Effects of Prior Footprints on the Bearing Capacity of Spudcan Foundations: A Case Study

Abstract

A jack-up rig is a self-elevating offshore drilling platform designed for mobile operations in the marine environment. The rig is supported by 3 or 4 legs, with spudcan foundations at the end. These spudcan provide the necessary stability and load-bearing capacity for rig foundation during drilling operations. The mobile rig sometimes revisits sites where prior installation, operation, and extraction activities have resulted in identifiable footprints on the seabed. The penetration of spudcan close to, or partially overlapping, footprints shall be carefully considered because the spudcan-footprint interaction may cause a change in the ultimate vertical bearing capacity (Fv). ISO 19905-1:2016 states that the interaction between a spudcan and a footprint is expected to be minimal when the edge-to-edge distance exceeds one spudcan diameter. This paper presents the effect of existing footprints on the vertical bearing capacity of a spudcan with various spacing variations from 0.5B to 1.75B, where B is the diameter of the spudcan. A 3D finite element method (3D FEM) is employed as the primary analysis tool in analyzing the effects of footprints on layered clay soil. The 3D FEM analysis is conducted because the analytical approach outlined in ISO 19905-1:2016 does not consider the influence of the footprint on the spudcan capacity. The analysis results show that bearing capacity may reduce by more than 50% when the center-to-center distance is less than 1B. Whereas, for distances greater than 1.5B, the influence of the footprint is relatively insignificant, only reducing less than 10% of the ultimate capacity of the spudcan.