Numerical investigation of dynamic installation of torpedo anchors in clay

This paper reports the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of torpedo anchors during dynamic installation in non-homogeneous clay. The large deformation finite element (LDFE) analyses were carried out using the coupled Eulerian–Lagrangian approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength using the Herschel–Bulkley model. The results were validated against field data and centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of anchor shaft length and diameter; number, width and length of fins; impact velocity and soil strength. The anchor velocity profile during penetration in clay showed that the dynamic installation process consisted of two stages: (a) in Stage 1, the soil resistance was less than the submerged weight of the anchor and hence the anchor accelerated; (b) in Stage 2, at greater penetration, frictional and end bearing resistance dominated and the anchor decelerated. The corresponding soil failure patterns revealed two interesting aspects including (a) mobilization of an end bearing mechanism at the base of the anchor shaft and fins and (b) formation of a cavity above the shaft of the installing anchor and subsequent soil backflow into the cavity depending on the soil undrained shear strength. To predict the embedment depth in the field, an improved rational analytical embedment model, based on strain rate dependent shearing resistance and fluid mechanics drag resistance, was proposed, with the LDFE data used to calibrate the model.

[1]  Majidreza Nazem,et al.  Dynamic analysis of a smooth penetrometer free-falling into uniform clay , 2012 .

[2]  M. Randolph,et al.  Soil strength estimation and pore pressure dissipation for free-fall piezocone in soft clay , 2014 .

[3]  Thomas C. Sheahan,et al.  Application of Recent Developments in Terrestrial Soft Sediment Characterization Methods to Offshore Environments , 2007 .

[4]  Mark Randolph,et al.  Rate effects on penetrometer resistance in kaolin , 2009 .

[5]  Mark Cassidy,et al.  Predicting the resistance profile of a spudcan penetrating sand overlying clay , 2014 .

[6]  M. Hossain,et al.  Effect of strain rate and strain softening on embedment depth of a torpedo anchor in clay , 2015 .

[7]  M. Randolph,et al.  Estimation of intact and remoulded undrained shear strengths from penetration tests in soft clays , 2010 .

[8]  R. Argiolas,et al.  Barracuda and Caratinga Integrated Deepwater Site Investigation, Offshore Brazil , 2003 .

[9]  Muhammad Shazzad Hossain,et al.  New Design Approach for Spudcan Penetration in Nonuniform Clay with an Interbedded Stiff Layer , 2015 .

[10]  Jon Tore Lieng,et al.  Deep Penetrating Anchor: Further Development, Optimization And Capacity Verification , 2000 .

[11]  C. J. Medeiros.  Low Cost Anchor System for Flexible Risers in Deep Waters , 2002 .

[12]  Mark Randolph,et al.  Characterisation of soft sediments for offshore applications , 2004 .

[13]  Alec Westley Skempton,et al.  The bearing capacity of clays , 1951 .

[14]  D. True.  Rapid Penetration into Seafloor Soils , 1974 .

[15]  G. Biscontin,et al.  Influence of Peripheral Velocity on Vane Shear Strength of an Artificial Clay , 2001 .

[16]  M. Randolph,et al.  Strength of fine-grained soils at the solid-fluid transition , 2012 .

[17]  M. Randolph,et al.  Effect of strain rate on mobilised strength and thickness of curved shear bands , 2006 .

[18]  John L. Tassoulas,et al.  Installation of Torpedo Anchors: Numerical Modeling , 2009 .

[19]  Mark Randolph,et al.  Resistance of full-flow penetrometers in rate-dependent and strain-softening clay , 2009 .

[20]  C. Gaudin,et al.  Experimental Investigation of Installation and Pullout of Dynamically Penetrating Anchors in Clay and Silt , 2014 .

[21]  M. Randolph,et al.  Variable rate full-flow penetration tests intact and remoulded soil , 2008 .

[22]  Paul W. Mayne,et al.  Rate Effects in Vane Shear Testing , 2007 .

[23]  F.E.N. Brandão,et al.  Albacora Leste Field Development- FPSO P-50 Mooring System Concept and Installation , 2006 .

[24]  M. Randolph,et al.  Setup Following Installation of Dynamic Anchors in Normally Consolidated Clay , 2009 .

[25]  Gokhan Saygili,et al.  Effect of soil variability on the penetration depth of dynamically installed drop anchors , 2011 .

[26]  Tor Inge Tjelta,et al.  Deep penetrating anchor: Subseabed deepwater anchor concept for floaters and other installations , 1999 .

[27]  Jairo Bastos de Araujo,et al.  High Holding Power Torpedo Pile: Results for the First Long Term Application , 2004 .

[28]  Mark Randolph,et al.  Effect of penetration rate on penetrometer resistance in clay , 2006 .

[29]  Jason T. DeJong,et al.  Closure of "Evaluation of Undrained Shear Strength Using Full-Flow Penetrometers" , 2012 .

[30]  S. Stegmann,et al.  In situ dynamic piezocone penetrometer tests in natural clayey soils — a reappraisal of strain-rate corrections , 2014 .

[31]  J. T. Lieng,et al.  Installation of Two Prototype Deep Penetrating Anchors at the Gjoa Field in the North Sea , 2010 .

[32]  Jürgen Grabe,et al.  Application of a Coupled Eulerian–Lagrangian approach on geomechanical problems involving large deformations , 2011 .

[33]  Philippe Coussot,et al.  Direct Determination of Rheological Characteristics of Debris Flow , 2000 .

[34]  Mark Randolph,et al.  Numerical analysis of a cylinder moving through rate-dependent undrained soil , 2011 .

[35]  M. Randolph,et al.  Combining upper bound and strain path methods for evaluating penetration resistance , 2005 .

[36]  Conleth O'Loughlin,et al.  Centrifuge tests on dynamically installed anchors , 2009 .

[37]  Zongrui Chen,et al.  Influence of overburden pressure and soil rigidity on uplift behavior of square plate anchor in uniform clay , 2013 .

[38]  Mark Randolph,et al.  In situ testing for design of pipeline and anchoring systems , 2007 .

[39]  ThoKee Kiat,et al.  Eulerian finite element simulation of spudcan–pile interaction , 2013 .

[40]  M. Randolph,et al.  Penetration of dynamically installed anchors in clay , 2013 .

[41]  N. Boylan,et al.  Full-Flow Penetrometer Testing in Bothkennar Clay , 2007 .