Performance of a Novel Dynamically Installed Fish Anchor in Calcareous Silt

Abstract

Diving upon pullout of an installed anchor leads to an increase in embedment depth and, hence, capacity under operational loadings. It is particularly critical for dynamically installed anchors (DIAs) in calcareous sediments where the embedment depth is generally relatively shallow. This paper proposes a novel fish DIA featuring an elliptically shaped shaft, which reduces hydrodynamic drag resistance. Every cross section of the anchor shaft is elliptical, and the widest part is in the middle. The shaft is shaped to be thicker near the head to lower the mass centroid and increase its diving potential. The padeye is fitted perpendicularly to the wider part of the shaft, so that the maximum resistance area can be mobilized during pullout loading. The dynamic installation and monotonic pullout performance of the fish DIA in calcareous silt were assessed through a series of beam centrifuge tests at 133.3 times Earth’s gravity, varying the drop height (hence the impact velocity), reconsolidation period after installation and load inclination angle at the mudline. The anchor tip embedment depths of 1.02–1.42 times the anchor length lay in the range for OMNI-Max and torpedo DIAs in calcareous silt. A dimensionless embedment depth–based model, total energy–based models, and a conventional shear resistance model were found to be adequate for the prediction of embedment depths. The tracked anchor trajectory confirmed that the fish DIA dove under inclined mudline loading (θ0<45°), as opposed to being pulled out for torpedo and OMNI-Max DIAs. The normalized vertical and horizontal holding capacities of the fish DIA were respectively 4.0 and 5.6 times those of a four-fin torpedo DIA.