Research on penetration depth calculation of anchors dropped into sand and gravel backfill layers for submarine pipelines

Objective Damages caused by dropping ship anchors have become a major factor contributing to third-party damages to submarine pipelines, indirectly affecting the operational safety of underwater structures. Therefore, determination of the penetration depth of dropped anchors into backfill layers using calculation models developed for anchor penetration into various backfill layers is of great significance.

Methods The influence of backfilling with coarse sand and gravel on penetration depths, focusing on Hall anchors as a case study, was studied using finite element analysis, experimental verification, and empirical formula methods. First, the DNV empirical formulas and numerical simulation methods were applied to calculate the penetration depths of Hall anchors with varying masses into coarse sand and gravel backfill layers. Next, a series of down-scaled physical model experiments for anchor dropping were conducted under the same conditions as those used for analyses with the DNV empirical formulas and numerical simulations based on a full-scale model. The results from the physical model experiments, numerical simulations, and calculations based on the DNV empirical formula were compared and analyzed.

Results Under coarse sand conditions, the penetration depth results generated by the three methods were in good agreement. In gravel conditions, the results from numerical simulations and physical model experiments were also in good agreement; however, there was a 45％ deviation between the DNV empirical formula and physical model experimental results. Since the DNV empirical formula is primarily based on circular tube dropping experiments, an empirical formula for equivalent front-end contact areas of anchors penetrating into gravel backfill layers was proposed. This formula, incorporating a modified front area factor, can more accurately predict the penetration depth of Hall anchors in gravel, with its calculation results closely aligning to those obtained from physical model experiments. Additionally, regression equations for maximum penetration depths of Hall anchors with different masses in coarse sand and gravel backfill layers were developed through fitting, based on the above calculation results.

Conclusion The research results provide a basis for the design and safety assessment of protection structures for submarine pipelines.