SOIL-STRUCTURE INTERACTION ANALYSIS OF EMBEDDED CAISSON ANCHOR UNDER TENSION LOAD

Capped structural cylinders, vertically embedded in the seafloor, may provide tension capacity for anchorage and bearing capacity for offshore structures. The capacity of such "inverted buckets" to resist combinations of static and cyclic loads may be predicted by reliable computational procedures. This has been demonstrated by model tests and field performance observations. The emphasis in this paper is on the soil-structure interaction and the determination of contact stresses between the embedded cylinder and the inside and outside soil. These stresses are required for the structural design. A 3-dimensional finite element program is used to analyze a one- cylinder unit embedded in soft clay. The anchor is subjected to a cyclic live load corresponding to that for a tension leg platform. In addition to the magnitude and distribution of the control stresses, the results of the analyses show how the anchor pull is shared among the four different kinds of load carrying mechanisms, (1) vertical underpressure under the cylinder top cap, (2) inside and (3) outside shear along the skirt walls, and (4) underpressure under the skirt tip circumference. The load sharing varies with the applied tension load level due to the non-linear soil behaviour, and it is very sensitive to the assumed bulk modulus (or Poisson's ratio) for the undrained soil.