Efficient Gravity Installed Anchor for Deepwater Mooring

Zimmerman, Evan H.; Smith, Matt; Shelton, John T.

doi:10.4043/otc-20117-ms

Cited by 7 publications

(2 citation statements)

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“…This gap remains somewhat constant ( $ 8.0%) as the impact velocity increases from 20 m/s to 30 m/s. A similar concept has been used at the Vøring Plateau, Troll Field and Gjøa Field in the North Sea (Lieng et al, 2010;Sturm et al, 2011) and in the Gulf of Mexico (Wodehouse et al, 2007;Zimmerman et al, 2009). Hossain et al (2014) provided a summary of these field tests.…”

Section: Effect Of Reference Strain Rate (_ γ Ref )mentioning

confidence: 94%

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

2015

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a b s t r a c tTorpedo anchors (of diameter $ 1 m) are released from a height of 50-100 m from the seabed, achieving velocities up to 35 m/s at impacting the sediment. The strain rates induced in the surrounding soil by this dynamic installation is therefore significantly higher than those associated with installation of other offshore foundations and anchoring systems. The high strain rates enhance the mobilised undrained shear strength compared to that measured by in-situ penetrometer or laboratory tests. This paper reports the results from dynamic installation of a torpedo anchor in strain softening, rate dependent soft clays, quantifying the effects relative to results for ideal Tresca material. The three-dimensional dynamic large deformation finite element (LDFE) analyses were carried out using the coupled EulerianLagrangian approach. The simple elastic-perfectly plastic Tresca soil model was modified to allow strain softening and strain rate dependency of the shear strength. Parametric analyses were undertaken varying the strain rate parameter, the sensitivity and ductility of the soil, and the soil undrained shear strength. Overall, embedment depth for rate dependent, strain softening clays lay below that for ideal Tresca material. Increased strain rate dependency of the soil led to marked reduction in embedment depth, only partly compensated by brittleness. Key results have been presented in the form of design charts, fitted by simple expressions to estimate the embedment depth of a torpedo anchor.

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Section: Effect Of Reference Strain Rate (_ γ Ref )mentioning

confidence: 94%

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

2015

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“…In order to select realistic anchor and soil parameters, a survey was carried out through reported case histories and field trials at various locations in the Campos Basin, offshore Brazil; Vøring Plateau, Troll Field and Gjøa Field in the North Sea, off the Western coast of Norway (Medeiros, 2002;de Araujo et al, 2004;Brandão et al, 2006;Zimmerman et al, 2009;Lieng et al, 2010) and offshore geotechnical characterisation report (Argiolas and Rosas, 2003;Randolph, 2004). Hossain et al (2014) provided a summary of these field tests.…”

Section: Geometry and Parametersmentioning

confidence: 99%

Numerical investigation of dynamic installation of torpedo anchors in clay

Kim¹,

Hossain²,

Wang³

et al. 2015

Ocean Engineering

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a b s t r a c tThis 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 EulerianLagrangian 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.

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