Applied Numerical Modeling in Geomechanics – 2020 – Billaux, Hazzard, Nelson & Schöpfer (eds.) Paper: 16-01 ©2020 Itasca International, Inc., Minneapolis, ISBN 978-0-9767577-5-7 1 INTRODUCTION In the offshore renewable industry, sources of natural energy are found in wind, waves and submarine currents which are generated by the tidal sea activity. This paper focuses on the numerical analyses that were carried out for the study of a very particular foundation, which will be adopted to hold a tidal turbine on hard rocky seabed. To place the study in the right context, it is important to keep in mind that it is necessary to have relatively steady water currents and sufficient energies (i.e. speed) to allow the harvesting of energy and achieve sufficient profitability of the machines. In such an environment, the continuous pres- ence of currents makes the sandy and clay seabed uncommon; on the contrary, rocky seabed are the most common case. For the foundation design, this poses several restrictions, mostly related to the fact that a) the drilling to carry out a geotechnical cored borehole is extremely difficult and impracticable; b) the drill- ing of rock to install any piles or tendons is not convenient, considering the water currents and the metocean harsh conditions. In this context, today the industry is focusing on the use of gravity foundations where self-weight of the foundation structure is increased by means of ballast. The reduction of the latter and the need to withstand significant horizontal loads to hold the turbine in place both require to increase the grip of the foundation on the rock. In general, the friction coefficient of the steel/rock contact is about 0.7-0.8, which in most cases and for the large offshore tidal turbines is not enough to keep the required ballast weight within acceptable, due consideration of the maximum lift capacity of the vessels available for the project. Once again, the increase of the global friction and grip on the seabed with a minimized ballast will benefit the overall design. 2 DESIGN AND ANALYSIS Since the early thoughts about the best suited foundation type to keep the tidal turbines safely in place, the idea of a statically determined, flat triangular structure was developed (Fig. 1a). At the three vertices of the triangle, the structure is lain on the hard seabed by means of so called “pins”, i.e. a pointy base formed by three plates crossed at the center (Fig. 1b), with one of the plates protruding downwards to form the first contact point with the rock surface. At first sight, such a foundation system should ensure the grip and stability of the entire system. However, if the vertical component of the force under the pin is not high enough to damage the rock and penetrate the seabed, the global friction of the system will not change. On the other hand, if the vertical force is strong enough to fracture the rock, the global friction will be reduced to that of granular soil, knowing that the horizontal load itself will contribute to further fragmentation. A numerical study of a pin foundation on hard, rocky seabed Emilio Nicolini 1 , Fabian Dedecker 2 & Raphaël Coquet 3 1 Cathie Associates S.A., Nanterre, France 2 Itasca Consultants S.A.S., Lyon, France 3 HydroQuest, France