The Influence of Chain Friction on Anchor Pile Design

Abstract

Abstract This paper presents a simple and versatile computer procedure for estimating the influence of chain friction on the pile load in an anchor pile-chain system. The chain-soil interaction is modeled in terms of the chain embedment depth, chain characteristics and soil strength profile. Example predictions are presented to illustrate the relative magnitudes of the vertical and horizontal components of the pile load, and the effects of the embedment depth, chain size and pretension. A limited comparison with some measurements of chain friction at shallow depth is also presented. Introduction Anchor piles have been used offshore to provide anchor points for mooring purposes for centenary-moored floating platforms (Figure 1) and are a leading candidate for anchoring the lines of guyed tower platforms. The mooring line has been connected at an intermediate position along the pile length (Figure 2(a)), or at the pile head with the head below or at the mud line (Figure 2(b) and 2(c». Positioning the pile head and chain connection at the muddling (Figure 2(c)) facilitates full inspection and refurbishment of the chain in long-term moorings. On the other hand, the bending stresses in the pile can be significantly reduced by connecting the anchor chain to the pile at a point below the pile head. The reduction results primarily from the additional end restraint provided by the soil above the chain connection. In practice, the chain connection point has been located as far down the pile as its mid-point, e.g., see Reference (1). The pile and chain are installed by driving the pile and the reconnected chain together. After some tensioning, the chain will assume a configuration as schematically shown in Figure 1. A pile-chain connection below the mud line implies that the load applied to the pile will have both a vertical and a horizontal component. The relative values of these two load components vary with the chain configuration, which in turn depends on the tension applied to the end of the anchor chain at the mud line. This paper discusses the influences of the chain soil interaction on the line tension at the pile connection. Example predictions from a proposed computational procedure are presented to illustrate the effects of embedment depth, chain size and pretension. 2. Previous Work Previous studies of the influence of "chain-soil interaction on chain tension can be divided into two groups: those related to the dragging friction and those related to chain embedment. Dragging friction on anchor chains was usually studied in the field or in laboratories by dragging a length of loose-ended chain in contact with soil. The results can be presented in terms of an "efficiency" ratio (i.e., dragging resistance divided by weight of chain in air) as summarized in Table 1. The chain in his case generally does not cut into the soil and may not be in full contact with the soil. Interaction between soil and embedded chain was considered by Reese (4) and Gaunt and Cox (5).