pile is embedded into the ocean bed. Three-dimensional analyses of the concrete pile are then performed. Based on which, the stress distributions throughout the concrete pile can be investigated.

A parametric study is also performed to investigate the effects of changing parameters in relation to the stress distributions. Upon completing the analyses for the first order solution, the resulting stress and strain distributions at the varying time intervals can be compared as well as the resulting displacements from each load case. The effects of wave characteristics and the geometry of the pile on the internal stress distribution of an offshore concrete pile can then be explored through the parametric study.

1

S. Zhu, "Diffraction of short-crested waves around a circular cylinder", Ocean Engng, 20(4), 389-407, 1993. doi:10.1016/0029-8018(93)90003-Z

2

G+D Computing. Strand 6 User Guide and Reference Manual, Sydney, Australia, 1996.

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