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Chain-soil interaction in carbonate sand

Chain-soil interaction in carbonate sand
Chain-soil interaction in carbonate sand

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.

2493-2519
Curran Associates
Frankenmolen, S.F.
d2965f2a-0ff2-4af2-9b44-01780d3ef3ff
White, D.J.
a986033d-d26d-4419-a3f3-20dc54efce93
O'Loughlin, C. D.
cd36a0df-e345-48fa-9f35-2a43f2cdda6f
Frankenmolen, S.F.
d2965f2a-0ff2-4af2-9b44-01780d3ef3ff
White, D.J.
a986033d-d26d-4419-a3f3-20dc54efce93
O'Loughlin, C. D.
cd36a0df-e345-48fa-9f35-2a43f2cdda6f

Frankenmolen, S.F., White, D.J. and O'Loughlin, C. D. (2016) Chain-soil interaction in carbonate sand. In Offshore Technology Conference 2016 (OTC 2016). vol. 3, Curran Associates. pp. 2493-2519 .

Record type: Conference or Workshop Item (Paper)

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.

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More information

Published date: August 2016
Venue - Dates: Offshore Technology Conference 2016, OTC 2016, , Houston, United States, 2016-05-01 - 2016-05-04

Identifiers

Local EPrints ID: 420512
URI: http://eprints.soton.ac.uk/id/eprint/420512
PURE UUID: ee30491e-bad2-4e08-9b97-6df1e1b832a0
ORCID for D.J. White: ORCID iD orcid.org/0000-0002-2968-582X

Catalogue record

Date deposited: 09 May 2018 16:30
Last modified: 08 Jun 2022 01:51

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Contributors

Author: S.F. Frankenmolen
Author: D.J. White ORCID iD
Author: C. D. O'Loughlin

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