Prelude FLNG Driven Anchor Piles in Australian Carbonate Soils: Free-Fall Risk Management by Design

Abstract

Abstract The Prelude FLNG facility anchoring system is based on a turret with 16 anchor legs, each connected to driven anchor piles in generally very weak and variable carbonate soils. Driven piles have a chequered history offshore Australia, due to the very low skin friction that is typically mobilized. At Prelude, the combination of weak soils with some harder layers and low expected skin friction led to numerous installation challenges such as free-fall, overpenetration, soil blow-out and plug depression risks. Mitigating these risks was achieved in a joint risk-based approach resulting in the unique combination of floating plate parachute, bearing V-shaped cruciform and "pushing" weight tool and installation methodology. Post-treatment of the lifting crane data allowed taking benefit of the experienced soil behavior during the campaign to improve the methods with every pile, leading to the successful installation of the 16 anchors. It also improved the knowledge of the heterogeneous Australian carbonate soils including sand/silt/clay layers through comparison with geotechnical original geotechnical investigations and laboratory tests. This paper describes the various design and installation challenges and the novel solutions which were applied to manage the substantial risk of failure to install these anchor piles. The successful installation and associated data acquired is unique and have led to invaluable improvements in the understanding of pile-soil interaction in carbonate soils.