Installation of Two Prototype Deep Penetrating Anchors at the Gjoa Field in the North Sea

Lieng, Jon Tore; Tjelta, Tor Inge; Skaugset, Kjetil

doi:10.4043/20758-ms

Cited by 42 publications

(31 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%

“…In this analysis, values for the various parameters were adopted as follows: C d ¼0.63 (Lieng et al, 1999(Lieng et al, , 2000(Lieng et al, , 2010Sturm et al, 2011;O'Loughlin et al, 2013), n ¼1.0, N c,bA ¼13.56, N c,bF ¼ 7.5, η¼1.0, β ¼0.1, δ rem ¼1/S t ¼1/3, and ξ 95 ¼ 20. For the frictional resistance, R f2 was taken as 2R f1 from the previous reports (Einav and Randolph, 2006;Chow et al, 2014;Steiner et al, 2014).…”

Section: Extended Bearing Resistance Methodsmentioning

confidence: 99%

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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%

Section: Extended Bearing Resistance Methodsmentioning

confidence: 99%

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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“…A new hybrid anchoring system, referred to as the dynamically embedded plate anchor (DEPLA), combines the capacity advantages of vertically loaded anchors (Murff et al 2005;Gaudin et al 2006;Wong et al 2012) with the installation advantages of dynamically installed anchors (Medeiros 2001;Zimmerman et al 2009;Lieng et al 2010). The DEPLA comprises a removable central shaft or "follower" that may be fully or partially solid and a set of four flukes (see Fig.…”

Section: Introductionmentioning

confidence: 98%

“…Predicting the final embedment of the DEPLA after free-fall is complicated by the very high penetration velocities, as this significantly enhances the available shear resistance and introduces inertial drag resistance. As the velocity of dynamically installed anchors when they impact the seabed is typically between 10 and 30 m/s (Lieng et al 2010), for a typical anchor with a shaft diameter of about 1 m, the strain rates in the soil during installation are three orders of magnitude higher than that in a vane test (0.029 s −1 , Einav and Randolph 2006) and seven orders of magnitude higher than the standard laboratory testing rate of 1%/h (2.8 × 10 −6 s −1 ). O'Loughlin et al (2013a) showed that, for anchor geometries similar to the DEPLA in normally consolidated clay, inertial drag resistance dominates for embedment depths up to 0.84 times the anchor length.…”

Section: Introductionmentioning

confidence: 99%

Installation of dynamically embedded plate anchors as assessed through field tests

Blake

O’Loughlin

2015

Can. Geotech. J.

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A dynamically embedded plate anchor (DEPLA) is a rocket-shaped anchor that comprises a removable central shaft and a set of four flukes. The DEPLA penetrates to a target depth in the seabed by the kinetic energy obtained through free-fall in water. After embedment the central shaft is retrieved leaving the anchor flukes vertically embedded in the seabed. The flukes constitute the load-bearing element as a plate anchor. This paper focuses on the dynamic installation of the DEPLA. Net resistance and velocity profiles are derived from acceleration data measured by an inertial measurement unit during DEPLA field tests, which are compared with corresponding theoretical profiles based on strain rate-enhanced shear resistance and fluid mechanics drag resistance. Comparison of the measured net resistance force profiles with the model predictions shows fair agreement at 1:12 scale and good agreement at 1:7.2 and 1:4.5 scales. For all scales the embedment model predicts the final anchor embedment depth to a high degree of accuracy.Résumé : Un système d'ancrage dynamique à ailerons (« dynamically embedded plate anchor » ou DEPLA) est une ancre en forme de fusée constituée d'un cylindre central et d'une empenne comportant quatre ailerons. Le DEPLA s'enfonce dans le lit de sable à une profondeur cible grâce à l'énergie cinétique acquise au cours de sa descente en chute libre dans l'eau. Après que le dispositif s'est enfoncé dans le sable, la partie cylindrique centrale est retirée, laissant l'empenne enfouie verticalement dans le lit de sable. Les ailerons constituent l'élément porteur de charge comme dans le cas d'un système d'ancrage à plaques. Le présent article décrit l'installation dynamique du DEPLA. Les profils de résistance nette et de vitesse sont obtenus à partir des valeurs d'accélération enregistrées par un appareil de mesure de l'inertie durant les essais du DEPLA sur place et comparés avec les profils théoriques correspondants, basés sur la résistance au cisaillement (qui augmente avec la vitesse de déformation) et la résistance de traînée propre à la mécanique des fluides. Une comparaison entre les profils des forces de résistance nette mesurés et les prédictions des modèles met en évidence une assez bonne concordance à l'échelle 1:12 et une bonne convergence aux échelles 1:7,2 et 1:4,5. Pour toutes les échelles, la modélisation de l'enfouissement de l'ancre permet de prédire la profondeur de pénétration du dispositif d'ancrage dans le sable avec un haut degré de précision. [Traduit par la Rédaction] Mots-clés : ancre, argile, dynamique, traînée dans un fluide, en mer, vitesses de déformation.

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Installation of Two Prototype Deep Penetrating Anchors at the Gjoa Field in the North Sea

Cited by 42 publications

References 1 publication

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

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

Numerical investigation of dynamic installation of torpedo anchors in clay

Installation of dynamically embedded plate anchors as assessed through field tests

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