Mechanical response of dropping a Hall anchor and its penetration depth in riverbed silt

Abstract

To ensure the safety of an underwater pipeline embedded within riverbeds, it is essential to evaluate its burial depth and penetration depth during the emergency anchoring of a ship. In this research, the mechanical behaviour of the entire anchoring process with considering the penetration depth of the Hall anchor in the riverbed silt, was holistically examined using experimental measurements, theoretical calculations and computational analysis. Mock-up Hall anchors (5 and 7.5 kg) were dropped at five different heights beforehand, and their transient velocities during the underwater motion were measured using a 2D image sonar. When the water is deep enough, the velocity of Hall anchor when it touches the ground is predominantly determined by its weight with regardless of the height of the drop. Moreover, a theoretical analysis was employed to investigate the behaviours of uniform motion at the state of equilibrium and infinite depths of water, and the exact coefficient of fluid resistance of the Hall anchor was presented. Computational analysis based on the formulation of the arbitrary Lagrangian–Eulerian method was proposed to examine the anchor in the riverbed silt and predict its penetration depth with considering the influence of boundary restrictions. In addition, the riverbed silt with a high moisture content was considered as a material with fluid features, which will be examined for fluid–solid coupling analysis with a Hall anchor by means of a constraint algorithm with a penalty function. The predicted value of the maximal penetration depth is reasonable and reliable compared with the empirical formula, which is crucial to evaluate the burial depth of underwater pipelines. A research approach for the penetration depth of a Hall anchor in riverbed silt was proposed and demonstrated by considering the entire anchor process through a combination of experimental, theoretical, and computational analyses.