Tamaki Ura and Yoshiyuki Yamamoto, University of Tokyo

Abstract Since soil-loading on an anchor embedded has not been clarified, modern mooring anchors have been obliged to be designed on the basis of previous experiences with bower anchors. In order to provide informations of soil-loading, the present authors have performed experimental studies on it acting on each structural member of an anchor dragged in sand. With the use of the experimental results, behaviors of anchors in sand can be calculated numerically. In order to realize effective modern anchors, the present paper proposes an adequate conception of absolute-roll-stability by considering the stabilizing moment acting on the anchor in a perturbed configuration The advantage of the absolute1y-roll-stable anchor is such that it recovers from a nil holding power state as it is dragged in the direction of the shank. New prototypes of bower anchors and single-fluke anchors are designed in order to realize the abso1ute-rollstability, and their behaviors in sand are demonstrated in the photographs. It is also shown that the single-fluke anchor yields satisfactory holding power even if it is dragged in an inclined direction upwards. Introduction Mooring anchors have been used to prevent the ship or the floating structure from moving under the influence of waves, wind and tide, and are characterized by the following items;stability,maximum holding power,penetration,easy handling, It is very difficult to determine the effectivity of mooring anchors, because maximum holding power, stability and penetration depends upon the characteristics of the sea bed. The behaviors of anchors in the sea bed cannot be numerically because of the lack of informations relating to the soil-loading acting on them. The present authors have investigated the characteristics of the soil-loading on each structural element of an anchor by laboratory tests in sand bed and derived simple formulas for calculating them in an arbitrary configuration of the anchor. Then it becomes possible to analyze the behavior of an anchor dragged in non-cohesive sand bed. The effectivity of improved traditional anchors and the advantage of newly designed prototypes can be also analyzed by the same method. The stability is a vital function of a reliable mooring system, although it has been rather ignored in comparison to the easy handling for conventional bower anchors. In this paper, therefore, a design procedure for a stable or absolutely-roll-stable anchor will be proposed on the basis of the formulas obtained. Soil-Loading On Anchors In Sand Bed As shown in Fig. 1, the rectangular coordinates x,y,z, are introduced so that the x,y-plane lies on the horizontal undisturbed sand surface and the z-axis points vertically upwards. An anchor in sand of the specific weight ? is moving ahead with a constand velocity V in the x-direction. Let dS be a surface element of the anchor with outward normal n=(nx,ny,nz). Disregarding the elevation of soil and the interactions of the anchor's elements,1 the soil-loading fdS=(fx,fy,fz)dS on dS is given byMathematical equation (Available in full paper)