Suction caissons and anchors are widely used for the foundation of both bottom-fixed and floating offshore structures. They are installed by means of self-weight and underpressure applied to the inside of the skirt compartment. The penetration resistance during installation is the sum of the tip resistance and wall friction of the skirt, both of which are functions of the foundation geometry, soil properties, soil-state and boundary conditions.
Several authors have proposed methods to predict the penetration resistance during installation. In general, high estimates of soil strength parameters are used to predict the installation resistance for design purposes. In addition, safety factors may be applied when predicting the achievable penetration depth and the corresponding loads acting on the structural components. However, current standards and guidelines lack a consistent approach for evaluating these loads and safety factors.
In order to apply a consistent safety concept in the structural caisson design, two methods for assessing the penetration resistance and hence required suction pressure are investigated; a simplified CPT-based method and a more advanced bearing-capacity-based method. For that purpose, a probability-based analysis has been performed, assuming statistical distributions of the corresponding input parameters, and a representative target failure probability. The performance of the two methods methods is investigated using the example of a generic sand and a generic clay profile. Based on these analysis, partial safety factors for the use in a deterministic design are proposed.