Loads on Driven Monopile Foundations due to Pore Pressures

Abstract

The preferred foundation type for offshore wind is still monopile foundations due to the relative ease of design, manufacturing, and installation. The wave loads (between seabed and wave crest) on present-day monopile foundations are inertia-dominated with Keulegan–Carpenter numbers rarely larger than 6–10. The design of foundations include external environmental factors such as aerodynamic loads on tower and blades, Morison-type loads due to waves and current, and slamming loads from breaking waves; however, the literature, international standards, and guidelines have so far ignored the loads on the monopile due to pore pressure in rigid or elastic soil skeletons. The present work will show that the loads due to pore pressure are intimately linked to the inertia wave loads in the water column (above the seabed). The loads will be quantified with a simple engineering approach including diffraction effects from regular and irregular waves, where the soil skeletons are considered as (i) rigid with incompressible pore water and (ii) elastic with partially saturated, compressible pore water based on the quasi-static formulation of the Biot equations. The importance of this new force contribution to the total hydrodynamic force is discussed as well as its importance for design of monopile foundations.