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The seismic response of pile-supported offshore structures is strongly affected by the nonlinear behavior of the supporting piles. Nonlinear response of piles is the most important source of potentially nonlinear dynamic response of offshore platforms due to earthquake excitations. It is often necessary to perform dynamic analysis of offshore platforms that accountsfor soil nonlinearity, discontinuity condition at pile soil interfaces, energy dissipation through soil radiation damping and structural non linear behaviors of piles.In this paper, an attempt is made to develop an inexpensive and practical procedure compatible with readily available structural analysis software for estimating the lateral response of flexible piles embedded in layered soil deposits subjected to seismic loading. In the proposed model a BNWF (Beam on Nonlinear Winkler Foundation) approach is used consisting of simple nonlinear springs, dash pots and contact elements. Gapping and caving-in conditions at the pile-soil interfaces are also considered using special interface elements. This model was incorporated into a Finite Element program (ANSYS), which was used to compute the response of laterally excited piles. A linear approach was used for seismic free field ground motion analysis. The computed responses compared well with the Centrifuge test results.This paper deals with the effects of free field ground motion analysis on seismic non linear behavior of embedded piles. Different parts of a BNWF (Beam on Nonlinear Winkler Foundation) model, together with quantitative and qualitative findings and conclusions for dynamic nonlinear response of offshore piles, are discussed and addressed in detail. The proposed BNWF model (only using the existing features of the available general finite element software) could easily be implemented in a more comprehensive model of nonlinear seismic response analysis of pile supported offshore platforms.

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Determination of allowable free span length plays a crucial role in design of offshore pipelines. The seabed intervention cost and safety of an offshore pipelines project are largely influenced by pipelines free spanning during the project life time. Different criteria are proposed by both the current designing guidelines and researchers there is however lack of comprehensive assessment of independent parameters affects the design length of free span. In this note, it is intended to investigate the effects of seabed formation along with axial force on Natural Frequency of offshore pipelines. Based on this assessment a new simple formula is proposed. Finally, to evaluate the result of this study, the allowable free span length of Qeshem Island pipelines is calculated as a case study and compared with those of the DNV (1998) and ABS (2001) guidelines and the modal analysis.