Chain-Soil Interaction in Carbonate Sand

Abstract

Abstract The design of mooring anchors of Floating LNG systems on Australia's North West Shelf (NWS) is challenging due to the high mooring loads and the seabed which comprises carbonate soils. Anchors with an embedded padeye require engineering of the chain inverse catenary and management of interfaces between the anchor and mooring system design teams. While considerable investigation of the anchor line performance in clay has been conducted, there has been limited investigation into the anchor line behavior in sand, and none in carbonate sand. The limited relevant information in the codes and standards and public domain may therefore result in over-conservative design for the ground chain, the anchors and the overall mooring system. In order to optimize the design of the mooring systems in relation to a developing Floating LNG project, a program of chain-soil interaction tests has been performed in a geotechnical centrifuge at the Centre of Offshore Foundation Systems in the University of Western Australia. The purpose of the testing was to provide data to allow better assessment of the shape and load distribution on the chain inverse catenary of a large mooring chain at high loads in carbonate sand. The testing achieved scaled loads equivalent to the 10,000 year return period storm experienced by an FLNG facility on the NWS, and included detailed profiling of the inverse catenary at different loading stages. The program spanned a range of chain sizes and soil densities as characterized by miniature cone penetrometer tests. This paper outlines the design considerations for chain-soil interaction and provides guidance for interface management, which sets the centrifuge modelling program in context. The results of the centrifuge program are presented and interpreted against the backdrop of conventional design assumptions and existing theories for chain-soil interaction. Using the interpreted results, a new method for the analysis of chain-soil interaction in carbonate sands is proposed, which includes the use of cone tip resistance profiles. The method and input parameters are calibrated via the centrifuge test results. The insights gained have resulted in improved design assumptions for the conditions modelled, and further refinements of the analysis approach are foreshadowed. These outcomes have led to improved estimates of the inverse catenary configurations and mooring anchor loads, and future work is anticipated to allow more general improvements to design practice.