Experimental Investigation of Installation and Pullout of Dynamically Penetrating Anchors in Clay and Silt

Abstract

This paper reports the results from a series of model tests undertaken to provide insight into the behaviour of torpedo anchors during dynamic installation and pullout in lightly overconsolidated kaolin clay and calcareous silt. The tests were carried out in a drum centrifuge at 200 g, varying the drop height, hence the impact velocity, and the time delay for consolidation prior to pullout. The pullout angle at the mudline was also varied to encompass various mooring systems, including catenary (0), taut leg (45) and tension leg (~80). Two geometries of torpedo anchors were explored, varying fin and tip geometry. The results demonstrated that the anchor embedment depth increased as the drop height (and hence the impact velocity) increased and the soil undrained shear strength decreased. In stronger silt, the cavity above the installing anchor remained open whereas in soft clay, it was fully backfilled and replenished. The corresponding anchor embedment depth was also about 0.63 times compared to that in clay. The anchor holding capacity was found to increase with increasing post-installation consolidation time, depth of embedment and soil undrained shear strength and with reducing pullout angle at the mudline. The anchor rotation during pullout and hence the pullout distance for attaining the maximum capacity reduced as the load

Damage Assessment of Free-fall Dropped Object on Sub-seabed in Offshore Operation

“…As shown in Figs. 9 and 11, a low estimation of the decrement slope of the velocity in the soil and the final embedment depth in the analytical results guarantee a conservative estimation and time consumption if the CEL method-based numerical analysis works well and reports precise values, as in several previous studies O'Loughlin, , 2013Hossain, 2014). In Fig.…”

“…It is chiefly based on Newtonian mechanics and assumes uniform clay. Several investigations have used similar approaches based on True's model for freefall anchor installation (de Araujo, 2004;Brandão, 2006;O'Loughlin, , 2013Hossain, 2014). A modified equation of motion is shown in Eq.…”

“…In a similar research field, there have been several studies on the dynamic installation of torpedo (freefall) anchors. These have mostly involved centrifuge and numerical modeling and a few field trials (Hossain, 2014). Fig.…”

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

“…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. In these regions, seabed sediments are mostly composed of normally consolidated clay, with strength increasing somewhat linearly with depth.…”

“…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.…”