Dynamic installation and monotonic pullout of a torpedo anchor in calcareous silt

Abstract

Challenges associated with dynamically installed anchors include prediction of the anchor embedment depth, which dictates the anchor's holding capacity. This is particularly true for calcareous sediments, as very little performance data exist for this anchor type in these soils. This paper reports results from a series of model tests undertaken to provide insight into the behaviour of a torpedo anchor during dynamic installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the drop height and consequently the impact velocity, and the consolidation period prior to anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge model test data were used to calibrate: (a) an analytical dynamic embedment model, based on conventional bearing and frictional resistance factors but with strain-rate-dependent undrained shear strength for the soil; and (b) an analytical quasi-static vertical pullout capacity model, accounting for reverse end bearing and frictional resistance. A total energy based expression, appropriate for calcareous silts, was proposed for predicting anchor embedment depth for a given anchor geometry, mass and impact velocity. For assessing anchor vertical holding capacity, a piezocone based direct design approach was also proposed, deriving anchor end bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Anchor capacity under inclined loading was presented as failure envelopes expressed in terms of dimensionless vertical and horizontal components of anchor net resistance, which agreed well with a finite-element based envelope developed for embedded foundations. The regain of anchor capacity was found to be in good agreement with predictions based on the cavity expansion framework.

Installation and Monotonic Pullout of a Suction Caisson Anchor in Calcareous Silt

“…where ). This concept is consistent with the method proposed by Nottingham (1975) and Schmertmann (1978) for assessing pile capacity in clay, and recently adopted for back calculating torpedo anchor vertical pullout capacity in calcareous silt (Hossain et al, 2015).…”

“…For this axisymmetric problem,  = 1/2 of plane strain value of ~0.08 (as was obtained for installation) = 0.04 was used (following Low et al, 2008). The strength mobilised during caisson pullout will be different to that mobilised during caisson installation due to significant inward flow that causes significant effective stress changes in the soil surrounding the caisson, strength regain following reconsolidation (Chen & Randolph, 2007;Hossain et al, 2015). These effects were captured by back-figuring o from the measured holding capacity and Equation 8 using the measured (intact) shear strength profile, su,p = 1.5 + 1.…”

Effect of strain rate and strain softening on embedment depth of a torpedo anchor in clay

“…Several studies (Medeiros, 2002;de Araujo et al, 2004;Brandão et al, 2006;Richardson et al, 2009;O'Loughlin et al, 2009O'Loughlin et al, , 2013Chow et al, 2014;Hossain et al, 2014Hossain et al, , 2015Kim et al, 2015) have adopted such an approach, with variations on the inclusion and formulation of the various forces acting on a torpedo anchor. A similar approach is adopted here…”

“…Extensive background information to installation of torpedo anchors can be found in Hossain et al (2014Hossain et al ( , 2015 and Kim et al (2015), which are not repeated here. For convenience, Figs.…”

Novel Anchoring Solutions for FLNG - Opportunities Driven by Scale

“…It has also been used for temporary facilities in the Gulf of Mexico (Zimmerman et al 2009), although the design used in the Gulf of Mexico is quite different to that used offshore Brazil. The majority of performance data are from centrifuge studies (O'Loughlin et al 2004Richardson et al 2006Richardson et al , 2009Gaudin et al 2013;Hossain et al 2015) although a growing body of field experience is becoming available (Zimmerman et al 2009, O'Beirne et al 2015.…”