دانلود رایگان ترجمه مقاله پاسخ چرخه ای جانبی پی های شمع در پلتفرم های دریایی – الزویر 2012

مقاله انگلیسی رایگان

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Abstract

Fixed offshore platforms supported by pile foundations are always subjected to lateral cyclic loads due to environmental conditions. In general, nonlinear pile–soil interaction is the most important source of nonlinear response of offshore platforms due to design environmental loads. Finite element models are high precision method in simulation of the pile soil interaction problems however these analyses are usually complex and computationally expensive. In contrast, Beam on Nonlinear Winkler Foundation (BNWF) models are versatile, efficient and can possess sufficient precision. In this paper a new robust and practical BNWF model is presented for lateral behaviour of pile foundations under cyclic lateral loads. This cyclic pile–soil interaction model is incorporated as a user element into a general finite element software (ABAQUS) and can be easily used for complicated nonlinear strength analysis of fixed platforms. Monotonic or cyclic loading, gap formation and development, drag force and different backbone curves recommended by American Petroleum Institute can be easily used in this BNWF model. This paper deals with the effects of cyclic pile soil interaction on lateral response of offshore piles. Different parts of this BNWF model are discussed and addressed in detail. The piles behaviour in an example fixed offshore platform are investigated under lateral cyclic and monotonic loadings.

 1. Introduction

Pile-supported coastal and offshore structures in marine soil deposits are always subjected to large lateral loads. Usually, the critical lateral forces on piles used in coastal structures are due to berthing and mooring forces, whereas piles in offshore jacket platforms are subjected to cyclic lateral loads due to waves. Nonlinearity of the soil stress–strain behaviour and geometric nonlinearities, such as disjointing and sliding between pile and soil, are the most dominant factors on the pile response. Bea [1] performed a series of static push-over analyses on a fixed offshore platform, and showed that the first nine nonlinear events were concentrated in the foundation piles. Nonlinear pile–soil interaction (PSI) is one of the main parameters that can deeply affect the overall response of the supported structures. Analyses of lateral pile response are categorized into three major distinct methods [2]: The limit analysis method in which the ultimate soil reaction is to be drawn from assumed ultimate displacement of pile [3]; Finite element (FE) or boundary element (BE) continuum-based models which are computationally expensive and practically difficult; Linear and nonlinear Winkler spring methods based on p–y curves. In direct numerical approaches, FE and BE methods are used for solving the pile–soil interaction problems in which the pile, surrounded by soil and the pile–soil interface are modelled all together in one integrated model. Yegian and Wright [4], Randolph [5], Trochanis et al. [6] and Bentley and El Naggar [7] used finite element method (FEM), whereas Kaynia and Kausel [8], and Sen et al. [9] implemented boundary element method (BEM) for the response analysis of piles. Millán and Domínguez [10] and Padrón et al. [11] used coupled finite and boundary element methods for dynamic response of pile supported structures under time harmonic excitations. Their presented models were able to take into account dynamic pile soil interaction in a rigorous manner. 3D finite element and boundary element techniques need excessive computational efforts for pile soil interaction analysis, and thus they are not frequently used in engineering practices. Modelling pilesoil separation, gap formation, gap developments and other interface nonlinearities, can prove to be complicated tasks in continuum-based models. In addition, 3D finite element or boundary element elastoplastic pile–soil interaction models cannot not be easily incorporated in commercially available structural programs to compute the lateral response of complex offshore platforms as a whole. Beam on Nonlinear Winkler Foundation (BNWF) models are computationally efficient and practically versatile, both for professional engineers in industrial design and in academia for research purposes. The method is of great use in comparison with continuum-based models and is continually the subject of further developments and modifications. BNWF assumes that pile–soil interaction behaviour and force emerged in each soil layer are only related to soil displacement in the same depth and direction. Hence, it facilitates soil modelling by separated springs along the pile shaft. Matlock et al. [12], Makris and Gazetas [13], Nogami et al. [14] and El Nagger and Novak [15,16] proposed several models on the BNWF premise. Trochanis et al. [17] adopted the BNWF model with viscoplastic effect to study interaction behaviour in quasi-static and static conditions. Badoni and Makris [18] carried out several laboratory experiments to verify results of Trochanis’s model. Wang et al. [19] proposed several configurations of nonlinear springs and a parallel dashpot to investigate the damping field of BNWF soil–pile interaction models, mainly used in seismic studies. Boulanger et al. [20] proposed a configuration of parallel and series springs as well as a dashpot to incorporate nonlinear soil behaviour, gap formation in cohesive soils, drag force, and soil damping into BNWF models.