Suction Anchor Piles for the Na Kika FDS Mooring System Part 1: Site Characterization and Design

Key aspects of the suction anchor pile design for the Na Kika Floating Development System are presented. Site characterization of the foundation soils, determination of the required pile size and structural design are emphasized. Soil parameters for suction anchor pile design are developed using an integrated approach, incorporating shallow geophysical data, geotechnical data obtained from soil borings and anchor installation records of mobile offshore drilling units. The locations of soil borings were selected based on the principal geology of the development area, and not on specific anchor locations. Spatial variation of soil properties is evaluated in the context of the geologic model and available data. A simple procedure is demonstrated to estimate distributions of overburden pressure and undrained shear strength at specific anchor locations. The required pile size is based on a target probability of failure of the overall mooring system. With probabilistic models of load and resistance, event tree analyses are used to evaluate design preferences with respect to failure modes and estimate the annual probability of failure. The design also accounts for spatial soil variability, shear strength anisotropy and cyclic load effects. Most of the pile cylinder experiences negligible stress when subjected to in-service loads. The section of the pile local to the mooring line connection, however, is limiting in terms of structural strength and fatigue. Ultimate strength analysis of the pile indicates an ultimate (structural) strength greater than the pile capacity (with respect to soil failure) and the chain break load.