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Video: Novel Dynamically Installed Fish Anchor - Diving Upon Loading in Calcareous Silt

Authors
K. Chang (The University of Western Australia) | M. S. Hossain (The University of Western Australia) | Y. H. Kim (The University of Western Australia) | M. F. Randolph (The University of Western Australia) | D. Wang (Ocean University of China)
Document ID
OTC-28901-PT
Publisher
Offshore Technology Conference
Publication Date
2018
Document Type
Presentation
Language
English
Copyright
2018. Copyright is retained by the author. This presentation is distributed by SPE with the permission of the author. Contact the author for permission to use material from this video.
Keywords
Centrifuge model test, Calcareous silt, Dynamically installed anchors, Floating facilities, Large deformation finite element analysis
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This paper proposes a novel dynamically installed ‘fish’ anchor, adopting a geometry taken from nature, for economic and safer tethering of floating facilities in deep water. Every cross section of the fish anchor shaft is elliptical, leading to very low drag resistance during free fall through the water column, and also low resistance in penetrating the seabed sediments. The padeye is fitted on the widest part of the shaft to mobilise the maximum resistance area under operational loading. The fish anchor embedment depth during dynamic installation, and capacity under both monotonic and cyclic operational loading in calcareous silt were assessed through centrifuge model tests and large deformation finite element (LDFE) analyses.

During dynamic installation, the normalised tip embedment depth of the fish anchor was typically three times that for the torpedo anchors and 50 % greater than that for the OMNI-Max anchors. Under operational loading, the fish anchor dived deeper, reaching penetrations 20 to 60 % greater than achieved during installation. By contrast the torpedo anchors (for all mooring mudline inclinations) and the OMNI-Max anchors (apart from a single test with mooring mudline inclination of 0°) pulled out directly without diving, reflecting insufficient free-fall penetration in calcareous soil. Regardless of the padeye offset ratio and mooring mudline inclinations, the diving efficiency of the fish anchor, which dictates the potential gaining capacity, was significantly higher than that of the OMNI-Max anchor. The normalised net capacity of the fish anchor was significantly higher than obtained with the torpedo anchors regardless of mooring mudline inclinations, and comparable to that obtained with an OMNI-Max anchor for mooring mudline inclination 0°, after allowing for loading-unloading cycles experienced by the OMNI-Max DIA prior to a nominally monotonic loading test.

Dynamically installed anchors have yet to be used in calcareous silty sediments (e.g. offshore Australia). This is primarily because the anchor tip embedment depth in calcareous silt has been found to be only half of that in clay due to the naturally higher undrained shear strength gradient and high dilation-induced bearing and shaft resistance. During subsequent loading, the anchor then pulls out of the seabed, without diving. To achieve adequate capacity under operational loading, deeper penetration that allows anchor diving and better diving potential are therefore critical in calcareous silt. The fish anchor was found to dive in calcareous silt for mooring mudline inclinations < 38°, while by contrast the OMNI-Max anchor generally did not dive. As such, the fish anchor has the potential for efficient anchoring to allow economic development of oil and gas reserves in deep water with calcareous seabed sediments.

