McDermott recently installed two platforms for a client offshore Bass Strait, in approximately 55 m of water. Due to formation conditions, the sub-structure’s foundations were originally designed as two stage piles comprising driven primary piles, and drilled and grouted insert piles.
During installation planning, the risk of drilled hole collapse was identified. The client elected to mitigate this risk by specifying that all insert pile installation works were to be completed whilst maintaining approximately 120 kPa of Positive Head (PH) pressure across the open hole. Additionally, the ability to install a secondary insert pile through and below the insert pile, was also specified.
To accommodate the requirement for positive head, McDermott elected to raise the water level inside the drilling caissons, using a spread of submersible lift pumps. To permit installation of a secondary insert pile, the insert pile was changed to a two piece, field welded design, along with the deletion of all internal grout ducting, and the provision of a cast in-situ concrete toe. Pre-cast, non-reinforced Concrete Spacer Blocks (CSBs) were installed inside the insert pile offshore, and provided a conduit for subsequent grouting operations.
For field assembly and installation of the insert piles a suite of pile handling tools were developed, which included external pile gripping and work platforms and a dedicated Pile Lowering System (PLS).
This paper discusses the key changes to the pre-existing installation methodology, details the multiple new build or modified installation aids, and identifies lessons learnt for future works.