Anchor-PiIe Design for Ocean-FIoor Environments Using Finite-Element Analysis

ABSTRACT This paper analyzes some aspects of anchor-pile design for marine environments, using a finite-element mathematical model of the pile, soil, and the pile/soil interface. Forces and stresses in the soil and pile are computed with a commercially-available finite-element program, ANSYS. A key feature of this analysis is the manner in which pile/soil interface phenomena are modelled, using gap elements. Load capacity of the anchor pile is determined through analysis of interface conditions, indicated by the gap elements. Example calculations are presented, based upon an 80-foot long anchor-pile with a 4-footdiameter, imbedded in a sandy-clay marine sediment. Stresses in the 'soil and at the soil/pile interface are plotted, showing a concentration at or near the interface. INTRODUCTION Piling system design for ocean floor applications is a subject of interest to those challenged with the engineering of offshore drilling and production platforms and mooring systems. The behavior of pile systems under bearing loads in marine sediments has been studied by Yegian and Wright (1973), Matlock (1970), and others. Reese (1973) has presented a design basis for anchor-piles used in mooring systems. The ocean-floor environment poses some interesting design problems for anchor-pile systems. In addition to overburden forces acting on the soil, one can have a significant hydrostatic pressure effect acting on the pile/ soil system in deep-water applications, such as the North Sea. Although there is undoubtedly a lateral component involved in the load on such deep-water anchor-piles, the principle component is vertical. Therefore, the present paper will look exclusively at axially-loaded anchorpiles. The purpose of this paper is to examine certain design features of the deep-water anchor-pile, using'a finitelement model of the pile/soil system. The intent is twofold:to stir up interest in the design of anchor-piles subject to such loads in deep-water applications, andto demonstrate the utility of commercially-available finite element programs for the analysis of pile/soil systems under load. The following sections of the paper detail the assumptions in the model and the interpretation of the calculation results. ANALYTICAL MODEL OF PILE/SOIL SYSTEM A. Pile/soil Model A pictorial model of the pile/soil system is shown in Figure 1. Due to the axial symmetry of this particular system, it is expedient to reduce the model to' a two-dimensional one, as shown in Figure 2. The centerline of the pile is one boundary. The other boundary is taken sufficiently far away- from the pile, such that it is beyond the influence of pile/soil interaction. This is assumed to be twice the pile length in the present study. As pointed but by Yegian and Wright, the extent of the pile " zone of influence" in the soil is related to the pile diameter, D. From their results, it appears that the limit of the pile " zone of influence" is about 8 times the pile diameter, D. In the present model, the pile length is 20 times the pile diameter. Thus, the pile/soil effects should be wholly contained within the boundaries of the model.