Pin Foundation on Hard Seabed-Paper

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The paper investigates a pin foundation for tidal-energy devices installed on hard, rocky seabeds. Strong and steady currents often occur where seabed conditions make conventional foundation installation difficult. The proposed system uses a gravity-based structure supported at three points; pointed steel pins use the structure’s vertical weight to resist horizontal current loads, while allowing relatively straightforward installation and decommissioning.<br /><br />The pin’s resistance is complex. High stresses beneath its tip can crush rock locally, while lower stresses farther away mobilize friction along joints. The authors therefore examine whether horizontal loading can cause the pin to penetrate the seabed and develop greater resistance than simple steel–rock friction.<br /><br />They modelled a fractured rock mass and pin using PFC3D, a particle-based numerical method. The rock mass was calibrated to represent good-quality rock, with three joint sets. Simulations applied a constant vertical load and moved the pin horizontally. At sufficiently high vertical load, the pin progressively penetrated the rock, mobilizing new material and producing horizontal-resistance peaks above the friction expected for a pin merely sliding over a flat surface. A load-controlled check found the foundation remained stable below a peak resistance. When the vertical load was reduced fivefold, the pin slid across the seabed without penetrating, mobilizing approximately the assumed steel–rock friction coefficient of 0.3.<br /><br />The results suggest that pin foundations can achieve higher equivalent friction than flat foundations on rock, but this depends nonlinearly on vertical load. The findings are preliminary: brittle failure and the imposed-velocity procedure make some numerical force fluctuations difficult to interpret. Further work should assess reversed loading and validate the model with full-scale or physical model tests.

Keywords

pin foundation

tidal-energy devices

rocky seabed

gravity-based structure

steel pins

horizontal current loads

seabed penetration

PFC3D simulation

steel-rock friction

fractured rock mass