Boukpeti, N. and White, D. J. (2011). Strength characterisation of a carbonate silt across the solid-fluid boundary report no. GEO: 11549, MERIWA project no. M395. Crawley, Western Australia: Geomechanics Group, Centre for Offshore Foundation Systems, The University of Western Australia.Brandão, F. E. N., Henriques, C. C. D., de Araujo, J. B., Ferreira, O. C. G. and dos Santos Amaral, C. (2006). Albacora Leste field development-FPSO P-50 mooring system concept and installation. Proceedings of the 38th Offshore Technology Conference, Houston, Texas, paper OTC 18243.Chang, K, Hossain, M. S., Wang, D. and Kim, Y. H. (2017a). Performance of a novel dynamically installed fish anchor in calcareous silt. submitted to Journal of Geotechnical and Geoenvironmental Engineering.Chang, K, Hossain, M. S., Kim, Y. H. and Wang, D. (2017b). Cyclic capacity and diving potential of novel fish anchor in calcareous silt. submitted to Journal of Geotechnical and Geoenvironmental Engineering.Coop, M. R., Sorensen, K. K., Freitas, T. and Georgoutsos, G. (2004). Particle breakage during shearing of a carbonate sand. Géotechnique 54, No. 3, 157–163.Dassault Systemes (2012). ABAQUS, Version 6.12 EFDocumentation. Rhode Island: Hibbitt, Karlsson and Sorensen, Inc.Gaudin, C., O'Loughlin, C. D., Hossain, M. S. and Zimmerman, E. H. (2013). The performance of dynamically embedded anchors in calcareous silt. Proceedings of ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering, OMAE2013, Nantes, France, paper OMAE2013-10115.Hossain, M. S., Kim, Y. H. and Gaudin, C. (2014). Experimental investigation of installation and pullout of dynamically penetrating anchors in clay and silt. Journal of Geotechnical and Geoenvironmental Engineering 140, No. 7, 040140261-04014026-13.Hossain, M. S., O'Loughlin, C. D. and Kim, Y. H. (2015). Dynamic installation and monotonic pullout of a torpedo anchor in calcareous silt. Géotechnique 65, No. 2, 77–90.Hossain, M. S., Kim, Y. H., Shim, J. U. and Ryu, M. C. (2017). Anchor for deep seabed. Korea Patent Application No. 102017-0018688.Kim, Y. H., Hossain, M. S. and Chang, K. (2017) Numerical investigation of novel dynamic installed anchors in clay and calcareous silt. submitted to Ocean Engineering.Kim, Y. H. and Hossain, M. S. (2017). Dynamic installation, keying and diving of OMNI-Max anchors in clay. Géotechnique 67, No. 1, 78–85.Lieng, J. T., Tjelta, T. I. and Skaugset, K. (2010). Installation of two prototype deep penetrating anchors at the Gj0a Field in the North Sea. Proceedings of the 42nd Offshore Technology Conference, Houston, Texas, paper OTC 20758.Mao, X. and Fahey, M. (2003). Behaviour of calcareous soils in undrained cyclic simple shear. Géotechnique 53, No. 8, 715–727.Medeiros, C. J.Jr. (2002). Low cost anchor system for flexible risers in deep waters. Proceedings of the 34th Offshore Technology Conference, Houston, Texas, paper OTC 14151.Miao, G. and Airey, D. (2013). Breakage and ultimate states for a carbonate sand. Géotechnique 63, No. 14, 1221–1229.Randolph, M. F., Jewell, R. J., Stone, K. J. L. and Brown, T. A. (1991). Establishing a new centrifuge facility. Proceedings of the International Conference on Centrifuge Modelling, Centrifuge' 91, Boulder, Colorado, pp. 3–9. Rotterdam, the Netherlands: Balkema.Randolph, M. F., Cassidy, M. J., Gourvenec, S. M., and Erbrich, C. J. (2005). Challenges of offshore geotechnical engineering. Proceedings of 16th International Conference of Soil Mechanics and Geotechnical Engineering, Osaka, Japan, 1, 123–176.Richardson, M. D. (2008). Dynamically installed anchors for floating offshore structures. PhD Thesis, The University of Western Australia, Crawley, Australia.Sharma, S. S. and Ismail, M. A. (2006). Monotonic and cyclic behavior of two calcareous soils of different origins. Journal of Geotechnical and Geoenvironmental Engineering 132, No. 12, 1581–1591.Shelton, J. T. (2007). OMNI-Max anchor development and technology. Proceedings of the 2007 Oceans Conference, Vancouver, Canada.Wei, Q., Tian, Y., Cassidy, M. J., Gaudin, C. and O'Loughlin, C. D. (2015). Behaviour of OMNI-Max anchors under chain loading. Proceedings of the 3rd International Symposium on Frontiers in Offshore Geotechnics (ISFOG), Oslo, Norway, 925–930.Young, D. F., Munson, B. R., Okiishi, T. H. and Huebsch, W. W. (2010). A brief introduction to fluid mechanics. John Wiley and Sons.Zimmerman, E. H., Smith, M. W. and Shelton, J. T. (2009). Efficient gravity installed anchor for deepwater mooring. Proceedings of the 41st Offshore Technology Conference, Houston, Texas, paper OTC 20117.

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Novel Dynamically Installed Fish Anchor - Diving Upon Loading in Calcareous Silt

Chang, K., The University of Western Australia
Hossain, M. S., The University of Western Australia
Kim, Y. H., The University of Western Australia
Randolph, M. F., The University of Western Australia
Wang, D., Ocean University of China
28901-MS OTC Conference Paper - 2018
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